V ariable Off-Time Control Loop for Current-Mode Floating Buck Converters in LED Driving Applications Vlad Anghel, Christopher Bartholomeusz, An ca Gabriela V asilica

Abstract

A versatile cont roller architecture, used in cur- rent-mode floating buck converters for LED driving, is developed. State-of-the-art controllers rely on a fixed switching period and variable duty cycle, foc using on current averaging circuits. In- stead, the proposed controller architecture is based on fixed peak current and adaptable OFF time as the average current control method. The control lo op is comprised of an averaging block, transconductance ampli fier, and an innovative time modulator. This modulator is intended to provide constant control loop response regardl ess of input voltage, cur rent storage inductor, and number of LEDs in order to improve converter applicability for LED drivers. Fabricated in a 5 V standard 0.5 mC M O S 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 be tween 11 and 20 V , and a wide inductor range of 100–1000 H. In all instances, the me asured average LED current variation was lower than 10% of the desired value. A maximum conversion ef ficiency of 91% is obtained when driving 50 mA through four LEDs (with 14 V input voltage and an inductor of 470 H). A stable CCM converter operation is also proven by simulation for nine LEDs and 45 V input voltage.