Ideally, a customized buck-boost solution is needed for each string, where the string voltage is derived from the car battery with a step-down conversion when possible (buck mode) and a step-up conversion when necessary. With an input voltage as low as possible the system switcihng losses are reduced and the efficiency is improved.
Another concern is current and voltage accuracy. The typical peak or valley current-mode buck converter controls the inductor peak current. However, the diode string current is the average current in the inductor. This peak-to-average current error is eventually eliminated by the outer voltage control loop but returns during transient conditions. For example, in Figure 2, the matrix manager may instantly raise the number of powered-up diodes from eight to twelve. The resulting output voltage step produces a current and voltage fluctuation at the output of the buck converter that takes tens of microseconds to extinguish. A high-ratio PWM dimming circuit will sample this current for only a few initial microseconds where the amplitude is dipping, which results in incorrect dimming brightness and color. A control loop that measures average current, as opposed to peak current, would naturally eliminate this problem.
Synchronous High-Power Buck-Boost LED Driver Solution
An ideal solution should meet the requirements of smooth buck-boost operation and fast transient response. The LED controller shown in Figure 3 enables such a solution.
Typically, the diode string is attached directly to VOUT. The IC integrates a high-side p-channel dimming MOSFET driver (DIMOUT/) for PWM dimming applications that require a current source with PWM dimming capability as shown in Figure 3.