Patent ID: 7468723

Claim:
A circuit for driving a string of series-coupled light-emitting diodes, comprising: a plurality of switch circuits that are arranged to selectively short separate portions of a plurality of portions of the string of series-coupled light-emitting diodes over time such that a cardinality of the light-emitting diodes in the string of series-coupled light-emitting didoes that are on is fixed while the circuit for driving is enabled, wherein the plurality of portions includes a portion and another portion; the portion includes a half of the series-coupled light-emitting diodes, the other portion includes the other half of the series-coupled light-emitting didoes, and wherein the plurality of switch circuits includes: a first switch circuit that is arranged to short the portion of light-emitting diodes if a first control signal is asserted, wherein the first control signal has a duty cycle of approximately fifty percent; and a second switch circuit that is arranged to short the other portion of light-emitting diodes if a second control signal is asserted, wherein the second control signal is approximately an inverse of the first control signal; and a boost converter that is arranged to regulate an output voltage at a first output node based, in part, on negative feedback based on a feedback voltage at a feedback node, wherein the first switch circuit is coupled between the first output node and a second output node, the second switch circuit is coupled between the second output node and the feedback node, the boost converter is arranged to convert an input voltage into the output voltage, the boost converter is a pulse-width modulation based regulator having a maximum duty cycle, the string includes 2* N light emitting diodes, first portion include N light-emitting diodes, the second portion includes N light-emitting diodes, the circuit for driving drives N of the 2*N light-emitting diodes at a time, and wherein N is pre-determined in accordance with the equation: N=Floor ([(VIN/(1−Dmax))−VFB]/VLED), where Floor( ) represents truncation such that N is an integer, VIN represents the input voltage, Dmax represents the maximum duty cycle, VFB represents the feedback voltage when the output voltage is at the value to which it is being regulated, and where VLED represents the forward voltage of the light-emitting diodes in the string having the highest voltage of the light-emitting diodes in the string.