Patent ID: 8203284

Claim:
A driver for a string of series arranged light emitting diodes of which at least three emit light having different spectra, the driver comprises: a main power supply having an output coupled across the string for supplying a main current to the string, a first secondary power supply coupled to a first junction between a first light emitting diode and a second light emitting diode and a second secondary power supply coupled to a second junction between the second light emitting diode and a third light emitting diode to supply or withdraw a delta current from the first and second junctions, respectively, the delta currents being smaller than the main current (Iai), the main power supply and the first and second power supplies arranged such that the main current is the current through at least one of said light emitting diodes and the sum of the main current and the delta currents is the current through at least one of said light emitting diodes, and a controller for controlling the first and second secondary power supplies to generate the delta current to obtain a desired spectral composition of the mixed light emitted by the string.