The LM5020 is employed here to design a Boost power supply where the input voltage rail is stepped up to create the output voltage rail. The LM5020 is a versatile pwm controller requiring an external FET switch which can be sized to fit the output specification. Here the output FET is driven especially hard through the driver circuit, Q1 & Q2, in order to optimize efficiency and turn-on/off transitions. Please note that this power supply can maintain steady state operation at 6V input but that it is necessary for the input voltage at startup to be above 13V. The input voltage, Vin, during the on-time of the switch charges the L1 inductor through the FET to ground. During the off-time of the FET switch the inductor reverses polarity to forward bias the diode D3 to charge the output capacitors to the required output voltage and provide current to the load. The current sense circuit, involving transformer T1, provides for the current mode operation of the control loop as well as a cycle to cycle current limit function. T1 is used instead of the more typical current sense resistor because of the high pulsed currents occurring at low input voltages. The output voltage is set through the voltage divider R1 & R2 and the loop transient response is governed by the resistor capacitor circuit across the feedback pin, VFB, and the compensation pin, COMP, of the controller IC. Output capacitors, C4 & C7, are selected to limit ripple and noise voltages while the input capacitance, C3, limits the noise ripple effects found on the input rail. The softstart ramp time is set via C6; the switch frequency is set by R7. After startup the voltage to power the LM5020 is taken from the output voltage through D1.
Notes: The FET M1 in the schematic is shown as Q3 in the board layout.
Schematic
Example Schematic Showing Connection for all Components.
Example Schematic Showing Connection for all Components.
