A High Accuracy Current-Balanced Control Technique for LED Backlight

Abstract

This paper proposes a new circuit to regulate current in parallel LED (Light Emitting Diodes) strings. The new technique not only enhances the current matching in each string but also reduces device numbers and external resistance requirements. In this new circuit, the LED current has a low correlation to its forward voltage (V-f). The LEDs should have constant driving current as LCD backlight for two reasons. Firstly, keeping the maximum current and maintain the reliability. Secondly, obtaining expected luminous intensity for each LED. Furthermore, this circuit matches the current of eight LED strings and keeps high accuracy by using only two operational amplifiers. In other words, it reduces the power consumption and die size. There are two benefits for having just one resistance outside. Firstly, it can reduce pin numbers of the driving circuit. Secondly, it is easy for user to determine the maximum current of LED strings instead of predefined by the driving IC. The current accuracy of LED strings with a total forward voltage variation (Delta V) around 1.1V is less than 0.1%.