Light source dimming control circuit

1. A light source dimming control circuit, comprising:

a voltage signal generator for generating a first voltage signal between a predetermined voltage level and a ground level;

a reference voltage signal generator for generating a reference voltage signal at an output end according to the first voltage signal and a second voltage signal lower than the predetermined voltage level, the reference voltage signal generator comprising:

a first diode with an anode end coupled to the voltage signal generator, and a cathode end coupled to the output end; and

a second diode with an anode end receiving the second voltage signal, and a cathode end coupled to the output end;

a first switch with a first end coupled to an output end of a light source;

a first resistor with a first end coupled to a second end of the first switch, and a second end coupled to a ground end;

a switch controller coupled to the first switch and the voltage signal generator for turning on the first switch when a sawtooth signal is lower than the first voltage signal, and for turning off the first switch when the sawtooth signal is higher than the first voltage signal; and

a current controller coupled to the output end of the reference voltage signal generator and the first end of the first resistor for controlling current flowing through the first resistor according to the reference voltage signal.

2. The light source dimming control circuit of claim 1 , wherein the current controller controls a power supply to provide power to the light source such that a voltage level at the first end of the first resistor is equal to a voltage level of the reference voltage signal.

3. The light source dimming control circuit of claim 1 , wherein the current controller comprises:

an N-type transistor with a first end coupled to the second end of the first switch, and a second end coupled to the first end of the first resistor; and

an error amplifier for generating a control voltage at a control end of the N-type transistor according to the reference voltage signal and a voltage level at the first end of the first resistor.

4. The light source dimming control circuit of claim 1 , wherein the switch controller comprises:

a sawtooth signal generator for generating the sawtooth signal; and

a comparator coupled to the sawtooth signal generator and the voltage signal generator, for outputting a logic high signal to the first switch to turn on the first switch when the sawtooth signal is lower than the first voltage signal, and for outputting a logic low signal to the first switch to turn off the first switch when the sawtooth signal is higher than the first voltage signal.

5. The light source dimming control circuit of claim 1 , wherein the voltage signal generator comprises:

an inverter with an input end for receiving a dimming signal, and an output end for outputting an inverse signal opposite to the dimming signal;

a P-type transistor with a first end receiving a third voltage signal with the predetermined voltage level, and a control end coupled to the output end of the inverter;

an N-type transistor with a first end coupled to a second end of the P-type transistor, a second end coupled to the ground end, and a control end coupled to the output end of the inverter;

a second resistor with a first end coupled to the second end of the P-type transistor, and a second end coupled to the anode end of the first diode; and

a capacitor with a first end coupled to the anode end of the first diode, and a second end coupled to the ground end.

6. The light source dimming control circuit of claim 5 , wherein the dimming signal is a pulse width modulation signal.

7. The light source dimming control circuit of claim 1 , wherein the reference voltage signal generator further comprises:

a second resistor with a first end coupled to the cathode end of the second diode, and a second end coupled to the output end; and

a third resistor with a first end coupled to the output end, and a second end coupled to the ground end.

8. The light source dimming control circuit of claim 1 , wherein the reference voltage signal generator further comprises:

a second resistor with a first end coupled to the cathode end of the second diode, and a second end coupled to the output end;

a third resistor with a first end coupled to the output end;

a fourth resistor with a first end coupled to a second end of the third resistor, and a second end coupled to the ground end; and

a second switch with a first end coupled to the second end of the third resistor, and a second end coupled to the ground end, for controlling an electrical connection between the second end of the third resistor and the ground end according to a control signal.

9. The light source dimming control circuit of claim 8 , wherein the second switch is an N-type transistor.

10. The light source dimming control circuit of claim 1 , wherein a maximum voltage level of the sawtooth signal is equal to a voltage level of the second voltage signal, and a minimum voltage level of the sawtooth signal is equal to the ground level.

11. A light source dimming control circuit, comprising:

a voltage signal generator for generating a first voltage signal between a predetermined voltage level and a ground level;

a reference voltage signal generator for generating a reference voltage signal at an output end according to the first voltage signal and a second voltage signal lower than the predetermined voltage level, the reference voltage signal generator comprising:

a first diode with an anode end coupled to the voltage signal generator, and a cathode end coupled to the output end; and

a second diode with an anode end receiving the second voltage signal, and a cathode end coupled to the output end;

a first resistor with a first end coupled to an output end of a light source, and a second end coupled to a ground end;

a current controller comprising:

a transistor coupled between the output end of the light source and the first resistor; and

an error amplifier for generating a control voltage at a control end of the transistor according to the reference voltage signal and a voltage level at the first end of the first resistor;

a first switch for turning on the transistor when the first switch is turned off, and for turning off the transistor when the first switch is turned on; and

a switch controller coupled to the first switch and the voltage signal generator for turning off the first switch when a sawtooth signal is lower than the first voltage signal, and for turning on the first switch when the sawtooth signal is higher than the first voltage signal.

12. The light source dimming control circuit of claim 11 , wherein a first end of the first switch is coupled to the control end of the transistor, and a second end of the first switch is coupled to the ground end.

13. The light source dimming control circuit of claim 11 , wherein the switch controller comprises:

a sawtooth signal generator for generating the sawtooth signal;

a comparator coupled to the sawtooth signal generator and the voltage signal generator, for outputting a logic high signal when the sawtooth signal is lower than the first voltage signal, and for outputting a logic low signal when the sawtooth signal is higher than the first voltage signal; and

an inverter with an input end for receiving the signals outputted from the comparator, and an output end coupled to the first switch for outputting inverse signals opposite to received signals from the comparator.

14. The light source dimming control circuit of claim 11 , wherein the voltage signal generator comprises:

an inverter with an input end for receiving a dimming signal, and an output end for outputting an inverse signal opposite to the dimming signal;

a P-type transistor with a first end receiving a third voltage signal with the predetermined voltage level, and a control end coupled to the output end of the inverter;

an N-type transistor with a first end coupled to a second end of the P-type transistor, a second end coupled to the ground end, and a control end coupled to the output end of the inverter;

a second resistor with a first end coupled to the second end of the P-type transistor, and a second end coupled to the anode end of the first diode; and

a capacitor with a first end coupled to the anode end of the first diode, and a second end coupled to the ground end.

15. The light source dimming control circuit of claim 14 , wherein the dimming signal is a pulse width modulation signal.

16. The light source dimming control circuit of claim 11 , wherein the reference voltage signal generator further comprises:

a second resistor with a first end coupled to the cathode end of the second diode, and a second end coupled to the output end; and

a third resistor with a first end coupled to the output end, and a second end coupled to the ground end.

17. The light source dimming control circuit of claim 11 , wherein the reference voltage signal generator further comprises:

a second resistor with a first end coupled to the cathode end of the second diode, and a second end coupled to the output end;

a third resistor with a first end coupled to the output end;

a fourth resistor with a first end coupled to a second end of the third resistor, and a second end coupled to the ground end; and

a second switch with a first end coupled to the second end of the third resistor, and a second end coupled to the ground end, for controlling an electrical connection between the second end of the third resistor and the ground end according to a control signal.

18. The light source dimming control circuit of claim 17 , wherein the second switch is an N-type transistor.

19. The light source dimming control circuit of claim 11 , wherein a maximum voltage level of the sawtooth signal is equal to a voltage level of the second voltage signal, and a minimum voltage level of the sawtooth signal is equal to the ground level.