A 1.2A buck-boost LED driver with 13% efficiency improvement using error-averaged SenseFET-based current sensing

Abstract

High-current LED drivers suffer from a significant efficiency loss due to the presence of a current regulation element (CRE) in series with the LED. In a conventional driver, either a series current source or a sense resistor acts as a CRE to regulate the LED current (l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LED</sub> ). In this paper, we seek to improve the efficiency by eliminating the series CRE. To this end, we employ a highly accurate current sensing scheme to directly regulate l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LED</sub> and achieve more than 13% efficiency improvement. The LED driver is fabricated in a 0.5μm CMOS process and operates at switching frequencies up to 2MHz with an off-chip 2.2μH inductor and a 10μF capacitor. The converter can drive up to 1200mA at an LED forward voltage of about 3.6V. This paper shows the converter start-up, l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> , and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SENSE</sub> in all the 3 modes of operation. For better illustration, l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> is displayed with a 50mA offset on all the plots. Because a different sense-FET is selected in each switching cycle, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SENSE</sub> differs from cycle to cycle by the amount of mismatch between the sense-FET and the power-FET. However, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SENSE</sub> averaged over 32 switching cycles, obtained from the LED current estimator, yields an accurate measure of the LED current.