Audio My op-amp is oscillating. What should I do? The frequency of the oscillation is a very important clue. Frequencies near or above the op-amp''s GBW generally show instability in the output stage, usually caused by capacitive loading on the output or poor supply bypassing. Also try adding a 0.1uF ceramic bypass capacitor to the power supplies. If the frequency is within the operating range of the circuit (below GBW and near GBWP), and the overall circuit has a relatively high gain, it could be input-to-output coupling causing the feedback. Try shielding the input sections from the output sections and move input and output components away from each other. If the frequency is below 10Hz (commonly referred to as "motorboating"), and the output is driving a relatively heavy load, it is usually an inadequate supply current, poor power supply bypassing, or the lack of a "star-ground" layout. This happens frequently with audio power amplifiers. Add larger supply bypass capacitors and make sure the load''s ground is returned directly to the power supply''s ground. Major causes of oscillations are poor grounding techniques, poor input to output isolation, and/or poor power supply bypassing. Can two or more LM4652 ICs be put in parallel to drive lower impedance loads for higher power? Yes. The PCB layout is a little more difficult but two different layouts have been tested on the bench. One layout has the two LM4652s side by side with a space of 0.5 inches between for easy mounting to a common heat sink. The other layout has the two LM4652s back to back with just enough space between to allow for a heat sink to be mounted. Both layouts were designed with minimal gate trace lengths as possible. Similar performance in Noise, THD, and power were observed for each board. Changes in the application circuit that were required for high power operation was the addition of 10 µ F capacitors on the supply pins (Vcc and Vee) of the LM4651 and 47 µ F capacitors on the supply pins (Vcc and Vee) of each LM4652. Larger capacitors are recommended for unregulated supplies. The dual LM4652 configuration can be used to drive two loads with the same input signal (i.e. dual voice coil woofer) with each LM4652 having its own output filter. The dual configuration can also be used to drive lower impedance loads connecting the outputs of the LM4652s in parallel and using a single, high power filter before the load. When using the dual configuration the current limit function is now current per LM4652. For example, if the current limit is set to by using a 100k Ω resistor for RSCKT then each LM4652 will be allowed to produce 11 Amps of peak current before current limit is activated. Dual configuration allows for total output power levels up to 300W for the LM4651/52 class D solution. A demo board for dual configuration is not available at this time. Relevant Part: LM4652 At high power the LM4651/LM4652 class D audio amplifier seems to be going into some protection mode. What''s happening? If driving a load that is smaller than 4 Ω t he current limit protection may be turning on at higher output power levels. The current limit set by RSCKT is the sum of the peak output current in the load and the ripple current in the inductors. For a 4 Ω load and 125W output the peak current in the load is 7.9 amps. The ripple current is affected by the inductor value, supply voltage and switching frequency. Some head room in the current limit should be designed in when setting the value of RSCKT. Normally RSCKT is set to 100k Ω  giving 11Amps of output current. Lowering this resistor will increase the peak output current allowed from the FETs. This symptom can also be due to noisy or low supply lines. Under voltage detection is activated when either supply is within 10.7 Volts of GND. If the supply is unregulated and momentarily droops near this level UVD may be activated unintentionally. Increasing the capacitors on the supply lines will often solve this problem. Relevant Part: LM4651;LM4652 Can the LM4651 be used at a separate supply voltage than the output FETs (LM4652)? Yes and No. The bootstrap capacitors (labeled CBT) are charged to 6 Volts. When the high side of the output FETs is turned on the gate voltage is 6 Volts above Vcc of the LM4651. For example if the Vcc of the LM4651 is 20 Volt, the gate voltage on the topside FETs will be 26 Volts giving a Vgs of 6 Volts. This value of 6 Volts was chosen because the LM4652 does not need a high gate voltage to turn fully on minimizing RDS(on). If the LM4651 is used at a lower supply voltage than the LM4652 the FETs may not turn fully on causing a higher RDS(on) resulting in lower efficiency. But the LM4651 can be used at a supply voltage that is HIGHER than the output FETs or the LM4652. This would result in a higher Vgs than 6 Volts. The limit for Vgs of the LM4652 is 10 Volts before damage can result. The low side FETs do not use bootstrap capacitors since the gate voltage is 6 volts above Vee (-20 Volts, Vgate = -14 Volts). Relevant Part: LM4651;LM4652 Can the LM4651/LM4652 be used for frequency ranges other than subwoofer? Yes. The audio frequency range is controlled by the external components and is not limited by the LM4651/52 chip set. Several external values need to be changed to increase or decrease the audio frequency range of operation. To increase the audio frequency range of operation the following values must change: L1 and CBYP (the output filter) must decrease, Cfl1, Cfl2 and Cf may need to decrease but normally this is not the case, R1, R2, and Rf may need to change to lower noise and gain to meet target specifications, ROSC may need to decrease, RDLY will need to be reduced or set to 0 Ω . A pre-amplifier with a gain of 10V/V and a 2 pole active input filter is recommended. The input filter will control the audio frequency range while the output LC filter will be set to high levels to allow the frequency range of interest to have a flat, linear response through the output filter. This pre-amplifier/filter can be implemented easily with a low cost dual op. amp. like the LM833. Relevant Part: LM4651;LM4652 Can the LM4651/LM4652 be used with a single supply instead of a dual supply? Yes. The performance with single supply is the same as with dual supply. The single supply voltage minimum is double the dual supply minimum. The dual supply minimum is +/-11V so the single supply minimum is 22V. A reference voltage of 12V up to ½Vcc must be provided as the GND connections to the LM4651/52 chip set. Vee connections then become 0V (single supply GND) and Vcc connections are the single supply voltage. This voltage reference can be created with a resistor divider, a resistor and a Zener diode or a voltage regulator. Bypass capacitors will need to change connections with capacitors now added on the reference voltage to the single supply GND. The voltage rating of these capacitors will need to be higher than a dual supply configuration. Relevant Part: LM4651;LM4652 How can I decrease the class D audio amplifier''s audible noise? The audible noise can be decreased several ways. Noise is mostly affected by the ratio of Rf/R2. For good performance it is recommended that this ratio be higher than 100. For example, Rf would be 1M Ω and R2 would be less than 10k Ω Then the value of R1 would be adjusted for the desired gain. As the ratio increases the noise will be reduced. Another way to lower noise is to lower the gain of the class D amplifier. Changing the value of R2 and R1 plus using a pre-amplifier will help lower audible noise. National’s LM833 dual audio operation amplifier is a good solution. Often a pre-amplifier is necessary already so adjusting the gain on the pre-amplifier to account for the decreased gain of the class D amplifier will maintain a constant system gain but will lower the audible noise. A third alternative is to decrease the output filter frequency response. If the application is a subwoofer or bass amplifier the output filter can be set to a lower 3dB frequency. This approach will often cost more due to larger values for the inductors and capacitor in the output filter. A fourth alternative is to lower the supply voltage. This may not be a possibility based on the target output power specification. Relevant Part: LM4651;LM4652 How do I calculate the ripple current for different inductor values used in the LM4651/LM4652 class D audio amplifier? Ripple current depends on the value of the inductor, the frequency of modulation (switching frequency) and the supply voltage. The general equation for calculating ripple current is: ½{(Vcc/L)(1/2*SWF)}. Relevant Part: LM4651;LM4652 How do you design the output LC filter for a bridge application verses a single-ended application? A 2-pole output filter consists of an inductor in series with the output and a capacitor going to ground after the inductor. This is the correct design for a single-ended configuration using a 2-pole output filter. When the design is a bridge tied load (BTL) often the same output filter is used; namely an inductor in series with each output and a capacitor to ground after the inductor to ground or four components. For a BTL configuration this is correct but the pole location is not the same. What the filter actually looks like to the output is a capacitor to ground from one output then from ground through a capacitor to the other output. In effect, two capacitors in series or half the capacitance value than originally calculated. This is easily solved by connected one capacitor across the load at half the calculated value. Each output sees a capacitor so the effect is double the value used is seen in the output filter. The BTL configuration therefore requires a smaller capacitor than the single-ended configuration for a filter response that is the same. Often times in the single-ended configuration a small capacitor is put in parallel with the output filter capacitor. This small capacitor is a high quality capacitor with the purpose filter the high frequency to ground. These capacitors are needed in the BTL design. A capacitor value around 10 - 20% of the large output filter capacitor should be used on each output to ground. A correctly designed output for a BTL configuration is two inductors, one large capacitor across the load and two capacitors that are 10 - 20% in size of the large capacitor across the load connected from each output to ground, five total components Relevant Part: LM4651;LM4652 Is there a recommended range for gain for the LM4651/LM4652 class D amplifier? Yes. Unlike Class AB amplifiers the class D amplifier can be run with unity gain to over 100 V/V. High gain is not recommended. The recommended range for gain of the LM4651/52 class D amplifier is 1V/V to 20 V/V Relevant Part: LM4651;LM4652 The LM4651/LM4652 demo board is a double-sided board. Can a single-sided board be used? Yes. For high frequency design a double-sided board is much better. Planes can be used for power and ground. If these planes are laid out over each other a small amount of parasitic capacitance (pF) is created. This small parasitic capacitance filters very high frequency off the supply planes to the ground plane giving a cleaner voltage supply for the LM4651/52. A double-sided board also allows for a smaller layout size. A single-sided board would need larger supply bypass capacitors close to the supply pins of the LM4651/52 than a dual sided board Relevant Part: LM4651;LM4652 The lower -3dB point of the LM4651/LM4652 class D amplifier is not low enough for my application. How can I decrease the lower -3dB point for more bass response? The lower 3dB point is controlled by the relationship of Cin, R1 and Rlp on the demo boards. Increasing the value of Cin is the easiest way to fix this problem. Alternately, R1 or Rlp can be increased but changing these values will effect the gain and the upper 3dB point of operation. Relevant Part: LM4651;LM4652 What is the damping factor of the LM4651/52 class D amplifier solution? Damping factor is the ratio of the output impedance to the load impedance. The higher the factor the better the amplifier can control the movement of the speaker. If the damping factor is too low the bass will not sound "tight". The output impedance of a class D amplifier also depends on the output filter. Typical values for damping factor for the LM4651/52 chipset are 40 to 100 at 100Hz. The value depends on the DC resistance of the inductors used in the output filter. To improve damping factor the lowest DC resistance inductor possible should be used. For a damping factor of 50 the DC resistance of the inductor should be less than 0.04 Ω . Relevant Part: LM4651;LM4652 What is the maximum supply voltage that can be applied to the LM4651 or LM4652 without any input signal or output power? The maximum supply voltage of the LM4651/52 is limited by the fabrication process. The +/-22 Volt guarantee is for full output power. The LM4651/52 chips can normally go to +/-25 Volts without damage but not with full output power in all cases. Because there is often confusion about operating and maximum ratings one specification is given. In most cases an unregulated supply is used in the application with the voltage rails increasing as the output power decreases. With no input signal or output power the supply rails must not go above +/-22 Volts or there is the possibility that a small number of parts will fail Relevant Part: LM4651;LM4652 What is the ripple current in the inductors on the LM4651/LM4652 demo board? The ripple current is approximately 0.8 Amps in each inductor, giving 1.6 Amps total ripple current Relevant Part: LM4651;LM4652 Why does the Application Circuit in the LM4652 Data Sheet (Figure 1, page 4) show two different capacitors on the feedback filter (Cfl1and Cfl2)? Why not just one larger capacitor? The purpose of using two capacitors in the feedback filter is to lower EMI. The first capacitor (Cfl1) is placed very near the output of the LM4652. The feedback traces to the LM4651 can be long and the longer a trace is the better it radiates electrical noise (EMI). Having the switching square wave output on a long trace will only increase EMI. Having a capacitor right near the output filters the audio signal from the switching square wave so an analog low frequency waveform is present on the longer feedback traces. The second capacitor (Cfl2) is used right near the input to the LM4651. This capacitor is placed near the LM4651 to filter off any additional high frequency noise picked up by the long feedback traces. The combination of these two capacitors lowers EMI and provides a cleaner feedback signal to the LM4651. Relevant Part: LM4651;LM4652 Why is Over-Modulation Protection necessary for an LM4651/LM4652 class D amplifier? The over-modulation protection’s main purpose is to create an artificial clipping point a little below where the actual clipping would occur with out the protection. This allows the LM4651/52 class D amplifier to control the output when clipping so no extra distortion is introduced from oscillations or other imperfections. Also, if there was no protection signal then the output FETs would stay on for an extended period of time increasing the heating of the FET which also increases the RDS(on). Relevant Part: LM4651;LM4652 With the LM4651/LM4652 class D audio amplifier, how can the DC offset seen at the load be reduced to eliminate clicks and pops at turn on/off? The DC offset seen at the load is due to the duty cycle error. When there is no input signal present the class D amplifier should show a 50% duty cycle square wave. Any error from exactly 50% creates some DC offset at the load. To minimize the DC offset the ratio of Rf/R2 should be above 100. A higher ratio will give a lower DC offset. If the DC offset is still unacceptable then adjusting the value of the resistor labeled ROFFSET shown in the Application Circuit in the Data Sheet (Figure 1, page4) will reduce the DC offset to 0V. This resistor puts a small DC voltage at the input so there is no duty cycle error. The value of this resistor will often be very large, several mega - ohms. Relevant Part: LM4651;LM4652 Can the LM4651 be used to drive discrete output FETs for higher output current and lower impedance loads for higher output power? As noted in other LM4651 FAQs, there can be some difficulty with driving discrete FETs with the LM4651. The major problem is that the bootstrap capacitors (CBT) charge to 6 Volts. Some discrete FETs specify a minimum of 10 Volts for Vgs to have the minimal RDS(on). Since the Vgs will be only 6 Volts the FETs may not turn fully on and RDS(on) will not be minimize. This will result in lower efficiency. If the discrete FETs use a slightly lower supply voltage than the LM4651 then Vgs can be increased. For example, if the LM4651 has a +/-22 Volt supply and the discrete output FETs have a +/-18V supply then Vgs will be 10 Volts. There are two other possible problems with using discrete FETs instead of the LM4652. One is that Thermal Shut Down (TSD) will have to be implemented separately. The LM4652 has a TSD output flag that goes back to the LM4651. This flag transitions to 6V when the die temperature exceeds approximately 150 ° C. The LM4651 needs a 5V signal to shut down the output stage. The second problem is that the current limit control of the LM4651 will not be accurate. The LM4652 and LM4651 have very closely matched output FETs. If discrete FETs are used they will not be matched as closely to the LM4651 output FETs. Current limit may still work but would need to be recalibrated with each different discrete FET. Relevant Part: LM4651 What is the output impedance of the LM4651 small signal FETs? The LM4651 FET output impedance would be the RDS(on) of one of the transistors since at any time one of them is fully on. At room temperature (25° C) the RDS(on) of the FETs in the LM4651 is typically 1.0 Ω . Relevant Part: LM4651 Does the Thermal Shut Down output from the LM4652 an analog signal or a digital? The output of the TSD pin on the LM4652 is at zero volts until the die temperature reaches about 140 ° C. The voltage then starts to increase quickly as the die temperature continues to increase to over 150 ° C. When the voltage reaches 5V the LM4651 goes into STBY mode. The TSD output will go as high as 6V. When the die temperature cools below 150 ° C the TSD voltage transitions down to less than 2V. Relevant Part: LM4652 How do I chose a value for Rsckt, the resistor setting the current limit for the LM4652? The LM4652 current limit is a function of the output current and the ripple current in the output inductors (L1). Unless a load impedance lower than 4 Ω is used a value of 100k Ω f or RSCKT should be fine. Lower impedance loads require more output current so a lower value for RSCKT like 39k Ω can be used setting the current limit around 13 Amps. The ripple current will increase as the value of the inductor is lowered. Relevant Part: LM4652 In LM4652 applications, what is the purpose of Rgate, the series gate resistors? The gate resistors increase the rise and fall time of the gate voltage. This helps remove some the highest frequencies in the square waveform. EMI is lowered due to lower frequency content in the square waveform Relevant Part: LM4652 What is the forward voltage on the body diode in the LM4652 FETs? It has not been measured exactly, but from the die design the forward voltage is around 0.55 V to 0.65 V Relevant Part: LM4652 What is the gate capacitance of the LM4652 FET H-bridge? The gate capacitance is typically 1000pF on each FET contained in the LM4652 Relevant Part: LM4652 What is the output impedance of the LM4652 FETs? The FET output impedance would be the RDS(on) of one of the transistors since at any time one of them is fully on. The value of RDS(on) depends on the temperature of the die. At room temperature (25 °C) the RDS(on) of the FETs in the LM4652 is typically 200m Ω . Relevant Part: LM4652 What is the purpose of the Schottky diodes in LM4652 applications? Can the Schottky diodes be integrated into the LM4652 die? If the supply voltage is lowered can lower current Schottky diodes be used or eliminated completely? The Schottky diodes protect the LM4652 from fly back voltages. The output filter contains inductors (L1) and driving a real work load speaker also has inductance. These inductors cause fly back voltages. If the fly back voltage is too high the LM4652 will be permanently damaged. The diodes also help clamp the output square waveform to have less overshoot. The recommended value for the Schottky diodes is 50 Volts and 3 Amps or 50 Volts and 1Amp with a high surge current (>20A) rating. The Schottky diodes cannot be integrated into the LM4652 die, because the fabrication process of the LM4652 does not have this ability. A larger diode can be put in parallel with the body diode but this is costly in die size. It is possible to use lower-current or no Schottky diodes in some low voltage applications. THD may increase due to the switching waveform have more overshoot and less ideal. EMI may also increase since the overshoot contains high frequency components. The topside diodes (from output to Vcc) may not be necessary for protection in most cases. The bottom side diodes (from output to Vee) are needed. The fly back voltages are really a function of the output stage and inductance of the load. The factors all contribute so the amount of diodes and current rating are all application dependent. Lower current diodes like 1Amp may be ok in most cases Relevant Part: LM4652 What is the threshold voltage, Vt, of the FETs in the LM4652 H-bridge? The VT of the FETs in the LM4652 FET H-bridge is right around 1.0 Volts. Relevant Part: LM4652 What is the difference from the LM4670 and LM4671? The LM4670 uses a Delta-Sigma architecture that offers better THD+N performance; where as the LM4671 uses a PWM architecture that has lower idle current. Relevant Part: LM4670;LM4671 When should I use the LM4670 instead of the LM4671? The LM4670 has better THD+N and PSRR performance in the audio band. The LM4670 gives the highest output power under the same operating conditions and provides lower output noise compared to the conventional PWM architecture. It is best for high power applications, where it offers stellar full range audio performance. Relevant Part: LM4670;LM4671 When should I use the LM4671 instead of the LM4670? In some cases, the LM4671 fits better into medium to low power applications (headphone/ earpiece) and when lower Iddq (quiescent current) is needed. While the LM4671 offers excellent overall performance, the LM4670 has exceptional full range audio fidelity. Relevant Part: LM4670;LM4671 How do I get an output power of 300W or more per channel for the LM4702? The output stage will need to be composed of 3 NPN/PNP Output Pairs. It is also possible to get 100W/Ch with 1 output pair, and 100W-200W/CH with 2 NPN/PNP output pairs. Relevant Part: LM4702 Do I need a heat sink for the LM4702? You do not need a heat sink on the LM4702 itself. However, it is necessary to use a heat sink on the output stage. The LM4702 does have internal thermal protection, but this does not monitor the output stage. Relevant Part: LM4702 Why do I need an output stage for the LM4702? The LM4702 both conditions the input signal and produces a very high output voltage swing. However, the output current is very small at 5.5mA typical. This means that it needs output (current driving) transistors to produce the current needed to supply high power to 3, 4 and 8 &8486; loads. Relevant Part: LM4702 The LM4906 is described as a "bypass capacitor-less audio power amplifier." What does this mean? Why is it important? Conventional single-supply audio amplifiers have an internal bias supply voltage that requires an external bypass capacitor. The LM4906 was designed so that this bypass capacitor is not required. The benefits of eliminating this bypass capacitor include reduction in space and cost associated with a high-quality tantalum capacitor, and improvement of power supply rejection (PSRR). In addition, the bypass capacitor affects the sound quality of the amplifier, the frequency response, susceptiblity to RF interference, click and pop performance as well as turn-on and turn-off times. Elimination of the bypass capacitor allows for greater consistency in design because these variables are removed. As with all audio amplifiers, the LM4906 does require power supply bypassing. For best performance (low noise, high power supply rejection), the power supply bypass capacitor should be connected as close to the power supply pin as possible. See the LM4906 datasheet for details. Relevant Part: LM4906 Can I use more than 3 LEDs with the LM4970? This depends on the voltage attached to the LED. If this voltage is high enough (boost probably needed), multiple LEDs may be strung in series to each of the LED driver outputs. This will cause all the LEDs in series to react to the same output driver they are attached to. (Please note that enough voltage must be supplied to turn on ALL LEDs.) Depending on the current drive requirement of the LEDs, it is possible to place them in parallel at the output of the LED driver. Relevant Part: LM4970 How do I change the intensity of the LEDs when I have the audio synchronization disabled and the I2C LED control enabled for the LM4970? The I2C LED control mode allows the user to directly control the LED driver through bypassing the audio sync mode including the PWM module. The current selection register (Table 6 in the datasheet) will allow the user to change the current drive of the LEDs, decreasing the current will also decrease the intensity of all the driver outputs at one time. Please note that the duty cycle changed via the PWMs only in the audio sync mode, not in this mode. Relevant Part: LM4970 What does changing the PWM frequency of the LM4970 accomplish? PWM frequency is analogous to the sampling rate of the audio input signal. A higher PWM frequency setting will result in a more accurate LED representation of the audio input signal in the audio synchronization mode. However, a PWM frequency that is set too high will decrease the ON time of the LED, which will result in reduced LED intensity. A PWM frequency setting of 60 Hz results in an optimal balance between LED accuracy and intensity (see pg.11-12 in the datasheet) Relevant Part: LM4970 Is it possible to parallel audio amplifiers together for additional current drive into a speaker? Yes. However a 0.1 Ω resistor should be tied directly to the output of each amplifier to prevent current hogging. National''s application note AN-1192 goes into great detail on higher current applications with Overture products, including bridged output, parallel output and bridge/parallel output configurations. What is Class D audio? Class D specifies a switch-mode amplifier circuit design. This design is much more efficient than linear amplifiers, with a trade off of slightly more noise. Can two or more Boomers be connected to a single speaker and the one driving the speaker be selected by putting the other parts in power down? Yes. When the Boomer goes into power down [using the shutdown pin function], the outputs go into a high impedance mode. When in this high impedance shutdown mode, voltages can be appliied to the output pins up to the 6V. abs max limit of the part. There is no specification on how high the high impedance is. This applies for most Boomers, with the exception of the LM4894 and LM4895. The output of these devices goes low impedance when in shutdown mode, preventing parallel connections on the output. Such limitations are noted in the application information section of the datasheet where applicable Where can I get a copy of the 1980 National Semiconductor Audio/Radio Handbook? The 1980 Audio/Radio Handbook offers information on the use of audio-quality op amps in audio circuits, including preamps, tone controls, filters, power amplification, radio, and others. In this book the term "floobydust" is explained and demonstrated as being a mixed bag of topics. A reprint of this useful Handbook can be obtained through audioXpress. Note that the reprint is faithful to the original, down to the contact information for National Semiconductor - as it was in 1980. Please see the National web site for current contact information. Also see National''s Audio Solutions home page for information about current audio technologies, products, and design tools. What is the minimum value that can be used for the DC blocking on the output of an audio amplifier? The minimum value of the input capacitor is determined by the input resistance and the minimum desired low frequency response. The formula is: Cmin = 1/[2(pi)(Rload)(F-3dB)] When would a Class D amplifier be used? Class D amplifiers are a great choice when heat and/or battery life is a prime consideration in the design. Where can I find information on Class D audio amplifiers? Visit audio.national.com for links to design guides, application notes, new products, and ebooks for Class D audio amplifiers.