Patent ID: 8680785

Claim:
A light emitting diode (LED) based illumination device, comprising: a first LED string including a first plurality of LEDs coupled in series; a second LED string including a second plurality of LEDs coupled in series; a current source coupled to the first LED string and the second LED string; and a variable master current minor circuit comprising a switching circuit coupled to the first LED string and the second LED string and a current minor circuit coupled between the switching circuit and the current source, wherein the switching circuit switchably connects the first LED string to the current minor circuit as a master LED string when the first LED string has a largest forward voltage and connects the second LED string to the current minor circuit as the master LED string when the second LED string has the largest forward voltage and the current mirror circuit maintains equal currents through the first LED string and the second LED string with reference to a current through the master LED string; wherein the switching circuit comprises: a first transistor having a drain coupled to the first LED string, a source and a gate; a second transistor having a drain coupled to the second LED string, a source coupled to the source of the first transistor, and a gate; wherein the current minor circuit comprises: a third transistor having a drain coupled to the drain of the first transistor, a source, and a gate; and a fourth transistor having a drain coupled to the drain of the second transistor, a source coupled to the source of the third transistor, and a gate coupled to the gate of the third transistor; wherein the gate of the first transistor is coupled to the second LED string, the gate of the second transistor is coupled to the first LED string, and the source of the third transistor is coupled to the current source.