Dynamic Output Resistance Optimization for Duty-cycle Control in Hybrid Capacitive LED Drivers

This paper proposes a technique to optimize dynamically the efficiency of hybrid switched capacitor converters, while changing their output voltage/current. Pulse width modulation can be used to change the output current in LED drivers based on hybrid converters. However, changes in duty-cycle cause a reduction in efficiency due to the increasing output resistance of the associated switched capacitor converter. The proposed solution re-configures dynamically the ratio among the flying capacitors and the size of the switches in the power train to minimize the output resistance in different duty-cycle ranges. Transistor level simulations, using 180nm CMOS technology, show the benefits of this technique in a 3:1 Dickson step-down hybrid capacitive LED driver using three duty-cycle optimization regions. A better efficiency and flatter output resistance over a variable conversion ratio was achieved with up to 8% efficiency improvement in this case study, without increasing the area occupied by power switches and flying capacitors.