Method and apparatus for a LED driver with high power factor

1. A control circuit of a LED driver comprising:

an output circuit, the output circuit generating a switching signal to produce an output current for driving at least one LED in response to a feedback signal, in which the switching signal is coupled to switch a transformer;

an input circuit, the input circuit coupled to sample an input signal for generating the feedback signal; and

an input-voltage detection circuit, the input-voltage detection circuit coupled to detect an input voltage of the LED driver and generate an input-voltage signal in response to the input voltage of the LED driver;

wherein the input signal is correlated to the output current of the LED driver; the input circuit will not sample the input signal when the input-voltage signal is lower than a threshold.

2. The control circuit as claimed in claim 1 , wherein the input circuit further comprises a low-pass filer to provide a constant on time for the switching signal.

3. The control circuit as claimed in claim 2 , wherein the low pass filter is hold in the previous state when the input-voltage signal is lower than the threshold.

4. The control circuit as claimed in claim 2 , wherein a bandwidth of the low-pass filer is lower than a line frequency.

5. The control circuit as claimed in claim 1 , wherein the output circuit generates the switching signal operated in a boundary current mode.

6. The control circuit as claimed in claim 1 , wherein the input circuit further comprises an integrator for a constant current control.

7. The control circuit as claimed in claim 1 , wherein the input circuit further comprises an error amplifier for developing a feedback loop, the error amplifier is disconnected when the input-voltage signal is lower than the threshold.

8. The control circuit as claimed in claim 1 is a primary-side controlled circuit.

9. The control circuit as claimed in claim 1 , further comprising a comparator for comparing the input-voltage signal with the threshold, in which the comparator generates a blanking signal to stop the input circuit sampling the input signal when the input-voltage signal is lower than the threshold.

10. The control circuit as claimed in claim 1 , wherein the input circuit comprises:

a current-detection circuit, the current-detection circuit measuring the input signal to generate a current-waveform signal, the input signal being a current-sense signal;

an integrator, the integrator integrates the current-waveform signal for generating the feedback signal, the feedback signal being a current-feedback signal;

an error amplifier, the error amplifier comparing the current-feedback signal with a reference signal to generate an amplified signal; and

a low-pass filter, the low-pass filter generating a current-loop signal in response to the amplified signal;

wherein the output circuit generates the switching signal in response to the current-loop signal, the error amplifier is disconnected when the input-voltage signal is lower than the threshold, the low-pass filter is hold in the previous state when the input-voltage signal is lower than the threshold.

11. The control circuit as claimed in claim 1 , further comprising a voltage-detection circuit, the voltage-detection circuit generating a demagnetizing-time signal in response to a voltage-sense signal correlated to an output voltage of the LED driver, the output circuit generating the switching signal in response to the demagnetizing-time signal.

12. The control circuit as claimed in claim 1 , wherein the input-voltage detection circuit detects the input voltage of the LED driver through a resistor and generates the input-voltage signal in response to the input voltage of the LED driver.

13. A method for controlling a LED driver comprising:

generating a switching signal to produce an output current for the LED driver in response to a feedback signal, in which the switching signal is coupled to switch a transformer;

sampling an input signal for generating the feedback signal, in which the input signal is correlated to the output current of the LED driver;

generating an input-voltage signal in response to the level of an input voltage of the LED driver; and

stopping the sample of the input signal when the input-voltage signal being lower than a threshold.

14. The method as claimed in claim 13 , wherein the feedback signal is a low bandwidth signal for achieving a constant on-time for the switching signal.

15. The method as claimed in claim 13 , wherein the switching signal is operated to achieve a boundary current mode of the power conversion.

16. The method as claimed in claim 13 , further error-amplifying the feedback signal, in which the error-amplifying is stopped when the input-voltage signal is lower than the threshold.

17. The method as claimed in claim 13 , further comprising a low-pass filtering for the loop compensation, in which the low-pass filtering is hold in the previous state when the input-voltage signal is lower than the threshold.

18. The method for controlling the LED driver as claimed in claim 13 is a primary-side controlled method.

19. The method as claimed in claim 13 , wherein the input-voltage signal is generated by detecting the input voltage of the LED driver through a resistor.

20. The method as claimed in claim 13 , further generating a demagnetizing-time signal in response to a voltage-sense signal correlated to an output voltage of the LED driver for generating the switching signal in response to the demagnetizing-time signal.