Variable Off-Time Control Loop for Current-Mode Floating Buck Converters in LED Driving Applications

A versatile controller architecture, used in current- mode floating buck converters for LED driving, is developed. State-of-the-art controllers rely on a fixed switching period and variable duty cycle, focusing on current averaging circuits. Instead, the proposed controller architecture is based on fixed peak current and adaptable OFF time as the average current control method. The control loop is comprised of an averaging block, transconductance amplifier, and an innovative time modulator. This modulator is intended to provide constant control loop response regardless of input voltage, current storage inductor, and number of LEDs in order to improve converter applicability for LED drivers. Fabricated in a 5 V standard 0.5 mu m CMOS technology, the prototype controller is implemented and tested in a current-mode floating buck converter. The converter exhibits sound continuous conduction mode (CCM) operation for input voltages between 11 and 20 V, and a wide inductor range of 100-1000 mu H. In all instances, the measured average LED current variation was lower than 10% of the desired value. A maximum conversion efficiency of 91% is obtained when driving 50 mA through four LEDs (with 14 V input voltage and an inductor of 470 mu H). A stable CCM converter operation is also proven by simulation for nine LEDs and 45 V input voltage.