Power How can I create a BOM (Bill of Materials) for a regulator, PLL, or amplifier application? Using the wireless WEBENCH online design tools, you can create a regulator, PLL, or amplifier application. The design process includes developing a bill of materials (BOM) and initial evaluation of circuit performance through simulation. Why does the LM4040 reverse voltage drift after temperature cycling? This is due to the annealing process used to purify the silicon. As a result, customers will notice a shift due to temperature cycling. The shift should still be within the initial tolerance specified for the reference, as long as the test conditions are similar to those given in the data shee Relevant Part: LM4040 How do I determine the value for a shunt reference''s series resistor? For a shunt reference to provide a stable voltage, a certain amount of current MUST be flowing THROUGH the reference. This current is determined by a resistor between the power source and the reference input. For example, for the LM385, there should be 50uA to 1mA of current flowing through the LM385. 100uA is the current at which all the specifications are made, and is a good operating current for the reference. To figure the value of the resistor Rs, you must also make allowances for any current shunted away from the reference by the circuit connected to the reference output. The equation is:    Rs = (Vsupply - Vreference) / (Iload + Ireference). For a 1.2V reference with a 5V supply, and no significant load, Rs = (5V - 1.24V) / 100uA = 37.6kohm. The actual value of the resistor is not very critical. 10k to 39k would work fine. To determine load current, be sure to check the "Reference Current" (Iref) spec, or calculate it from the "reference input resistance" spec, of your A/D or D/A converters! Rs values of a few kilohms are not uncommon for use on converter inputs, and should be buffered with an op-amp if possible to provide a low impedance source. Relevant Part: REFERENCE;LM385 What does "ppm" stand for? How can I convert the units to the ones that we are familiar with? PPM is "parts per million", or 1/1,000,000, or 0.000001. It''s also equivalent to 0.0001 percent. 1ppm is equivalent to 1uV out of 1V Relevant Part: REFERENCE Why use driver ICs for white LEDs? While the simplest LED drive circuit is a voltage and a current-limiting resistor, performance can be improved with specifically designed LED ICs. LED issues are solved by using a good controller IC: constant current LED source needs regulation multiple LED''s might have different forward voltage drop (causing different brightness) battery voltage droops lower than needed for LED poor regulation leads to early device failure For the LP2995 DDR Termination Regulator, is a 100uF capacitor required on the output when using a single DDR SRAM? Yes, the 100 uF output capacitor is a minimum value. Additional output capacitance may be required for improved transient response. But the 100 uF is a minimum that must be maintained to insure the stability of the LP2995. For more information, see the "Output Capacitor" discussion in the Component Selection portion of the LP2995 datasheet. Relevant Part: LP2995 What is the thermal resistance of the LM2825N? Thermal resistance junction-to-ambient (ThetaJA) depends on the PCB copper area thermally connected to the LM2825, and can be improved by airflow. With no airflow, ThetaJA is 32C/W for 1 square inch of 1-ounce copper, decreasing to 29C/W for 6 square inches of copper. Adding 500lfm of airflow reduces ThetaJA to 24C/W (for 1 square inch of copper) or 22C/W (for 6 square inches of copper) Relevant Part: LM2825 What is the translation between PCB copper thickness in ounces, inches, and mm? National datasheets often describe PCB copper thickness in terms of ounces. This can be translated as follows:    1 ounce copper = 0.0014 inches = 0.0356mm    2 ounce copper = 0.0028 inches = 0.0711mm. What is Current-Limit Sense Voltage? Current-Limit Sense Voltage is the voltage across the current-limit terminals required to cause the regulator to current-limit with a short circuited output. This voltage is used to determine the value of the external current-limit resistor, when current limit is not controlled internal to the device (such as in the case of many regulator controllers). Relevant Part: LM105;LM205;LM305;LM376;LM723;LM2524D;LM3524D;LM1578A;LM2578A;LM3578A What is Feedback Sense Voltage? Feedback Sense Voltage is the voltage, referred to ground, on the feedback terminal of the regulator while it is operating in regulation Relevant Part: LM105;LM305 What is a Quasi-LDO? A Quasi-LDO is a type of linear regulator. A linear regulator uses a transistor or FET, operating in its linear region, to subtract excess voltage from the applied input voltage, producing a regulated output voltage. Dropout voltage is the minimum input to output voltage differential required for the regulator to sustain an output voltage within 100mV of its nominal value. Quasi-LDO (Low Drop-Out) regulators often use an NPN power transistor as the pass device, driven by a PNP. The dropout voltage for a Quasi-LDO is Vsat (PNP) + Vbe (NPN), typically around 1V total. This value is between the LDO typical dropout of 200mV and the traditional linear regulator dropout of about 2V. National''s QLDO products include the LM3480 and LM3490, for load currents of up to 0.1A, and the LM1117, LM1084, LM1085, and LM1086 for load currents of 0.8A - 5A. The LM3480 and LM3490 are supported by National''s Power WEBENCH online design environment. Relevant Part: LM1084;LM1085;LM1086;LM3480;LM3490;LM1117 Can the LM1084 generate 2.5V output at 3A load from a 3.3V input source? The LM1084 minimum dropout voltage at a 3A load is 0.95V; therefore it is not recommended for this application, which requires a dropout of less than (3.3V-2.5V)=0.8V at the full 3A load current. Instead, we recommend using the LM3478 switching regulator in the SEPIC topology. An example of a SEPIC topology circuit is shown in Figure 12 in the LM3478 datasheet. WEBENCH Power Designer supports the SEPIC application for LM3478 and other devices; enter your design requirements to obtain an appropriate solution. Relevant Part: LM1084;LM3478 Does the LM1117''s 10mA maximum supply current specification apply over the full load? The supply current for this device does not change with load. 10mA is the maximum supply current, over temperature, for any load within the operating limits Relevant Part: LM1117 The LM117 linear regulator in the WG package has separate Output and Output Sense pins. Are those two pins equivalent? These two pins must be tied together to provide the full functionality of the device . The Output pin (pin 12) is the emitter of the pass transistor, and the Output Sense pin (pin 13) is a Kelvin-sensing internal feedback node Relevant Part: LM117WG/883 The LM117 die has four connections, Vin, Vout, ADJ, and sense. What do you do with sense connection? The sense pad is the Vout sense pad and it (as well as the back of the die) gets connected to Vout. It is bonded out separately so that changes in load current will affect only the drop in bond wires to the power device and the sense line will maintain the voltage at the point where it is connected constant. Two things can happen if sense connection is left floating: 1. The most likely occurance is that there is a connection through the substrate from Vout sense to Vout. In this case any drops in the Vout bondwire will not be compensated for. This could result in Load Regulation values of over 20mV depending on size of bond wire and load. 2. Vout sense is isolated from Vout. In this case we would not expect the part to regulate at all. It could oscillate, go to a rail or sit at some arbitrary voltage depending on the die parasitics. Relevant Part: LM117 For a LM120 die, what voltage potential should the back side of the die be connected to? For the LM120, the backside bias should be Vin (the most negative potential on the device) Relevant Part: LM120 How do Buck-Boost (Inverting) Switching Converters work? See the Switching Regulator Overview for some helpful material which covers various types of switching regulator theories of operation. The Buck-Boost or Inverting regulator topic may be found on Page 37 of the document. Additional information may also be found in AN-1157, "Positive to Negative Buck-Boost Converter Using LM267X SIMPLE SWITCHER Regulators." Relevant Part: LM2574;LM2575;LM2576;LM2590;LM2591;LM2592;LM2593;LM2594;LM2595;LM2596;LM2597;LM2598;LM2599;LM2670;LM2671;LM2672;LM2673;LM2674;LM2675;LM2676;LM2677;LM2678;LM2679 Do you need a protection diode between output and input of the LM257x step-down regulator? The concern here is reversing biasing the switch. There is, like with all FETs and BJTs in an IC, a body diode, or reverse breakdown associated with the switch. If you indeed have any instance where the output voltage is higher than the input voltage with these regulators then damage to the switch can occur. If Vout-Vin is less than 7V there shouldn''t be a problem since the NPN BJT in this IC has a reverse breakdown of about 7V. But if so, then the only way to safeguard against it for sure is to put a protection diode in series with the input. This of course is only feasible if the extra 0.5V drop across it will not drop the input to the IC too low. This is not a problem limited to National Semiconductor switching regulators. All switchers trying to maximize efficiency using NPN switches will need this protection under those circumstances. Relevant Part: LM2574;LM2575;LM2576 How do I configure the LM2576HV for shutdown control of a -12V output inverting regulator? Referring to the LM2596(HV) datasheet, Figure 27 shows the shutdown configuration for a -5V inverting regulator. The shutdown transistor pulls the ON/OFF pin below 1.3V (with respect to the ground pin) to turn the regulator ON. Conversely, when the shutdown control transistor turns ON, the voltage drop across R4 is approx. 3.7V (assumung logic "1" input is +5V), giving 1.3V at the ON/OFF pin, thus shutting the regulator OFF. For the -12V output, the value of R1 should be changed to 21.1k to maintain the 3.7V drop across R4. The value of R4 should remain unchanged (6.2k). As for the input diode, D1, it may need to be changed to a higher voltage rating such as the 1N5822 which can tolerate the total voltage applied across the LM2596 (Vin - Vout) Relevant Part: LM2576-12 How can I calculate the peak current rating of the switch in a flyback dc-dc converter? A method of calculating flyback regulator design may be viewed in the online document "Switching Power Supply Design: Discontinuous Mode Flyback Converter". It contains calculation details needed for designing a 3-output flyback converter, including switch peak current. This item is contained in Section (3) Primary Current. The peak current required for a particular flyback design may also be estimated using National''s Power WEBENCH online design environment, which includes calculated performance as well as online electrical simulation. Relevant Part: LM2577;LM2585;LM2586;LM2587;LM2588;LM3478;LM3488 Why does a voltage drop on the input (Vin) sometimes cause the LM2577 boost/flyback regulator to lock up? If the voltage drops to a minimum voltage between 2.0 and 3.5 volts and then rises to its normal operating level, the internal oscillator will shut down and the output voltage will drop to about 0 volts. This condition will persist until the input voltage is dropped to 0 volts and powered back up again. This lock-up problem can be eliminated by adding a 10-100 Ω resistor between the input source and the Vin pin. Relevant Part: LM2577 How can I achieve an isolated power supply with 12 to 15V dc input, 12V dc output @0.6W or 50mA? The LM2585-12 flyback regulator should be able to satisfy your power requirement. There is an application note available, AN-1095, which provides design guidelines about building an isolated power supply using the LM2587 (the same family as the LM2585). Feedback voltage for controlling the duty cycle of the regulator to maintain its output constant is through an opto-isolator. The appnote makes mention of the Switchers Made Simple software which is used to design the circuit (less the feedback loop). This software is also part of WEBENCH, National''s on-line design tool set. Either tool can be used to design your flyback circuit. Relevant Part: LM2585-12 Are evaluation boards available for the SIMPLE SWITCHER converters? Pre-built evaluation boards are available for most of our newer switching regulators, at the web page for Evaluation and Demo Boards for Power Management Products. Although some of the older devices such as LM2595 no longer have pre-built evaluation boards available, most of the SIMPLE SWITCHER converters are supported by "BuildIt." "BuildIt" provides customized evaluation board kits available through National's Power WEBENCH. Relevant Part: LM2585;LM2586;LM2587;LM2588;LM2590;LM2591;LM2592;LM2593;LM2594;LM2595;LM2596;LM2597;LM2598;LM2599;LM2670;LM2671;LM2672;LM2673;LM2674;LM2675;LM2676;LM2677;LM2678;LM2679 What would happen if the input voltage to a boost regulator exceeded the desired output voltage? If the input voltage on a 12V-output boost regulator increased to more than about 12.5V, the output voltage would rise and track the input, less about a diode drop. Better regulation would result from adding a low dropout linear regulator to the output of an adjustable-output boost regulator, which has its output voltage set to 12.5-12.7V. The low-dropout regulator, of an appropriate current rating, can be used to drop the output to 12V, even when the boost regulator input is too high. For example, the LM2940-12 is a low-dropout regulator suitable for regulating a 12V output at up to 1A Relevant Part: LM2585;LM2586;LM2587;LM2588;LM2940 How do I determine the values of the compensation resistor and capacitor needed for the LM2588 comp pin? What is the function of the comp pin? The compensation network consisting of Rc and Cc form a pole-zero pair which stabilizes the regulator. The values of Rc and Cc are mainly dependent on the regulator''s voltage gain, maximum load current, inductor size, and output capacitor size. To determine suitable values of Rc and Cc, you can use the online design tools on power.national.com''s WEBENCH, or use the stand-alone design tool "Switchers Made Simple". Alternately, the "Rc&Cc Equations" file contains equations for calculating the values of Rc, Cc and Cout which will ensure the regulator''s stability. However, this procedure does not necessarily result in Rc and Cc that provide optimum compensation. In order to guarantee optimum compensation, one of the standard procedures for testing loop stability must be used, such as measuring Vout transient response with a pulsing load. Relevant Part: LM2585;LM2586;LM2587;LM2588 What are the effects of changes in the input voltage, Vin, on the compensation pin (Pin 1) of the LM2587 as featured in AN-1095? In AN-1095, "Design of Isolated Converters Using SIMPLE SWITCHERs", the LM3411 always supplies a regulated +5V reference during normal operation, causing the LED to excite the base of the opto-isolator''s NPN transistor. The NPN functions as variable impedance element. Thus, as Vin varies, the voltage at Pin 1 also varies, adjusting the duty cycle higher or lower accordingly to maintain Vout constant. When Vout goes lower to a point where the LED shuts OFF, the C-E junction of the NPN "opens", which in turn makes Vin to appear directly at Pin 1, thus increasing the Duty Cycle to raise Vout. The opto transistor is biased to operate in the linear range. It is part of the control loop. It is confusing to think of it in instantanous terms. Instead, think of it as a feedback system whose sole goal is to keep the output at 5V. If the 5V wavers, the opto-isolator will react and bring Vc at pin 1 up or down accordingly, which in turn conrols the duty cycle to regulate the output voltage, and the system returns to equilibrium. If Vin changes, the same thing will happen, except this time the duty cycle is reacting to a change in Vin. Relevant Part: LM2585;LM2586;LM2587;LM2588 Does the input capacitor of an LM258x-based boost or flyback regulator need to be a polarized type? No, the input capacitor does not need to be polarized. Ceramic is fine. However, if you do use a polarized tantalum capacitor, it does need to be connected with the polarity shown in the application circuits on the LM258x datasheet Relevant Part: LM2586;LM2585;LM2587;LM2588 Using WEBENCH for an LM2586 power supply design, I want to run it at 200kHz instead of the 100kHz default switching frequency. How can I make it automatically recalculate the other component value? National''s Power WEBENCH online design environment can be used to develop a power supply design based on your particular requirements. If the design uses a SIMPLE SWITCHER regulator, for example the LM2586 boost/flyback converter, values and part numbers of the external components will be automatically found, and the circuit performance will be estimated. In addition, the design can be simulated electrically and/or thermally. While many SIMPLE SWITCHER regulators operate at a default, fixed frequency, the operating frequency of the LM2586 is set by the value of the resistor connected at Pin 1 (ON/OFF and Freq. Adj). As shown in Fig. 39 on Page 21 of the LM2586 data sheet, for 200kHz operation, the resistor value to use at Pin 1 should be 22kΩ (at 25°C ambient) . WEBENCH calculates the component values based on the default switching frequency, 100kHz. At this time it does not recalculate the values of the other components, e.g., inductor and output capacitor sizes, which are both affected by operating frequency. Note that in many cases, the ideal values of L and C at 200kHz are very close to those needed at 100kHz, so it may not be necessary to select different components. For boost applications, the value of the inductor is given by:   L = Vin(Vo - Vin) / (2Io x Fosc x Vo), and the output capacitor value is   C > Io(Vo - Vin) / (Fosc x Vo x Vripple), where Vripple is approx. 0.01 x Vout. Relevant Part: LM2586;LM2588 How can a regulator circuit deliver 1.8V at 320mA from a negative 12V/110mA power supply? See "Neg-to-Pos Converter" for an example of a negative to positive converter using any flyback IC or controller. This same "buck-boost" approach can be used for your design. For your low power requirements, you will need only one flyback IC/controller, instead of the two shown in the higher-power example. The only complexity is how the feedback is created...this involves a PNP transistor, as shown in the example. Relevant Part: LM2588;LM2585;LM2586;LM2587;LM2577 Why does a voltage drop on the input (Vin) sometimes cause the LM258x boost/flyback switcher to lock up? If the voltage drops to a minimum voltage between 2.4 and 3.9 volts and then rises to its normal operating level, the internal oscillator will shut down and the output voltage will drop to about 0 volts. This condition will persist until the input voltage is dropped to 0 volts and powered back up again. Adding a 10-100 Ω resistor between the input signal and the Vin pin can eliminate this lock-up problem. Relevant Part: LM2588;LM2585;LM2586;LM2587 How can I use the LM258x type switching regulator in an isolated multiple output design? For single isolated output, please refer to application note AN-1095, "Design of Isolated Converters Using SIMPLE SWITCHERs". For multiple outputs, please refer to AN-777, "LM2577 Three Output, Isolated Flyback Regulator". Relevant Part: LM2588;LM2587;LM2585;LM2586 Can the LM2587/LM2588 handle a 5A load since it is rated at 5A switch current? No. In the Boost topology design, the output load current and the switch current are not equal The maximum available load current is always less than the current rating of the switch transistor. The maximum total power available for conversion in any regulator is equal to the input voltage multiplied by the the maximum average input current (which is less than the current rating of the switch transistor). Since the output voltage of the Boost is higher than the input voltage, it follows that the output current must be lower than the input current. The easiest way to determine the necessary switch current rating (and the appropriate boost/flyback regulator IC) is to use the online design tools of National''s Power Webench, or the offline design tool of "Switchers Made Simple". One way to get around this limitation is to use flyback topology with an external power transistor to boost the load current handling capability of the converter. Relevant Part: LM2588;LM2587 What is the newest version of "Switchers Made Simple" and where can I download it off the web? "Switchers Made Simple" is a switching regulator design software that''s used within the online WEBENCH power supply design environment, and is also available as a downloadable tool. For additional electrical and thermal analysis tools and access to customized evaluation boards, as well as recommendations for newer products, check out WEBENCH online. Switchers Made Simple v6.24, and supports the following Buck, Boost, and Flyback SIMPLE SWITCHER converters:    -- Buck topology: LM259x and LM259xHV, LM267x   -- Boost/Flyback topologies: LM2698, LM258x, LM2577 SMS6.24 functionally combines the calculation tools of the earlier Switchers Made Simple v4.3 and LM267x Made Simple v2.01 with the "Solution Selector" tool originally developed for online use in WEBENCH. Released 12/6/01, Version 6.24 includes all the features and solutions of v6.1, plus LM259xHV and LM2698 designs, reduced download size and an improved install procedure. The software can be downloaded from National''s Switchers Made Simple web page. Relevant Part: LM2590;LM2591;LM2592;LM2593;LM2594;LM2595;LM2596;LM2597;LM2598;LM2599;-;LM2670;LM2671;LM2672;LM2673;LM2674;LM2675;LM2676;LM2677;LM2678;LM2679;-;LM2698;LM2588;LM2585;LM2586;LM2587;LM2577 What are crossover frequency and phase margin, and what is their relationship? Crossover frequency is the bandwidth of the regulator control loop (i.e. the frequency at which the frequency response gain crosses zero dB). It should not be greater than 10-25% of the switching regulator''s switching frequency. Higher crossover frequency causes reduction in gain margin and causes unstable operation. Crossover frequencies below 1kHz for the LM2597 are not recommended, as this indicates a very slow transient response. Phase margin is the frequency response phase at the crossover frequency. For the LM259x regulators, it is usually set around 45 degrees, but can be as low as 25 degrees and still result in a stable frequency response. Relevant Part: LM2590;LM2591;LM2592;LM2593;LM2594;LM2595;LM2596;LM2597;LM2598;LM2599 What is the equation for the LM259x AC losses during switch turn-on and turn-off? AC (switching) losses depend on the transistor switch transition times, and the currents and voltages being switched. Power dissipation while the switch turns on is:   Pdac(on)= ton x (Ipk- Ipp) x f x Vin. Power dissipation while the switch turns off is:   Pdac(off)= toff x Vin x f x Ipk. In these equations, f is the frequency, Ipk is the switch peak current, Ipp is the peak to peak ripple, ton (the switch turn on time) = 20ns, and toff (the switch turn off time) = 30ns. Relevant Part: LM2590;LM2591;LM2592;LM2593;LM2594;LM2595;LM2596;LM2597;LM2598;LM2599 What should be done if the LM2594HV inverting regulator does not start up with relatively large input capacitance? In this specific situation, the LM2594HV SIMPLE SWITCHER switching regulator was being used in an inverting circuit to generate -12V from +28V. The circuit was based on the -5V output circuit (Figure 25) in the LM2594HV datasheet. The regulator would not start up with 3 paralleled 6.8uF/600mOhm capacitors on the input, although with less capacitance, it started OK. Two things needed to be checked: (1) Make sure the delayed startup circuit is being used as recommended - R1, R2, C1 of Figure 25 in the LM2594HV datasheet. This is intended to allow startup from input voltage sources which cannot deliver adequate current. If it is being used, you might have to tinker with the values. If not, you should try it. (2) Make sure the diode is between the source and the input capacitor (see D1 in Figure 25 of the LM2594HV datasheet). This helps the circuit look like a buck regulator to the IC. An inductor in place of the diode would also work - and it reduces your output ripple. Relevant Part: LM2590;LM2591;LM2592;LM2593;LM2594HV;LM2597HV Why does a warning message come up stating "Insufficient Phase Margin" when I am designing a circuit using SMS 6.24? In Switchers Made Simple 6.24 (SMS6.24), the warning message for "Insufficient Phase Margin" occurs when the phase margin is less than 23 degrees. Most often, designs with low phase margin occur when using the LM259x and LM259xHV series of products. To eliminate the stability problems, use a -ADJ part and set the Cff capacitor to about 2.2nF to 10nF. For high output voltage (>12V), select a cheaper output capacitor with higher ESR. You may not find one in the list of choices, so check "Custom" and enter an ESR > 300mohm. If you are using the LM259x products at Vin<=40V, then try the SMS4.3 software (an older software platform that works a little better for these devices at this time) Relevant Part: LM2590HV;LM2591HV;LM2592HV;LM2593HV;LM2594;LM2595;LM2596;LM2597;LM2598;LM2599 When using two LM2599 switching regulators in parallel, one LM2599 turns off completely, while the second one, driving the same load came up but at a higher voltage. Is this normal? This is perfectly normal. The regulator with the lower output voltage will drop to 0% duty cycle in trying to pull down the output voltage of the higher regulator. This is due to voltage feedback control. The only way to parallel two voltage controlled switchers is with ballasting resistors (and possibly or''ing diodes) to allow their outputs to be of different voltages, just as you would with two linear regulators. To set the regulators up to be redundant, we suggest setting the two outputs to different voltages (maybe 500mV different) and feedback for the higher Vout from the actual load and the lower output from the anode of its or''ing diode. This way the lower voltage regulator will be on but not conducting any current until the higher output goes away. When they switch over, the load will see a different Vout but maybe not too much of a shift to matter. The other thing we suggest is using a hold-up cap on the load to maintain the output voltage while the second switcher comes to life when it has to. Relevant Part: LM2594;LM2595;LM2596;LM2597;LM2598;LM2599 What part is recommended to convert Vin=5V to an output of Vout= -22V@10mA? See the LM2611, an inverting switching regulator which uses the Cuk topology. Relevant Part: LM2611 Can two of the LM2621 switching regulators be put in parallel to boost the output current? It is easy to parallel two of the LM2621. All that is needed is to tie the two outputs together. They will most likely not have the exact same bandgap and the resistors used will not be exactly equal, but whichever one ends up with a slightly higher output voltage will pump current more often. When it is at its current limit and the output starts dropping slightly, the other one will start switching to supply the extra current needed. This will cause many strange frequencies on the output. Therefore an LC filter may be a good idea if noise becomes an issue. If paralleling two LM2621s, a current sharing resistor could be used, but is not needed. If connected to the right resistor, it could divide the supply current more equally, but because we could never know which will have a slightly higher bandgap or hysteresis window, adding a resistor to the output may not help and therefore is not required. Consequently, one of the ICs will do most of the work at lower load currents, which is okay. Relevant Part: LM2621 Measuring efficiency using our LM2621 demo board resulted in 10-15% lower than the typical efficiency value claimed in the datasheet. Why? We have found some things on the LM2621 evaluation board that might be of interest. We took some readings on an LM2621 board that was reported to have low efficiency, and we we still got the high efficiency numbers that we had measured on other LM2621 boards. We tried using other methods of measuring the current, another multimeter and a current probe, and got different results. We found out that the first DMM we used was not averaging correctly, and because of the large input ripple on the LM2621, we suspected that the others might not be as well. So to eliminate averaging problems, we used a large LC filter between the lab supply and the board and measured the voltage directly at the board and the current into the LC filter. This gave a much smoother current waveform and all of the meters and probes agreed. With this we got efficiency measurements very close to the datasheet. We believe two things are happening to get strange readings. First, the input current waveform is tough for DMM''s and scopes to get an accurate RMS measurement, so different equipment often gives quite different results. Second, we don''t believe the efficiency measurements for the datasheet were taken with the input capacitor used on the eval. board. We believe there is an additional efficiency hit due to the small input cap with high ESR. By just adding another 33uF, low ESR cap to the input we saw a significant improvement in the efficiency. This is reducing the input current ripple which, in turn, reduces the power dissipated by the capacitor ESR which is also lower. We believe the efficiency will be very close to the datasheet specs when this part is located next to a low ESR battery or has better input capacitance with lower ESR. For testing purposes it may be worth while to use a large LC filter (C across the input C and L between the lab supply and input C) to simulate a low ESR source or better input capacitance. This will give a better idea of the LM2621 efficiency without the loss and averaging problems due to the undersized cap. Relevant Part: LM2621 How can the output ripple of an LM2621 boost converter be reduced? First confirm that the circuit components were selected using instructions in the datasheet, and the PCB layout follows the datasheet guidelines. If the output ripple voltage is too large, for example 500mV on a 5V output, there are a few more things to check: (1) Check the circuit grounding and lead lengths: If the objectionable ripple occurs at the switching frequency (measure 200kHz per the datasheet typical application circuit), then make sure that the negative terminals of C1, C2, Icpin1 and the output return terminal are located closely together. Output should be taken from the terminals of C2. Don''t use more than one measurement grounding point. (2) Add "bead" to reduce diode ringing: Some high frequency noise (10 to 25MHz) may be generated in the diode when it is switched off. If the objectionable noise occurs synchronous to turn-off in the switching cycle, a small bead on the diode lead may reduce it. (3) Check timing capacitor, CF1 connections: If the objectionable ripple occurs at a subharmonic of the switching frequency, or at the hysteretic period (light load), then check that the timing capacitor and 0.1 VDD decoupler are connected directly to the signal return pin. Recheck the size of CF1. Relevant Part: LM2621 What would happen to an LM2621-based boost converter if the input goes above the programmed output, then returns to normal range? The LM2621 switches safely and smoothly between standby and normal operation when the input voltage rises above the output, then recovers. The shutdown feature will be quite smooth as well, regardless of the input voltage. When the input is above the output, the switch will stop working and the only current draw will basically be the Vdd and the comparator monitoring the output, probably about 80uA. When the input falls below the output, the LM2621 will start switching with its 70% fixed duty cycle. It is a hysteretic converter which helps make the transition smooth, but it also means that the switch will most likely not be switching all of the time with the high input voltage and low load currents. This improves efficiency quite a bit Relevant Part: LM2621 How does the LM2621 feedback pin leakage current affect the output voltage setting? The feedback leakage current is typically less than 1 µA. Therefore, it will not significantly affect the output voltage setting, unless RF2 resistor is in the megohm range. Relevant Part: LM2621 Can two LM2621 boost circuits use the same frequency-setting resistor so that their switching frequencies are synchronized? Using the same resistor (tied to the FREQ pins of two LM2621s) to set matching or synchronized frequencies will not work. Both ICs will have a slightly different tolerance on the oscillator so using the exact same resistor may not produce the same results. If you could guarantee the same oscillator frequency for a given resistor, you would still have differences since the the LM2621 runs hysteretically at lower load currents. You could never guarantee that they would have a switching burst at the same time or for the same duration. So the lower frequency content will be at a different frequency and phase for each depending on many lossy factors Relevant Part: LM2621 How can a 3.3V source be changed to a low-noise 5V/300mA output? The LM2622 boost converter is ideal. Use the typical boost circuit in Figure 5 of the datasheet. Then add a post filter to reduce ripple to 1mV. The post filter inductor can be a small 1uH 400mA part, then use the appropriate output capacitor. Relevant Part: LM2622 Is it better to use the LM2630/LM2631 or the LM2636 in a regulator to deliver a 2.5V/8A load from a 5V source? The LM2636 switching regulator controller is the better choice. The LM2636 uses voltage-mode current sensing in its current limit scheme. This technique is more tolerant of noise associated with high-current switching (especially at start-up) than the current-mode sensing scheme used in the LM2631/LM2631, which requires the use of a small (< 10 mΩ ) sense resistor in the same application. The LM2636-based regulator is then able to handle higher load currents without difficulty, such as 8-10A. Relevant Part: LM2630;LM2631;LM2636 What happens when the LM2630 Sync pin goes high, but a negative pulse on the Sync pin doesn''t arrive soon enough? The LM2630 SYNC pin is falling edge triggered and not level triggered. It will run from the internal oscillator if the SYNC pin is tied high or low. Note that the SYNC pin must not be pulled to more than 10V; otherwise the LM2630 will be damaged Relevant Part: LM2630 Why is the LM2630 output voltage limited to 6V max, even though Vin max is 30V? The LM2630 switching regulator controller was designed for use in microprocessor-based systems, to provide supply voltages of 1.8 to 5V. The current sense amplifier has an Absolute Maximum input range of 7V, which limits the output voltage to about 6V. In addition, the regulator starts up at Vin=4.5V. If the output voltage does not reach steady-state fast enough, the device will "think" it is in an undervoltage condition and activate undervoltage protection (UPV), turning the regulator off. The higher the output voltage, the worse this gets. If you need a buck regulator controller with a higher output voltage range, consider the LM3477. Relevant Part: LM2630 How the output of an LM2637-based switching regulator be set to less than 1.3V? The output voltage of a LM2637-based regulator cannot be brought down lower than 1.3V. This is internally limited by the DAC input low voltage which provides duty cycle control of the regulator, which in turn, sets the output voltage. Therefore, you cannot set the output voltage externally. There is a similar limitation on regulators using the LM2638 and LM2645. For motherboard-type power supply designs requiring output voltage of less than 1.3V, consider the LM2633, with a programmable output voltage of 0.95V to 2.0V. Two new products which allow output voltages down to 0.6V are the LM2647 and LM2727/LM2737. Relevant Part: LM2637;LM2638;LM2645;LM2647;LM2737;LM2727 When an external 8MHz clock is supplied to the LM2639, what is the allowable range of the clock duty cycle? The maximum duty cycle for the LM2639 multiphase controller is around 78%, regardless of the source of the clock Relevant Part: LM2639 The output capacitor of the LM2639 buck regulator circuit is labelled as "22u 10v 1210 Y5V X12". What does this mean, and who makes it? "22u 10V" is the capacitance and voltage rating of the capacitor. "1210" is the physical size of the capacitor in millimeters. "Y5V" indicates the capacitor is of the standard Y5V series. For this application, the more expensive X5R series could be used for more consistent performance over temperature. "X12" means 12 capacitors are paralleled together. The vendor of this particular output capacitor is Taiyouden. Relevant Part: LM2639 Why do the LM2640 performance graphs show results at -40C yet the minimum temperature rating for the device is 0C? The performance parameters as shown in the Electrical Characteristics table are only guaranteed from 0C to +125C. The typical curves show what a typical part did at -40C. The typical curves performance is not guaranteed, and does not indicate the device will operate consistently and reliably at temperatures below 0C Relevant Part: LM2640 When setting the output voltage for the regulator, what voltages should appear at FB1 and FB2? When the system is in regulation, the feedback pins FB1 and FB2 will be at the bandgap reference voltage (1.238V typical). This may not be obvious from the datasheet, since only the bandgap reference voltage is specified. But if you look at the block diagram, it shows that one input of the gm error amplifier is the FB pin (for output feedback), and the other is the bandgap reference voltage BG; the error amplifier''s role is to make sure its inputs are at the same voltage. Relevant Part: LM2641;LM2640 How is the input capacitance determined for an LM2641-based regulator? The response time of the regulator will be the same regardless of the input capacitance value. It does not care if the input voltage dips, as long as it does not drop below the minimum input voltage required for operation (or regulation if the output is higher than 5V or so). It will supply the instantaneous current if there is a fast load increase, but only until the source catches up. So the input capacitance depends a lot on the source. If using a battery with high ESR, another regulator with slow response time, or some other source with high source resistance, you will need a larger capacitor if a large voltage dip cannot be tolerated. If the source is very low ESR or has fast response, the input capacitor value can be reduced. For an approximation, start by making sure the RMS current rating of the capacitor is at least 75% of the max load current (50% if the ambient will not exceed 40 degC). This is only to help in sizing, this will often give a minimum value if cross referenced with the required voltage rating. Then assume at an instantaneous load increase that the capacitor will initially supply all the current. You will have to estimate the response time of the source to supply the input current needed, that is the time when the voltage will stop dipping. Then just use i=Cdv/dt to size the cap for a certain voltage drop. An easier way if it''s an option is simply to use a large value. The larger the input cap is the better. In most cases with this part, 22uF is ok for a 4A output. In just about any case you should be fine using 47uF to 100uF. Relevant Part: LM2641 How is the LM2653/LM2655 enabled with the shutdown pin if a soft-start capacitor is not being used? Ground the shutdown pin to disable, and open the shutdown pin to enable Relevant Part: LM2653;LM2655 Is the softstart capacitor from the LM2655 shutdown/softstart pin to ground required? The Shutdown/Softstart pin has two functions, i.e., shutdown command and softstart. The softstart capacitor is highly recommended for proper operation of the regulator (it limits the peak current drawn during startup). Omitting the softstart capacitor may result in a regulator that will not start up. The Shutdown/Softstart pin can still be used to shut down the regulator with active TTL Low Relevant Part: LM2655 How can the LM2662/LM2663 be used as a negative voltage splitter? Referring to the LM2662/LM2663 datasheet, use the voltage splitter circuit on the front page with the following modifications: 1. Connect pin 1 (SD) to pins 5 and 6 (OUT & LV); 2. Connect the ground to pin 8 (V+); 3. Connect the negative voltage to pin 5 (and all other points showing a ground on the schematic). You will now get a negative voltage at pin 3 (GND) of the IC that is 1/2 of the negative voltage applied to pin 5. Relevant Part: LM2662;LM2663 Can two LM2662s be paralleled to get greater output ? Yes, they can be hooked up in parallel to multiply the load current by as many devices connected in parallel, i.e., 400mA for 2 LM2662 Relevant Part: LM2662 What would happen if the LM2662 FC pin is grounded rather than left floating (as recommended for 20kHz operation)? There is an internal pull down resistor on the FC (frequency controller) pin. Grounding the FC pin is the same as leaving it open, and will set the operation to 20kHz Relevant Part: LM2662 What is the source of the output impedance equation for inverting switched-capacitor converters? The equation in the LM2664 and similar data sheets is specific to all switched-capacitor inverters with a four switch topology, which is basically all of them. The equation was derived from the capacitor characteristics and the average switch resistance assuming around 50% duty cycle, which is how all of the inverters now operate. All the inverter data sheets should have this equation somewhere but some people write it in a slightly different form, which can be confusing. Assuming all four switches are identical, the output impedance of an inverting switched capacitor converter is:    Rout = approx. 2 x Rsw + 2/(fosc x C1) + 4 x ESRc1 + 4 x ESRc2, where Rsw is the sum of the four switch on-resistances, fosc is the switching frequency, C1 is the capacitance of the central capacitor, and ESRc1 and ESRc2 are the series resistances of C1 and C2. Relevant Part: LM2664;LM2660;LM2661;LMC7660;LM2663;LM2662;LM828 Can I use 4.7uF instead of the 3.3uF capacitor value recomended for the LM2664? You can use nearly any capacitor value you would like. The Application Information section in the data sheet contains equations that show that using higher value capacitors is better, and lower ESR is better as well, because this produces lower output ripple. The only penalty to using larger capacitors is the associated greater size and cost Relevant Part: LM2664 In the voltage doubler application, how do you force the output to go to zero during shutdown? Normally, when the application is shut down using the SD pin, the output voltage drops to the input voltage less a diode drop. Even if the external diode is removed, the internal parasitic diodes will still keep the output at Vin when the part is shut down. The way to get around this problem of output voltage during shutdown is to use an external switch (FET). Place the FET switch either before the input capacitor, or after the output capacitor. This allows for completely cutting off the input voltage or the load and allows the output at the load to fall to 0V. The Rdson of modern FETs is small enough that it should not present a significant voltage drop Relevant Part: LM2665;LM2661;LM2663;LM2685;LM2765;LM2766;LM2792 In the LM2665 voltage doubler application, is the external diode required? The LM2665 will work OK without the external diode in this application, but long-term reliability may degrade. Without an external diode, the circuit will use the parasitic diodes internal to the IC. The input voltage needs to be higher for startup, but a 5V input is more than plenty to start the IC even after the parasitic diode drops. The next problem is that you will be stressing the IC and possibly lowering the lifetime or reliability. The parasitic diodes will only conduct during startup, but these brief peak currents at startup could possibly cause damage to the device (while this is rare, it must be mentioned) Relevant Part: LM2665 What is the penalty for using an inductor with lower value than is recommended? While lower-value inductors can be used with some success, there are three types of problems that may be encountered: Higher inductor ripple current, so that the current limit is engaged at a lower-than-expected load current Higher ripple voltage on the output, which may be seen as excessive noise by the load; The possibility of pushing the crossover frequency above 40kHz, where the switching regulator becomes unstable. Inductor values recommended in the LM267X datasheets and in the associated design software (Switchers Made Simple, WEBENCH) are based on controlling the inductor ripple current, output ripple voltage, and frequency response to avoid these problems. Relevant Part: LM2670;LM2671;LM2672;LM2673;LM2674;LM2675;LM2676;LM2677;LM2678;LM2679 How does the LM267X current limit protection work? The LM267X switching regulator family has a foldback type of current limit protection. When there is an overcurrent fault condition, the output voltage sags as the output current is self limited. The current limiting protection circuitry is designed to hold the device in this fault mode until the output load current is "folded" back to a low enough level (load hysteresis) that the protection circuit is reset and the device is allowed to resume normal operation. See additional information about the device operation in current limit in the product datasheet, for example as shown in the LM2676 datasheet application information. Relevant Part: LM2670;LM2671;LM2672;LM2673;LM2674;LM2675;LM2676;LM2677;LM2678;LM2679 What happens if the output and input pins of the LM267X switching regulator are shorted? The LM267X device should have no problem with this. The main area of concern would be the external passive components. If the output capacitor does not have a high enough voltage rating then it could be destroyed. If the short is to the switch output and not to the output capacitor cathode, then the capacitor is still in danger and the inductor could overheat. In either case the output voltage will go up and the IC will simply stop switching. There should be no reason to be concerned about the LM267X. If the condition is removed and the passives have survived, then the chip should just resume normal operation Relevant Part: LM2670;LM2671;LM2672;LM2673;LM2674;LM2675;LM2676;LM2677;LM2678;LM2679 When using WEBENCH to design an LM267x-based power supply, why does the inductor value change with max. load current? The Webench Circuit Calculator used for LM267x-based regulators calculates the inductor value based on maximum load current, allowing for greater ripple current (less inductance) as load current decreases. If you want to test a given circuit at various load currents, design the circuit for the expect maximum load current. Then evaluate performance using either the "Operating Values" tab or "Analyze a Design", selecting Vin and Iout as desired Relevant Part: LM2670;LM2671;LM2672;LM2673;LM2674;LM2675;LM2676;LM2677;LM2678;LM2679 How do I minimize or eliminate noise on the LM267X switch pin? The main cause of noise on the Switch pin wave forms is usually the board layout (typically, long thin traces). However, here are few guidelines for reducing noise assuming cleaning up the layout does not help:   (A) Put in the following snubber: 4.7 nF (0.0047µF) in series with a resistor of 100 Ω . Put this very close to the IC between output pin (Pin 1) and gnd pin (Pin 4). Try also moving it directly between anode and cathode of the catch diode. Keep very short lead lengths on the snubber for it to work well. Try putting it on the solder side of the board since you can usually get space there to keep small lead lengths. Don''t use a wirewound resistor, as its inductance will make the problem worse.   (B) Check the switch pin waveform using an oscilloscope. If the waveform shows a switching frequency of 260kHz with no noise, all is fine. However, if it shows a jittery 260kHz, go to the next stage.   (C) Put a 0.1 µF cap very close to the Input Pin and Gnd pin (with short leads again). This will ensure proper decoupling in cases of poor layout.   (D) The ON/OFF pin (Pin 7) may be picking up noise if left hanging. To confirm this problem, place a small cap from this pin to gnd. You may also wish to bias this pin high through a 1-10K resistor by connecting to Vin. However if you try this, we recommend you put in a 5.1V zener from Pin 7 to Pin 4 to protect this pin.   (E) You can try to reduce the noise by slowing the switching transitions slightly. Try a 10-22 Ω resistor in series with the boostrap capacitor. Relevant Part: LM2670;LM2671;LM2672;LM2673;LM2674;LM2675;LM2676;LM2677;LM2678;LM2679 The LM267x datasheet recommends the voltage rating for the output capacitor to be 1.3 times the maximum output voltage of the power supply. Exactly which voltage is this? The recommended voltage rating equation for the output capacitor refers to the regulated switching converter circuit output voltage. If the regulator output voltage is programmed for 10V, the voltage rating for the capacitor should be 13V (1.3 x 10V) or higher Relevant Part: LM2670;LM2671;LM2672;LM2673;LM2674;LM2675;LM2676;LM2677;LM2678;LM2679 How should the drive to the LM267X ON/OFF pin be configured? The ON/OFF pin (pin5) of the LM2675 is a normally ON switch (or active OFF). Internally to the voltage regulator, a Logic HI is provided to the regulator`s active circuits via a 7V zener which is powered through Vin (pin7). The minimum threshold voltage required to switch the regulator ON is 1.4V. The ON/OFF pin may be left open (no connection) to activate the regulator upon application of input voltage within the specified range of 6.5V to 40V. Switching this pin to GND (Logic LO) turns the regulator OFF Relevant Part: LM2670;LM2671;LM2672;LM2674;LM2675;LM2676;LM2677;LM2678 Can the LM267x ON/OFF pin be left floating? It is okay to leave the ON/OFF pin open, because there is an internal current source that forces the voltage on this pin to well above the 2.0V level required to turn it on. It may also be a good practice to pull the ON/OFF pin high using a 100k resistor. Relevant Part: LM2670;LM2671;LM2672;LM2674;LM2675;LM2676;LM2677;LM2678 What is the minimum operating frequency when driving the LM267x synch pin with an external oscillator? The maximum switching frequency supported by the LM267x SIMPLE SWITCHER converters is 280 kHz, when the SYNC pin is driven by an external oscillator Relevant Part: LM2671;LM2672;LM2670 Will the LM267X softstart function be affected by a widely-varying regulator load? Softstart is not affected by the load current drawn from the LM267X switching regulators. A softstart capacitor connected from the Softstart pin to ground allows for a slow turn-on of the IC . The softstart capacitor controls the rate at which the regulator starts up at power ON, by allowing the duty cycle of the internal power switch to gradually increase to its final value. This can significantly reduce the amount of surge current required from the input supply during an abrupt application of the input voltage Relevant Part: LM2671;LM2672;LM2673;LM2679 Can the LM2673/LM2679 soft start pin be used as a shutdown (On/Off) pin? We strongly recommend not using the soft-start pin as an on/off pin, as doing this may cause damage to the device. A better approach is use a device that has an on/off pin such as the LM2678 or use a FET in series with the LM2679 input or output. Relevant Part: LM2673;LM2679 When designing an LM2674-based inverting switching regulator to deliver -15 V output, what is the allowable maximum input voltage? When a buck switching regulator is used to develop a negative output voltage from a positive input, the maximum input voltage becomes limited by the value of the negative output. For example, if the output voltage is -15V, and the Maximum Operating Rating for the input voltage is 40V, then in the application the maximum input voltage becomes (40V-15V) or 25V. The reason for this limitation is that the negative output voltage is normally seen at the ground pin of the IC, so the IC total supply voltage is the sum of the magnitudes of the input and output voltages. See Application Note AN-1157 "Positive to Negative Buck Boost Converter Using LM267X Simple Switcher Regulators" for more information. The issue is noted in the "IC Device Ratings" section. Relevant Part: LM2674 When using an LM2675 to provide 5V power for a lamp which blinks at 1Hz, drawing 250mA, the regulator goes into discontinuous mode when the lamp is off. When the lamp turns on, the 5V rail sometimes dips 2V for 100ms before "catching back up." Why? Would applying a minimum load fix the problem? When the load is removed, such as when the lamp blinks off, the LM2675 goes into discontinuous mode because of the zero amp load current. Since it takes 1 second for the lamp in your application to blink ON, the energy previously stored in the capacitor is discharged through the catch diode. On reapplication of the load, it takes time for the output voltage to ramp up, which explains the transient dip of approximately 100ms you observed. A resitive load (250 Ω ) drawing a minimum of 20mA should suffice to maintain the specified load regulation. Relevant Part: LM2675 What is limiting the load current on my LM2678 evaluation board? If the LM2678 evaluation board cannot deliver the full rated load current of 5A, the input ceramic capacitor may be too far away from switcher input pin. It needs to be as close as is physically possible to the Vin and Ground pins, with wide, short traces Relevant Part: LM2678 What are the requirements for voltage regulators in battery-powered systems? Requirements for voltage regulators in battery-powered systems support the small size, light weight, and low cost characteristic of those systems. The voltage-regulator IC should require very few small, low-cost external components. To minimize size and weight, it should be housed in a small-outline or leadless leadframe package. If the regulator is a switch-mode type, it should operate at its highest useable switching frequency, to minimize the size and weight of its associated surface mount capacitors and inductors. Relevant Part: LM2734;LM2736 Why do you need a voltage regulator when operating from a battery? The battery output voltage declines as the battery discharges, so you need a regulator to maintain the required system voltage. The regulator must be selected to provide the system voltage when the battery is at its lowest charge level before the battery is declared fully discharged. If the system voltage is lower than the battery voltage range, a low-voltage step-down switching regulator can be used. If the battery voltage range is narrow (e.g. from NiCd cells), a low-dropout linear regulator may be suitable to produce a regulated lower output voltage. If the system voltage is higher than the battery voltage range, or within the range, then a switching regulator in a boost or buck-boost configuration can be used. Relevant Part: LM2734;LM2736 Why use a switch-mode voltage regulator for battery-powered systems? The switch-mode regulator can provide efficiencies of 90% or greater. The linear low-dropout (LDO) regulator efficiency is directly related to the ratio of the output voltage to the input voltage. If the battery voltage is 4.2V, and the output voltage is 2.5V, the LDO efficiency will be 60% at best. Higher efficiency means longer battery run time. In addition, switching regulators are available to convert the battery voltage to a higher output (boost), a lower output (buck), or an output in the middle of the battery voltage range (buck-boost). It is possible to achieve multiple output voltages from a single switching regulator IC. In contrast, the LDO output voltage is constrained to be less than the minimum battery voltage. Relevant Part: LM2734;LM2736 Why would you need a step-down (buck) regulator for a battery powered application? A step-down (buck) regulator reduces the battery voltage to a lower, regulated system supply voltage. A single Li-ion battery output can reach 4.2 V at full charge, and three new alkaline cells can produce 4.5 V. Typical required IC operating voltages range from 3.3 V to 1V, so the step-down function is necessary. When the maximum battery voltage is much greater than the desired system voltage, a buck switching regulator will operate with greater efficiency (lower power loss) than will a low-dropout regulator. Relevant Part: LM2734;LM2736 How can the switching frequency of a switching regulator be controlled? While many switching regulators have a non-adjustable, fixed switching frequency, some have provisions for changing or controlling this frequency. There are three techniques used to change the switching frequency of a switching regulator IC. (1) One approach uses an external resistor to change a capacitor charge current within the oscillator. (2) Some regulators can be synchronized with an external oscillator. (3) A few regulators have multiple pre-set switching frequencies that can be selected by connecting a pin to either ground or the Vcc supply line. Relevant Part: LM2742;LM2743;LM2744 What are the typical features of buck-regulator ICs? Buck (or step-down) regulator ICs will often have many of these features. Fixed or adjustable output voltage; some ICs have both capabilities. Single-ended or synchronous rectifier outputs. Soft-start that causes the output to come up gradually and limit inrush current. Power-Good output indicates if the output voltage is in regulation. Undervoltage lockout that prevents operation if the input voltage is too low. Thermal shutdown that cuts off regulator operation if the IC exceeds a specific temperature threshold. Overcurrent protection that deactivates the regulator if the load current exceeds a specific threshold. (This feature requires the regulator to sense its output current.) Overvoltage protection that prevents regulator operation if the output voltage exceeds a specific threshold. Relevant Part: LM2742;LM2743;LM2744 What is a buck converter? A buck converter, or stepdown voltage regulator, provides either isolated or nonisolated, switch-mode dc-dc conversion, reducing an input voltage to a regulated, lower output voltage. The figure shows a simplified non-isolated buck converter that accepts a dc input and uses pulse-width modulation (PWM) of switching frequency to control the output of an internal power MOSFET (Q1). An external Schottky rectifier diode, together with external inductor and output capacitors, produces the rectified, filtered dc output. The regulator IC compares a portion of the rectified dc output with a voltage reference (VREF) and varies the PWM duty cycle (on-time vs total switch period) to maintain a constant dc output voltage. If the output voltage increases slightly due to a reduced load, the PWM briefly lowers its duty cycle to reduce the regulated output, keeping it at its proper voltage level. Conversely, if the output voltage tends to go down, the feedback causes the PWM duty cycle to briefly increase and maintain the proper output. Relevant Part: LM2742;LM2743;LM2744 What is the design criteria for choosing the external component parts of a buck regulator circuit? Refer to the figure above. All required and optional external components will be specified or described by the regulator IC manufacturer. For most applications, the external components to be selected will include the input and output capacitors, the inductor, and sometimes compensation resistors and/or capacitors for the error amplifier. Here are typical guidelines used to select these components: Error Amplifier: For fast response and tight regulation, the error amplifier should have a high gain-bandwidth product, preferably above 1 MHz. Some regulator ICs allow external passive components to control this, while others depend on internal parts. Input capacitor: Select the input capacitor according to suggestions in the IC manufacturer''s data sheet. Ceramic X7R or X5R types are preferred. A low-ESR (equivalent series resistance) ceramic capacitor provides the best noise filtering of the input voltage spikes caused by rapidly changing input current. Place the input capacitor as close as possible to the IC''s VIN pin. In some cases, a larger value could improve filtering. Output capacitors: Output filter capacitors smooth current flow from the inductor to the load, which helps to maintain a steady output voltage during transient load changes. The output capacitor also aids in reducing output ripple. To perform these functions, the capacitors chosen must have sufficient value and low ESR. Output inductor: Choose an inductor that does not saturate at the rated output current. Saturation current ratings are usually specified at 25°C, so ask the manufacturer for ratings at the maximum application temperature. Also, make sure the inductor current ripple is low enough to achieve the required output voltage ripple. Relevant Part: LM2742;LM2743;LM2744 What''s the difference between non-isolated and isolated regulators? An isolated converter employs a transformer to provide dc isolation in the energy transfer between the input and output voltage. The non-isolated converter usually employs an inductor, and there is no dc voltage isolation between the input and the output. The vast majority of applications do not require dc isolation between input and output voltages, because isolation is often provided in the input voltage source, or the input source is a battery. Relevant Part: LM2742;LM2743;LM2744 What’s the effect of switching frequency on the design of a switching regulator? Switching frequency determines the physical size and value of external filter inductors and capacitors. The higher the switching frequency, the smaller the physical size and component value. However, there is an upper frequency limit where either magnetic losses in the inductor or switching losses in the regulator IC and power MOSFET reduce efficiency to an impractical level. In addition, at higher switching frequencies, PCB layout must use RF techniques, to avoid introducing parasitics that would otherwise impair the regulator efficiency and performance. Relevant Part: LM2742;LM2743;LM2744 Why does the LM2931 draw excessive ground current when the input drops, forcing the output to an out of regulation condition? The LM2931 is one of a group of LDO regulators that has a characteristic in their ground pin current referred to as the "carrot". The carrot is a point in the ground pin current that spikes up as the input voltage is reduced. The error amplifier in the regulator always tries to force the output to be the right voltage by adjusting the current through the pass device (in this case, the PNP transistor). As the input voltage is reduced (and the voltage across the pass transistor decreases) the current gain of the PNP begins to drop. To maintain the correct output voltage, the error amplifier has to drive the base of the PNP harder to supply the same load current. The PNP base drive current leaves the regulator as ground pin current. As the input voltage drops further the regulator will approach dropout, causing the error amplifier to drive the PNP base with maximum current (this is the top of the carrot). This value of current may be 3 or 4 times the maximum ground pin current that is required to drive full rated load current with 5V across the pass transistor. The carrot is recognized as an undesirable characteristic, since the additional ground pin current must be supplied by the source, but does not power the load (it just heats up the regulator). In the newer LDO regulators, circuitry was built in to prevent this ground pin spike from occurring. For example, the LP2951 (and all of the products in that family) have only a negligible increase in ground pin current as the input voltage crosses through the range where dropout is occurring. Relevant Part: LM2931;LM2926;LM2927;LM2935;LM2936;LM2937;LM2940;LM2941 How should the LM2931 ON/OFF pin be driven to put the regulator into standby mode? The ON/OFF pin must be pulled high to disable the circuit and leave the regulator in the standby mode. The application note AB-11, "High-Efficiency Regulator Has Low Dropout Voltage", states (in the fourth paragraph) that keeping the ON/OFF pin open disables the circuit. However, this is for the application circuit shown in the document, which includes a pullup resistor (R4) which pulls the LM2931 ON/OFF pin high. Relevant Part: LM2931 What are the implications of floating the ground pin of the LM2937 or LM2940 LDO? It is difficult to pinpoint exactly what will happen inside the part because it depends on possible parasitic structures which allow current to flow through small geometries. It is highly likely that some weak junction will malfunction. The best recommendation we can give about leaving a ground pin floating is: Never do it Relevant Part: LM2937;LM2940 Can the LM2941 output voltage be adjusted to lower than 5V? No. The output voltage should not be set to less than 5V because some devices will work and others won`t. We do not guarantee the performance of the chip for outputs <5V. Because the regulator output voltage is used in the IC''s internal biasing, a low output voltage will cause the regulator to not be properly biased for normal operation. The symptoms can be failure of the current limit or thermal shut down circuitry, or the bandgap reference may not come up leading to no output Relevant Part: LM2941 The LM2941 output voltage is programmed to 5V, but it never exceeds 4V when the input voltage slowly rises. Why? If the input voltage is not applied rapidly, the output voltage may not reach the programmed value. We believe the misoperation must be due to one of two things (the first one is more likely):   1) The input source is being loaded down because it can''t supply enough current to get the LM2941 through the saturation region. When the output is "stuck" at about 4V, what is the input voltage? If the input is 4 - 5V, that means you need a supply source with more current capability. It is not a latch-up, rather the infamous "carrot" spike in quiescent current associated with first generation LDOs (see the data sheet Typical Performance Characteristics, quiescent current vs input voltage).   2) Part is oscillating. Check the output when it is stuck at 4V. If you have an oscillation, you need to check the ESR on the output capacitor to be sure it is within the range specified in the product datasheet. Relevant Part: LM2941 Will the device be damaged if a voltage greater than Vin is applied at the output? The device will not be damaged if an external voltage is applied at the output that is greater than the input as there is no current path from output to input. However, the external voltage should not exceed the Absolute Maximum input voltage of 26V Relevant Part: LM2941 What are the implications of not connecting the LM317 die sense pad to the Vout pad? (leaving sense pad floating). One of two things will happen. Neither situation is good and will result in extremely poor performance of the regulator, if it works at all. (1) The most likely is that there is a connection through the substrate from Vout sense to Vout. In this case any drops in the Vout bondwire will not be compensated for. This could result in Load Regulation values of over 20mV depending on size of bond wire and load. (2) Vout sense is isolated from Vout. In this case we would not expect the part to regulate at all. It could oscillate, go to either Vin or 0V, or sit at some arbitrary voltage depending on the die parasitics. Relevant Part: LM317 What is the easiest way to design a constant current source or current limit into a circuit? If the current is between 1 ua and 10 ma, the LM334 can be used. The load current is determined by resistor Rset according to the formula: Iset = Ir + Ibias = Vr /Rset + Ibias. Please refer to application notes in LM334 datasheet for further information. If the current is between 5 ma and 100 ma, the LM317L can be used. If the current is between 10 ma and 1.5 A, the LM317 can be used. The LM317 and the LM317L are used with a resistor between the output pin and the adjust pin to set the current according to the formula: Iout = 1.25/R where R is between 0.8 Ω to 120 Ω for the LM317 and R is between 12 Ω to 240 Ω for the LM317L. Relevant Part: LM334 What is the importance of efficiency for ultra-low voltage DC-DC converters? Efficiency is critical for battery life, the higher the IC converter efficiency, the longer the usable battery operation before recharging. High efficiency and minimum current are important for the converters standby, shutdown and operational modes. Relevant Part: LM3370;LM3670;LM3671;LM3674 How can a converter IC protect its associated system when battery voltage drops? Many converter ICs have undervoltage lockout (UVLO) that turns off the converter if the battery voltage drops below a specific threshold, protecting the associated equipment. Most UVLO functions exhibit a hysteresis between the voltage that activates IC and the voltage when it cuts off. For example, a converter IC may turn on if the battery voltage goes above 3.5V and turn off if the battery voltage drops below 2.7V. Relevant Part: LM3370;LM3670;LM3671;LM3674 How can the converter IC improve its operational efficiency? Many converter IC can operate in two different modes, one for high load currents and another for lower load currents. They operate in the conventional PWM (pulse width modulation) at normal loads and switch to a second mode for low current loads. Switching at low current loads can be with pulse frequency mode (PFM) or pulse skip mode in which the IC skips clock cycles. Relevant Part: LM3370;LM3670;LM3671;LM3674 How can you change the output voltage of a switch-mode converter IC? Some fixed-output converter ICs have the ability to reduce there output voltage and still maintain regulation. This requires two external resistors across the output; the voltage at the intersection of the two resistors is then fed back to the error amplifier input of the converter IC. The high-low range of output voltage adjustment depends on the resistors, internal voltage reference, and characteristics of the converter IC. Another output voltage adjustment approach is to provide pin selectable output voltages, that is, the designer can connect different IC pins together to change the output voltage. Relevant Part: LM3370;LM3670;LM3671;LM3674 How do you ensure that the converter IC does not draw excessive current? Preventing excessive output current requires a means for monitoring that current. One approach is to use an external low-resistance current sensing resistor in the converter''s output circuit. Another approach is to use a power MOSFET whose drain current can be monitored. In either case, the converter is disabled if the output current exceeds a preset threshold. Relevant Part: LM3370;LM3670;LM3671;LM3674 How is the converter IC design impacted by battery voltage in portable systems? The battery type of choice for many portable systems is now the lithium-ion type whose typical output can range from 2.7V to 4.2V. Therefore, any IC powered by an li-ion battery must operate efficiently when stepping down its output to the sub-volt range. Relevant Part: LM3370;LM3670;LM3671;LM3674 What is done in the DC-DC converter IC that aids efficiency? Highest efficiency is obtained if the MOSFET power switch and synchronous rectifiers are integrated into a single monolithic IC. Synchronous rectifiers using MOSFETs provides higher efficiency than conventional Schottky rectifiers at ultra-low voltages. Relevant Part: LM3370;LM3670;LM3671;LM3674 What is the effect of switching frequency on converter IC efficiency? A high switching frequency allows use of physically smaller external components. However, the converter IC''s power MOSFETs must operate efficiently at the switching frequency, which means a fast rise and fall times as well as a low on-resistance. In addition, the external inductor employed by the converter IC must exhibit minimal losses at the switching frequency and load current. Relevant Part: LM3370;LM3670;LM3671;LM3674 How are the output voltage programmed on the LM3370? The outputs of the LM3370 can be programmed through Buck 1 & Buck 2 registers via I2C. Buck 1 has output voltage ranges of 1V to 2V in 50mV steps and Buck 2 has output voltage ranges of 1.8V to 3.3V in 100mV steps. If the I2C feature is not used, output voltages will default to the pretrimmed set voltages. Relevant Part: LM3370 How does the PFM-PWM mode operate in the LM3370 The LM3370 has a PFM-PWM topology where the device automatically switches between PWM (Pulse Width Modulation) and PFM (Pulse Forced Modulation) depending on the value of the output current. Under low load conditions, the device goes into PFM mode, which implies that it is not switching at all times. This is the feature that allows the LM3370 to achieve high efficiency at very low output currents. When the device is operating at higher loads, the device is in PWM mode and delivers very efficient performance (greater than 90%). Additionally users can force the device into PWM mode through the use of I2C. Relevant Part: LM3370 Can a fixed output regulator such as the LM340-5.0 be used with an adjustable output? If so, how? In theory it can be done, but it is not generally recommended because of the difference in temperature coefficients beween the internal resistive divider and the external resistors. This may not yield precise output voltage setting; therefore a better solution is to use an adjustable output regulator Relevant Part: LM340 With a 12V-80V input source, how are isolated 12V/200mA and 5V/200mA outputs developed? A current-mode flyback controller such as LM3478 is suitable for this application. Because the input voltage exceeds the maximum input rating of the LM3478, you will need a separate supply for the LM3478. Application Note AN-1095, "Design of Isolated Converters Using SIMPLE SWITCHERS", has additional information on isolated supply design. Relevant Part: LM3478 How can a regulated 3.3V at 1.5A be produced from a source of 3V to 5V? This application involves an output voltage that is within the range of the available input voltage. A switching regulator using the SEPIC topology would be the best choice. WEBENCH Power Designer supports the SEPIC configuration - just enter your design requirements and select a suggested solution. Relevant Part: LM3478 What type and values of capacitors should be used at the LM3480 output? The LM3480 is a very simple regulator, in the heritage of the classic LM317 type linear regulators. It uses an NPN pass device like the LM317, so it is also very easy to stabilize. Capacitor ESR does not affect this regulator''s stability, therefore, there are no ESR curves. MLCC (multi-layer ceramic) capacitors work well. We recommend the use of a 0.1uF capacitor as shown on the datasheet front page as a minimum capacitance. Larger output caps will simply cut down the peaks at the edges of load transients, but the regulator will be stable with any value 0.1uF or above Relevant Part: LM3480;LM3490 How can I develop two regulated voltages, +30V at 3A and -30V at 3A, to power a high-powered amplifier? For the positive output, use the LM3488. Use the Boost Converter design information contained in the Typical Applications. For the negative output, use the LM3477. Start from the buck application circuit in the datasheet. Tie the buck regulator output to ground. Tie everything that was ground to -Vout. Connect the input capacitor between Vin and GND (rather than Vout). The average inductor current is Iout/(1-D). For more information on this approach, see AN-1157, "Positive to Negative Buck-Boost Converter Using LM267X SIMPLE SWITCHER Regulators". Relevant Part: LM3488;LM3477 What does "OCthresh / Over-Current Threshold" specification mean? For the USB switches, how does the "OCthresh/OverCurrent Threshold" specification relate to the "Isc/Short Circuit Current" protection? Since the "Isc / Short Circuit Current" protection limits the maximum current to 1A (typical), it would seem the current could never exceed the "Over-Current Threshold" of 2A (typical). There are actually two current limits depending on the status of the Error Flag timer. When the device is first Enabled, and before the Error Flag has timed out, the current limit is "Isc / Short-Circuit Current", typically 0.8A to 1.2A depending on the specific device. When the Error Flag timer successfully times out, and the device is in ‘normal operation’, the limit becomes "OCthresh / Over-Current Threshold", typically 2.0A to 2.25A depending on the specific device. If, during normal operation, the output current exceeds the OCthresh limit (about 2A), the output current will be disabled for a few milliseconds, the Error Flag timer will be reset, the Current Limit will be reset to the lower Isc limit (about 1A), and then the output current will attempt to restart just as if the Enable pin had been toggled Off then back On. Relevant Part: LM3525;LM3526;LM3543;LM3544 What is the battery temperature measurement feature? Li-based batteries cannot safely charge above 50°C or below 0°C. The LM3658 or LP3947 provides the bias and measurement of the voltage across an external thermistor that is embedded in the battery for accurate battery temperature measurement. Charging will stop when the battery temperature falls outside the 0-50 degree window. Relevant Part: LM3658;LP3947 How does the PFM-PWM mode operate in these devices? The LM3670/LM3671 have a PFM-PWM topology where the device automatically switches between PWM (Pulse Width Modulation) and PFM (Pulse Forced Modulation) depending on the value of the output current. Under low load conditions, the device goes into PFM mode, which implies that it is not switching at all times. This is the feature that allows the LM3670/3671 to achieve high efficiency at very low output currents. When the device is operating at higher loads, the device is in PWM mode and delivers very efficient performance (greater than 90%). The LM3674 has a fixed PWM topology. This is useful for those applications that are more sensitive (like RF circuits) to the slight increase in voltage ripple that the PFM has relative to PWM mode. Also the fixed PWM mode works well in application where there is no low power save modes. Relevant Part: LM3670;LM3671;LM3674 Are the LM376, LM105, LM205, or LM305 regulators currently available from National? The entire family of voltage regulators has been discontinued as of 2001. However, the LM305AH may still be available from Rochester Electronics. The LM105 was the military-temperature range device, the LM205 operated over the industrial temperature range, and the LM305/LM305A was commercial. LM376 was an LM305 in a plastic DIP package. These devices were upgrades of the LM100, National''s first integrated product from approx. 1968. Relevant Part: LM376;LM105;LM205;LM305 For the LM4041CIM, what is the polymer type and percentage of total polymers, weight in grams, and oxygen index? For LM4041CIM (8-pin SOIC): Polymer type is epoxy; % polymers = 25%; Weight of polymer = 0.015g; Oxygen (O2) index = 28% minimu Relevant Part: LM4041 Where can I find information about the LM5000 series of high voltage switching regulators? General information about the LM5000 series of high voltage switching regulators can be found at http://www.national.com/appinfo/power/hv.html . The LM5000 high-voltage boost/flyback converter and the LM5007 high-voltage buck converter are supported in the online design tools of the Power WEBENCH. Further information and Mathcad files for the LM5000 flyback application are available at http://www.national.com/appinfo/power/lm5000.cgi, which also includes information about the LM5030 push-pull and LM5041 cascaded power converter applications. Also, refer to Application Brief 126, "High voltage, single chip DC/DC regulator optimized for flyback, boost, or forward power converter applications, " featuring the LM5000. Relevant Part: LM5000;LM5030;LM5041;LM5007;LM5642;LM5110 How is the switching frequency selected for the LM5000? The LM5000-3 operates at 300 kHz or 700 kHz selectable with an external pin, the LM5000-6 operates at 600 kHz or 1.25 MHz. High switching frequencies are generally needed in xDSL boost applications, and permit the use of smaller external components. In flyback and forward applications, lower switching frequencies at 300 kHz are generally preferred, and enable the reduction of core losses of the magnetics. Relevant Part: LM5000 Is it possible to use ceramic output capacitors with the LM5000? Yes, we recommend using low ESR output capacitors (e.g. ceramic) to obtain better performance and lower output ripple. However, the external compensation pin does allow for the use of higher ESR capacitors as well. Relevant Part: LM5000 Since the LM5000 is based on a PWM current mode architecture, are there any problems with subharmonic oscillation? The LM5000 has internal ramp compensation, to avoid sub-harmonic oscillations that occur when the duty cycle is greater than 50%. A maximum duty cycle of 80% and an external compensation pin additionally permit both continuous and discontinuous operation modes. Relevant Part: LM5000 Can LM5020 be used in an offline AC/DC converter? Yes. An external shunt regulator can be used to drop the rectified 400V DC line down to 15V DC input for a start-up regulator in the LM5020 controller. Shortly after the AC/DC converter is up and running, an auxiliary feedback winding off the transformer will provide VCC voltage to the LM5020, which shuts off the startup regulator and eliminates the load on the external shunt regulator. Relevant Part: LM5020 Do I need an external opto coupler if I am designing a non-isolated solution with the LM5020? No, the LM5020 has an internal error amplifier with a GBW of 4 MHz on board that can be used to close the feedback loop of non-isolated converters. Relevant Part: LM5020 How does the internal start-up regulator of the LM5020 work? The LM5020 contains an internal high voltage start-up regulator allowing it to be attached directly to the input voltage bus. The input pin of the regulator (VIN) is connected directly to the line voltage and operates over a wide range (15V to 100V). The output of the regulator (VCC pin) is decoupled using a 0.1 µF to 50 µF filter capacitor. When the voltage on the VCC pin reaches the regulation point of 7.7V, the controller output is enabled and the regulator begins to softstart. After starting, the controller will operate and remain enabled until the VCC pin voltage drops below 6.2V. Relevant Part: LM5020 How does the line under-voltage protection circuitry of the LM5020 work? The LM5020 contains a line Under-Voltage Lockout (UVLO) comparator and precision 1.25V voltage reference. An external resistor divider connected from the line input to ground programs the UV threshold and hysteresis. The UVLO comparator holds the controller in an off state until the UVLO pin exceeds 1.25V. When the UVLO threshold is achieved, a 20 µA hysteresis current source is activated to flow out of the UVLO pin through the external resistor network. This current source raises the voltage at the UVLO pin to avoid false triggering due to noise. The UVLO pin can also be used to implement a remote enable/disable function. The converter is disabled by simply pulling UVLO below the 1.25V comparator threshold using an external switch or transistor. Relevant Part: LM5020 How is the switching frequency selected? The LM5020 oscillator is set by a single external resistor connected between the RT pin and ground. The resistor value to set the oscillator frequency is calculated as follows: LM5020-1:      LM5020-2: RT= 1 F x 152 x 10-12      RT= 1 F x 304 x 10-12 The LM5020 can also be synchronized to an external clock that is at a higher frequency than the free running frequency set by the RT resistor. The sync signal is capacitively coupled to the RT pin and must have a peak amplitude of at least 3V and pulse-width between 15 ns and 150 ns. Relevant Part: LM5020 Can the Pulse-Width Modulator (PWM) frequency be synchronized to an external clock? Some PWM converters provide the ability to synchronize the oscillator to an external clock with a frequency that is either higher or lower than the frequency of the internal oscillator. If there is no requirement for synchronization, it is typically recommended to connect the sync pin to GND to prevent noise interference. Consult the product datasheet for details on an individual product''s synchronization function. Relevant Part: LM5025;LM5020;LM5030;LM5033;LM2586;LM2588;LM2670;LM2608;LM2612;LM2614 What is the purpose of the soft-start circuit in a power converter? The soft-start feature allows the power converter to gradually reach the initial steady state operating point, thus reducing start-up stresses and surges. In most PWM ICs an internal current source charging an external capacitor establishes the soft-start time. Relevant Part: LM5025;LM5020;LM5030;LM5033;LM2655;-;LM2671;LM2672;LM2673;LM2679;LM2716;LM2711;LM2710;LM3211;-;LM2743;LM2744;LM2642;LM2577;LM2585;LM2586;-;LM2587;LM2588;LM5642;LM2597;LM2598;LM2599;LM2590;LM2593;LM2633 What is the difference between voltage-mode and current-mode PWM controllers? The circuit shown below is a voltage-mode PWM controller in which the error amplifier output is compared to a voltage ramp from the oscillator to determine the output pulse width. A current mode PWM replaces the oscillator ramp with a ramp that is proportional to the current in the magnetic element. Relevant Part: LM5025;LM5020;LM5030;LM5033;LM2733;LM2731;LM2577;LM2585;LM2586;LM2587;LM2588;LM2648;LM3477 What sets the switching frequency of a pulse-width modulated (PWM) controller? Many PWM controller ICs operate at a fixed switching frequency, set by a single external resistor or capacitor. To set a desired oscillator frequency, use the equation in the controller datasheet to calculate the resistor value. For example, see the equation in LM5025 datasheet. Some PWM controller ICs operate with fixed on-time or fixed-off time, often controlled by an external resistor (see the LM5010 for example). The resulting switching frequency varies with input voltage. Other PWM controller ICs operate in a completely hysteretic mode. The switching frequency of a hysteretic converter depends on values of the output inductor and capacitor, as well as operating voltages and currents. An example is the LM3485 hysteretic controller. Relevant Part: LM5025;LM5020;LM5030;LM5033;LM3485 How is output current limiting implemented in Pulse-Width Modulator (PWM) controllers? Most PWM controller ICs provide current-limiting protection by sensing the power switch current. If the current-sense input exceeds a specific threshold, it terminates the present cycle (cycle-by-cycle current limit). Circuit layout is critical when using a current-sense resistor, which must be a low inductance type. Locate the capacitor associated with the current-sense filter capacitor very close to, and connected directly to, the PWM IC pin. Also, all the noise-sensitive, low-power ground connections should be connected together near the IC ground pin, and a single connection should be made to the power ground (sense resistor ground point). Relevant Part: LM5025;LM5020;LM5030;LM5033;LM5041;LM3477;LM3478;-;LM5642;LM3485;LM2633;LM2642;LM2645;LM2727;LM2737;LM2744;LM3475;LM2636;LM2638;LM2639 Why use Pulse-Width Modulation (PWM)? Switch-mode converters employ a power semiconductor switch (often a MOSFET) to drive a magnetic element (transformer or inductor) whose rectified output produces a dc voltage. Pulse-width modulation (PWM) is used to control the switch output power, by varying its ON time vs OFF time. The ratio of ON time to the switching period time is the duty cycle, and is determined by the regulator feedback. Like any regulator, a switch-mode converter uses feedback to maintain its target output voltage in response to load current changes. Because the switching action causes power dissipation in the switch to be low (little current when voltage is present, and small voltage when handling high current), power conversion efficiencies exceeding 90% are common, about twice that of a linear regulator. Figure 1 shows three different variations of the PWM duty cycle: 10%, 50%, and 90%. Relevant Part: LM5025;LM5020;LM5030;LM5033 What does the Undervoltage-Lockout (UVLO) circuit of a Pulse-Width Modulator controller do? The undervoltage-lockout (UVLO) circuit sets the minimum operational dc input voltage of the pulse-width modulated (PWM) controller. There are two UVLO thresholds. When the UVLO turn-on threshold is exceeded, the PWM controller turns ON. If dc input voltage falls below the UVLO''s lower turn-off threshold, the PWM controller turns off. Relevant Part: LM5025;LM5020;LM5030;LM5033 What is the configuration of a PWM regulator circuit? The figure below shows a simplified pulse-width modulated (PWM) controller employed in a switch-mode converter. In operation, a fraction of the dc output voltage feeds back to the error amplifier, which causes the comparator to control the ratio of the PWM ON and OFF times. The ratio of the ON time to the total period is the duty cycle. If the power drawn at the output changes, the feedback network adjusts the duty cycle to maintain the output voltage at the desired level. To generate the PWM signal, the error amplifier takes the difference of the feedback signal input and a stable voltage reference to produce an error voltage signal. The comparator compares the error amplifier output voltage with the sawtooth-shaped ramp from the oscillator, producing a modulated pulse width. The comparator output is applied to the switching logic, whose output goes to the output driver for the external power MOSFET. The switching logic provides the capability to enable or disable the PWM signal applied to the power MOSFET. Relevant Part: LM5025;LM5020;LM5030;LM5033 Can the LM5030 or LM5033 push-pull controller be used for half-bridge or full-bridge converters ? Yes. By mating the push-pull outputs of the LM5030 or LM5033 to the control inputs of National''s LM5100 half-bridge drivers both the full and half-bridge DC/DC converters can be configured. Configuration Controller MOSFET Driver Benefit/application Push-pull LM5030/33 Not required High-efficiency medium-power Half-bridge LM5030/33 (1) LM5100A High-efficiency medium/high power Full-bridge LM5030/33 (2) LM5100A High efficiency high-power Relevant Part: LM5030 Can the LM5030 push-pull controller be used in AC/DC off-line power supplies ? Yes. The LM5030 is integrated with a 100V start-up regulator for DC/DC converters (for operation on a 48 Vdc telecom bus) which can be bypassed and used on a rectified AC/DC line by connecting the VIN to VCC pin and use an external linear regulator to provide bias to the controller (needs 8V higher). Relevant Part: LM5030 Since LM5030 does not have an external reference available for an optocoupler pull-up resistor, what methods are available when utilizing the LM5030? The internal error amplifier within the LM5030 is configured with an internal resistor pull-up to an internal 5V reference. In isolated applications the FB pin can be grounded and the optocoupler connected directly to the COMP pin A. Relevant Part: LM5030 Can the LM5033 push-pull controller be used in AC-DC off-line power supplies? The LM5033 can be powered from either an unregulated 15V-100V input applied to the VIN pin for operation on a 48V DC telecom bus, or from a regulated 10-14V input supply sourced from the rectified AC line and applied to the VCC pin. Relevant Part: LM5033 What is the intermediate bus architecture? The use of intermediate bus converters (IBC) is becoming very popular in distributed power architectures. This implementation continues to distribute 48V along the system backplane. The intermediate architecture differs from conventional distributed power architecture at the card level. On each card in the system, an intermediate power bus voltage is established rather than converting directly to multiple point-of-use voltages. The intermediate power bus converter ground isolates and steps down the backplane voltage to a nominal 8V to 12V. Non-isolated point of load converters (typically buck regulators) receives power from the intermediate bus and regulate down to point-of-use voltages, which completes the power system. Relevant Part: LM5033 Do I need an external error amplifier if I am designing a non-isolated solution with the LM5041 or LM5041A? No, the LM5041 and LM5041Ahave internal error amplifiers with a GBW of 4MHz on board that can be used. Relevant Part: LM5041;LM5041A How is the LM5041/LM5041A oscillator synchronized to an external clock ? The external clock must be of higher frequency than the free running frequency set by the Rt resistor. The clock signal should be capacitively coupled into the Rt pin with a 100 pF capacitor. A peak voltage level greater than 3V is required for detection of the sync pulse. The sync pulse width should be set in the 15 to 150 ns range by the external components. Relevant Part: LM5041;LM5041A How is the switching frequency of the LM5041 and LM5041A selected? The LM5041/LM5041A oscillator is set by a single external resistor connected between the Rt pin and return. The Buck regulator switches at twice the frequency of the push-pull outputs. To set a desired oscillator frequency the necessary Rt resistor can be calculated as: RT= (1/F) - 172 x 10-9 182 x 10-12 Rt resistor should be located very close and connected directly between the Rt and RTN pins Relevant Part: LM5041;LM5041A How much current can be safely be drawn from the VCC and REF pins? The current limit of VCC is set at a maximum of 15 mA and the current limit of the REF pin is set at a minimum of 10 mA. Care must be taken to load these pins at much less than the current limits to limit the power dissipation on the die. The current drawn from these pins is a load on the input supply and as such the power dissipation can be significant even at a load of 10 mA depending on the VIN voltage. There is on die thermal protection that will shutdown the part if the on die temperature is above 165 °.C. Relevant Part: LM5041;LM5041A Since the LM5041 and LM5041A are based on PWM current mode architectures, are there any problems with subharmonic oscillation? The LM5041 and LM5041A have internal ramp compensation, to avoid sub-harmonic oscillations that occur when the duty cycle is greater than 50%. Relevant Part: LM5041;LM5041A What is the input voltage range of the LM5068? The LM5068 has a wide negative input voltage range which can be anywhere from -10V to -100V. The LM5068 controls current in the return path and is intended for negative supply applications. If used with a positive supply, it would interrupt the ground connection which is usually undesirable. Relevant Part: LM5041;LM5041A Does the LM5068 have under-voltage and over-voltage protection? Yes, the LM5068 has user-programmable UV and OV protection with <2% threshold set point accuracy and programmable hysteresis. Relevant Part: LM5068 Does the LM5068 limit current during start-up? Yes, the LM5068 has a user-programmable active current limit with a multi-function programmable timer capacitor and provides three modes of overcurrent protection. First level current threshold initiates a time-delayed circuit breaker. Second level current threshold with a feedback loop for active current limiting. Third level threshold with fast gate discharge path that limits peak currents under worst case catastrophic fault conditions. Relevant Part: LM5068 Does the LM5068 require supply line filtering to eliminate voltage spikes from entering the VDD supply pin? Yes. The LM5068 includes an internal bias regulator that can be damaged if input transients exceed 100V. An RC filter between the line input and VDD pin is recommended to ensure any ringing attributed to load current interruptions through the supply line inductance does not exceed the 100V absolute maximum rating specification. Relevant Part: LM5068 How many versions of the LM5068 are available and what are the differences? There are four versions available for the LM5068: LM5068-1 Latches off after a fault condition until power is removed. Power Good (PGOOD) output polarity is active high. LM5068-2 Successively retries after a fault condition at intervals determined by the CT capacitor. PGOOD output polarity is active high. LM5068-3 Latches off after a fault condition until power is removed. PGOOD output polarity is active low. LM5068-4 Successively retries after a fault condition at intervals determined by the CT capacitor. PGOOD output polarity is active low. Relevant Part: LM5068 What is unique about the LM5100A and LM5101A compared to the original LM5100/01? 3A vs. 2A gate drivers. These devices offer the highest peak gate current of any half-bridge driver available on the market today as well as a newly developed high-speed bootstrap diode. The drivers outperform competition in speed and efficiency in high frequency switching regulator applications where robust gate drive and switching speed are critical. Relevant Part: LM5100;LM5100A;LM5101;LM5101A What is a power-MOSFET gate driver? A power MOSFET gate driver is a power amplifier that accepts a low-power input from a controller IC and produces the appropriate high-current gate drive for a power MOSFET. A gate driver is used when a pulse-width-modulated (PWM) controller cannot provide the output current required to drive the gate capacitance of the associated MOSFET used in the power converter circuit. Gate drivers may be implemented as dedicated ICs, discrete transistors, or transformers. They can also be integrated within a controller IC. Partitioning the gate-drive function off the PWM controller allows the controller to run cooler and be more stable by eliminating the high peak currents and heat dissipation needed to drive a power MOSFET at very high frequencies. Relevant Part: LM5100;LM5101;LM5102;LM5104;LM5110;LM5112;LM5100;LM5100A;LM2726;LM2725;LM2722;LM2724A How do discrete gate drivers and gate driver ICs differ? Discrete gate drivers constructed with bipolar npn and pnp emitter-followers can achieve reasonable drive capability, but they''re not as space-efficient as gate-driver ICs. Implementing delay and other housekeeping functions needed for safe operation is cumbersome and costly with a discrete circuit. The gate-driver IC overcomes these limitations. Relevant Part: LM5100;LM5101;LM5102;LM5104;LM5110;LM5112;LM5100;LM5100A;LM2726;LM2725;LM2722;LM2724A How does one select between single and dual channel gate driver ICs? Single gate-driver ICs offer flexibility in circuit configuration and PCB placement, while dual gate-driver ICs may require less PCB area. The output of a single gate-driver IC output is either inverting or non-inverting with respect to the controller input. Some single-channel gate drivers provide both inverting and non-inverting inputs. Dual gate-driver ICs may have either two inverting, two non-inverting, or one inverting and one non-inverting channel. Relevant Part: LM5100;LM5101;LM5102;LM5104;LM5110;LM5112;LM5100;LM5100A;LM2726;LM2725;LM2722;LM2724A How does the gate driver affect MOSFET switching speed? The Miller effect produced by MOSFET CGD is what predominantly limits switching speed. A MOSFET responds instantaneously to changes in gate voltage and begins to conduct when the gate reaches the threshold voltage (VGS). To address a wide range of applications, suppliers offer a variety of power MOSFETs that transition at different gate thresholds, such as logic-level MOSFETs with lower threshold voltage. Gate waveforms indicate a plateau at a gate voltage above the threshold voltage. The amount of drive current available from the gate driver determines the MOSFET drain-voltage rise and fall times, and thus the time required to drive the gate through this plateau region. Relevant Part: LM5100;LM5101;LM5102;LM5104;LM5110;LM5112;LM5100;LM5100A;LM2726;LM2725;LM2722;LM2724A What are the benefits of integrated circuit (IC) gate drivers? Gate-driver ICs include a logic input buffer that drives sufficient current-gain stages to produce a high-current output. In addition, the dedicated gate-driver IC can include the housekeeping functions need for safe operation. Also, it can be easily placed closer to the power MOSFET, thereby reducing noise interference and voltage-distribution drops across the pc-board trace. Relevant Part: LM5100;LM5101;LM5102;LM5104;LM5110;LM5112;LM5100;LM5100A;LM2726;LM2725;LM2722;LM2724A What are the considerations when designing MOSFET gate drive for synchronous rectification switching converters? Shoot-through current is a potential problem for MOSFETs used in synchronous rectification. Because the gate driver must turn on and off not only the power switch but also the rectifier switch, a low impedance may be presented to the input voltage source during switching transitions. This low transition impedance can allow a shoot-through current to be conducted through both the power-switching MOSFET and the synchronous-rectifier MOSFETs. High shoot-through currents result in greater electromagnetic interference, more noise on the input voltage source, lower efficiency, and reduced reliability. Some gate-driver ICs include a non-overlap circuit that prevents shoot-through current. Other ICs specify a minimum amount of nonoverlap, or dead-time. That is, a minimum time at the switching transitions (two per operating cycle) where both MOSFETs are turned off. Maintaining the dead-time prevents the problem of shoot-through current but reduces circuit efficiency from its optimum value. Relevant Part: LM5100;LM5101;LM5102;LM5104;LM5110;LM5112;LM5100;LM5100A;LM2726;LM2725;LM2722;LM2724A What are the primary gate-driver design considerations? An important attribute for the gate driver is its ability to provide sufficient drive current to quickly pass through the Miller Plateau Region of the power-MOSFET''s switching transition. This interval occurs when the transistor is being driven on or off, and the voltage across its gate-to-drain parasitic capacitor (CGD) is being charged or discharged by the gate driver. The figure below plots total gate charge as a function of the gate-drive voltage of a power MOSFET. Total gate charge (QG) is how much must be supplied to the MOSFET gate to achieve full turn-on. It is usually specified in nanocoulombs (nC). Relevant Part: LM5100;LM5101;LM5102;LM5104;LM5110;LM5112;LM5100;LM5100A;LM2726;LM2725;LM2722;LM2724A What influences gate-driver-IC lifetime and performance? Load power requirements, thermal characteristics of the semiconductor package and its cooling method determine the lifetime and performance of a gate-driver IC. The device''s junction temperature must be kept within the rated limit at all times. Relevant Part: LM5100;LM5101;LM5102;LM5104;LM5110;LM5112;LM5100;LM5100A;LM2726;LM2725;LM2722;LM2724A What''s the circuit model for a gate driver and power MOSFET? The figure below shows the simplified model, including the parasitic components that influence high-speed switching, gate-to-source capacitance (CGS), the gate-to-drain capacitance (CGD), and drain-to-source capacitance (CDS). Values of the source inductance (LS) and drain inductance (LD) depend on the MOSFET package. The other parasitic component is RG, the resistance associated with the gate signal distribution within the MOSFET that affects switching times. Relevant Part: LM5100;LM5101;LM5102;LM5104;LM5110;LM5112;LM5100;LM5100A;LM2726;LM2725;LM2722;LM2724A Can the LM5105 and LM5107 drivers tolerate load transients voltages (at HS pin) more than 1V below ground which can occur when driving inductive loads? Yes. Unlike any competitive half-bridge driver, the LM5107 and LM5105 specify transient voltages capability down to -5V. Relevant Part: LM5107;LM5105 Can the LM5107 be used for DC motor drivers? No, the bootstrap capacitor must be occasionally refreshed by a pulse-width modulation switching event. Relevant Part: LM5107 If motion is initiated in velocity mode, does the LM629 need to stop the motor before the mode is changed to position mode? What about changing from position mode to velocity mode? The motor does not have to be stopped to change from velocity mode to position mode, or from position mode to velocity mode. Relevant Part: LM628;LM629 If the LM628 or LM629 velocity mode is programmed and motion is initiated, can the motor velocity be changed at any time? The velocity can be changed on the fly, but you cannot program a negative value for velocity. You can program a lower value than the previous value. The motor is accelerated or decelerated at the max accelaration rate until the new velocity is reached. Motor direction can be changed on the fly by changing the direction bit, NOT by changing velocity sign Relevant Part: LM628;LM629 What happens during a velocity mode sequence if a velocity of "0" is programmed? Does the LM629 revert to position mode? If a velocity of "0" is programmed, the motor is decelerated until zero velocity is reached, which means the motor will stop. The device does not revert to position mode but it maintains the position stopped at. Relevant Part: LM628;LM629 What is the "functionality test" referred to on page 4 of the LM628/LM629 Programming Guide (AN-693)? The functionality test referred to under Software Reset Considerations in AN-693, "LM628 Programming Guide", is the code represented by the section of Flow Diagram 2 that starts at Reset interrupts. There is a small chance (1/500,000) the LM628/9 will not reset properly even though a valid hardware reset signal has been applied. This is due to the asynchronicity between the hardware reset signal and the clock. The "functionality test" is simply a way of catching this 1 in 500,000 miss and looping back to execute another hardware reset to correct the problem. See Flow Diagram 2. While it may seem likely that there would be a hidden test mode for the LM628 and LM629, in reality there are no undocumented commands for these devices. Relevant Part: LM628;LM629 Is it possible to change the LM628 proportional, integral, and differential (PID) filter coefficients to zero while operating the device (i.e. on the fly change) assuming a steady state condition has been reached prior to setting the coefficients to zero? Yes, you can change your LM628 filter parameters on the fly using LFIL and UDF commands. The integral summation term will remain the same if you don''t have any position error, but the PID output will be zero because you have set the multiplier coefficient to zero. Relevant Part: LM628 What is the correction to the typographical error found on page 21 of AN-706, which relates to the LM628 and LM629? In the upper left corner of page 21 in AN-706, there is a formula S=at2/2, followed by 2x166,667/58,600. This should be 2x166,667/ 58,600x58,60 Relevant Part: LM628 What is the difference between the LM628 6MHz and 8MHz devices? The 6MHz and 8MHz versions of the LM628 have the same DC and AC electrical specification except for the supply current (Idd). Idd at 8MHz increases to 135mA Relevant Part: LM628 With the LM628/LM629, must the Write data really be held for 120ns after the Write signal goes away? The datasheet shows (page 4) that the Write Data Hold Time is 120ns min, and the Write Timing diagram on page 6 shows this period to be after the 100ns Write pulse goes away. It is not a typo. When the device was tested, it was found that for a write operation to be successful, the data to be witten must be held for at least 120ns after the write signal goes high Relevant Part: LM629;LM628 What is the state of the LM629 PWM magnitude output during power up and reset? During power-up, before the power supply is valid (5V +/- 10%), there is a chance the PWM output will go high. Some customers have complained about this because it can cause an uncontrolled spin of the motor. Once the LM629 has a valid power supply, it gains control of the PWM output and pulls it low. The motor will then stop due to loading and friction. At this point, the control loop is open and there is no signal to cause the motor to move. You can control this by gating the PWM signal between the LM629 and the power amplifier Relevant Part: LM629 How can I put the LM629 back into normal operating mode after an "excessive error" interrupt has been tripped? There are two possible approaches to get the motor moving again after using the RSTI command (see command set for usage) to reset the excessive position error flag. (1) Reset the particular device with RST command, depending on how this may affect your application. (2) Read the current position of the stopped motor using the RDRP command. Use the read position as your new destination position. Start the motor (STT command). Now, change the destination position again to the actual final destination the motor was originally shooting for and start (STT) the motor again Relevant Part: LM629 How can the output of the LM629 be put in Tri-State? The LM629 cannot tri-state its own outputs (sign and magnitude outputs). One way to add tri-state to the outputs is to put tri-stateable logic gates (like noninverting buffers/latches) in series with the outputs and control them (the logic gates) with the system microprocessor Relevant Part: LM629 Can I parallel two LM78xx linear regulators? Paralleling of 3-terminal regulators is generally not recommended because the devices will not share current equally. If, for instance, you try to make a 2 Amp regulator using two LM7812s or two LM7815s, the device with the higher output voltage could be carrying more load than the other. Or even worse, the second regulator may be totally off. The reliability of such a system is poor because of the combination of high temperature and high current in the first regulator. A simple way to improve sharing is to insert a low value resistor (about 0.1 Ω ) in series with each output. The problem with this approach is that load regulation is poor, because the voltage drop across each resistor will vary as the load current varies, in this case 0.1V error for full load of 1A per regulator. A better solution is to use either a linear regulator with a sufficiently high current limit, or a linear regulator controller with an appropriate pass transistor. For this example, the LM1085 quasi-lowdropout (QLDO) linear regulator would be a good integrated solution, or the LP2975 controller could be used for a more customized design. See the online design environment of the Power WEBENCH for recommended regulators to meet your system specifications. Relevant Part: LM7812;LM7815 The surface mount LM79Lxx (and 78Lxx) has multiple V- (gnd) pins. Do I have to connect all of them, or can I just connect to one of them? It is not necessary to connect to all V- pins, but it is necessary to have these pins connected to PC board copper traces for thermal dissipation. Heat is conducted out of the device through the leads. We recommend connecting all pins to as large a block of copper trace as much as possible Relevant Part: LM78LXX;LM79LXX Do both Vout pins on the LM78Lxx micro-SMD package have to be connected? On the data sheet, the micro-SMD connection diagram shows two seperate Vout pins. The pins are connected inside the package. So either or both may be used as a connection to the load. However, for maximum power dissipation, all 8 pins [solder bumps] should be soldered to the pcb using the pattern provided on the data sheet Relevant Part: LM78LXX Is it safe to apply a voltage at the output of a standard linear regulator like the LM78Mxx if there is no power applied at the the input? The LM78M05 is not designed to sink current. If the input pin (Vin) is at ground, then forcing the output to any voltage can cause damage to the internal low current paths in the control circuit. A protection diode connected between the input and output terminals is required to reroute the energy to the Vin pin, thus preventing damage to the regulator. If the input pin (Vin) is an open circuit, then a voltage applied to the output pin will not damage the regulator. The load on the external voltage source will be the regulator internal feedback resistors, 5k-15kohms (depending on voltage option) Relevant Part: LM78M05;LM78M15;LM78M12;LM341 Why is a low Iq so important for automotive applications? There are several applications within a car which need to be kept powered even though the car is not running (i.e. door locking systems, alarm systems, etc). These "keep alive" systems must not drain the battery so regulators with low or ultralow Iq specs are preferred. Relevant Part: LM9076 When using an LMD18200 to control a DC motor, will the back EMF generated by the motor damage the LMD18200? The DC motor will generate back EMF. This is normal and not a problem for the LMD18200 in general. If the load drum has a lot of rotational inertia, however, the back EMF may become a problem if the drum is spinning fast and the system tries to brake or change the direction of the drum abruptly. Assume the drum reaches high speed by having one set of switches ON (e.g., upper left and lower right) for a near 100% duty cycle. The back EMF may reach a voltage near the input supply voltage (depends on the motor characteristics). Braking or changing directions abruptly by swapping the set of ON switches (now upper right and lower left) will abruptly apply a voltage equal to the input supply voltage plus the back EMF across the motor. A load with high rotational inertia will take a long time to slow down (long as in mechanical time constant vs electrical). This means the motor will see the input supply voltage plus the back EMF across its terminals for a long time. This voltage could be close to twice the input supply voltage. Such a voltage for such a time may cause the motor current to shoot past the 6A absolute maximum of the LMD18201 (again, depends on the motor characteristics, inertia, supply voltage, etc.). Violating an absolute maximum specification may damage the LMD18201. The LMD18201 has internal current limiting circuitry that can help protect the device, but this cannot guarantee violating the 6A absolute maximum will not damage the device. The internal current limiting circuitry is really a last-ditch measure to help the device survive occasional unintended fault conditions. It cannot protect against repetitive violations of absolute maximum ratings that may occur as ''normal'' operating conditions imposed by the customer''s system. Relevant Part: LMD18200;LMD18201 How many LMD18201 drivers are needed for each stepper motor (two windings in motor)? Two drivers are required for driving stepper motors Relevant Part: LMD18200;LMD18201 What is the reverse recovery time for the LMD18200/LMD18201 sourcing and sinking power device diode? The reverse recovery time for the sourcing power device diode is typically 70ns with a reverse recovery current of 1A when tested with a full 6A of forward current through the diode. For the sinking power devices diode, it is typically 100ns with 4A of reverse current when tested with a full 6A of forward current through the diode Relevant Part: LMD18200;LMD18201 Would the LMD18200 charge pump be turned off when the PWM is set to low and Brake is set to high? No. Only the four DMOS switches will be turned off. All other circuitry will remain ON Relevant Part: LMD18200;LMD18201 What circuitry is affected when the LMD18200/LMD18201 over-temperature circuit is activated? When the over-temperature limit is detected, the over-temperature circuit turns off only the output module while the rest of the chip remains running. When the output device is turned off at high temperature, it is turned back on when the chip cools down Relevant Part: LMD18200;LMD18201 Using a 48V supply, why are there unexpected failures of the LMD18200? Inductive loads can generate high voltage spikes at turn on, turn off and even during operation. Any time the supply voltage is higher than 36V, unknown failures are usually voltage related. 48V operation is a red flag! The only solution is Transient Voltage Suppressors(TVS). Usually, a TVS on the supply pin is sufficient. But on occasion, depending on the nature of the inductive load, a TVS will also be required on each output pin Relevant Part: LMD18200 Can the LMD18200 and LMD18201 be paralleled to boost the output current? The LMD18200 and LMD18201 can be paralleled, following these guidelines. If you follow (5) through (9) below, the devices will be very close together (physically). The need to keep the devices very close together limits the number that can be paralleled in any practical manner. Except for the BOOTSTRAP 1 pins and BOOTSTRAP 2 pins, connect all pins of the same type together For example, if you''re paralleling 2 devices, drive both PWM inputs with the same signal, drive both DIRECTION inputs with the same signal, connect both OUTPUT 1 pins together and so forth. For each device, connect a bootstrap capacitor between BOOTSTRAP 1 and the paralleled OUTPUT 1 pins. 2 devices require 2 capacitors. 3 means 3, and so forth. For each device, connect a bootstrap capacitor between BOOTSTRAP 2 and the paralleled OUTPUT 2 pins. 2 devices require 2 capacitors. 3 means 3, and so forth. Bypass each device as described in the data sheet. Do not skimp on bypassing. Doing so may destroy the devices. When connecting the OUTPUT 1 pins together, use short, fat, wide traces. Inductance between the OUTPUT 1 pins will kill the devices. When connecting the OUTPUT 2 pins together, use short, fat, wide traces. Inductance between the OUTPUT 2 pins will kill the devices. When connecting the GROUND pins together, use short, fat, wide traces (i.e., a ground plane). When connecting the Vs pins together, use short, fat, wide traces (i.e., a power plane). All devices should be coupled thermally. Coupling them well thermally is critical. Failing to do so may destroy the devices. One method is to sink them to the same heat sink. Relevant Part: LMD18201;LMD18200 To improve LMD18200/18201 system efficiency, is it okay to use external Schottky diodes in parallel with the FET internal diodes? The internal body diode has a forward voltage drop of 1.5 Volts. It is OK to use high powered Schottky diodes across the upper FETs to remove the heat caused by using the internal diodes. The Schottky diodes also have excellent reverse recovery, which reduces the heating of the lower FETs Relevant Part: LMD18201;LMD18200 What is the difference between the LMD18201 and LMD18200? The LMD18200 requires an external resistor for current sensing, while the LMD18201 has a built in current sense and no external resistor is require Relevant Part: LMD18201 How should the LMD18201 driver current be measured while the motor is running? The current via pin 8 is a measure of the ratio of instantaneous current going through the power device. It''s not AC or DC; it depends on the current state of the power device. The ratio is 377uA per 1A of current going through the power device. The power device is on only during the duty cycle of the PWM signal; so as duty cycle varies, so will the voltage you measure across the resistor connected to pin 8. Using a DC ammeter will not give a relevant answer - the reading will be the V/Z of the system it is connected to Relevant Part: LMD18201 When using the LMD18245, how do I pick the Rs value and power rating? The value of the voltage across Rs is a function of the load current level which determines the sensed current value. Rs is used to set the gain of the current sense amplifier. In this case, zero load current will produce one volt across Rs. Remember that the current passing through the 10k Ω resistor is only 500µA(max), the power loss due to the resistor is about 2.5mW and therefore a 1/4 Watt resistor is ok. Relevant Part: LMD18245 Can the LMD18245 be used to PWM control a servo motor? It is not advisable to use the LMD18245 to PWM control a servo motor because of the extraneous circuitry required to accomplish the task Relevant Part: LMD18245 What is the safe minimum value for the LMD18245 off-time? The guaranteed operating range for the off-time is 10 us to 100 ms. This specification can be found as "MONOSTABLE Pulse Range" under "Operating Ratings" in the LMD18245 datasheet Relevant Part: LMD18245 With supply voltage of 36V, why are there troublesome failures on the LMD18245? Unseen voltage spikes are the likely problem. Anytime the supply voltage is around 30V or higher, we recommend a transient voltage suppressor from each output to ground. The TVS can be rated for any voltage between the actual supply voltage and the 55V maximum recommended operating voltage. Motors sometimes generate random high voltage high speed spikes during stopping, starting and speed changing. Subsequent failure analysis usually indicates an overvoltage condition. A TVS will eliminate this failure mode Relevant Part: LMD18245 What is the approximate efficiency of the LMD18245? There is actually no hard data on this, but using the data sheet parameters provided as Iq = 15mA(max)@Vcc = 20V, Iload = 3A (max Load), Rds(on) = 0.6 Ω (max) , the worst case efficiency of the device (not including external components) can be calculated to be approximately 80%. Relevant Part: LMD18245 For the LMD18245, do Pins 4 through 8 specify a four bit digital number that corresponds to an analog voltage? Yes, the four-bit digital inputs determine the fractional of portion VDAC REF voltage (eg +5.0V) as (1111 = 5.0V, 0000 = 0V) that is fed to the comparator input Relevant Part: LMD18245 If the LMD18245 VDAC REF = 5.0 V, does M4-M1 = 1111 correspond to Imax = Vcc/R (the maximum current level obtainable)? Not necessarily. It means that depending on the gain set for the current sense amp the sensed current should develop a 5V voltage across Rs before chopping is initiated. See "TYPICAL OPERATION OF A CHOPPER AMPLIFIER" and "THE CURRENT SENSE AMPLIFIER" sections in the LMD18245 datasheet on how the the chopper amp operates and sense current is set up, respectively Relevant Part: LMD18245 When using the LMD18245, is a negative current reference achieved by switching the DIRECTION level at pin 11? No. Switching DIRECTION level changes the direction of current flow across the load in the H-Bridge. See figures 1, 2 and 3 in the LMD18245 datasheet. The current is still sensed as positive Relevant Part: LMD18245 How is the LMD18245 chopping frequency calculated? The RC sets the time the bottom switch is off and current recirculates in the motor winding and decays towards zero. The time the bottom switch is on and current ramps up is the other part of the chopping period. This time depends on a number of factors such as the input voltage, the motor L and R, the off time set by the RC, the current trip point, etc. During the off time, current in the motor winding decays away from the trip point. How long it takes the current to ramp back up to the trip point depends on how far the current decayed, the motor LR characteristics, the input voltage, etc. The LMD18245 only controls the off time. Relevant Part: LMD18245 Should the LMD18245 control inputs be given additional protection, in case of signal applied when power is off, or in case of ESD? The control inputs are high impedance inputs and their input current spec is +/-10uA. Most microcontroller output pins can source 5 or 6 mA or less out of most of their I/O pins. But most microcontrollers also have a few high current I/O pins. When a logic high is on the microcontroller output and the microcontroller sourcing current into the control pins of the LM18245 is less than 5 or 6 mA, no damage whatever will be done. If the current can be 8 to 10mA [or more] then a current limiting resistor should be used. If the output of the microcontroller is low, then there is no issue because a low output connected to an unpowered input will produce little current flow. The same comments apply to all the DAC inputs. The inputs are not particularly ESD sensitive so no additional protection is needed for ESD. But noise or voltage spikes on the input lines [below the 12V maximum rating, of course] always have the chance to make the part act differenly from what you intended. So if you can see noise or hash on the inputs, a small R-C filter is a good idea. You can determine the need just by looking at the inputs with an oscilloscope. These are not fast parts so noise that can affect you will tend to be in the low microsecond range, not in the nanosecond range. One last caution. Althought the part is rated at 55V, if the supply voltage is above 30V, we recommend Transient Voltage Suppressors on the supply pins. The reason is that many or most motors will generate a hefty turn-off and sometimes a turn on or operating voltage spike that is far about what you expect. V= L di/dt is a powerful thing if di/dt is really fast. Relevant Part: LMD18245 What happens if the LMD18245 loop bit is not set when all 8 instructions in the instruction RAM are being used? When all 8 instructions are used, the program will loop back to instruction 0 whether loop bit is set or not Relevant Part: LMD18245 When using the LMD18245, can I use a separate digital controller to update the reference current level? Yes, it is o.k. to use a separate controller to update the digital inputs to change the load current level (in effect, Rs voltage level) before chopping, but make sure that there are no timimg issues Relevant Part: LMD18245 How do you set up the adjustable output voltage of the LMS1585 or LMS1587 using external resistors? To set up the output voltage using the adjustable version, please apply the formula: Vo= Vref(1+ R2/R1) + Iadj x R2, where R2= 100 Ω and Vref=1.250V (see product datasheet) R2 is the resistor which connects between the adjust pin and ground . The value of R1, which connects between the output and the adjust pin, can be calculated using the formula rearranged to solve for R1: R1 = (R2 x Vref)/(Vo - Vref - (Iadj x R2)) Relevant Part: LMS1585;LMS1587 What is the difference between "Vout" and "OUTPUT" for a low-dropout regulator? The "Output" is physically and identically the same as "Vout". [There was an unfortunate change of terms in the figures on the LMS1585A data sheet. The center pin (Output) is physically the same as Vout shown on the Tab of both the TO-220 and the TO-263 packages. Relevant Part: LMS1585A Does the LMS1585 have an internal diode from output to input? If so, what is its current-handling capacity? National''s LMS1585 is equipped with an internally-connected diode between the input and output pins, like the other "1585" devices made by our competitors. It can handle 20 A for short durations (microseconds). Our LMS1585 can handle 50 to 100 A for one microsecond and about 7 A of continuous current Relevant Part: LMS1585 Why is an input capacitor necessary when a battery is used as an input source to an LDO? Any low-dropout (LDO) regulator requires low source impedance to assure stability. A battery looks fairly reactive at high frequencies; thus a capacitor at the input of the LDO is necessary. Can you use the same 1uF capacitor for three regulators? Yes, as long as the capacitor is physically located within about one cm of the input pin of all of the LDO regulators. This is so that trace impedance does not isolate the capacitor from the input. If that happens, the LDO will not have the low source impedance it needs to be stable. In general, both input and output caps on an LDO should be within one cm of the pin they are connected to. Relevant Part: LP2950;LP2951;LP2952;LP2953;LP2954;LP2980 At what temperature will the LP2950/LP2951 LDO regulator shut down, how will it shut down, and at what temperature will it resume operation? Does it affect the life of the part to be put in thermal shut down frequently? The LP2951 will go into thermal shutdown at approximately 170 to 180degC. It will turn back ON when it cools down 5 to 10 degrees lower than the shutdown temperature. Shutdown is accomplished by switching the base of the series power transistor to OFF. Repeated thermal overload condition is not a recommended operating scenario for the LP2951 because this will decrease the reliability of the device. Relevant Part: LP2951;LP2950 How can the LP295x series LDOs be used for current limiting (i.e. as a current source)? The LP2951 can be used as a current SINK or a current limiter (see pg 12 of the LP2951 datasheet). The same technique can be used for the LP2952/3 (pg 13 of the LP2952/3 datasheet) Relevant Part: LP2951;LP2952;LP2953 When the LP2951 is used as an adjustable regulator, what do you do with pin 2 (Sense) and pin 6 (Vtap)? When used in the Adjustable Regulator application, pin 2 (Sense) and pin 6 (Vtap) should be left open. These pins are connected to the internal resistor divider used to program the output voltage to its standard value Relevant Part: LP2951 What controls the precision of the LP2951 output voltage? The precision of the output voltage setting is limited by the tolerance spec of the LP2951 and not by the tolerance of the external resistive feedback divider. If perfect resistors are used, the output voltage tolerance will still be that specified for the regulator, i.e., 0.5%. However, if higher tolerance resistors are used, the resulting output voltage tolerance can be estimated as the sum of the tolerance of each resistor and the tolerance of the regulator for the grade selected. Relevant Part: LP2951 Can two LP2951''s be put in parallel to boost current? Paralleling the LP2951 to get more current is a poor solution as this requires balancing the current between the two devices via small resistor (0.1Ω) in series with each of the outputs. This results in poor regulation because of the voltage drop across the resistor. A better approach is, of course, to use a higher current rating device. The other approach we suggest is using PNP transistor to boost the current, the down side to this approach is the addition of Vbe drop which means the minimum input must be higher than output by about 1.5V. Relevant Part: LP2951 How would the LP2951 error output behave during a 100ms power dip? The output capacitor will have to try to hold the output up long enough to prevent it from falling too low in order to prevent the flag from going low. For example, assume the output is set to 3.7V. The error flag can activate at a voltage as high as 25 mV below the 1.23V reference, which is 75 mV below the 5V output. So, the output capacitor should allow the output to droop NOT MORE than 75 mV in the 10 ms interval at the rated load current. If the load current, for example, is 50 µA: I = C dV/dT, 50 µA = C x 0.075/0.01 It follows that the output capacitor must be not smaller than about 7 microfarads. Since the output capacitor is 100µF, the output should stay up long enough that the flag will not activate during the transient. There is one word of caution: If the voltage dip on the Vin line is very fast (fast fall time), it will couple through the internal circuitry and cause a deviation in the reference voltage, which can cause a brief activation of the error flag. The way to prevent this is to filter the Vin to prevent very fast voltage spikes from reaching the Vin pin. Relevant Part: LP2951 Can the LP2953 auxiliary comparator input be left floating? Can this cause oscillation? Comparator inputs should never be left floating. If not used, the input should be tied to ground. Not terminating the input could cause oscillation, depending on how noisy the environment is, as well as other things unique to the specific part (like what the impedance looking into the comparator input is). However, we do know that the input of the Auxiliary Comparator is the same circuit topology as the Shutdown input. Since the input pin is a PNP base, one would think that it would be OFF if left unconnected (since no base drive current could flow). However, all devices have some leakage current (increasing with temperature) which can turn ON this PNP. This is why it must be actively terminated to operate in a predictable manner. In summary, the Auxiliary Comparator input must be terminated (either pulled up to a fixed voltage or tied to ground) to be sure that the output is not toggling between states. Based on the operation of the Shutdown input, it is fairly certain that some devices will have enough leakage current through the PNP device to cause it to switch states. Relevant Part: LP2953 How can I configure the LP2954 for 2.5V output and 1A load using an external pass transistor? Although it can be done, it is not generally recommended to use an external pass transistor to boost the LP2954 output current capabilities to 1A. This design approach yields an inefficient regulator and the pass device will not benefit from the LP2954 thermal and overload potection features. The use of the LM2941, which is also an LDO and rated at 1A, is a much better and more practical solution. Relevant Part: LP2954;LM2941 How do you eliminate or reduce overshoot on the LP2975 LDO regulator output? Overshoot is typically caused by marginal stability. A good way to test stability is to put a scope on the output and load step the output (go from no load to full load abruptly by tapping the load resistor lead down onto the output terminal). Marginal stability will show itself as ringing and overshoot on a load step. Optimum compensation is achieved when the output transient dies out after only one ring. In the question that prompted this FAQ, the load current was 500mA maximum. The output capacitor should be at least 250µF (330µF would be OK) to set the first pole at around 100 Hz. The ESR of the output cap should be about 0.25 - 0.30 Ω to set the main compensation zero at around 2k - 3k Hz. Best results would be seen with a 330µF solid Tantalum output capacitor with an external resistor of about 0.2 Ω placed in series with it. Aluminum electrolytics are not recommended because their ESR varies too much with temperature. The feedforward capacitor should be about 1000 pF - 1200 pF. Relevant Part: LP2975 How can I adjust the output voltage of the LP2980 to 6.5V? The LP2980 is available in an adjustable version. Just type in LP2980-ADJ on the product search and the part will come up in the search results. The output can be adjusted from 1.23-15.0V. Using a fixed-output-voltage version of the LP2980 as an adjustable regulator will compromise the stability of the regulator Relevant Part: LP2980;LP2980-ADJ How is the LP2989 regulator turn-on time affected by a capacitor on the bypass pin? The Typical Performance Characteristics in the LP2989 datasheet show Turn-ON Waveforms for Cbypass=0 and Cbypass=10nF. With no bypass capacitor (Cbypass=0) the typical turn-on time is 50us, while with 10nF the turn-on time is typically 1-2msec Relevant Part: LP2989 Is the LP3878-ADJ stable with ceramic capacitors? Yes, the LP3878-ADJ is designed to be used with low ESR ceramic capacitors. Relevant Part: LP3878-ADJ When a 10 nF external capacitor is applied to the bypass pin, what noise improvement can I expect? Typically, the output noise will reduce from 85 µV to 18 µV. Relevant Part: LP3878;LP3878-ADJ How does the dual rail feature work? A. Fabricated on a CMOS process, these devices operate from two input voltages: Vbias provides voltage to drive the gate of the N-MOS power transistor, while Vin is the input voltage which supplies power to the load. The use of an external bias rail allows the part to operate from ultra low Vin voltages, and there by reducing the wasted power loss that would occur converting from a higher voltage. Relevant Part: LP3881;LP3882;LP3883;LP38841.LP38841-ADJ;LP38842;LP38842-ADJ;LP3883 What is a dual-input LDO? A dual-input low-dropout linear regulator (LDO) uses two separate input voltages for power (Vin) and control (Vbias) circuitry. The low-current Vbias input, typically 5V, powers the control circuitry, while the high-current regulator output is powered from very low Vin (e.g. 2.5V or 1.8V). The allows the regulator to operate with relatively high efficiency, as the lower the power input voltage, the less power is dissipated by the regulator Relevant Part: LP3891;LP3892;LP3983 What is LDO mode? In the case where there is no battery in the portable device, the device can still operate with the wall adapter plugged in the outlet or with the car adapter activated. The LP3947 detects the absence of a battery and automatically configures the internal charger circuit as a linear regulator that acts as a power supply to the portable device. The supply voltage is equal to the termination voltage of the battery. This feature is valuable as manufacturers don''t require a battery during assembly test. Relevant Part: LP3947 Does the LP396X CMOS LDO require a minimum load for proper regulation? No, the LP396x CMOS LDO works well without any external load. Although it does requires a small load, this is provided by the internal circuitry and the quiescent current of the regulator Relevant Part: LP3961;LP3962;LP3963;LP3964;LP3965;LP3966 What is the specification for the LP3962/4/6 adjust pin current? The LP3962, LP3964, and LP3966 are low-dropout regulators using CMOS technology. The current on the adjust pin is in the order of pico-amps (mostly due to the leakage of the ESD structure on this pin). This current is so small that it is neglible in most applications compared to the current drawn by the feedback resistors, and it is impossible to cost-effectively measure or test it Relevant Part: LP3962;LP3964;LP3966 In the LP396x adjustable output Typical Application Circuit, what should be the value for Cf (across R2)? We recommend 68pF to 100pF for this feedforward capacitor, with R1=10k Ω . Relevant Part: LP3964;LP3965;LP3966 Can I use the sense pin of the LP3964, LP3965, or LP3966 fixed-voltage LDOs to adjust the output voltage? No. The LP3964, LP3965 and LP3966 fixed voltage regulators are designed to deliver their specified output voltage. The function of the sense pin is to maintain a precise output voltage in order to compensate for possible voltage drops in the high current output path. If the sense pin is driven to a value below its internal sense voltage, a diode clamp will conduct and the output will not be properly controlled. If voltages other than the standard fixed voltage options are desired, consider using the adjustable voltage versions of these regulators Relevant Part: LP3964;LP3965;LP3966 What are the requirements for an LDO regulator in a multi-function power management IC? The LDO must be able to provide a stable output low voltage output with minimum dropout at the rated output current. It should exhibit very low noise and a high power supply rejection ratio (PSRR). Its input voltage should be in the range of a single Li-ion or three NiMH batteries. Furthermore, it should have fast turn-on and turn-off to allow output power sequencing that does not affect system operation. Relevant Part: LP3970;LP3971 What is a typical application for a multi-function power management IC? Multi-function power management ICs are members of a new generation of devices that manage the power for special application processors. An example is Intel''s XScale® processor designed to optimize low power consumption and high performance processing. Its micro-architecture state-of-the-art processing technology enables it to produce mW/MIPS performance. This processor supports mobile handheld devices, including: pocket PCs, smartphones, MP3 players, and portable GPS devices. Relevant Part: LP3970;LP3971 What is the possible mix of power sources in a multi-function power management IC? One approach can include a combination of switch-mode dc-dc converters and LDO (low dropout) linear regulators. They can be controllers that employ external power semiconductor switches or regulators with an internal power MOSFET switch. One possibility is a 3.3V output and a 1.8V output for processor and logic applications. Furthermore, present integration techniques make it possible to include CPU core and memory power as well as other functions, such as power-on and power-off sequencing, and an I2C serial interface for external control. Relevant Part: LP3970;LP3971 What are the requirements for a switch-mode buck converter IC use in a multifunction power management IC? The IC should be optimized for powering ultra-low voltage circuits operated from a single Li-ion cell or three NiMH batteries. For most applications it should be capable of a 500mA to 1A output. To minimize power drain and conserve battery life, the IC should have some form of power switching at low power, i.e., either PFM (pulse frequency modulation) or pulse skipping. By switching at frequencies from 1MHz and higher, the converter can conserve space by employing small physical size capacitors and inductors. Relevant Part: LP3970 What does low dropout mean? "Low dropout" refers to the smallest difference between the input and output voltages of a voltage regulator that allows the IC to still regulate the output voltage. That is, the LDO device regulates the output voltage until its input and output approach each other at the dropout voltage. Ideally, the dropout voltage should be as low as possible, to allow the input voltage to be relatively low and still assure regulation. This keeps the regulator input-to-output voltage difference low, minimizing power dissipation and maximizing efficiency. And because of this low dropout voltage, the LDO extends battery life by permitting the battery to be discharged all the way down to a few hundred millivolts of the desired output voltage. Relevant Part: LP3990;LP3991;LP3992;LP3993;LP3994;LP3997;LP3999 What does power-supply ripple rejection (PSRR) mean for an LDO regulator? Power-supply ripple rejection (PSRR) is a measure of the LDO’s ability to prevent output-voltage fluctuations caused by variations in input voltage. PSRR is usually specified at a specific frequency, such as 60-dB rejection at 120 Hz. Low-ESR output capacitors and added bypass capacitors improve the PSRR performance. Battery-based systems should use an LDO that maintains high PSRR at low battery voltages. Relevant Part: LP3990;LP3991;LP3992;LP3993;LP3994;LP3997;LP3999 What is the source of noise in an LDO regulator? The LDO''s internal bandgap voltage reference is the source of noise, usually specified as microvolts rms over a specific bandwidth, such as 30 µVrms from 1 to 100 kHz. This low-level noise causes much less of a problem than the switching transients and harmonics from a switch-mode converter. In the figure below, the LDO has a voltage-reference bypass pin to filter noise using a capacitor to ground. Adding the datasheet-specified input, output, and bypass capacitors usually results in a non-problematic noise level. Relevant Part: LP3990;LP3991;LP3992;LP3993;LP3994;LP3997;LP3999 What output voltages do LDOs provide? LDOs are available with either an adjustable or fixed output voltage. To assure regulation, the output voltage must be less than the minimum input voltage less the dropout voltage. Typically, fixed-output types have an output voltage that has been pre-set between 1V and 5V, with a tolerance of ±2% to ±6%. Adjustable-output LDOs have an output voltage that''s set with two external resistors, also typically between 1V and 5V. Some LDO families are available in a full range of output voltages in 100- or 50-mV steps, for example, 2 to 6 V in 100-mV steps. This wide range of output voltages is made possible by laser-trimming the ICs during their manufacture. Relevant Part: LP3990;LP3991;LP3992;LP3993;LP3994;LP3997;LP3999 What''s the impact of a regulator''s low dropout on its input voltage? A regulator''s dropout voltage determines the lowest usable input supply voltage. That is, although the specifications may show a broad input-voltage range, the input voltage must still be greater than the dropout voltage plus the desired output voltage. For a 200-mV-dropout LDO, the input voltage must be above 3.5 V to produce a regulated 3.3-V output. Relevant Part: LP3990;LP3991;LP3992;LP3993;LP3994;LP3997;LP3999 What''s the impact of low dropout on the LDO regulator''s output voltage? With an LDO, the difference between input voltage and output voltage is often kept small, to minimize power loss, so the output voltage must be tightly regulated to be sure the input voltage is sufficient to assure regulation. Plus, transient response must be fast enough to handle loads that can go from zero to tens of amperes in nanoseconds, and still keep the output in regulation. The accuracy of the output voltage is affected by changes in input voltage, output load current, and temperature. Temperature changes cause output voltage variation because of the effects of temperature on LDO voltage reference, error amplifier, and its sampling resistors (R1 and R2). Relevant Part: LP3990;LP3991;LP3992;LP3993;LP3994;LP3997;LP3999 Is external component placement important for an LDO regulator? Input and output capacitors should be placed as close as possible to the input and output pins, respectively. If the LDO employs external current sensing, board-trace resistance should be as low as possible because it increases the apparent dropout voltage. This is more of a factor at higher output currents. LDOs with an external MOSFET should maintain as short as possible distance between the gate pin and the LDO, because a long trace can create a parasitic inductor, resulting in ringing of the output voltage. Relevant Part: LP3990;LP3991;LP3992;LP3993;LP3994;LP3997;LP3999 How does packaging affect LDO selection? LDO selection involves meeting the required performance of the specific application with regulators having package types which can fit into the available PCB space. This tradeoff includes the LDO''s power dissipation, output current, dropout voltage, and pin count. Normally, the higher the dissipation and output current, and the lower the dropout voltage, the larger the die becomes and its required package. Also, LDO reliability depends on the package’s thermal resistance from junction-to-ambient (RJA), which determines its temperature rise per watt of dissipation. The lower the thermal resistance, the more efficiently the package can eliminate heat and the better the reliability. Relevant Part: LP3990;LP3991;LP3992;LP3993;LP3994;LP3997;LP3999 Is PowerWise just for cell phones, or for PDAs and other electronic devices as well? National''s PowerWise technology can be used in all electronic handheld devices. It can also be used in any applications where energy conservation is needed. Relevant Part: LP5550 What is adaptive voltage scaling? What are the benefits? Adaptive voltage scaling relies on an embedded Adaptive Power Controller (APC) that tracks the performance variations of the system processor. The APC accurately communicates the performance (frequency), temperature, and process variations of the system processor over a PowerWise high speed low power interface to an external compliant power management chip. The power management unit then adaptively adjusts the supply voltage of the system processor based on performance requirements. Relevant Part: LP5550 What is the PowerWise® technology? National Semiconductor''s PowerWise technology is a radically new system-level solution that promises to increase battery life in mobile phones by 25% to 400% in several stages. The technology will be developed in phases. The first phase of this technology targets ARM Powered® system-on-a-chip devices. PowerWise technology is a three-part solution: embedded intelligence in the mobile phone''s digital processors such as baseband and applications processor; an open standard interface; and companion power management chips. Relevant Part: LP5550 I need to convert 5V into 3.3V. Should I use a switching regulator or linear regulator? The most appropriate regulator for a 5V to 3.3V conversion depends on the total current you need to supply, cost constraints, output noise, and thermal restrictions in the end product. The efficiency of a linear regulator in this application is 60% up to an ideal of 66% (3.3/5.0). The more current you need to supply the more heat you generate, so linear regulators usually make the best sense when currents are low. If cost is the primary concern, then use a linear regulator. National has linear regulators which will perform this function with ratings of 50mA to 5Amps (LP298X, LP295X, and LM3940) and a controller (LM3411-3.3) which would allow designs in excess of 5Amps. Switching regulators are much more efficient than linear regulators. Switchers will vary in efficiency. An LM2595 Simple Switcher would have an efficiency of around 75% at 1Amp. Use synchronous rectification (where the diode is replaced by an active FET) and efficiency can reach 90% at 1Amp. The LM2653 is a part that works like this. With the improved efficiency you may be able to design without a heat sink or operate to a higher ambient. These parts cost much more than a linear regulator and are more difficult to design. But 5V to 3.3V is a real common design request and canned solutions are available in the datasheets or by using design software (Switchers Made Simple or WEBENCH). Another solution is the LM3350, a switched capacitor fractional converter which converts 5V to 3.3V at up to 50mA . These simple switched capacitor converters are unregulated so the output will droop as load is applied. Output impedance is around 4ohms. The LM3352, LM3354, and LM3355 are regulated switched capacitor buck-boost converters which can take an input voltage of 2.5V to 5.5V and convert it to a regulated 3.3V at up to 50mA-200mA (depending on device). One thing you do need to worry about with switching regulators or switched capacitor converters is noise. The output noise can be reduced with a post filter in extremely sensitive applications. Noise will also be conducted to the input, so use proper input capacitors. The datasheets of the parts mentioned above discuss proper design procedure. Relevant Part: REGULATOR;LDO;SWITCHERS;SWITCH-CAP What is the "quiescent current" of a linear regulator? The quiescent current, Iq, is the difference between the input current Iin and the load current Iout, measured at minimum load. In other words, Iq = Iin - Iout. For fixed-voltage regulators, Iq is the ground pin current Ignd. For 3-terminal adjustable-voltage regulators, such as the LM317, Iq is the sum of the Adjust pin current and the current through the feedback resistors. The databook definition of quiescent current is: "That part of positive input current that does not contribute to the positive load current. The regulator ground lead current." Quiescent is defined as "at rest, peaceful, motionless", so it follows that for linear regulators, it makes sense to define quiescent current as the current that the part consumes when it is connected to VIN but held OFF by the S/D pin. If it does not have S/D pin, then it is the current it draws with VIN connected but supplying no load current. If load current is being supplied, the term "ground pin current" is typically used. Relevant Part: REGULATOR;LDO What is the difference between a "standard" 78xx/79xx/LMx40/LMx20 type regulator and an "LDO" type regulator? A "standard" regulator (78xx/340 series) requires Vin to be at least 3V higher than the output voltage. For a standard 5V regulator to maintain a 5V output, Vin must be at least 8V. However, a "Low Dropout" regulator (LDO) only requires a difference of about 0.7V between Vin and Vout to maintain a regulated output. With an LDO, Vin can fall all the way to 5.7V before the LDO regulator starts to fall out of regulation. For a battery operated circuit, the battery voltage now can fall to 5.7V, versus 8V for the standard regulator before regulation is lost. This will extend the life of the battery, since you can "squeeze" the last remaining charge from a battery. The difference between the minimum input voltage and the desired output voltage is the dropout voltage. Dropout voltages for standard and LDO regulators depend on the type of pass device used in the regulator. Standard regulators use an NPN pass device, and to keep the NPN and its drive circuitry active, it needs at least 2Vbe + Vsat, or about 2V, from collector (Vin) to emitter (Vout). The LDO regulators use a PNP pass device, which can be operated near saturation [i.e. with only a few hundred millivolts from emitter (Vin) to collector (Vout)]. To allow for variation from device to device, these regulators have dropout voltages guaranteed to be less than some higher number, typically 3V for standard regulators and 700mV for LDOs. Relevant Part: REGULATOR;LDO What is an LDO (Low Drop-Out) Regulator? An LDO is a type of linear regulator. A linear regulator uses a transistor or FET, operating in its linear region, to subtract excess voltage from the applied input voltage, producing a regulated output voltage. Dropout voltage is the minimum input to output voltage differential required for the regulator to sustain an output voltage within 100mV of its nominal value. LDO (Low Drop-Out) regulators for positive output voltages often use a PNP for the power transistor (also called a pass device). This transistor is allowed to saturate, so the regulator can have a very low drop-out voltage, typically around 200mV compared with around 2V for traditional linear regulators using an NPN composite power transistor. A negative-output LDO uses an NPN for its pass device, operating in a manner similar to that of the positive-output LDO`s PNP device. Newer developments using a CMOS power transistor can provide the lowest drop-out voltage. With CMOS the only voltage drop across the regulator is the ON resistance of the power device times the load current. With light loads this can become just a few tens of millivolts. Several low-dropout regulators are supported by the Power WEBENCH online design environment. In addition, LDOs can be selected by specification using the parametric selection guide. Additional information about power management products and applications can be found at Power.National.Com. Relevant Part: REGULATOR;LDO Can I directly replace a "standard" regulator with a "low dropout" (LDO) regulator? For the most part, yes. An LDO can replace a standard linear regulator, but there are some things to watch out for:   (1) Some LDOs may have a lower maximum input voltage than their comparable "standard" regulators. Carefully compare all of the datasheet specifications.   (2) The outputs of most LDOs require a specific type of capacitor with the proper ESR rating placed on the regulators output, otherwise they WILL oscillate. Be sure to read the "External Capacitors" section in the Application Hints section of the LDO''s datasheet. Ceramic capacitors are NOT recommended.   (3). With some 5V LDOs, if the input voltage falls below 5V, the current drawn by the regulator may start to increase sharply. Be sure to look over the "Quiescent Current vs. Input Voltage" graphs in the datasheet Typical Characteristic Curves. Relevant Part: REGULATOR;LDO Why is there a difference in the bandwidth and stability of the LDO, Quasi-LDO, and standard NPN regulators? The standard NPN linear regulator tends to have wide bandwidth and be insensitive to output capacitive loading. The LDO (PNP) regulator tends to have lower bandwidth and require specific output capacitors to maintain stability. A Quasi-LDO regulator has characteristics somewhere between the NPN and LDO. The reason for these differences has to do with the orientation and type of the output transistor. In an NPN regulator, the input voltage is applied to the collector and the load is placed on the emitter. In an LDO, or PNP-based design, the input voltage is applied to the emitter and the load is applied to the collector. If you mentally turn the circuit on its side, you will see that the standard, or NPN-based regulator forms an emitter follower. The overall circuit generally has a gain of less than 1. With the PNP LDO circuit, the load impedance acts as a collector resistance. The LDO circuit creates a common collector amplifier, with the load impedance acting as a collector gain resistor. At some combination of load current (re) and load impedance (Rc), you will create an amplifier out of the output stage - and a poor quality one at that. This extra gain and phase shift by the output stage creates the condidtions for severe oscillations. The capacitor on the output is required to shunt the load impedance (Rc) and keep the gain below 1. This requires the output capacitor to have very low series resistance. Since we are only talking about a few ohms, standard off-the-shelf electrolytics cannot be used because of their high internal resistance (ESR). Tantalum, Ceramic or special low ESR output capacitors must be used. Because not every regulator design is the same, some regulators require certain types of capacitors with certain ESR ranges. The regulator manufacturer will usually specify the type of capacitor that should be used, or the range of acceptable ESR for the capacitor. Heed their warnings and use the recommended capacitor types. When replacing a "Standard" regulator with a LDO device in an exisiting design, make sure the exisiting output capacitors meet the regulator manufacturer''s requirements. Relevant Part: REGULATOR;LDO What are the important points in heat management of regulators? There are three key points to heat management that are especially critical in linear regulators: (1) Don''t burn yourself (the surface of the regulator can get hot, especially the metal tab!); (2) Don''t melt something next to the regulator (same reason); (3) Don''t plan to run the regulator hotter than its Operating Maximum Temperature (it will shut down). Heat affects more than just the regulator. Heat conducted down a poorly heatsinked regulator''s leads can damage the PCB material and solder connections over time. Nearby electrolytic capacitors may become damaged at the high temperatures. Heat management of a regulator follows a simple equation:    Tj = Ta + (Pd x ThetaJA) where Tj is the internal (junction) temperature of the IC, Ta is the ambient temperature near the IC, Pd is the power dissipated by the IC, and ThetaJA is the thermal resistance from the internal junction of the IC to its ambient environment. "Heatsinking" is the process of reducing ThetaJA so that the IC internal temperature stays within its legal range during normal operation. Note that if enough power is dissipated in the IC, depending on package choice, it may not be possible to heatsink the regulator enough to keep it within its Operating Ratings. See the datasheet of the regulator in question for details about heat sinking and thermal resistance. This information may be in the notes that follow the Electrical Characteristics, or may be in a separate section in Applications Information. For an example of thermal calculations and heatsinking for an LDO in a TO-220 package, see the LP2957 datasheet (Application Hints, Heatsink Requirements section). For an example with the dual-in-line and standard surface-mount packages, see the LP2952/2953 datasheet (Application Hints, Heatsink Requirements section). The online design environment of the Power WEBENCH provides recommendations for heatsinking of LDOs and switching regulators based on your design requirements. In addition, WEBENCH''s WebTHERM offers online thermal modelling for many switching regulator designs. Relevant Part: REGULATOR;LDO What is efficiency? Efficiency is the proportion of input power delivered to the load: N= Pout/Pin = Pout/(Pout + Ploss). Pout is the output power, Vout x Iout. Ploss is the power dissipated by the regulator (including any necessary components external to the regulator IC). For a linear, quasi-LDO, or LDO regulator, Ploss = (Vout x Iout)/(Vin x Iout + Vin x Ignd). Ignd is the ground pin current of the regulator IC at the specified load current. If Ignd is much less than Iout, Ploss = approx. Vout/Vin. For switching regulators, the value for Ploss should be calculated based on losses during the switch on-time, losses during the switch off-time, and losses during the switch transitions, factoring in their relative time durations. Relevant Part: REGULATORS How do I calculate the values needed for the feedback resistors of adjustable linear regulators, such as the LM317? Two resistors are used to set the value of the output voltage for adjustable regulators. For adjustable standard linear regulators, such as LM317 and LM337, R1 is the resistor between Vout and ADJ, while R2 goes from ADJ to ground. For adjustable low-dropout regulators such as LM2941 and LP2951, as well as the SIMPLE SWITCHER converters, R1 is the resistor between ADJ and ground, while R2 goes from ADJ to Vout. (The difference in these configurations is due to variation in how the regulator''s internal reference and error amplifier are arranged.) Check the datasheet for your specific product for the arrangement of its feedback resistors. The values of the resistors are determined from this formula: Vout = Vref (1 + R2 / R1). R1 is generally specified on the datasheet, and you must solve for R2. A more precise form of the equation allows for adjust pin bias current: Vout = Vref (1 + R2 / R1) + (Iadj * R2). Vref usually is 1.25V for linear (LM317/337 type) regulators and 1.23V for SIMPLE SWITCHER converters. Verify the typical reference voltage value for your device on its datasheet. For linear regulators, R1 is usually 240 Ω , and SIMPLE SWITCHER converters can have a value between 1K and 4K, with 2K preferred. Iadj is typically 50µA for the LM317/337. See Application Note AN-181, "3-Terminal Regulator is Adjustable", for more information about the LM317. Relevant Part: REGULATOR What is/are the disadvantage(s) of paralleling two or more linear regulators? There are many disadvantages to paralleling multiple linear regulators: - Higher cost because of duplication of components - More board space required - Terrible load regulation because of the required series resistance - Stability problems - Potential to blow up under short-circuit as the two regulators short circuit control loops may try to buck each other. To avoid these disadvantages, it is much better to use a higher current device or use an external pass device to increase load current capability. Relevant Part: REGULATOR How do current limit and thermal protection work together to protect the regulators? If you "short" or "overload"` a linear regulator, it will go into constant-current mode (dropping the output voltage to maintain a constant load current). However, if the regulator is not heatsinked well, it will soon overheat. As the regulator exceeds its maximum junction temperature ("overheats"), the thermal limiting will then engage, and it will now start cutting back on the supplied load current. As the load current drops, the heat dissipated by the regulator drops, and it eventually reaches a point of equilibrium between heat generated and output current limiting. This all depends on the heatsinking and output current vs. temp curves. The regulator never completely "shuts off" (like a circuit breaker). It just starts "backing off" into a safe region, trying to protect itself (and the load). It will always be trying to source something into the load. When the load current is reduced below current limit AND the device cools below the thermal limit, the regulator will raise the output voltage until normal operation is resumed. Relevant Part: REGULATOR What is "On/Off" or "Shutdown" on a regulator? An On/Off or Shutdown feature allows the regulator to be turned on or off while power is applied to its input. While in the Off or Shutdown mode, the regulator''s supply current drops to a low level as the output stage is disabled but the internal bias circuitry is still operational. When turned On again, the regulator regains regulation of the output voltage much more quickly than it would if the input supply voltage had been switched off then on. If there is a bar shown over the "Off" part of On/Off or over "Shutdown", the regulator is turned on with a logic high. Otherwise, a logic low enables the regulator Relevant Part: REGULATOR What is Dropout Voltage? Dropout Voltage is minimum voltage required across the regulator (Vin - Vout) to maintain a constant, specified output voltage. At some point, the difference between Vin and Vout will be low enough that the regulator cannot maintain regulation. The "dropout voltage" listed in the datasheet is the Vin-Vout voltage at which the output voltage drops a specified amount. The actual dropout voltage will vary over temperature, input voltage and load condition Relevant Part: REGULATOR What is Ripple Rejection? Ripple rejection is the change in output voltage due to a small signal change in the input voltage, usually expressed in mV/V or dB. It is often improved by the use of an input capacitor with low series resistance (ESR) at the ripple frequency. For many low-dropout linear regulators, it is also improved by the use of a larger output capacitor with low ESR. Consult the product datasheet for capacitor selection guidelines Relevant Part: REGULATOR Why does my linear regulator get hot? The regulator gets hot because it regulates the energy delivered to its load. A linear regulator drops an input voltage to a lower, regulated output, while conducting a current nearly equal to the load current. It must dissipate the excess energy, which is:    Pd = (Vin - Vout)*Iout (Pd is in Watts) This energy is dissipated in the form of heat - the regulator die heats up above the ambient temperature with a temperature rise that''s proportional to the power dissipated, and to the thermal resistance from inside the die to the ambient environment:    Trise = Pd*ThetaJA (ThetaJA is in degC/Watt) The die has an allowed Operating Rating for its internal temperature (junction temperature or Tj). If the temperature rise is too great, and the ambient temperature (Ta) is high, the junction temperature can exceed this rating, and the device will shut itself down. For most regulators, this maximum temperature is 125C (257F). Tj = Ta + Trise. Relevant Part: REGULATOR How can I keep the regulator cool? The temperature rise can be minimized by reducing the thermal resistance. Thermal resistance is dealt with differently depending on whether the regulator is in a package that is designed to have a heat sink, or not. In both cases, the total thermal resistance is the sum of two parts:   1.Resistance from inside the die to the package surface or leads.   2.Resistance from the package surface or leads to the environment. #1 is fixed, based on the die size and package type. #2 can be reduced by external heat sinking. If the package is dual-in-line or surface-mount, the PC board is providing some heat sinking. This can reduce the total thermal resistance to about 60% of the thermal resistance seen with just a minimum of board area. If the package is a power package with a metal tab, such as the TO-220, a heat sink can reduce the total thermal resistance to about 5-10% of the resistance seen with no heat sink. Relevant Part: REGULATOR What is a Switched-Capacitor Converter? A typical switched-capacitor converter contains four large MOS switches, which are switched in a sequence to typically invert, double, or halve the input supply voltage. Energy transfer and storage are provided by external capacitors. During the first part of the switching cycle, the input voltage is applied across one capacitor (C1). During the second part of the switching cycle, the charge from C1 is transferred to a second capacitor C2. The most traditional switched-capacitor converter configuration is an inverter, where C2 has its positive side referred to ground, and its negative side delivers the negative output voltage. After a number of cycles, the voltage across C2 will be pumped up to the input voltage. Assuming no load on C2, no loss in the switches, and no series resistance in the capacitors, the output voltage will be exactly the negative of the input voltage. In reality, the charge transfer efficiency (and thus the output voltage accuracy) depends on the switching frequency, the on-resistance of the switches, and the capacitor value and series resistance. A similar topology, the doubler, uses the same set of switches and capacitors, but changes the connection of ground and the input voltage. Other more complex variations use additional switches and capacitors for other ratios of Vin to Vout, and in some cases specialized switching sequences are used to generate fractional relationships (3/2, for example). In its simplest forms, the switched-capacitor converter is unregulated. Some newer National Semiconductor switched-capacitor converters have automatically-adjusted gain stages to produce a regulated output; others follow the unregulated output with an internal low-dropout linear regulator. Relevant Part: SWITCHED-CAP What is Load Transient Response? Load transient response is the change of the regulator output voltage to a sudden change in load current. Ideally, the regulator output voltage would be independent of load current, and would not show any overshoot or ringing as it adapted to a new load current. However, all regulators have an output resistance and some sort of output voltage control that cause the output voltage to change with load. The output resistance causes the output voltage to drop a little with load current, and regulators designed for higher loads will have lower output resistance. The lower the resistance, the smaller the difference between the steady-state output voltages at different loads. Overshoot and ringing due to a sudden load change (transient) are related to the large-signal and small-signal frequency responses of the output voltage control loop. Slower large-signal response produces a slower recovery to the new output voltage. Lower phase margin in the small-signal response produces more overshoot and ringing in the output voltage; 45 degrees produces an ideal response, and more phase margin gives an overdamped response. The regulator bandwidth gives the ring frequency. Load transient response is usually improved using the same techniques as for improving frequency response. Relevant Part: SWITCHERS;LDO What is an Error Flag? An Error Flag is an open-collector output that provides a signal when the regulated output voltage drops more than 5% (typically) from the nominal output voltage. On start up, the Error Flag is low until the output voltage reaches 95% of the nominal output voltage. In some cases, a delay is added to the Error Flag''s power-OK transition. This delay is set by an external capacitor, and can be used as a Power-On-Reset function to reset a microprocessor on power-up. If there is a bar shown over the term "Error", a low output voltage condition causes the open collector output to be high (flag transistor OFF). When the output voltage is within 5% of nominal, this Flag output is low. Relevant Part: SWITCHERS;LDO What is Sync (synchronization) or Frequency Adjust? In switching regulators and switched-capacitor converters, an internal oscillator is used to set the switching frequency of the output transistors. The value of this switching frequency determines some of the external components used in the converter, which determines the frequencies of noise generated by the converter, and affects the performance of the converter. Some converters allow the switching frequency to be changed either by adjusting the internal oscillator frequency ("Frequency Adjust") or by synchronizing the oscillator to an external source ("Sync"). In general, by increasing the switching frequency, smaller components (capacitors, inductors) can be used in the converter output stage. This may reduce the efficiency of the converter, because of an increase in switching losses, unless higher-quality components are also used. A well-performing higher-frequency converter will have faster transient response than will one of a lower frequency. If there are several converters on a board, it is often a good practice to synchronize them to a common source. This controls the noise generated by the whole lot, and minimizes any "beat frequency" that could otherwise be generated. The concern is usually important with higher-power converters, e.g. 5W and up. In many cases, the switching frequency can only be increased from its default. The product datasheet will indicate the frequency range for this feature. Relevant Part: SWITCHERS;SWITCHED-CAP What happens when the input goes below and above the programmed output voltage in a buck-boost topology? When the input voltage dips below Vout, the regulator functions in a boost (step-up) mode. When the input gets higher than Vout, it functions as a buck (or step-down) regulator. In other words, the regulator will maintain the output constant at the programmed Vout, regardless of the variation of the input within a specified range Relevant Part: SWITCHERS What is Stability (as it applies to regulators)? A control loop is stable if, at the frequency when the gain has reduced to 0dB (unity gain), the total phase shift is less than 360º. Phase shift usually includes a 180º phase shift due to use of negative feedback, leaving a maximum of 180º shift due to feedback and other elements in the control loop. The difference between 360º and the total phase shift at the unity gain frequency is the phase margin. Stability is typically considered good if the phase margin is at least 45º. For regulators, we usually aim for a greater phase margin, typically 60-80º, to allow for variation in component values that affect the frequency response. When a switching regulator is unstable, the average output voltage will often be between 40% and 90% of the target. This is because the switch duty cycle is swinging between 0% and nearly full cycle, at a frequency approximately the bandwidth of the regulator. The output voltage will show this cyclical pattern, with the switching frequency superimposed. The regulator may even produce an audible response. Typical switching regulator bandwidths (and oscillation frequencies) are between 1000Hz and 20kHz. To fix an unstable switching regulator, the design process needs to be revisited. It may be necessary to change the inductor or output capacitor value or ESR; it may be necessary to change the circuit compensation, if it is accessible to the user. Stability can be evaluated using "Switchers Made Simple" design tools, as well as the Power WEBENCH. When a linear regulator is unstable, the output voltage will show an oscillation at the unity gain frequency, typically between 1000Hz and 100kHz. To fix an unstable linear regulator, first check the datasheet for output capacitor selection guidelines - a capacitor value that''s too low, or has an ESR outside the guidelines, is often the culprit. It may also be necessary to improve the input capacitor, increasing its value or reducing its ESR. Relevant Part: SWITCHERS What is Frequency Response (as it applies to regulators)? Frequency response is the response of a control loop to small-signal perturbations. In the case of regulators, the perturbations are often caused by noise in the system. The control loop has the IC reference voltage as its input, and the regulator output voltage as its output. Regulator input voltage is one of the things that modifies the control loop gain. Frequency response is usually measured in terms of gain in dB (decibels) and phase shift over frequency, and it is often displayed in a Bode Plot. In many cases, the unity-gain crossover frequency and phase margin are given as figures of merit for a given regulator design. The Power WEBENCH Online Design tools include an electrical simulator that can be used to show the frequency response of a switching regulator design. Relevant Part: SWITCHERS How do Linear Regulators compare to Switching Regulators? Linear Voltage Regulators and Switching Voltage Regulators both have advantages and disadvantages when used in system power supplies. Either may be a suitable choice, depending on the requirements of the system. In general, linear regulators are a better choice when the input voltage is a few Volts higher than the output (but not closer than the regulator''s dropout voltage), and when the load current is less than about 3A. Switching regulators are a better choice when the input voltage is less than or much greater than the desired output, when multiple outputs are desired, and when power dissipation must be kept low. Linear Voltage Regulators have these advantages: Simple Low output ripple voltage Excellent line and load regulation Fast response time to load or line changes Low electromagnetic interference (EMI). They have these disadvantages: Low efficiency, especially with higher input voltages Large space requirement if heatsink is needed. Switching Voltage Regulators have these advantages: High Efficiency (reduces source power requirements and need for heat sinking) Capable of handling higher power densities Topologies available to deliver single or multiple output voltages, greater than or less than or spanning the input voltage. They have these disadvantages: Greater output noise Slower transient recovery time EMI is produced. Relevant Part: SWITCHERS What is a switching regulator? Switching regulators are featured power supply solutions in the WEBENCH Power online design environment. A switching regulator uses an output stage, switched repetitively on and off, together with energy storage components (capacitors and inductors) to generate an output voltage. Regulation is achieved through the adjustment of the switch timing based on a fed-back sample of the output voltage. In fixed-frequency regulators, the switch timing is adjusted through modulation of the pulse width of the switch voltage - this is known as PWM control. In gated-oscillator or burst-mode regulators, the switch pulse width and frequency is kept constant, but the output switch is gated on or off by the feedback control. There are other variations on the market, including fixed-on-time and fixed-off-time control. Depending on the arrangement of switches and energy storage components, output voltages can be generated that are greater than or less than the input voltage, and multiple output voltages can be generated with one regulator. In most cases, a buck (step-down) switching regulator will convert the source power more efficiently than will a linear regulator, given the same input voltage and output power requirements. Relevant Part: SWITCHERS What are the common switching regulator topologies? Switching regulators are configured into specific topologies, for achieving different types of voltage or power conversion. Many of these topologies can be seen in power supply solutions developed in the WEBENCH Power Designer online design environment. The most common topologies are these: Buck: Used to reduce a DC voltage to a lower DC voltage. Boost: Provides an output voltage that is higher than the input. Buck-Boost (inverting): an output voltage is generated opposite in polarity to the input. Flyback: Uses a transformer to generate an output voltage that is less than or greater than the input, as well as multiple outputs. Sepic: A two-switch, two-inductor converter that produces an output voltage that can be either greater than or less than the input voltage. Sepic converters are often described as offering a noninverting buck-boost converter function. Many other topologies have been developed for high-power conversion, or other less common conversion requirements. These often use two or more switches (e.g. bipolar transistors or MOSFETs). Some of the speciality topologies that are most often seen are these: Buck-Boost (non-inverting): A four-switch converter that combines the functions of a buck and a boost converter, this topology is an alternative to the Sepic configuration. Push-Pull: A two-switch transformer-based converter that is especially efficient at low input voltages. Half-Bridge: A two-switch converter used in many off-line applications. Full-Bridge: A four-switch converter (usually used in off-line designs) that can generate the highest output power of all the types listed. For additional information about the operation of these switching regulator topologies, see the following resources: AN-556, "Introduction to Power Supplies" "Linear and Switching Voltage Regulator Fundamentals, Part II" AN-1484, "Designing a SEPIC Converter" Additionally, you can enter the topology name into the "Search" window of National web pages, and products that can operate in the given topology will be presented in the search results. You can also find reference designs that incorporate specific topologies. Go to the Power Management Reference Designs, and sort the resulting table by Topology. What is Input Voltage? Input Voltage is the DC voltage applied to the input terminal with respect to ground. What is Input-Output Voltage Differential? Input-Output Voltage Differential is the voltage difference between the unregulated input voltage and the regulated output voltage, for which the regulator will operate within specifications What is Line Regulation? Line Regulation is the change in output voltage for a change in the input voltage. The measurement is made under conditions of low dissipation or by using pulse techniques such that the average chip temperature is not significantly affected What is Long Term Stability? Long Term Stability is the stability of the output voltage under accelerated life-test conditions at 125 degrees Celsius with maximum rated voltages and power dissipation for 1000 hours What is Maximum Power Dissipation? Maximium Power Dissipation is the maximum total device dissipation for which the regulator will operate within specifications (including maximum junction temperature). When a value is given for Maximum Power Dissipation, it is typically valid for room temperature operation. When a device has internal over-temperature limiting, Maximum Power Dissipation is given as "Internally Limited". You can estimate the maximum power that can be dissipated in an application, while keeping the junction temperature within its limit, by using the following equation:   Pdiss = (Tjmax - Ta)/ThetaJA, where Tjmax is the maximum junction temperature specified for the device, Ta is the ambient temperature, and ThetaJA is the thermal resistance (junction to ambient) of the device package. What is Output Noise Voltage? Output Noise Voltage is the RMS ac voltage at the output, with constant load, and no input ripple, measured over a specified frequency range What is Output Voltage Range? Output Voltage Range is the range of regulated output voltages over which the specifications apply What is Standby Current Drain? Standby Current Drain is the part of the operating current of the regulator which does not contribute to the load current. (See Quiescent Current What is Temperature Stabiltity? Temperature Stability is the percentage change in output voltage for a thermal variation from room temperature to either temperature extreme What is Thermal Regulation? Thermal Regulation is the percentage change in a regulator''s output voltage for a given change in power dissipation over a specified time period What is a voltage regulator? As part of a system power supply, a voltage regulator is an electronic circuit which provides a steady supply voltage to a load. The load is typically a circuit which performs other functions needed in the system. The voltage regulator circuit may be a linear type (operating with DC voltages) or switching (converting the input voltage into a pulse-width modulated signal which is then filtered). The input voltage to the regulator is allowed to vary within a specified range, and the load may vary up to a specified maximum, but the regulator controls the output supply voltage so that it remains constant. Additional resources: - Select regulators according to their system requirements using the Power WEBENCH online design environment. - Select regulators by specification using the parametric selection guide. - Learn about linear and switching regulators in Analog University (click on Power Management to see the lessons). - Read a tutorial on the operation and use of voltage regulators, "Linear and Switching Voltage Regulator Fundamentals", Part 1 (Linear Regulators) and Part 2 (Switching Regulators). - Get additional information about power management products and applications at Power.National.Com. What happens to a linear regulator or LDO when the minimum input-output dropout voltage specification is violated? When the input voltage is close enough to the output voltage so that the regulator is in "dropout", the output voltage will start to track the input: Vo=Vin-Vdropout. Also, the PSRR will go to zero, meaning any noise on the input will be seen at the output. On bipolar LDOs, the ground pin current will be at its maximum during dropout operation. Check the "Quiescent Current" graph in the regulator''s datasheet for details Is National Semiconductor a manufacturer of IGBT products? National Semiconductor Corp. is not a manufacturer of IGBT, or Insulated-Gate Bipolar Transistor, products. National does offer a wide variety of switching power supply regulators and controllers, as well as high-speed MOSFET drivers. See our Power Management Solutions here. What are the benefits of variable switching frequency? Some switching regulator applications, such as an AM radio in a car, are sensitive to particular frequency bands. Choosing the best switching frequency is crucial in these applications for avoiding Electro-Magnetic-Interference (EMI). The inductor value in a switching regulator is often inversely proportional to the switching frequency, but the losses are directly proportional. Therefore, having a choice of switching frequency allows optimization of size versus efficiency. What are the key advantages to Current Mode Regulation? Current mode control of a switching regulator offers fast response to input line transients. This is important when there is a large input to output voltage differential (e.g.: 66V to 3.3V) and the input voltage is likely to change. In addition, current mode control offers fast response to load current transients. See Power Designer #106 for more information about current mode control as used in switching regulators. What is the "P+ Product Enhancement" reliability program? The "P+ Product Enhancement" program ensures improved reliability of many selected power management devices. This program involves dynamic tests that screen out devices subject to infant mortality or damage under high-stress conditions What is the benefit of synchronous control of regulator switching frequency? Synchronous control, or "synch frequency input," offers a means of choosing a switching frequency using an external clock source. The choice of frequency could be crucial in EMI management. In addition, the synch input allows multiple switching regulators to use a common frequency by having a common input going to all of them or by self-synchronization (if available) where one switcher sets the frequency of the whole set. Having a common switching frequency allows for one common frequency band for EMI management. This avoids having to manage beat frequencies that can arise due to multiple frequencies in the system. What tools are available from National for simulation and design of power supply circuits (including switching regulators)? National offers online and off-line tools to support power supply design based on National''s switching regulators and LDO voltage regulators. The WEBENCH online power supply design environment offers component selection (voltage regulator IC and other components), electrical and thermal simulation, and customized evaluation boards. "Switchers Made Simple" is an off-line software tool for switching-regulator based power supply design. It contains component selection and performance calculations that are also a part of WEBENCH. Why is the output current of a boost switching regulator lower than the regulator''s rated current? The current ratings for boost switching regulators like the LM2585 SIMPLE SWITCHER are for the peak switch current, not the output current.  The switch current is dependent both on the output current and the amount of boost in voltage that is required in the design.  So for designs which require a large increase in voltage, the switch current is much greater than the output current.  Energy must be conserved by the following equation: Pout = Pin x Efficiency    or Iin = (Vout x Iout)/(Vin x Efficiency) which is approximately = Iswitch For an example, let Vin=12V and Vout=48V at 0.5A. If we assume 85% efficiency, then the peak switch current rating will need to be at least: Iswitch = (48 * 0.5)/(12 * 0.85) = 2.4A (choose device with at least 3A rating) Higher load current or greater boosting ratios will require proportionally higher switch ratings. To determine appropriate solutions for your boost regulator design, use the online design tool WEBENCH Power Designer. Why use a SIMPLE SWITCHER product? The SIMPLE SWITCHER products are supported with an on-line Webench tool suite that helps with the design by choosing the components, allowing for optimization, electrical and thermal simulations and building a prototype board. In addition, these products are typically a member of the family of products with similar definitions, often the same package and variety of features offering flexibility to meet changing needs of the power supply.

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