A High-Dimming-Ratio LED Driver for LCD Backlights

The grayscale resolution of liquid crystal displays (LCDs) can be improved by modulating the grayscale of the pixels in LCDs or by dimming the LCD backlight source. Changing the voltage or turned-on time of a pixel to achieve high-resolution grayscale requires either an accurate digital-to-analog converter or a very high frequency pulsewidth modulation (PWM) signal, which increases hardware cost and power consumption. The fast transient response of light-emitting diodes (LEDs) makes them useful as a backlight source for improving the grayscale resolution of an LCD without additional overhead. This paper presents an eight-channel LED backlight driver that can improve conventional dimming control circuits for LCDs. The proposed design was fabricated in a TSMC 0.25-μm 60-V bipolar-CMOS-DMOS process, and the chip area is 2.7 mm
²
. A dimming ratio of 50 000:1 is achieved with a 500-Hz PWM signal. The maximum power conversion efficiency, including power consumption of the LED current regulators, is 89.3%, when the load is a 12 × 8 LED array, the input voltage is 20 V, and the LED current is set to 25 mA.