Patent ID: 8847501

Claim:
An apparatus for driving a plurality of LEDs, comprising: a plurality of LEDs divided into a plurality of LED segments connected in series, each of said plurality LED segments having a positive end and a negative end; an input voltage connected to the positive end of a leading LED segment of said plurality of LED segments; a plurality of voltage controlled current limiting devices, each of said plurality of voltage controlled current limiting devices being associated with one of said plurality of LED segments, and having a first terminal connected to the negative end of the associated LED segment, a second terminal being applied with a bias voltage and a third terminal being connected to a common node, said bias voltage being a respective constant voltage independent of variation of said input voltage; and a current source having a first end connected to said common node and a second end connected to ground; wherein each of said plurality of voltage controlled current limiting devices further has a threshold voltage V th , a saturation voltage V sat , a voltage V bc between said second terminal and said common node, and a voltage V ac between said first terminal and said common node, and each of said plurality of voltage controlled current limiting devices is cut off when the voltage V bc of the voltage controlled current limiting device is less than the threshold voltage V th of the voltage controlled current limiting device, the voltage controlled current limiting device behaves as a resistor when the voltage V bc is greater than the threshold voltage V th and the voltage V ac of the voltage controlled current limiting device is less than the saturation voltage V sat of the voltage controlled current limiting device, and the voltage controlled current limiting device conducts a constant current when the voltage V bc is greater than the threshold voltage V th and the voltage V ac is greater than the saturation voltage V sat .