Toshiba LED Backlighting for LCD Televisions Schematic



Page 1 of 6

LED Backlighting for LCD Televisions

Purpose

The intent of this paper is to familiarize Electronic Servicers with what will be the central component in the next generation of flat screen televisions. With the ever growing popularity of LCD televisions, research and development has produced a method of back lighting an LCD panel that is superior to the Cold Cathode Fluorescent (CCFL) Backlighting, currently the most popular amongst television manufacturers. The new method employs a Light Emitting Diode Array, commonly referred to as “LED Array” or, an in-line strip of individual LED’s, referred to as “Edge Lit”.

This paper is a basic explanation of how LED backlighting works. There are many algorithms applied to the IC’s involved in the LED Dimming process. This paper does not intend to go into the engineering behind this very complicated process.

Background

LED backlighting is an evolution of basic LCD technology. For a comprehensive understanding of basic LCD technology, it is recommended you review Toshiba White Paper, 2.0, “LCD Television Technology”.

Assuming basic LCD technology is understood, we will discuss the need for evolving to LED backlighting.

There are inherent difficulties with the CCFL method of backlighting:

1. CCFL requires an inverter to supply 400-700 volts AC as an operating voltage for the lamps. This results in heavy power requirements.

2. True “Black” areas of the picture are not attainable because the CCFL lamps are always on. This allows for leakage of light through the liquid crystal materials even when the crystals are “turned off”. The contrast ratio (true white to true black) suffers greatly because of this.

3. Motion blur is difficult to minimize using the CCFL method of backlighting

Page 2 of 6

(constantly on backlighting) due to the relatively slow response time (transition time from off to on to off) of the TFT circuitry in the panel.

4. CCFL average life span is 20,000 hours.

Physical Comparison

Figure 1 is a graphic of a conventional LCD panel utilizing a standard CCFL backlight assembly. You will notice the inverter, lamps, and diffuser.



Figure 1

Figure 2 is a drawing of two LED lighted panels; one Edge Lit, one LED array.

Figure 2

All three panels shown require lighting and a diffuser. At this stage, the main difference is the inverter (required by CCFL method). Of the two LED backlight designs, the LED

+ 4 hidden pages