Patent ID: 8525436

Claim:
A light-emitting diode (LED) current balance circuit for driving a plurality of lightbars, each lightbar comprising a plurality of LEDs coupled in series, each lightbar having a first terminal coupled to a lightbar voltage and a second terminal, the LED current balance circuit comprising: a current mirror for balancing currents through the lightbars by generating a plurality of sink currents according to a reference current and causing each sink current to sink a current from the second terminal of a corresponding lightbar while being enabled, and causing the currents through the lightbars to be zero while being disabled; a reference current generator coupled to the current mirror and supplied power from a first supply voltage for providing the reference current and a second supply voltage, the reference current varying according to the first supply voltage while the first supply voltage is less than a constant-current threshold value to implement an analog dimming by enabling the current mirror and employing the first supply voltage with variable voltage as a dimming signal, the reference current being constant while the first supply voltage is greater than the constant-current threshold value to implement a digital dimming by employing a pulse-width modulation (PWM) signal with a variable pulse width as the dimming signal to alternatively enable and disable the reference current generator or the current mirror; and a voltage compensation circuit coupled to the second terminals of the lightbars and supplied power from the second supply voltage for adjusting the lightbar voltage down while a voltage at the second terminal of one of the lightbars is greater than a compensation threshold value and adjusting the lightbar voltage up while voltages at the second terminals of the lightbars are less than the compensation threshold value under the condition of the turn-on of all lightbars.