Switching DC-to-DC converter with discontinuous pulse skipping and continuous operating modes without external sense resistor

• US 6,396,252 B1
• Filed: 12/14/2000
• Issued: 05/28/2002
• Est. Priority Date: 12/14/2000
• Status: Expired due to Term

• Current Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Original Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Inventors: Suzuki, Hidehiko, Culpepper, Barry James
• Primary Examiner(s): HAN, YOUNGHUIE JESSICA
• Application Number:
  US09/738,609
• Publication Number:
  US 20020074975A1
• Time in Patent Office: 530 Days
• Field of Search: 323/222,225,282,283,284,285
• US Class Current: 323/285
• CPC Class Codes: H02M 1/0032 Control circuits allowing low power mode operation, e.g. in standby mode
• CPC Class Codes: H02M 1/0041 Control circuits in which a clock signal is selectively enabled or disabled
• CPC Class Codes: H02M 3/156 with automatic control of output voltage or current, e.g. switching regulators
• 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

A switching DC-to-DC converter having at least one power channel including an inductor and a controller which generates at least one power switch control signal for at least one power switch of each power channel. The converter is configured to operate in a continuous mode when the inductor current remains above zero, to enter a discontinuous pulse skipping mode of operation when the inductor current falls to zero (which occurs when the load current is below a threshold value), and to leave the discontinuous pulse skipping mode and resume continuous mode operation when the inductor current rises above zero. The main difference between the continuous and discontinuous pulse skipping modes is that in the continuous mode, a power switch has a duty cycle determined by a feedback signal indicative of the converter'"'"'s output potential V_out (so that the duty cycle is independent of the current drawn from the converter by the load), and in the discontinuous pulse skipping mode the power switch has a duty cycle which is the longer of a minimum duty cycle and a discontinuous (non-pulse-skipping) mode duty cycle. The discontinuous pulse skipping mode is more efficient than the continuous mode under conditions of low load current. Preferably, the controller includes cycle-skipping circuitry operable in the discontinuous pulse skipping mode and optionally also the continuous mode to cause the power switch to remain off for at least one cycle under the condition that the converter'"'"'s output potential rises above a threshold. Preferably, the cycle-skipping circuitry includes a comparator which compares an error amplifier output (indicative of the converter output potential) with a threshold potential, and logic circuitry (e.g., an AND gate coupled to the comparator output) which asserts a latch-clearing signal once per switching cycle when the comparator output indicates that the converter'"'"'s output has risen above the threshold. Other aspects of the invention are a switching controller for use in such a converter and a method for generating power switch control signals for such a converter in a discontinuous pulse skipping mode of operation.

Claims

1. A switching DC-to-DC converter which generates an output potential V_out at an output node in response to an input potential, said converter comprising:

a switching controller configured to generate at least one switch control signal including a first switch control signal in response to a first feedback signal indicative of the output potential V_out and a second feedback signal indicative of kV_out, where k is a constant, the switching controller having a switching period; and

external circuitry including a first power switch and an inductor, wherein the first power switch has an input coupled to receive the input potential and an output coupled to a first node and is coupled to receive the first switch control signal, the inductor is coupled between the first node and the output node, and an inductor current flows through the inductor during operation of the converter,

wherein the switching controller is configured to operate in a continuous mode in which the inductor current remains above zero and the first switch control signal causes the first power switch to operate with a continuous mode duty cycle determined by the first feedback signal, and the switching controller is configured to enter a discontinuous pulse skipping mode in response to the inductor current falling to zero, wherein in the discontinuous pulse skipping mode, the first switch control signal causes the first power switch to operate with a duty cycle which is the longer of a minimum duty cycle and a discontinuous mode duty cycle, wherein the discontinuous mode duty cycle is a duty cycle at which the converter would operate in response to said first feedback signal when said converter generates the output potential in response to the input potential during a discontinuous mode without pulse skipping.

2. The converter of claim 1, wherein the controller includes mode control circuitry coupled to the output node and configured to trigger entry into the discontinuous pulse skipping mode upon detecting that the inductor current is zero and to trigger entry into the continuous mode when the inductor current rises from zero to a level above zero.

3. The converter of claim 2, wherein the external circuitry also includes a second power switch having an input coupled to a first node and an output coupled to a second node, and the mode control circuitry includes:

a first comparator having one input coupled to the first node, another input coupled to the second node, and an output at which the first comparator asserts a comparator output; and

a mode signal generation circuit having an input coupled to receive the comparator output, and being configured to produce in response to said comparator output a mode signal indicative of whether or not the inductor current is above zero.

4. The converter of claim 3, wherein the controller is a current mode switching controller, and wherein the controller also includes:

first switch control signal generation circuitry coupled to receive the first feedback signal;

an attentuator having an input coupled to the output node and an attentuator output at which the attenuator asserts the second feedback signal;

a second comparator having an input coupled to receive the second feedback signal, another input coupled to receive a periodic ramped voltage having period equal to the switching period and peak level proportional to the input potential, and an output; and

logic circuitry having an input coupled to receive the mode signal, another input coupled to the output of the second comparator, and an output coupled to the first switch control signal generation circuitry.

5. The converter of claim 4, wherein the logic circuitry comprises:

a NAND gate having an input coupled to receive the mode signal, another input coupled to the output of the second comparator, and an output; and

an AND gate having an input coupled to the output of the NAND gate, and another input and an output coupled to the first switch control signal generation circuitry.

6. The converter of claim 1, wherein the controller includes cycle-skipping circuitry operable in at least the discontinuous pulse skipping mode to cause the first power switch to remain off for at least one said switching period in response to the first feedback signal indicating that the output potential is above a predetermined threshold.

7. The converter of claim 6, wherein the controller includes an error amplifier having an input coupled to receive the first feedback signal and an output, and the cycle-skipping circuitry includes:

a comparator having an input coupled to the output of the error amplifier, another input maintained at a threshold potential, and an output at which the comparator asserts a comparator output signal; and

logic circuitry, having an input coupled to receive the comparator output signal, and configured to generate a control signal for causing the first power switch to remain off for at least one said switching period when the comparator output signal indicates that the output potential is above the predetermined threshold.

8. The converter of claim 7, wherein the logic circuitry is an AND gate having a first input coupled to receive the comparator output signal and a second input coupled to receive a periodic pulse train whose pulses occur with said switching period.

9. The converter of claim 7, wherein the logic circuitry is configured to assert a latch-clearing signal once during each said switching period when the comparator output signal indicates that the output potential exceeds the predetermined threshold.

10. The converter of claim 6, wherein the cycle-skipping circuitry is operable in both the discontinuous pulse skipping mode and the continuous mode to cause the first power switch to remain off for at least one said switching period in response to the first feedback signal indicating that the output potential is above a predetermined threshold.

11. The converter of claim 1, wherein the switching controller is a current mode switching controller, the at least one switch control signal includes a second switch control signal, and the external circuitry is buck converter circuitry, said buck converter circuitry including:

a second power switch having an input coupled to the first node, an output coupled to a second node, and a control terminal coupled to receive the second switch control signal.

12. The converter of claim 11, wherein the controller includes mode control circuitry coupled to the output node and configured to trigger entry into the discontinuous pulse skipping mode upon detecting that the inductor current is zero and to trigger entry into the continuous mode when the inductor current rises from zero to a level above zero, wherein the mode control circuitry includes:

a first comparator having one input coupled to the first node, another input coupled to the second node, and an output at which the first comparator asserts a comparator output; and

a mode signal generation circuit having an input coupled to receive the comparator output, and being configured to produce in response to said comparator output a mode signal indicative of whether or not the inductor current is above zero.

13. The converter of claim 11, wherein each of the first power switch and the second power switch is an NMOS transistor.

14. The converter of claim 13, wherein the controller is implemented as an integrated circuit, and the external circuitry is external to said integrated circuit.

15. The converter of claim 11, wherein the controller is configured so that the continuous mode duty cycle is proportional to a ratio of the input potential and the output potential.

16. The converter of claim 1, wherein the controller is implemented as an integrated circuit, and the external circuitry is external to said integrated circuit.

17. A switching DC-to-DC converter which generates an output potential V_out at an output node in response to an input potential, said converter comprising:

a switching controller configured to generate at least a first switch control signal and a second switch control signal in response to a first feedback signal indicative of the output potential V_out and a second feedback signal indicative of kV_out, where k is a constant, the switching controller having a switching period; and

external circuitry including a first power switch, a second power switch, and an inductor, wherein the first power switch has an input coupled to receive the input potential, an output coupled to a first node, and a control terminal coupled to receive the first switch control signal, the second power switch has an input coupled to the first node, an output coupled to a second node, and a control terminal coupled to receive the second switch control signal, the inductor is coupled between the first node and the output node, and an inductor current flows through the inductor during operation of the converter,

wherein the switching controller is configured to operate in a continuous mode in which the inductor current remains above zero and the first switch control signal causes the first power switch to operate with a continuous mode duty cycle determined by the first feedback signal, and the switching controller is configured to enter a discontinuous pulse skipping mode in response to the inductor current falling to zero, wherein in the discontinuous pulse skipping mode, the first switch control signal causes the first power switch to operate with a duty cycle which is the longer of a minimum duty cycle and a discontinuous mode duty cycle, wherein the discontinuous mode duty cycle is a duty cycle at which the converter would operate in response to said first feedback signal when said converter generates the output potential in response to the input potential during a discontinuous mode without pulse skipping.

18. The converter of claim 17, wherein the controller includes cycle-skipping circuitry operable in at least the discontinuous pulse skipping mode to cause the first power switch to remain off for at least one said switching period in response to the first feedback signal indicating that the output potential is above a predetermined threshold, and wherein during the continuous mode when the first feedback signal indicates that the output potential is not greater than the predetermined threshold, the first power switch is on when the second power switch is off, and the first power switch is off when the second power switch is on, and the first power switch switches on and off once per each said switching period.

19. The converter of claim 18, wherein the controller includes an error amplifier having an input coupled to receive the first feedback signal and an output, and the cycle-skipping circuitry includes:

a comparator having an input coupled to the output of the error amplifier, another input maintained at a threshold potential, and an output at which the comparator asserts a comparator output signal; and

logic circuitry, having an input coupled to receive the comparator output signal, and configured to generate a control signal for causing the first power switch to remain off for at least one said switching period when the comparator output signal indicates that the output potential is above the predetermined threshold.

20. The converter of claim 19, wherein the logic circuitry is an AND gate having a first input coupled to receive the comparator output signal and a second input coupled to receive a periodic pulse train whose pulses occur with said switching period.

21. The converter of claim 19, wherein the logic circuitry is configured to assert a latch-clearing signal once during each said switching period when the comparator output signal indicates that the output potential exceeds the predetermined threshold.

22. The converter of claim 18, wherein the cycle-skipping circuitry is operable in both the discontinuous pulse skipping mode and the continuous mode to cause the first power switch to remain off for at least one said switching period in response to the first feedback signal indicating that the output potential is above a predetermined threshold.

23. The converter of claim 17, wherein each of the first power switch and the second power switch is an NMOS transistor.

24. The converter of claim 17, wherein the controller is configured so that the continuous mode duty cycle is proportional to a ratio of the input potential and the output potential.

25. The converter of claim 17, wherein the controller includes mode control circuitry coupled to the output node and configured to trigger entry into the discontinuous pulse skipping mode upon detecting that the inductor current is zero and to trigger entry into the continuous mode when the inductor current rises from zero to a level above zero.

26. The converter of claim 25, wherein the mode control circuitry includes:

a first comparator having one input coupled to the first node, another input coupled to the second node, and an output at which the first comparator asserts a comparator output; and

a mode signal generation circuit having an input coupled to receive the comparator output, and being configured to produce in response to said comparator output a mode signal indicative of whether or not the inductor current is above zero.

27. The converter of claim 26, wherein the controller is a current mode switching controller, and wherein the controller also includes:

first switch control signal generation circuitry coupled to receive the first feedback signal;

an attentuator having an input coupled to the output node and an attentuator output at which the attenuator asserts the second feedback signal;

a second comparator having an input coupled to receive the second feedback signal, another input coupled to receive a periodic ramped voltage having period equal to the switching period and peak level proportional to the input potential, and an output; and

logic circuitry having an input coupled to receive the mode signal, another input coupled to the output of the second comparator, and an output coupled to the first switch control signal generation circuitry.

28. The converter of claim 27, wherein the logic circuitry comprises:

a NAND gate having an input coupled to receive the mode signal, another input coupled to the output of the second comparator, and an output; and

an AND gate having an input coupled to the output of the NAND gate, and another input and an output coupled to the first switch control signal generation circuitry.

29. The converter of claim 17, wherein the switching controller is a current mode switching controller, the external circuitry is buck converter circuitry, and the controller includes mode control circuitry coupled to the output node and configured to trigger entry into the discontinuous pulse skipping mode upon detecting that the inductor current is zero and to trigger entry into the continuous mode when the inductor current rises from zero to a level above zero, wherein the mode control circuitry includes:

a first comparator having one input coupled to the first node, another input coupled to the second node, and an output at which the first comparator asserts a comparator output; and

a mode signal generation circuit having an input coupled to receive the comparator output, and being configured to produce in response to said comparator output a mode signal indicative of whether or not the inductor current is above zero.

30. A switching controller having a switching period for use with power channel circuitry of a switching DC-to-DC converter, wherein the power channel circuitry generates an output potential V_out at an output node in response to an input potential, the power channel circuitry includes a first power switch and an inductor, the first power switch has an input coupled to receive the input potential and an output coupled to a first node, and the inductor is coupled between the first node and the output node so that an inductor current flows through the inductor during operation of the converter, said controller comprising:

switch control signal generation circuitry configured to generate at least one switch control signal including a first switch control signal in response to set and reset signals; and

additional circuitry coupled to the switch control signal generation circuitry and to receive a first feedback signal indicative of the output potential V_out and a second feedback signal indicative of kV_out, where k is a constant, wherein the additional circuitry is configured to generate the set signals and the reset signals in response to the first feedback signal and the second feedback signal, wherein the controller is configured to operate in a continuous mode in which the inductor current remains above zero and the first switch control signal causes the first power switch to operate with a continuous mode duty cycle determined by the first feedback signal, and the controller is configured to enter a discontinuous pulse skipping mode in response to the inductor current falling to zero, wherein in the discontinuous pulse skipping mode, the first switch control signal causes the first power switch to operate with a duty cycle which is the longer of a minimum duty cycle and a discontinuous mode duty cycle, wherein the discontinuous mode duty cycle is a duty cycle at which the converter would operate in response to said first feedback signal when said converter generates the output potential in response to the input potential during a discontinuous mode without pulse skipping.

31. The controller of claim 30, wherein the additional circuitry includes:

mode control circuitry configured to trigger entry of the controller into the discontinuous pulse skipping mode upon detecting, when coupled to the output node, that the inductor current is zero, and configured to trigger entry into the continuous mode upon detecting, when coupled to the output node, that the inductor current rises from zero to a level above zero.

32. The controller of claim 31, wherein the power channel circuitry also includes a second power switch having an input coupled to the first node and an output coupled to a second node, and wherein the mode control circuitry includes:

a first comparator having one input configured to be coupled to the first node, another input configured to be coupled to the second node, and an output at which the first comparator asserts a comparator output; and

a mode signal generation circuit having an input coupled to receive the comparator output, and being configured to produce in response to said comparator output a mode signal indicative of whether or not the inductor current is above zero.

33. The controller of claim 32, wherein the controller is a current mode switching controller, and wherein the additional circuitry also includes:

an attentuator having an input configured to be coupled to the output node and an attentuator output at which the attenuator asserts the second feedback signal;

a second comparator having an input coupled to receive the second feedback signal, another input coupled to receive a periodic ramped voltage having period equal to the switching period and peak level proportional to the input potential, and an output; and

logic circuitry having an input coupled to receive the mode signal, another input coupled to the output of the second comparator, and an output coupled to the first switch control signal generation circuitry.

34. The controller of claim 33, wherein the logic circuitry comprises:

a NAND gate having an input coupled to receive the mode signal, another input coupled to the output of the second comparator, and an output; and

an AND gate having an input coupled to the output of the NAND gate, and another input and an output coupled to the first switch control signal generation circuitry.

35. The controller of claim 30, wherein the additional circuitry also includes:

cycle-skipping circuitry operable in at least the discontinuous pulse skipping mode to assert to the switch control signal generation circuitry a control signal when the output potential is above the predetermined threshold, and wherein the switch control signal generation circuitry is configured to cause the first power switch to remain off for at least one said switching period in response to said control signal.

36. The controller of claim 35, wherein the additional circuitry also includes an error amplifier having an input coupled to receive the first feedback signal and an output, and the cycle-skipping circuitry includes:

a comparator having an input coupled to the output of the error amplifier, another input maintained at a threshold potential, and an output at which the comparator asserts a comparator output signal; and

logic circuitry, having an input coupled to receive the comparator output signal, and configured to assert the control signal to the switch control signal generation circuitry when the comparator output signal indicates that the output potential is above the predetermined threshold.

37. The controller of claim 36, wherein the logic circuitry is an AND gate having a first input coupled to receive the comparator output signal and a second input coupled to receive a periodic pulse train whose pulses occur with said switching period.

38. The controller of claim 35, wherein the cycle-skipping circuitry is operable in both the discontinuous mode and the continuous mode to assert the control signal to the switch control signal generation circuitry when the output potential is above the predetermined threshold.

39. The controller of claim 30, wherein said controller is configured so that the continuous mode duty cycle is proportional to a ratio of the input potential and the output potential.

40. The controller of claim 30, wherein said controller is a current mode switching controller, the at least one switch control signal includes a second switch control signal, and the power channel circuitry is buck converter circuitry including a second power switch having an input coupled to the first node, an output coupled to a second node, and wherein said additional circuitry includes:

mode control circuitry configured to be coupled to the output node, to trigger entry into the discontinuous pulse skipping mode upon detecting that the inductor current is zero, and to trigger entry into the continuous mode when the inductor current rises from zero to a level above zero.

41. The controller of claim 40, wherein the mode control circuitry includes:

a first comparator having one input configured to be coupled to the first node, another input coupled to the second node, and an output at which the first comparator asserts a comparator output; and

a mode signal generation circuit having an input coupled to receive the comparator output, and being configured to produce in response to said comparator output a mode signal indicative of whether or not the inductor current is above zero.

42. A method for generating a power switch control signal for a DC-to-DC converter which generates an output potential V_out at an output node in response to an input potential by switching at least a first power switch having an input coupled to receive the input potential and an output coupled to a first node, wherein the converter has an inductor coupled between the first node and the output node so that an inductor current flows through the inductor during operation of the converter, said method including the steps of:

(a) in a continuous mode in which the inductor current remains above zero, generating the power switch control signal in response to a feedback signal indicative of the output potential V_out, such that said power switch control signal causes the first power switch to operate with a continuous mode duty cycle determined by the feedback signal; and

(b) entering a discontinuous pulse skipping mode when the inductor current falls to zero, and in the discontinuous mode, generating the power switch control signal in response to a second feedback signal indicative of kV_out, where k is a constant, and in response to the feedback signal, such that said power switch control signal causes the first power switch to operate with a duty cycle equal to the longer of a minimum duty cycle and a discontinuous mode duty cycle, wherein the discontinuous mode duty cycle is a duty cycle at which the converter would operate in response to said feedback signal when said converter generates the output potential in response to the input potential during a discontinuous mode without pulse skipping.

43. The method of claim 42, wherein the converter also includes a second power switch having an input coupled to the first node and an output coupled to a second node, and wherein step (b) includes the step of:

determining when the inductor current falls to zero by monitoring voltage across the second power switch.

44. The method of claim 42, wherein step (b) includes the step of:

causing the first power switch to remain off for at least one switching period in response determining that the output potential is above a predetermined threshold.

45. The method of claim 44, wherein step (a) includes the step of:

causing the first power switch to remain off for at least one switching period in response determining that the output potential is above the predetermined threshold.