Circuits and methods for driving light sources

1. A dimming controller for controlling power of a light emitting diode (LED) light source, comprising:

a monitoring terminal operable for receiving a current monitoring signal indicating a current flowing through said LED light source;

a dimming terminal operable for receiving a ramp signal, wherein a voltage of said ramp signal increases if a power switch coupled between a power source and said LED light source is turned on;

a control terminal operable for providing a control signal to control a control switch coupled in series with said LED light source based on said current monitoring signal and said ramp signal;

a pulse-width modulation signal generator operable for generating a pulse-width modulation signal based on said ramp signal, wherein a duty cycle of said pulse-width modulation signal is determined by said ramp signal;

a sawtooth signal generator operable for generating a sawtooth signal; and

a comparator operable for comparing said ramp signal with said sawtooth signal to generate said pulse-width modulation signal, wherein an average current flowing through said LED light source increases as said ramp signal increases until said average current reaches a predetermined level.

2. The dimming controller of claim 1 , further comprising:

a pulse generator for generating a pulse signal,

wherein if said pulse-width modulation signal is in a first state, said dimming controller is operable for turning on said control switch in response to said pulse signal and is operable for turning off said control switch if said current monitoring signal increases to a reference signal which determines a peak value of said current flowing through said LED light source,

wherein if said pulse-width modulation signal is in a second state, said dimming controller is operable for turning off said control switch.

3. The dimming controller of claim 2 , further comprising:

a frequency setting terminal coupled to said pulse generator and operable for determining a frequency of said pulse signal.

4. The dimming controller of claim 1 , further comprising:

a voltage output terminal operable for providing a DC voltage to charge an energy storage element to generate said ramp signal.

5. A driving circuit for controlling power of a light emitting diode (LED) light source, comprising:

a power converter coupled to a power source and said LED light source and for receiving power from said power source, and operable for providing a regulated power to said LED light source, said power converter comprising a control switch coupled in series with said LED light source; and

a dimming controller coupled to said power converter and operable for controlling said control switch based on a ramp signal to adjust a current flowing through said LED light source, wherein a voltage of said ramp signal increases if a power switch coupled between said power source and said driving circuit is turned on, and wherein an average current flowing through said LED light source increases as said ramp signal increases until said average current reaches a predetermined level,

wherein said dimming controller is operable for generating a pulse-width modulation signal based on a comparison of said ramp signal and a sawtooth signal, wherein said ramp signal determines a duty cycle of said pulse-width modulation signal, and wherein said pulse-width modulation signal controls said control switch.

6. The driving circuit of claim 5 , wherein said LED light source comprises a light emitting diode (LED) string.

7. The driving circuit of claim 5 , wherein said dimming controller comprises:

a pulse generator for generating a pulse signal,

wherein if said pulse-width modulation signal is in a first state, said dimming controller is operable for turning on said control switch in response to said pulse signal and operable for turning off said control switch if a current monitoring signal indicating said current flowing through said LED light source increases to a reference signal, wherein said reference signal determines a peak value of said current flowing through said LED light source,

wherein if said pulse-width modulation signal is in a second state, said dimming controller is operable for turning off said control switch.

8. A method for adjusting power of a light emitting diode (LED) light source, comprising:

powering said LED light source by a regulated power from a power converter;

increasing a voltage of a ramp signal when a power switch coupled between a power source and said power converter is turned on; and

increasing an average current flowing through said LED light source as said ramp signal increases until said average current reaches a predetermined level;

generating a current monitoring signal indicating a current flowing through said LED light source;

comparing said ramp signal with said current monitoring signal;

generating a control signal based on said comparing; and

controlling a control switch coupled in series with said LED light source based on said control signal.

9. The method of claim 8 , further comprising:

generating a pulse signal;

turning on said control switch in response to said pulse signal; and

turning off said control switch if said current monitoring signal increases to said ramp signal.