Frequency jittering control for varying the switching frequency of a power supply

• US 20080279324A1
• Filed: 07/09/2007
• Published: 11/13/2008
• Est. Priority Date: 05/11/2007
• Status: Active Grant

• Current Assignee: Richtek Technology Corporation (MediaTek, Inc.)
• Original Assignee: Richtek Technology Corporation (MediaTek, Inc.)
• Inventors: Chou, Hung-Che, Lin, Tzu-Chen, Huang, Pei-Lun
• Granted Patent: US 7,701,305 B2
• Time in Patent Office: Days
• US Class Current: 375/376
• CPC Class Codes: H02M 1/44 Circuits or arrangements for compensating for electromagnetic interference in converters or inverters
• CPC Class Codes: H02M 3/156 with automatic control of output voltage or current, e.g. switching regulators
• CPC Class Codes: H03K 3/84 Generating pulses having a predetermined statistical distribution of a parameter, e.g. random pulse generators
• CPC Class Codes: H03K 7/06 Frequency or rate modulation, i.e. PFM or PRM
• CPC Class Codes: H04B 15/005 Reducing noise, e.g. humm, from the supply

Abstract

A frequency jittering circuit modulates a hysteretic band of an oscillator such that the clock generated by the oscillator has a jitter frequency, and thus a switching mode power supply operative on the clock will have a jittering switching frequency.

Claims

1. A frequency jittering circuit for varying the switching frequency of a power supply, comprising:

an oscillator for providing a clock generated based on a hysteretic band for the power supply operative thereon; and

a hysteretic band modulator for providing a modulating signal to modulate the hysteretic band such that the clock has a jitter frequency.

2. The frequency jittering circuit of claim 1, wherein the oscillator comprises:

a charger circuit to be charged and discharged for providing a voltage;

a first comparator for comparing the voltage with an upper limit of the hysteretic band to thereby determine a first comparison signal; and

a second comparator for comparing the voltage with a lower limit of the hysteretic band to thereby determine a second comparison signal;

whereby the first and second comparison signals are used to generate the clock.

3. The frequency jittering circuit of claim 2, wherein the upper limit of the hysteretic band varies with the modulating signal.

4. The frequency jittering circuit of claim 3, wherein the oscillator further comprises an adder for combining the modulating signal with a reference signal to thereby determine the upper limit of the hysteretic band.

5. The frequency jittering circuit of claim 4, wherein the modulating signal is a triangle wave.

6. The frequency jittering circuit of claim 3, wherein the upper limit is varied in such a manner that once it increases to a first threshold, it will start to decrease to a second threshold.

7. The frequency jittering circuit of claim 6, wherein the frequency of the clock decreases as the upper limit of the hysteretic band increases, and increases as the upper limit of the hysteretic band decreases.

8. The frequency jittering circuit of claim 6, wherein the upper limit of the hysteretic band increases or decreases by a difference in steps in each cycle of the clock.

9. The frequency jittering circuit of claim 8, wherein the difference is related to a duty cycle of the clock.

10. The frequency jittering circuit of claim 2, wherein the oscillator further comprises:

a flip-flop for controlling the charger circuit to be charged and discharged in response to the first and second comparison signals; and

a one shot circuit for generating the clock in accordance with an output of the flip-flop.

11. The frequency jittering circuit of claim 2, wherein the oscillator further comprises a current modifying circuit for controlling the currents to charge and discharge the charger circuit.

12. The frequency jittering circuit of claim 11, wherein the current modifying circuit comprises:

an error amplifier for generating an error signal depending on the difference between a first reference signal and a feedback signal proportional to an output power of the power supply;

a clamp circuit for clamping the maximum and minimum values of the error signal to thereby generate a clamped signal; and

an adder for combing the clamped signal with a second reference signal to generate a current modifying signal to control the currents to charge and discharge the charger circuit.

13. The frequency jittering circuit of claim 1, wherein the hysteretic band modulator comprises:

a capacitor;

a first current source for determining a charging current;

a first switch connected between the capacitor and the first current source;

a second current source for determining a discharging current; and

a second switch connected between the capacitor and the second current source;

whereby the first and second switches are switched to charge and discharge the capacitor to thereby generate a low-frequency signal to determine the modulating signal.

14. The frequency jittering circuit of claim 13, wherein the clock switches the first switch once the low-frequency signal is lower than a first threshold, and switches the second switch after the low-frequency signal increases to be higher than a second threshold.

15. The frequency jittering circuit of claim 13, wherein the hysteretic band modulator further comprises a gain circuit for amplifying the low-frequency signal to generate the modulating signal.

16. The frequency jittering circuit of claim 13, wherein the low-frequency signal is increased or decreased by a difference in steps in each period of the clock.

17. The frequency jittering circuit of claim 16, wherein the, difference is related to a duty cycle of the clock.

18. The frequency jittering circuit of claim 13, wherein the low-frequency signal is a triangle wave.

19. A frequency jittering method for varying the switching frequency of a power supply operative on a clock provided by an oscillator, comprising the steps of:

modulating a hysteretic band of the oscillator;

generating the clock based on the hysteretic band such that the clock has a jitter frequency.

20. The frequency jittering method of claim 19, wherein the step of modulating a hysteretic band of the oscillator comprises the steps of:

providing a low-frequency signal; and

varying an upper limit or a lower limit of the hysteretic band by the low-frequency signal.

21. The frequency jittering method of claim 20, wherein the step of providing a low-frequency signal comprises the step of charging and discharging a capacitor for generating the low-frequency signal.

22. The frequency jittering method of claim 21, wherein the step of charging and discharging a capacitor for generate the low-frequency signal comprises the steps of:

controlling the capacitor to be charged in steps by the clock once the low-frequency signal is lower than a first threshold and until the low-frequency signal increases to be higher than a second threshold; and

controlling the capacitor to be discharged in steps by the clock after the low-frequency signal is higher than the second threshold and until the low-frequency signal decreases to be lower than the first threshold.

23. The frequency jittering method of claim 20, wherein the low-frequency signal is a triangle wave.

24. The frequency jittering method of claim 20, further comprising the step of amplifying the low-frequency signal for generating a modulating signal to modulate the upper limit or lower limit of the hysteretic band.