The LM5020 is employed here to design a Buck-Boost type power supply where the absolute value of the negative input voltage rail can be above or below the positive output voltage rail. The LM5020 is a versatile pwm controller requiring an external FET switch, Q101, which is sized to fit the output current requirement. The input voltage, Vin, during the on-time of the switch charges the L100 inductor through the FET from ground to the negative rail. During the off-time of the FET switch the inductor reverses polarity to forward bias the diode D100 to charge the output capacitors to the required output voltage and provide current to the load. The output voltage is set by the loop using a current sourcing circuit, Q100 & R103, to develop a feedback voltage across R111. 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, C1,106,C107 are set to limit ripple and noise voltage while the input capacitance limits the noise ripple effects found on the input rail. The softstart ramp is optimized through C108 and the switching frequency is set via R105. The current in the loop is sensed across resistor R108 to afford current mode operation and cycle to cycle current limiting.
Schematic
Example Schematic Showing Connection for all Components.
Example Schematic Showing Connection for all Components.
