Switched mode power converter controller with ramp time modulation

1. A controller for use in a power converter, comprising: a drive circuit coupled to generate a drive signal to control switching of a power switch to control a transfer of energy from a power converter input to a power converter output;

an input for receiving an enable signal including enable events responsive to the power converter output, wherein the drive circuit is coupled to turn ON the power switch in response to the enable events and turn OFF the power switch in response to a power switch current reaching a current limit threshold; and

a current limit threshold generator coupled to receive the drive signal and vary the current limit threshold in response to the enable events of the enable signal,

wherein the current limit threshold generator is coupled to increase, within a current limit threshold range, the current limit threshold by a fixed increase amount during a fixed time period in response to an end of each ON time of the power switch, and

wherein the current limit threshold generator is coupled to begin decreasing, within the current limit threshold range, the current limit threshold at a decrease rate after the fixed time period after the end of each ON time of the power switch.

2. The controller of claim 1 , wherein the controller is coupled to receive the drive signal from the enable events of the enable signal and vary the current limit threshold in response to the enable events of the enable signal.

3. The controller of claim 1 , wherein a time between successive enable events of the enable signal is responsive to the power converter output.

4. The controller of claim 1 , wherein the enable events include transitions from one logic state to another logic state in the enable signal.

5. The controller of claim 1 , wherein the current limit threshold generator is coupled to vary the current limit threshold in response to a time between successive enable events of the enable signal over a range of output loads coupled to the power converter output.

6. The controller of claim 1 , wherein the fixed increase amount is less than the current limit threshold range such that a plurality of switching cycles of the power switch is required to cause the current limit threshold generator to vary the current limit threshold from a minimum current limit threshold to a maximum current limit threshold of the current limit threshold range.

7. The controller of claim 1 , wherein the current limit threshold generator is coupled to increase the current limit threshold up to a maximum current limit threshold in response to an end of each ON time of the power switch.

8. The controller of claim 1 , wherein the current limit threshold generator is coupled to decrease the current limit threshold, within the current limit threshold range, as a time between successive enable events increases.

9. The controller of claim 1 , wherein the fixed time period is substantially zero.

10. The controller of claim 1 , wherein the decrease rate is adjusted in response to an input voltage coupled to the power converter input.

11. The controller of claim 10 , wherein the decrease rate is selected from one of a first constant decrease rate and a second constant decrease rate in response to the input voltage.

12. The controller of claim 1 , wherein the drive circuit includes a latch coupled to generate the drive signal and coupled to be set in response to the enable events, wherein the drive circuit further includes a comparator coupled to reset the latch in response to a comparison of a current sense signal representative of the power switch current and the current limit threshold.

13. The controller of claim 1 , wherein the current limit threshold generator includes a monostable multivibrator coupled to generate a one shot pulse to determine the fixed time period, wherein the monostable multivibrator is coupled to receive the drive signal to generate the one shot pulse in response to the end of each ON time of the power switch.

14. A controller for use in a power converter, comprising:

a drive circuit coupled to generate a drive signal to control switching of a power switch to control a transfer of energy from a power converter input to a power converter output;

an input for receiving an enable signal including enable events responsive to the power converter output, wherein the drive circuit is coupled to turn ON the power switch in response to the enable events and turn OFF the power switch in response to a power switch current reaching a current limit threshold; and

a current limit threshold generator coupled to vary the current limit threshold in response to the enable events of the enable signal, wherein the current limit threshold increases in response to an end of each ON time of the power switch,

wherein the current limit threshold generator is coupled to increase, within a current limit threshold range, the current limit threshold by a fixed increase amount during a fixed time period in response to the end of each ON time of the power switch, and

wherein the current limit threshold generator is coupled to begin decreasing, within the current limit threshold range, the current limit threshold at a decrease rate after the fixed time period after the end of each ON time of the power switch.

15. The controller of claim 14 , wherein the controller receives the drive signal and varies the current limit threshold in response to the enable events of the enable signal.

16. The controller of claim 14 , wherein a time between successive enable events of the enable signal is responsive to the power converter output.

17. The controller of claim 14 , wherein the enable events include transitions from one logic state to another logic state in the enable signal.

18. The controller of claim 14 , wherein the current limit threshold generator is coupled to vary the current limit threshold in response to a time between successive enable events of the enable signal over a range of output loads coupled to the power converter output.

19. The controller of claim 14 , wherein the fixed increase amount is less than the current limit threshold range such that a plurality of switching cycles of the power switch is required to cause the current limit threshold generator to vary the current limit threshold from a minimum current limit threshold to a maximum current limit threshold of the current limit threshold range.

20. The controller of claim 14 , wherein the current limit threshold generator is coupled to increase the current limit threshold up to a maximum current limit threshold in response to the end of each ON time of the power switch.

21. The controller of claim 14 , wherein the current limit threshold generator is coupled to decrease the current limit threshold, within the current limit threshold range, as a time between successive enable events increases.

22. The controller of claim 14 , wherein the fixed time period is substantially zero.

23. The controller of claim 14 , wherein the decrease rate is adjusted in response to an input voltage coupled to the power converter input.

24. The controller of claim 23 , wherein the decrease rate is selected from one of a first constant decrease rate and a second constant decrease rate in response to the input voltage.

25. The controller of claim 14 , wherein the drive circuit includes a latch coupled to generate the drive signal and coupled to be set in response to the enable events, wherein the drive circuit further includes a comparator coupled to reset the latch in response to a comparison of a current sense signal representative of the power switch current and the current limit threshold.

26. The controller of claim 14 , wherein the current limit threshold generator includes a monostable multivibrator coupled to generate a one shot pulse to determine the fixed time period, wherein the monostable multivibrator is coupled to receive the drive signal to generate the one shot pulse in response to the end of each ON time of the power switch.