Application enhancement for driving large strings of LEDs

A technique for increasing the number of series connected LEDs driven by a current-mode floating buck converter is analyzed, simulated and implemented. The converter uses an external power switch, driven by a previously introduced low-cost variable frequency controller. The increase in input voltage and number of driven LEDs is achieved by the addition of a level shifter at the controller´s output which enhances its capability to drive a power MOSFET switch. Measurements performed on the improved converter biasing a maximum of 16 LEDs at up to 75V emphasize a peak efficiency of 94.6%. An efficiency over 87% can be obtained in all cases.