Control circuit and method for maintaining high efficiency over broad current ranges in a switching regular circuit

• US 6,304,066 B1
• Filed: 09/14/1999
• Issued: 10/16/2001
• Est. Priority Date: 03/23/1993
• Status: Expired due to Term

• Current Assignee: Analog Devices International Unlimited Company (Analog Devices, Inc.)
• Original Assignee: Linear Technology Corporation (Analog Devices, Inc.)
• Inventors: Flatness, Randy G., Wilcox, Milton E.
• Primary Examiner(s): Wong, Peter S.
• Assistant Examiner(s): Patel, Rajnikant B.
• Application Number:
  US09/395,895
• Time in Patent Office: 763 Days
• Field of Search: 323/282, 323/284, 323/285, 323/287, 323/222, 363/80, 363/97, 363/98, 363/21
• US Class Current: 323/282
• CPC Class Codes: H02M 1/0032 Control circuits allowing low power mode operation, e.g. in standby mode
• 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 circuit and method for controlling a switching voltage regulator having (1) a switch including one or more switching transistors and (2) an output adapted to supply current at a regulated voltage to a load including an output capacitor. The circuit and method generates a control signal to turn said one or more switching transistors OFF under operating conditions when the voltage at the output is capable of being maintained substantially at the regulated voltage by the charge on the output capacitor. Such a circuit and method increases the efficiency of the regulator circuit particularly at low average current levels.

Claims

1. A circuit for controlling a switching voltage regulator, the regulator having (1) a switch coupled to receive an input voltage and including a pair of synchronously switched switching transistors and (2) an output for supplying current at a regulated voltage to a load which includes an output capacitor, the circuit comprising:

a first circuit for monitoring the output to generate a first feedback signal;

a second circuit for generating a first control signal during a first state of circuit operation, the first control signal being responsive to the first feedback signal to vary the duty cycle of the switching transistors to maintain the output at the regulated voltage; and

a third circuit for generating a second control signal during a second state of circuit operation to cause both switching transistors to be simultaneously OFF for a period of time longer than a switching cycle of the switching transistors, during which period of time the output capacitor maintains the output substantially at the regulated voltage.

2. Circuitry for controlling a switching regulator having an inductive element coupled between a first node and an output of the switching regulator, a first switch coupled between an input of the switching regulator and the first node, and a second switch coupled between the first node and ground, the control circuitry comprising:

a voltage feedback path for monitoring a voltage at the output;

a current feedback path for monitoring a current through the inductor;

a first comparator coupled to the current and voltage feedback paths;

a second comparator coupled to the voltage feedback path; and

a control circuit operational in first and second states responsive to an output of the second comparator, wherein in the first state the control circuit controls the duty cycle of the first and second switches responsive to the output of the first comparator, and in the second state the control circuit keeps both switches OFF regardless of the output of the first comparator.

3. The circuitry of claim 2 wherein the control circuit provides a first control signal that controls the first and second switches so that one of the switches is OFF when the other switch is ON.

4. The circuitry of claim 3 wherein the control circuit provides a second signal that causes the second switch to be OFF even when the first transistor is OFF.

5. The circuitry of claim 4 wherein the second control signal overrides the first control signal to the second switch.

6. The circuitry of claim 5 further comprising a logic circuit coupled to the first and second control signals and to the second switch.

7. The circuitry of claim 5 wherein the control circuit further comprises:

a capacitor;

a current source turned on or off responsive to the output of the second comparator; and

a current sink coupled to the capacitor, wherein the capacitor is alternately charged by the current source and discharged by the current sink.

8. The circuitry of claim 6 further comprising a third comparator coupled to the capacitor, wherein the third comparator provides the second control signal responsive to a voltage on the capacitor.

9. The circuitry of claim 2 further comprising:

a transconductance amplifier having an input coupled to the voltage feedback path and an output coupled to an input of the first comparator; and

a current sink coupled to the output of the transconductance amplifier.

10. The circuitry of claim 2 wherein the second comparator comprises a hysteretic comparator.

11. A method of regulating a voltage at the output of a switching regulator having first and second switching transistors arranged in a push-pull configuration and an inductor coupled to the first and second switching transistors, the method comprising:

monitoring the voltage;

monitoring a current through the inductor; and

controlling the first and second switching transistors so that during a first period of time the first switching transistor is cycled OFF-ON-OFF and the second switching transistor is cycled ON-OFF-ON, and during a second period of time the first and second switching transistors are kept OFF, wherein the second period of time has a duration that is longer than the period of a single OFF-ON-OFF cycle of the first and second switching transistors.

12. The method of claim 11 wherein controlling the first and second switching transistors during the first period of time comprises turning the first switching transistor OFF and the second switching transistor ON when the inductor current reaches a threshold level.

13. The method of claim 12 wherein controlling the first and second switching transistors during the first period of time further comprises turning the first switching transistor ON and the second switching transistor OFF a predetermined interval after the first switching transistor was turned OFF and the second switching transistor was turned ON.

14. The method of claim 11 further comprising determining if the voltage exceeds a first threshold, wherein the first period corresponds to when the voltage is below the first threshold and the second period corresponds to when the voltage is above the first threshold.

15. The method of claim 11 wherein controlling the first and second switching transistors during the first period of time further comprises charging a control capacitor when the first switching transistor is ON and discharging the control capacitor when the first switching transistor if OFF.

16. The method of claim 15 further comprising:

determining if the voltage exceeds a first threshold; and

keeping the first switching transistor OFF when the voltage exceeds the first threshold.

17. The method of claim 16 comprising:

monitoring a voltage on the control capacitor; and

turning the second switching transistor OFF when the voltage on the control capacitor falls below a second threshold.

18. The method of claim 15 further comprising:

monitoring a difference between an input voltage and the output voltage of the switching regulator; and

discharging the control capacitor at a rate determined by the monitored voltage difference.