A highly efficient switched-capacitor LED driver with switching frequency hopping technique

A novel frequency hopping technique is proposed to increase the efficiency in a switched-capacitor LED driver, based on analyzing the dependence of power loss on switching frequency. LEDs' load current is featured uniquely by jumping periodically from a constant value (20mA) to zero for dimming function. Developed on an improved VCO, the hopping technique makes the switching frequency vary discretely at pre-defined levels in accordance with load current, whose basic principle is that heavier the load, higher the frequency (200KHz to 1MHz). By elimination of current-sensing resistors and other optimization measures, the efficiency is further improved. The circuit is designed and simulated in TSMC 0.35μm CMOS process. Simulation results show the overall efficiency is increased by 5% on average in comparison with fixed high frequency ones.