Abstract: This paper presents a high dimming ratio light emitting diode (LED) drive controller chip with digital mode dimming (DMD). The chip is composed of a boost power converter and a dimming control block. A novel constant on time (COT) control strategy is proposed for boost converter to achieve high dimming ratio. In addition, a fast enough load transient response of the converter power stage ensures its high dimming ratio. The COT control circuit operates mainly based on two current-capacitor timers and a finite state machine (FSM). The LED drive controller chip is designed and fabricated in 1.5 mm bipolar CMOS-DMOS (BCD) process with a die area of 1.31×1.43 mm2. Experimental results show that the proposed LED drive system works well. And, as expected, the minimum LED dimming on time of 1.0 ms and the corresponding dimming ratio of 1000:1 at 1 kHz dimming frequency are successfully achieved.

Key words: BCD process,  boost converter,  COT control,  LED drive,  high dimming ratio