Power supply soft start controller with no output voltage undershoot when transitioning from skip cycle mode to normal mode

1. A soft start circuit of a power supply controller comprising:

a soft-start reference generator configured to form a soft-start reference signal responsively to an output voltage of a power supply system increasing to a first value that is greater than a regulated value of the output voltage; and

a soft-start override circuit configured to receive a first signal that is representative of the output voltage and configured to override a value of the soft-start reference signal responsively to the output voltage decreasing to a second value that is less than the regulated value.

2. The soft start circuit of claim 1 wherein the soft-start reference generator is configured to generate a ramp signal as the soft-start reference signal.

3. The soft start circuit of claim 1 wherein the regulated value is less than the first value and greater than the second value.

4. The soft start circuit of claim 1 wherein the soft-start override circuit is configured to couple a voltage source to the soft-start reference signal responsively to the output voltage decreasing to the second value.

5. The soft start circuit of claim 1 wherein the soft-start override circuit includes a comparator coupled to receive the first signal and a first reference signal and form a control signal to cause the soft-start override circuit to change the value of the soft-start reference signal.

6. The soft start circuit of claim 5 wherein the control signal is coupled to switch a voltage source to an output of the soft-start reference generator.

7. The soft start circuit of claim 5 wherein the comparator has a first input coupled to receive the first signal, a second input coupled to receive the first reference signal from a reference voltage generator, and an output and further including a transistor having a control electrode coupled to the output of the comparator, a first current carrying electrode coupled to receive the first reference signal, and a second current carrying electrode coupled to an output of the soft-start reference generator.

8. The soft start circuit of claim 1 wherein a clamp circuit is configured to receive the soft-start reference signal and form a current sense reference signal that is used by a current sense comparator.

9. The soft start circuit of claim 1 wherein the power supply controller includes a PWM controller and a current sense comparator.

10. A method of forming a power supply controller comprising:

configuring the power supply controller to form a drive signal to control a power switch to regulate an output voltage to a regulated value;

configuring the power supply controller to receive a feedback signal that is representative of the output voltage;

configuring the power supply controller to form a soft-start reference signal responsively to a first value of the output voltage that is greater than the regulated value; and

configuring the power supply controller to inhibit the soft-start reference signal responsively to a second value of the output voltage that is less than the regulated value.

11. The method of claim 10 wherein configuring the power supply controller to inhibit the soft-start reference signal includes configuring the power supply controller to force the soft-start reference signal to a first reference value.

12. The method of claim 11 wherein configuring the power supply controller to force the soft-start reference signal to the first reference value includes configuring the power supply controller to force the soft-start reference signal to the first reference value in order to enable the power supply controller to regulate the output voltage to the regulated value.

13. The method of claim 11 wherein configuring the power supply controller to force the soft-start reference signal to the first reference value includes configuring a comparator to receive the feedback signal and a first reference signal and to enable a transistor to couple the first reference value to the soft-start reference signal responsively to the second value of the output voltage.

14. The method of claim 10 wherein configuring the power supply controller to form the soft-start reference signal responsively to the first value of the output voltage includes configuring the power supply controller to form the drive signal in bursts and use the soft-start reference signal to limit a peak value of current through the power switch.

15. The method of claim 14 wherein configuring the power supply controller to form the soft-start reference signal includes configuring the power supply controller to form a ramp shaped soft-start reference signal.

16. A power supply control method comprising:

forming a drive signal to operate a power switch to regulate an output voltage of a power supply system to a regulated value;

forming a soft-start reference signal to limit a peak value of current through the power switch responsively to a first value of the output voltage that is greater than the regulated value; and

changing a value of the soft-start reference signal responsively to a second value of the output voltage that is less than the regulated value.

17. The method of claim 16 wherein forming the soft-start reference signal to limit the peak value of current through the power switch responsively to the first value of the output voltage that is greater than the regulated value includes using the soft-start reference signal to control the drive signal to limit the peak value of current through the power switch.

18. The method of claim 16 wherein changing the value of the soft-start reference signal responsively to the second value of the output voltage includes coupling a voltage source to the soft-start reference signal.

19. The method of claim 18 wherein coupling the voltage source to the soft-start reference signal includes comparing a signal representative of the output voltage to a first reference signal and enabling a switch to couple the voltage source to the soft-start reference signal.

20. The method of claim 16 wherein changing the value of the soft-start reference signal responsively to the second value of the output voltage includes increasing the value of the soft-start reference signal.