Backlight Application Notes

LEDs are the most common type of backlight used on character and small graphic LCD modules. These backlight systems are current dependent, but they are often supplied by voltage sources, usually with a current limiting resistor. A constant current source is a much better match for a current dependent load. The forward current through the LED backlight array determines the brightness. Maintaining a constant, uniform brightness for the LCD display is often an important requirement for many applications. This paper presents a simple, low cost, efficient circuit to drive the LED backlight system, offering much greater accuracy, control, and repeatability. This circuit operates off the LCD logic supply voltage and delivers a constant current independent of both supply voltage variations and variations in the forward voltage drop of the LED backlight array.