Constant On-Time Regulator With Internal Ripple Generation and Improved Output Voltage Accuracy

• US 20080088292A1
• Filed: 12/12/2007
• Published: 04/17/2008
• Est. Priority Date: 09/11/2006
• Status: Active Grant

• Current Assignee: Microchip Technology Incorporated
• Original Assignee: Micrel Incorporated (Microchip Technology Incorporated)
• Inventors: Weng, Matthew, Vinn, Charles, Stoichita, Ioan
• Granted Patent: US 7,482,791 B2
• Time in Patent Office: Days
• US Class Current: 323/285
• CPC Class Codes: H02M 1/0025 Arrangements for modifying reference values, feedback values or error values in the control loop of a converter
• CPC Class Codes: H02M 3/156 with automatic control of output voltage or current, e.g. switching regulators

Abstract

A buck switching regulator formed on an integrated circuit receives an input voltage and provides a switching output voltage on a switch output node using a constant on-time, variable off-time feedback control loop. The buck switching regulator includes an amplifier comparing a feedback voltage to a reference voltage and generating an output voltage on an output terminal, a first capacitor and a first resistor connected in series between the switch output node and the output terminal of the amplifier, and a second capacitor coupled between the DC output voltage node and the output terminal of the amplifier. The first capacitor and the first resistor generate a ripple voltage signal which is injected onto the output terminal of the amplifier for use in the constant on-time, variable off-time feedback control scheme. The magnitude of the ripple voltage signal is a function of the capacitance value of the second capacitor.

Claims

1. A buck switching regulator formed on an integrated circuit and receiving an input voltage, the buck switching regulator controlling a first switch and a second switch using a constant on-time, variable off-time feedback control loop to drive a switch output node for generating a switching output voltage, the switch output node being coupled to an LC filter circuit external to the integrated circuit to generate a regulated output voltage having a substantially constant magnitude on an output node, the regulated output voltage being fed back to the buck switching regulator to a voltage divider for generating a feedback voltage on a feedback voltage node, the buck switching regulator comprising:

an amplifier with a high output impedance having a first input terminal coupled to receive the feedback voltage, a second input terminal coupled to receive a first reference voltage and an output terminal providing a first output voltage indicative of the difference between the feedback voltage and the first reference voltage;

an error comparator having a first input terminal coupled to receive a second reference voltage and a second input terminal coupled to receive the first output voltage of the amplifier, the error comparator generating an output voltage signal for controlling the constant on-time, variable off-time feedback control loop of the buck switching regulator;

a first capacitor and a first resistor formed on the integrated circuit of the buck switching regulator, the first capacitor and the first resistor being connected in series between the switch output node and the output terminal of the amplifier; and

a second capacitor coupled between the output node and the output terminal of the amplifier,

wherein the first capacitor and the first resistor generate a ripple voltage signal being related to the switching output voltage and provide the ripple voltage signal to the output terminal of the amplifier for use in the constant on-time, variable off-time feedback control loop, the magnitude of the ripple voltage signal being a function of the capacitance value of the second capacitor.

2. The buck switching regulator of claim 1, wherein the amplifier comprises a low-transconductance operational transconductance amplifier.

3. The buck switching regulator of claim 2, wherein the amplifier has a high DC gain and an AC gain of 1.

4. The buck switching regulator of claim 1, wherein the amplifier is formed on the integrated circuit of the buck switching regulator.

5. The buck switching regulator of claim 1, wherein the amplifier is formed external to the integrated circuit of the buck switching regulator.

6. The buck switching regulator of claim 1, wherein the second capacitor is formed external to the integrated circuit of the buck switching regulator.

7. The buck switching regulator of claim 1, wherein the second capacitor is formed on the integrated circuit of the buck switching regulator.

8. The buck switching regulator of claim 6, wherein the LC filter circuit includes a first inductor and an output capacitor and the second capacitor has a capacitance value related to the equivalent series resistance (ESR) value of the output capacitor.

9. The buck switching regulator of claim 8, wherein the second capacitor has a large capacitance value when the output capacitor has a large ESR value and the second capacitor has a small capacitance value when the output capacitor has a small or has a zero ESR value.

10. The buck switching regulator of claim 9, wherein the second capacitor has a large capacitance value of about 2.2 nF and a small capacitance value of about 220 pF.

11. The buck switching regulator of claim 9, wherein the ripple voltage signal has a small magnitude when the second capacitor has a large capacitance value and the ripple voltage signal has a large magnitude when the second capacitor has a small capacitance value.

12. The buck switching regulator of claim 7, wherein the LC filter circuit includes a first inductor and an output capacitor and the second capacitor has a capacitance value related to a range of equivalent series resistance (ESR) values of the output capacitor.

13. The buck switching regulator of claim 1, wherein the ripple voltage signal is a divided down voltage of the switching output voltage.

14. A method in buck switching regulator receiving an input voltage and controlling a first switch and a second switch using a constant on-time, variable off-time feedback control loop to drive a switch output node for generating a switching output voltage, the switch output node being coupled to an LC filter circuit to generate a regulated output voltage having a substantially constant magnitude on an output node, the regulated output voltage being fed back to the buck switching regulator to a voltage divider for generating a feedback voltage on a feedback voltage node, the method comprising:

coupling the feedback voltage to a first input terminal of an amplifier having a high output impedance;

coupling a first reference voltage to a second input terminal of the amplifier;

generating a first output voltage at an output terminal of the amplifier indicative of the difference between the feedback voltage and the first reference voltage;

generating a ripple voltage signal from the switching output voltage;

injecting the ripple voltage signal to the output terminal of the amplifier for use in the constant on-time, variable off-time feedback control loop;

coupling the first output voltage of the amplifier with the injected ripple voltage signal to a first input terminal of a comparator;

coupling a second reference voltage to a second input terminal of the comparator; and

adjusting the magnitude of the ripple voltage signal at the output terminal of the amplifier using a capacitive divider.

15. The method of claim 14, wherein:

coupling the feedback voltage to a first input terminal of an amplifier having a high output impedance comprises coupling the feedback voltage to a first input terminal of a low-transconductance operational transconductance amplifier; and

coupling a first reference voltage to a second input terminal of the amplifier comprises coupling the first reference voltage to a second input terminal of the low-transconductance operational transconductance amplifier.

16. The method of claim 15, wherein the amplifier has a high DC gain and an AC gain of 1.

17. The method of claim 14, further comprising:

providing a first capacitor and a first resistor connected in series between the switch output node and the output terminal of the amplifier, the first capacitor and the first resistor generating the ripple voltage signal from the switching output voltage and injecting the ripple voltage signal to the output terminal of the amplifier.

18. The method of claim 17, wherein the ripple voltage signal is a divided down voltage of the switching output voltage.

19. The method of claim 17, wherein adjusting the magnitude of the ripple voltage signal at the feedback voltage node using a capacitive divider comprises:

providing a second capacitor between the output voltage node and the output terminal of the amplifier, the magnitude of the ripple voltage signal being a function of the capacitance value of the second capacitor.

20. The method of claim 19, wherein the LC filter circuit includes a first inductor and an output capacitor and the second capacitor has a capacitance value related to the equivalent series resistance (ESR) value of the output capacitor.

21. The method of claim 20, wherein providing a second capacitor comprises providing a second capacitor having a large capacitance value when the output capacitor has a large ESR value and providing a second capacitor having a small capacitance value when the output capacitor has a small or has a zero ESR value.

22. The method of claim 21, wherein the second capacitor has a large capacitance value of about 2.2 nF and a small capacitance value of about 220 pF.

23. The method of claim 19, wherein the ripple voltage signal has a small magnitude when the second capacitor has a large capacitance value and the ripple voltage signal has a large magnitude when the second capacitor has a small capacitance value.