LED BACK LIGHTING IN LCD TELEVISION
TOSHIBA LED BACK LIGHTING FOR LCD TELEVISIONS
The intent of this paper is to familiarize
Electronic Servicers with what will be the central component in the new
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.
WHY LED
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 (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
shows a graphic of a conventional LCD panel utilizing a standard CCFL
backlight assembly. You will notice the inverter, lamps, and diffuser.
Figure showed is a drawing of two LED lighted panels; one
Edge Lit, one LED array.
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 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-determined
brightness 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.
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
In Figure 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 level, we could achieve
normalcy on the LCD Panel in 1/5th the 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, let’s apply the PWM theory to an LCD Panel
using an LED Array for back lighting. Refer to following Figure.
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
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.
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 (TFT”s) that drive the LCD’s 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 LED’s are instantaneous therefore, eliminates any
chance of light being passed through the LCD Material of those pixels. This
produces much darker areas of black, improving the Contrast Ratio. For a
refresher, see Figure bellow (LCD Matrix with TFT’s and capacitors).
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 dynamic contrast 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 ½ 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.
