LED driving system for driving multi-string LEDs and the method thereof

A LED driving system, comprising:

• an input port configured to receive an input signal;
• a power converter comprising a main switch operating at on and off states, wherein the power converter is coupled to the input port to receive the input signal, and wherein based on the on and off states of the main switch and the input signal, the power converter generates a driving voltage;
• n output lines coupled in parallel to form a first terminal and a second terminal, wherein n is a natural number, and wherein the first terminal is coupled to the power converter to receive the driving voltage, and each output line comprises:
• a LED string and an output switch coupled in series between the first terminal and the second terminal of each output line;
• and a capacitor coupled in parallel with the LED string;
• a current sense resistor coupled between the second terminal of the n output lines and a reference ground to generate a current sense signal indicative of a current flowing through the LED string;
• and
• a controller coupled to the current sense resistor to receive the current sense signal, and wherein based on the current sense signal, the controller generates control signals to control the main switch and the output switches operate at on and off states to regulate the current flowing through each LED string to be balanced;
• and wherein the controller further comprises:
• a transconductance amplifier having a first input terminal, a second input terminal, and an output terminal, wherein the first input terminal is coupled to the current sense resistor to receive the current sense signal, the second input terminal is coupled to the reference ground, wherein based on the current sense signal, the transconductance amplifier generates an amplified signal;
• an integrated capacitor coupled to the output terminal of the transconductance amplifier to receive the amplified signal, to generate an integrated signal;
• a reset switch coupled in parallel with the integrated capacitor to reset the integrated signal to be zero when the integrated signal reaches a current reference signal;
• a comparator having a first input terminal, a second input terminal and an output terminal, wherein the first input terminal is coupled to integrated capacitor to receive the integrated signal, the second terminal is configured to receive the current reference signal, wherein based on the integrated signal and the current reference signal, the comparator generates a comparison signal;
• and
• a logical circuit coupled to the comparator to receive the comparison signal, and wherein based on the comparison signal, the logical circuit generates control signals to the reset switch, the main switch, and the output switch.

A method for driving multi-string LEDs, comprising:

• deriving a current sense signal indicative of a current flowing through a LED string with a current sense resistor;
• generating a control signal in response to the current sense signal;
• converting an input signal to a switching signal by controlling a main switch to operate between on and off states;
• converting the switching signal to n driving voltages by controlling n output switches to operate between on and off states, wherein n is the LED string number, and wherein each output switch is coupled in series with each LED string;
• and
• driving each LED string by the driving voltage;
• wherein the main switch and the output switches are controlled by the control signal;
• and wherein the step of generating a control signal in response to the current sense signal comprises:
• amplifying the current sense signal to generate an amplified signal;
• integrating the amplified signal to generate an integrated signal;
• comparing the integrated signal with a current reference to generate a comparison signal;
• resetting the integrated signal if the integrated signal reaches a current reference;
• and
• generating the control signal in response to the comparison signal.