Patent ID: 7550934
Filing Date: 2009-06-23
Classification: H05B,Y02B

Abstract:
1. A light emitting diode (LED) driver for driving a plurality of LEDs connected in series, the driver comprising: a current regulator controller receiving a first feedback voltage corresponding to a current through the LEDs; a first transistor coupled in series with the LEDs, the first transistor being controlled by the current regulator controller to maintain a substantially constant current through the LEDs when the LEDs are conducting; a second transistor having a control terminal coupled to receive a brightness control signal, the second transistor being coupled between the current regulator controller and the first transistor for selectively decoupling an output of the current regulator controller from the first transistor to turn the LEDs on and off at a duty cycle corresponding a duty cycle of the brightness control signal; the plurality of LEDs connected in series having a first end coupled to an output voltage of a voltage regulator and having a second end providing a second feedback voltage to the voltage regulator, the voltage regulator maintaining the second feedback voltage at a substantially constant level by adjusting the output voltage, the voltage regulator comprising a voltage regulator controller having an output for providing pulse-width modulated (PWM) pulses, the voltage regulator further comprising a switching transistor coupled to receive the PWM pulses, a duty cycle of the switching transistor determining the output voltage coupled to the first end of the LEDs; and an open circuit detection circuit connected to receive the first feedback signal, the detection circuit detecting a level of the first feedback signal, wherein if the first feedback signal is above a first threshold, the detection circuit couples the PWM pulses to the switching transistor, and if the first feedback signal falls below a second threshold, the detection circuit decouples the PWM pulses from the switching transistor, such that the switching transistor is not switched by the PWM pulses in the event of an open circuit in the plurality of LEDs connected in series.