Patent ID: 7880404
Filing Date: 2011-02-01
Classification: H05B,Y02B

Abstract:
1. A system for driving a plurality of light emitting diodes (LEDs) in series comprising: a voltage regulator for receiving an input voltage and outputting a regulated output voltage at an output terminal sufficient to drive a series string of LEDs, the regulator having a first terminal for receiving a first pulse width modulation (PWM) signal having a first state to terminate the output voltage and having a second state to cause the output signal to be generated; a series string of LEDs having a first terminal at one end of the series string and a second terminal at an opposite end of the series string, the first terminal of the series string being electrically coupled to the output terminal of the regulator; a first transistor electrically coupled in series to the second terminal of the series string of LEDs for conducting a current through the series string of LEDs when the first transistor is on and when the voltage regulator is outputting the regulated output voltage; and a control circuit having an input terminal for receiving a second PWM signal for controlling an average brightness level of the series string of LEDs, the control circuit comprising logic circuitry providing the first PWM signal to the first terminal of the voltage regulator and providing a third PWM signal to turn the first transistor on and off at a certain duty cycle, the logic circuitry being configured to generate the first PWM signal and the third PWM signal so that the first transistor is always on prior to the regulated output voltage of the voltage regulator being applied to the string of LEDs and so that the regulated output voltage is not applied to the string of LEDs when the first transistor is off.