DC-DC CONVERTERS

• US 20110018515A1
• Filed: 07/22/2010
• Published: 01/27/2011
• Est. Priority Date: 07/22/2009
• Status: Active Grant

• Current Assignee: Cirrus Logic Incorporated
• Original Assignee: Wolfson Microelectronics plc
• Inventors: McCloy-Stevens, Mark
• Granted Patent: US 8,519,691 B2
• Time in Patent Office: Days
• US Class Current: 323/284
• CPC Class Codes: H02M 1/0009 Devices or circuits for detecting current in a converter
• CPC Class Codes: H02M 1/0032 Control circuits allowing low power mode operation, e.g. in standby mode
• CPC Class Codes: H02M 1/38 Means for preventing simultaneous conduction of switches
• CPC Class Codes: H02M 3/1588 comprising at least one synchronous rectifier element H02M3/1582, H02M3/1584 take precedence
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

Apparatus and method for providing current limiting in a DC-DC converter and especially to methods and apparatus suitable for a start-up mode of operation. The invention monitors the inductor (L) current when the high side supply switch is on against a peak current limit threshold. If the inductor current reaches the peak current limit threshold the high side switch is turned off. The inductor current when the low side switch is off is monitored against a valley current threshold. As long as the inductor current is above the valley current threshold turn on of the low side switch is inhibited. In this way current limiting is provided and the problems of stair-stepping are avoided. Embodiments employing lossless current sensing are described. The invention may be implemented in a start-up mode of operation wherein the converter is controlled purely by the peak current limit and valley current threshold which are increased over time.

Claims

1. A method of current limiting in a DC-DC converter comprising:

monitoring, when a high side switch is turned on, whether a first signal indicative of the current in the converter is above a first threshold;

in the event that the first signal is above the first threshold, turning the high side switch off;

monitoring, when the high side switch is turned off, whether a second signal indicative of the current in the converter is above a second threshold; and

in the event that the second signal is above the second threshold, inhibiting turn on of the high side switch.

2. A method as claimed in claim 1 comprising the step of, in the event that the first signal is above the first threshold, turning the low side switch on.

3. A method as claimed in claim 2 wherein, when the low side switch is turned on in response to the first signal being above the first threshold, the low side switch is maintained on until the high side switch turns on or until a low side off control signal is received.

4. A method as claimed in claim 1 further comprising the step of monitoring, when the low side switch is turned on, whether the second signal or a third signal indicative of the current in the converter is below a third threshold and, in the event that the third signal is below the third threshold, generating a low side off control signal.

5. A method as claimed in claim 1 wherein, when turn on of the high side switch is not inhibited, the high side switch turns on in response to a first control signal.

6. A method as claimed in claim 5 wherein the first control signal is derived from a clock signal.

7. A method as claimed in claim 6 wherein the clock signal has a constant frequency.

8. A method as claimed in claim 1 wherein the method is used during a start up mode of operation.

9. A method as claimed in claim 8 wherein, during the start-up mode of operation, a voltage loop of the converter is ignored.

10. A method as claimed in claim 8 wherein, during the start up mode of operation, at least one of the first threshold and the second threshold is increased over time.

11. A method as claimed in claim 10 wherein the first and/or second threshold levels are increased in a plurality of steps.

12. A method as claimed in claim 10 wherein the initial threshold level for the second threshold during start up mode is substantially zero current.

13. A method as claimed in claim 8 comprising operating in a start-up mode of operation until the output voltage of the converter is beyond an output voltage threshold and thereafter operating the DC-DC converter in a normal mode of operation.

14. A method as claimed in claim 12 wherein if the output threshold is not reached within a first period of time after the beginning of start up mode the converter is deactivated.

15. A method as claimed in claim 1 wherein, in normal mode of operation, the converter is operated in a valley current mode control mode.

16. A method as claimed in claim 15 wherein, in normal mode of operation, the second threshold is set above the expected operating valley current.

17. A method as claimed in claim 1 wherein the step of inhibiting turn on of the high side switch in response to the second signal being beyond the second threshold is only performed if the low side switch has been turned on in response to the first signal being beyond the first threshold.

18. A method as claimed in claim 1 wherein the first signal is determined from the electric properties of the high side switch when turned on and the second signal is derived from the electrical properties of the low side switch when turned on.

19. A method as claimed in claim 1 wherein at least one of the first threshold and the second threshold varies according to at least one of the high side supply voltage, the output voltage of the converter, the switching frequency and the inductance of the inductor.

20. A DC-DC converter comprising:

an inductor operably connected between a first node and an output node;

a high side switch operably connected between a high side supply input node and the first node;

a low side switch operably connected between a low side supply input node and the first node;

switch control circuitry operable to control the said high side and low side switches;

wherein the switch control circuitry comprises;

first current limit circuitry for turning the high side switch off if a first signal indicative of the current in the inductor is beyond a first threshold; and

second current limit circuitry for inhibiting turn on of the high side switch if a second signal indicative of the current in the inductor is beyond a second threshold.

21. A DC-DC converter as claimed in claim 20 wherein the high side switch comprises a p-channel transistor and the first current limit circuitry comprises a first comparator arranged to compare the source-drain voltage of the p-channel transistor with the first threshold.

22. A DC-DC converter as claimed in claim 20 wherein the low side switch comprises an n-channel transistor and the second current limit circuitry comprises a second comparator arranged to compare the source-drain voltage of the n-channel transistor with the second threshold.

23. A DC-DC converter as claimed in claim 20, wherein the switch control circuitry is configured to, in a start-up mode of operation, control turn on of the high side switch based on a clock signal.

24. A DC-DC converter as claimed in claim 20 wherein the first current limit circuitry is configured, in a start-up mode of operation, to increase the first threshold over time and/or wherein the second current limit circuitry is configured, in said start-up mode of operation, to increase the second threshold over time.

25. A DC-DC converter as claimed in claim 20 wherein the switch control circuitry is configured to turn the low side switch off if the current in the inductor when the low side switch is on reaches a third threshold.

26. A DC-DC converter as claimed in claim 20 wherein the switch control circuitry, in a normal mode of operation, is configured to provide valley current mode control.

36. A power management integrated circuit comprising a DC-DC converter as claimed in claim 20.

37. An electronic device comprising a DC-DC converter as claimed in claim 20.

38. An electronic device as claimed in claim 37 wherein the device is one of:

a portable computing device;

a laptop computer;

a personal data assistant;

a personal media player;

an mp3 player;

a portable television;

a mobile communications device;

a mobile telephone;

a navigation aid;

a GPS device;

a game console.

27. (canceled)

28. A method of starting a DC-DC converter comprising the steps of:

setting a first current limit and a second current limit;

receiving a clock signal comprising a series of clock pulses; and

monitoring at least one signal indicative of the current in the converter;

wherein the method comprises the steps of repetitively;

turning a high side switch off when the at least one signal indicative of the current in the inductor reaches the first current limit and keeping the high side switch off until at least the next clock pulse and

subsequently turning the high side switch on after the at least one signal indicative of the current in the inductor reaches the second current limit.

29. A method as claimed in claim 28 wherein, in a first mode of operation, the step of subsequently turning the high side switch on comprises turning the high side switch on at the first clock pulse following the time at which the at least one signal indicative of the current in the inductor reaches the second current limit.

30. A method as claimed in claim 29 wherein, in a second mode of operation, the step of subsequently turning the high side switch on comprises turning the high side switch on at whichever occurs later of said next clock pulse or the time at which the at least one signal indicative of the current in the inductor reaches the second current limit.

31. A method as claimed in claim 30 wherein the method comprises operating in said first mode of operation during at least a first period and operating in said second mode during at least a second period.

32. A method as claimed in claim 28 further comprising the step of turning the low side switch off if the inductor current is below a third threshold.

33. A method as claimed in claim 28 comprising the steps of increasing at least of the first current limit and the second current limit over time.

34. A method of providing a current limit for a DC-DC converter comprising a high side supply switch and a low side supply switch, the method comprising:

providing a first control signal for turning the high side supply switch on in normal operation; and

monitoring the current in the inductor against a first threshold when the high side supply switch is on and against a second threshold when the high side supply is off;

wherein the high side switch is turned off if the first threshold is exceeded and

wherein the high side switch is inhibited from being turned on whilst the second threshold is exceeded;

and wherein, when the high side switch is not inhibited from being turned on the switch is turned on in response to the first control signal.

35. A method of start up for a DC-DC converter comprising:

applying a first current limit to provide a peak current limit by turning a high side supply switch off; and

applying a second current limit to inhibit turn on of the high side supply switch;

wherein, when not inhibited from turning on the high side supply switch is turned on in response to a constant frequency clock signal.

39-41. -41. (canceled)