Fixed-frequency control circuit and method for pulse width modulation

1. A fixed-frequency control circuit for pulse width modulation triggering an on-time or an off-time of a pulse width modulation signal and determining a width of the on-time or the off-time according to a first current, the fixed-frequency control circuit comprising:

a frequency to current converter operative to convert a frequency of the pulse width modulation signal into a second current;

a current generator providing a reference current;

a subtractor connected to the frequency to current converter and the current generator, for subtracting the second current from the reference current to generate a differential current;

a current amplifier connected to the subtractor for amplifying the differential current to generate an error current; and

an adder connected to the current amplifier for adding the error current to the first current to adjust the width of the on-time or the off-time to stabilize the frequency of the pulse width modulation signal at a target frequency determined by the reference current.

2. A fixed-frequency control circuit for pulse width modulation triggering an on-time or an off-time of a pulse width modulation signal and determining a width of the on-time or the off-time according to a first current, the fixed-frequency control circuit comprising:

a clock generator providing a clock signal having a fixed-frequency;

an up/down counter connected to the clock generator for calculating a frequency difference between the pulse width modulation signal and the clock signal to generate a count;

a programmable current source connected to the up/down counter, operative to determine an error current according to the count; and

an adder connected to the programmable current source for adding the error current to the first current to adjust the width of the on-time or the off-time to stabilize the frequency of the pulse width modulation signal at the frequency of the clock signal.

3. The fixed-frequency control circuit of claim 2 , wherein the programmable current source comprises a plurality of common output switch current sources controlled by a plurality of bits in the count respectively.

4. A fixed-frequency control circuit for pulse width modulation triggering an on-time or an off-time of a pulse width modulation signal and determining a width of the on-time or the off-time according to a first current, the fixed-frequency control circuit comprising:

a clock generator providing a clock signal having a fixed-frequency;

a first one shot generator connected to the clock generator, triggered by the clock signal to generate a first pulse signal;

a first current mirror connected to the first one shot generator, responsive to the first pulse signal to generate a reference current related to a frequency of the clock signal;

a second one shot generator triggered by the pulse width modulation signal to generate a second pulse signal;

a second current mirror connected to the second one shot generator, responsive to the second pulse signal to generate a second current related to a frequency of the pulse width modulation signal; and

an adder connected to the first and second current mirrors for adding a differential current between the reference current and the second current to the first current to adjust the width of the on-time or the off-time to stabilize the frequency of the PWM signal at the frequency of the clock signal.

5. The fixed-frequency control circuit of claim 4 , wherein the first current mirror comprises:

a current source;

a first transistor having a gate and a drain connected to each other;

a switch connected between the current source and the drain of the first transistor, controlled by the first pulse signal;

a second transistor having a drain connected to the adder, for mirroring a current of the first transistor to generate the reference current; and

a low-pass filter connected between the gate of the first transistor and a gate of the second transistor.

6. The fixed-frequency control circuit of claim 4 , wherein the second current mirror comprises:

a current source;

a first transistor having a gate and a drain connected to each other;

a switch connected between the current source and the drain of the first transistor, controlled by the second pulse signal;

a second transistor having a drain connected to the adder, for mirroring a current of the first transistor to generate the second current; and

a low-pass filter connected between the gate of the first transistor and a gate of the second transistor.

7. A fixed-frequency control method for pulse width modulation triggering an on-time or an off-time of a pulse width modulation signal and determining a width of the on-time or the off-time according to a first current, the fixed-frequency control method comprising the steps of:

converting a frequency of the pulse width modulation signal into a second current;

subtracting the second current from a reference current to generate a differential current;

amplifying the differential current to generate an error current; and

adding the error current to the first current to adjust the width of the on-time or the off-time to stabilize the frequency of the pulse width modulation signal at a target frequency determined by the reference current.

8. A fixed-frequency control method for pulse width modulation triggering an on-time or an off-time of a pulse width modulation signal and determining a width of the on-time or the off-time according to a first current, the fixed-frequency control method comprising the steps of:

calculating a frequency difference between the pulse width modulation signal and a clock signal to generate a count;

determining an error current according to the count; and

adding the error current to the first current to adjust the width of the on-time or the off-time to stabilize the frequency of the pulse width modulation signal at the frequency of the clock signal.

9. The fixed-frequency control method of claim 8 , wherein the step of determining an error current according to the count comprises the steps of:

determining a configuration of a plurality of switches according to the count; and

common outputting a plurality of current sources according to the configuration to generate the error current.

10. A fixed-frequency control method for pulse width modulation triggering an on-time or an off-time of a pulse width modulation signal and determining a width of the on-time or the off-time according to a first current, the fixed-frequency control method comprising the steps of:

triggering a first pulse signal by a clock signal;

responsive to the first pulse signal, generating a reference current related to a frequency of the clock signal;

triggering a second pulse signal by the pulse width modulation signal;

responsive to the second pulse signal, generating a second current related to a frequency of the pulse width modulation signal; and

adding a differential current between the reference current and the second current to the first current to adjust the width of the on-time or the off-time to stabilize the frequency of the pulse width modulation signal at the frequency of the clock signal.