Design Techniques in Constant on/off-Time Peak Current Controlled Boost LED Drivers for Fast Start-up and Dimming Transient Performance

Abstract

A boost converter-based LED driver is predominantly used in portable display devices, where the high voltage gain makes it difficult to achieve fast start-up and dimming performance, because of the detrimental effect of the right-half-plane (RHP) zero. In the majority of commercial products, a fixed-frequency current mode control (CMC) technique is used. This requires an over-compensated ramp for current-loop stability, which would tend to degrade the phase margin using a type-II compensator. Constant off-time (COFT) CMC may not require any ramp compensation and can achieve superior performance. However, suitable design methods are not readily available to identify performance limits using fixed-frequency and COFT CMC architectures. This paper presents the small-signal design and performance limitations of a type-II controller in CMC architectures. Thereafter, a trajectory-based design approach is developed to identify critical performance limits using a resettable PI controller, which can achieve near time-optimal performance with and without a peak current limit. Start-up performance is shown to be further improved by using a low voltage LED string, however, at the cost of an increasing pin count. Finally, a method is identified to achieve smooth controller transitions in a peak current-based constant on/off-time multi-mode controller. Experimental results of a few commercial LED drivers with their performance limits are presented, and for the same specs, the performance improvement using the proposed design framework is demonstrated using simulation results.