Maintaining output voltage of DC-DC converter in discontinuous conduction mode

1. A method for maintaining an output voltage of a DC-DC converter, configured to operate in a discontinuous conduction mode, within a predetermined voltage range, the method comprising:

adjusting a duty cycle of the DC-DC converter based on an output voltage to maintain the output voltage within a predetermined voltage range;

wherein:

the duty cycle of the DC-DC converter is adjusted by switching between a first switching frequency to a second switching frequency;

the first switching frequency and the second switching frequency are selected such that the first switching frequency and the second switching frequency fall outside of at least one predefined disallowed frequency band; and

the DC-DC converter is configured for use in a device and the at least one predefined disallowed frequency band comprises switching frequencies that would give rise to interference or noise that would adversely affect operation of other components in the device.

2. The method of claim 1 wherein the second switching frequency is equal to 1/N of the first switching frequency, wherein N is a positive integer number.

3. The method of claim 1 further comprising:

adjusting the duty cycle of the DC-DC converter by decreasing an on time of the DC-DC converter.

4. The method of claim 3 further comprising: responsive to the output voltage being above the predetermined voltage range when using the first switching frequency, first decreasing the duty cycle by decreasing the on time of the DC-DC converter until a minimum duty cycle for the first switching frequency is reached, and

responsive to the output voltage remaining above the predetermined voltage range when using the first switching frequency, switching to the second switching frequency.

5. The method of claim 4 further comprising:

responsive to the output voltage remaining above the predetermined voltage range when using the second switching frequency, switching to a third switching frequency wherein the third frequency is 1/N+1 times the second frequency, wherein N is a positive integer number.

6. The method of claim 4 further comprising:

responsive to the output voltage being below the predetermined voltage range when using the second switching frequency, first increasing the duty cycle by increasing an on time of the DC-DC converter until a maximum duty cycle for the second switching frequency is reached, and

responsive to the output voltage remaining below the predetermined voltage range when using the second switching frequency, switching to the first switching frequency.

7. The method of claim 6 further comprising:

responsive to the output voltage remaining below the predetermined voltage range when using the first switching frequency, switching to a third switching frequency wherein the third frequency is greater than the first frequency.

8. The method of claim 3 further comprising: responsive to the output voltage being above the predetermined voltage range when using the first switching frequency, first switching to the second switching frequency; and

responsive to the output voltage remaining above the predetermined voltage range when using the second switching frequency, decreasing the duty cycle by decreasing the on time of the DC-DC converter until a minimum duty cycle for the second switching frequency is reached.

9. The method of claim 8 further comprising:

responsive to the output voltage remaining above the predetermined voltage range when using the second switching frequency, switching to a third switching frequency wherein the third frequency is 1/N+1 times the second frequency, wherein N is a positive integer number.

10. The method of claim 3 further comprising:

responsive to the output voltage being below the predetermined voltage range when using the second switching frequency, switching to the first switching frequency; and

responsive to the output voltage remaining below the predetermined voltage range when using the first switching frequency, increasing the duty cycle by increasing an on time of the DC-DC converter until a maximum duty cycle for the first switching frequency is reached.

11. The method of claim 10 further comprising:

responsive to the output voltage remaining below the predetermined voltage range when using the first switching frequency, switching to a third switching frequency wherein the third frequency is greater than the first frequency.

12. The method of claim 1 further comprising:

alternating between the first switching frequency and the second switching frequency, wherein the first switching frequency is higher than the predefined disallowed frequency band and the second switching frequency is lower than the predefined disallowed frequency band.

13. The method of claim 1 wherein the at least one predefined disallowed frequency band comprises two or more separate predefined disallowed frequency bands.

14. A DC-DC converter configured to in a discontinuous conduction mode and to maintain an output voltage within a predetermined voltage range, the DC-DC converter comprising a controller, wherein the controller comprises:

an input configured to receive an indication of the output voltage; and

an adjustment block configured to adjust a duty cycle of the DC-DC converter based on the output voltage to maintain the output voltage within a predetermined voltage range;

wherein:

the duty cycle of the DC-DC converter is adjusted by switching between a first switching frequency to a second switching frequency;

the first switching frequency and the second switching frequency are selected such that the first switching frequency and the second switching frequency fall outside of at least one predefined disallowed frequency band; and

the DC-DC converter is configured for use in a device and the at least one predefined disallowed frequency band comprises switching frequencies that would give rise to interference or noise that would adversely affect operation of other components in the device.

15. The DC-DC converter of claim 14 wherein the second switching frequency is equal to 1/N of the first switching frequency, wherein N is a positive integer number.

16. The DC-DC converter of claim 14 wherein the adjustment block is further configured to adjust the duty cycle of the DC-DC converter by decreasing an on time of the DC-DC converter.

17. The DC-DC converter of claim 16 wherein the adjustment block is further configured to: responsive to the output voltage being above the predetermined voltage range when using the first switching frequency, first decrease the duty cycle by decreasing the on time of the DC-DC converter until a minimum duty cycle for the first switching frequency is reached, and

responsive to the output voltage remaining above the predetermined voltage range when using the first switching frequency, switch to the second switching frequency.

18. The DC-DC converter of claim 17 wherein the adjustment block is further configured to: responsive to the output voltage remaining above the predetermined voltage range when using the second switching frequency, switch to a third switching frequency wherein the third frequency is 1/N+1 times the second frequency, wherein N is a positive integer number.

19. The DC-DC converter of claim 16 wherein the adjustment block is further configured to:

responsive to the output voltage being below the predetermined voltage range when using the second switching frequency, first increase the duty cycle by increasing an on time of the DC-DC converter until a maximum duty cycle for the second switching frequency is reached, and

responsive to the output voltage remaining below the predetermined voltage range when using the second switching frequency, switch to the first switching frequency.

20. The DC-DC converter of claim 19 wherein the adjustment block is further configured to: responsive to the output voltage remaining below the predetermined voltage range when using the first switching frequency, switch to a third switching frequency wherein the third frequency is greater than the first frequency.

21. The DC-DC converter of claim 16 wherein the adjustment block is further configured to: responsive to the output voltage being above the predetermined voltage range when using the first switching frequency, first switch to the second switching frequency; and

responsive to the output voltage remaining above the predetermined voltage range when using the second switching frequency, decrease the duty cycle by decreasing the on time of the DC-DC converter until a minimum duty cycle for the second switching frequency is reached.

22. The DC-DC converter of claim 21 wherein the adjustment block is further configured to: responsive to the output voltage remaining above the predetermined voltage range when using the second switching frequency, switch to a third switching frequency wherein the third frequency is 1/N+1 times the second frequency, wherein N is a positive integer number.

23. The DC-DC converter of claim 16 wherein the adjustment block is further configured to: responsive to the output voltage being below the predetermined voltage range when using the second switching frequency, switch to the first switching frequency; and

responsive to the output voltage remaining below the predetermined voltage range when using the first switching frequency, increase the duty cycle by increasing an on time of the DC-DC converter until a maximum duty cycle for the first switching frequency is reached.

24. The DC-DC converter of claim 23 wherein the adjustment block is further configured to:

responsive to the output voltage remaining below the predetermined voltage range when using the first switching frequency, switch to a third switching frequency wherein the third frequency is greater than the first frequency.

25. The DC-DC converter of claim 14 wherein the adjustment block is configured to alternate between the first switching frequency and the second switching frequency, wherein the first switching frequency is higher than the predefined disallowed frequency band and the second switching frequency is lower than the predefined disallowed frequency band.

26. The DC-DC converter of claim 14 wherein the at least one predefined disallowed frequency band comprises two or more separate predefined disallowed frequency bands.