LED Backlighting Whitepaper

8/12/2019 LED Backlighting Whitepaper

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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 newmethod employs a Light Emitting Diode Array, commonly referred to as LED Array or,

an in-line strip of individual LEDs, referred to as Edge Lit.

This paper is a basic explanation of how LED backlighting works. There are many

algorithms applied to the ICs 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 WhitePaper, 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 lampsare 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.

2009 Toshiba America Consumer Products, LLC LED Backlighting Whitepaper 10.0Page 1 of 6


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

(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



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array offers more precise brightness/contrast control and will therefore, be the method

discussed in this paper.

An LED Array obviously requires a power source (Driver Board). An LED Array that is

not intended for use as a brightness/contrast controller can be powered by a simple DC

power source producing a constant current. This method will produce a pre-determinedbrightness level uniformly throughout the entire panel. The end result would be

comparable to using a CCFL or Edge Lit Backlight. Therefore, another type power

source must be used to achieve more precise control when using an LED Array. Figure

3 is a close-up of an LED array showing a driver and the driver PCB.

Figure 3

Pulse Width Modulation (PWM)

By changing the On time and Amplitude of a DC voltage, we can achieve control of

the current flow through an LED Array at specific times. See Figure 4.

Figure 4

In Figure 4, we can see that the average current flow through the array in a given time

period is 100ma. However, by using PWM, we are only driving the array for 1/5th the

time with 5 times the current. If 100ma was the desired current for a normal brightness



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level, we could achieve normalcy on the LCD Panel in 1/5 ththe time. The human eye

has a built-in retention for the highest brightness level applied. This retention gives the

perception of a brightness level closer to the 500ma drive than the 100ma drive.

Therefore, when a brightness level higher than what is considered normal (desired) is

required by the signal, PWM can provide that level (perceived by the human eye). In

order to prevent noticeable changes in applied light (flicker), the pulse repetition rate

must be higher than 100hz and less than 1khz. One of the side benefits of PWM is

decreased power requirements due to the decreased duty cycle (up to 30% less power

compared to CCFL).

Applying the Theory

Now, lets apply the PWM theory to an LCD Panel using an LED Array for back lighting.

Refer to Figure 5.

LCD DISPLAY PANEL

LED SECTION OF ARRAY

Figure 5

If the LED array is divided into sections (grids) and each section lights a small portion of

the entire LCD panel (also sectioned into grids), the brightness level in one particular

section can easily be controlled using PWM in conjunction with data from the T-Con and

Video Processing circuitry. For example, if the processed video is calling for a bright



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picture in the upper right portion of the LCD panel, the backlight section(s) in that area

will be told to provide the required brightness level. This synchronization between the

Video Processor, T-Con, and LED circuitry will occur across the entire LCD panel thus,

improving brightness (where called for) across the display.

What about Dark Scenes

The refresh rate of the panel is either 60, 120, or 240 times per second. Those are the

rates that scene changes may occur. When the processed video requires pixels to

become dark (black), during a refresh cycle, the Thin Film Transistors (TFTs) that drive

the LCDs for those pixels are told to shut-off. Because of the charging capacitor in the

TFT circuitry, there is a slight delay in shutting down the LCD. To compensate for that

delay, the LED array section in that particular grid will also be told to turn-off. The

response time of the LEDs are instantaneous therefore, eliminates any chance of lightbeing passed through the LCD Material of those pixels. This produces much darker

areas of black, improving the Contrast Ratio. For a refresher, see Figure 6 (LCD Matrix

with TFTs and capacitors).

Figure 6



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Summary

Having the ability to increase the brightness of the backlight during bright scenes and

shut-off (or decrease) the backlight during dark (black) scenes can produce dynamiccontrast ratios of up to 2,000,000 to 1. In comparing the life expectancy of LED to that

of CCFL, the LED backlighting lasts up to 2 times longer. The fact the response time

is almost instantaneous in an LED, motion blur is reduced when the backlight is shut-off

at the same time the TFT is commanded to shut-off. There can be no brightness lag if

there is no backlight applied to illuminate the liquid crystal material.

Resources

Pacific Display Devices

Video Imaging Design Line (NXP Semiconductors)

U. of Southern Cal Department of Engineering

Gadgetfolder.com

ConsumerReports.org

Sound & Vision


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