High dimming ratio LED driver with fast transient boost converter
In this paper, a high dimming ratio LED drive controller without color shift issue was proposed, fabricated and tested. It is composed of a boost power stage and some dimming control circuits. To fit high ratio dimming, it is necessary for the power stage to have fast enough transient response during the LED load switching. Therefore, a constant on time (COT) controlled boost converter was proposed. The boost switch on time is fixed by a current capacitor timer and the inductor current is just sampled during the off time modulation with COT control. This control scheme simplifies the current sensing since in this case the equivalent series resistor (ESR) of the output capacitor samples the inductor current directly. The chip is implemented with 1.5 mum BiCMOS process, and the final die area is 1.31 x 1.43 mm2. Simulation and experiment results have shown that the proposed LED drive system features the minimum LED dimming on interval as small as 1 mus and the corresponding dimming ratio is 1000:1 at 1K Hz dimming frequency. In addition, the PFM operation comes free at dimming off interval.