Introduction to LED Backlight Driving Techniques for Liquid Crystal Display Panels

Abstract

Liquid crystal display (LCD) is widely used in various display applications such as cellular phones, PC monitors, televisions (TVs), multimedia products, among others. An LCD backlight module usually includes backlight sources, a light-diffusion plate, a reflector, a brightness-enhancement film (BEF) and a light-guide plate (LGP). Conventionally, cold cathode fluorescent lamps (CCFLs) are required to provide sufficient backlighting for LCD panels [1-5]. Growing concerns about environmental issues will inhibit the use of CCFLs that contain poisonous mercury. Owing to improvement in long operative life, wider operation temperature range, and the simplicity of driver circuit work with low and safe voltages, light emitting diode (LED) has gradually substituted the CCFL as backlight [6-9]. This chapter will introduce some LED backlight driving techniques for LCD panels. Some dimming control methods will also be introduced and compared for regulating the LED current and brightness of the LED backlight system. The principal goal of this chapter is to ensure that readers become familiar with LED backlight driving techniques for LCD panels. We begin this chapter with a look at three LCD backlight structures: edge-light type, bottom-light type and hollow type. Figure 1(a) shows the bottom-light structure. Because of its high-luminance feature, the bottom-light structure is commonly used for PC monitors and TVs. With regard to the luminance uniformity on an LCD panel, a light-mixed zone is necessary between the diffusion plate and the light sources. This zone causes undesirable thickness for large-sized TV applications. Figure 1(b) shows the hollow type structure in which an LGP is used to reduce the thickness of the light-mixed zone. The good qualities of this type pertain to its compact shape, high luminance and good thermal dissipation. Figure 1(c) shows the edge-light structure commonly used in a small-scale LCD panel. This type is of compact shape and low power consumption, so it is suitable for notebook PCs and personal digital assistant (PDA) products. There are two types of LEDs for backlight sources, the white-light LEDs and RGB LEDs. The white-light LED is composed of a blue LED coated with yellow phosphor. Simple driving feature make it as a popular choice for new generation of LCD backlight sources in portable display products. Its color filter divides the emitted white light into RGB sub-pixels to present color pictures. Thicknesses of O pe n A cc es s D at ab as e w w w .in te ch w eb .o rg