FILE NO.
REFERENCE NO. TI5110LCD
Training Manual
Principle of LCD Display
CONTENTS
Pages
1. Construction of LCD Display ----------------------------------------------------------------------- 2 - 5
1-1 Principle of LCD Display ------------------------------------------------------------------------------- 2
1-2 Construction of LCD Display --------------------------------------------------------------------- 2 - 3
1-3 Main Component of LCD Display --------------------------------------------------------------- 4 - 5
2. Principle of Liquid Crystal --------------------------------------------------------------------------- 6 - 8
2-1 Liquid Crystal --------------------------------------------------------------------------------------------- 6
2-2 Rubbing-process------------------------------------------------------------------------------------- 6 - 7
2-3 Operation of Liquid Crystal ---------------------------------------------------------------------------- 8
3. Principle of LCD --------------------------------------------------------------------------------------- 9 - 11
3-1 Operation of Polarized Board for LCD Panel (Shutter)----------------------------------------- 9
3-2 Operation of Alignment Film------------------------------------------------------------------------- 10
3-3 Operation of LCD Panel ------------------------------------------------------------------------ 10 - 11
3-4 Transparent Electrode -------------------------------------------------------------------------------- 11
4. Type of LCD Display Construction ------------------------------------------------------------ 12 - 13
4-1 Twisted Nematic (TN) Type -------------------------------------------------------------------- 12- 13
4-2 Super TN (STN) Type---------------------------------------------------------------------------- 12- 13
4-3 Triple STN (TSTN) Type / Film STN (FSTN) Type --------------------------------------- 12- 13
5. System of LCD Display ---------------------------------------------------------------------------- 14 - 20
5-1 Dot-Matrix System ------------------------------------------------------------------------------------- 14
5-2 Colorization ---------------------------------------------------------------------------------------------- 15
5-3 Drive System -------------------------------------------------------------------------------------------- 16
5-4 Passive Matrix System-------------------------------------------------------------------------- 16 - 17
5-5 Active Matrix System---------------------------------------------------------------------------- 18 - 19
5-6 Drive of Active Matrix System----------------------------------------------------------------- 19 - 20
6. Improvement Technology of LCD Display -------------------------------------------------- 21 - 27
6-1 Subject of LCD Display ------------------------------------------------------------------------------- 21
6-1-1 Angle of View----------------------------------------------------------------------------------------- 21
6-1-2 Response Characteristic--------------------------------------------------------------------------- 21
6-2 Angle of View-------------------------------------------------------------------------------------------- 22
6-3 Multi-Domain System --------------------------------------------------------------------------------- 23
6-4 MVA (Multi-domain Vertical Alignment) System ----------------------------------------------- 24
6-5 IPS (In-Plain Switching) System ------------------------------------------------------------------- 25
6-6 Optically Compensated Film ------------------------------------------------------------------------ 26
6-7 OCB (Optically Compensated Birefringence) System ---------------------------------------- 26
6-8 Improvement of Response Speed ----------------------------------------------------------------- 27
6-8-1 Inpulse System--------------------------------------------------------------------------------------- 27
6-8-2 FFD (Feed Forward Driving) System----------------------------------------------------------- 27
7. Appendix ----------------------------------------------------------------------------------------------- 28 - 31
7-1 Backlight-------------------------------------------------------------------------------------------------- 28
7-2 LVDS Circuit--------------------------------------------------------------------------------------------- 29
7-3 Block Diagram Example------------------------------------------------------------------------ 30 - 31
-2-
Training Manual Principle of LCD
■ Construction of LCD Display
1. Construction of LCD Display
1-1 Principle of LCD Display
The LCD (Liquid Crystal Device) Display is used to display the electric signal, converted from picture
data similar to a CRT display. The transistor (TFT) switched by the electric signal changes the transmis-
sion to light in small picture elements (pixels) of the LCD. The LCD display makes the picture by grouping
these elements of each RGB color.
1-2 Construction of LCD Display
LCD Display
Liquid Crystal is packed between the board modules (TFT and Common) and the LCD panel (or LCD
shutter) is constructed. A back light is attached to the LCD panel for LCD Display.
Board Module (Common Electrode)
The Common Electrode consists of a polarized board, a color filter, and a transparent electrode on a
glass plate. An alignment film is formed on the transparent electrode.
Board Module (TFT Electrode)
The TFT Electrode consists of a polarized board and a transparent electrode (pixel electrode and drive
transistor) on a glass plate. An alignment film is formed on the transparent electrode.
Backlight
A fluorescent light is used for the Backlight.
✐ TFT:
Thin Film Transistor
✐ LCD Panel and LCD Shutter: They are the same things, but in the explanation LCD panel is used
for structure and LCD shutter is used for function.
-3-
Training Manual Principle of LCD
Construction of LCD Display
Board
Module
(TFT side)
Board Module
(Common side)
The light of each picture element is transmitted
by switching the drive transistor (TFT) on and off.
LCD Layer
Backlight
Polarized
Board
Polarized
Board
Pixel
(Picture Element)
TFT
Transparent
Electrode
(Pixel, TFT)
Color Filter
Transparent
Electrode
(Common)
Glass
Plate
Glass
Plate
Note: Alignment film is not
shown in this figure.
Fig. 1 Construction of LCD Display
(Transparent Type TFT LCD)
-4-
Training Manual Principle of LCD
Construction of LCD Display
1-3 Main component of LCD Display
LCD Shutter
Supplying voltage to the transparent electrodes between the pixel and common sides changes the
arrangement of liquid crystal. By assembling two polarized boards, the transfer of light from the backlight
can be controlled by the transparent ratio of the LCD Shutter.
Liquid Crystal
Liquid Crystal is a material whose state is between a solid and a liquid. It has both characteristics of
solids and liquids, and generally it is a white turbid liquid. Its molecules are normally arranged compara-
tively opaque and change to transparent with the application of voltage or heat.
Transparent Electrode (Film)
An LCD shutter is operated by supplying voltage derived from the video signal. Transparent film is used
for its electrode.
Alignment Film
This is a film for arranging liquid crystal molecules and is made of Polymid resin.
Polarized Board
The light with a specified direction passes through a polarized board.
Drive Transistor
The thin film transistor (TFT) is used to drive the LCD shutter of each pixel.
Color Filter
It is a filter with three colors (R, G, B) arranged for each pixel.
Backlight
Liquid crystal does not emit light. A light source is needed for display. The light source placed on the
reverse side of the LCD panel is called “Backlight.”
-5-
Training Manual Principle of LCD
Construction of LCD Display
Backlight
Polarized
Board
Polarized
Board
Glass
Plate
Glass Plate
Alignment Film
Alignment Film
Liquid Crystal
Transparent Electrode
(Pixel, TFT)
Transparent Electrode
(Common)
Color Filter
Module
(Back)
Module
(Front)
LCD Layer
LCD
Shutter
TFT Display
Drive Circuit
(with IC)
LCD
Display
Backlight
LCD
Module
LCD Panel
(LCD Shutter)
Fig. 2 Construction of LCD Display
(Cross Section)
Fig. 3 Assembly of LCD Display
-6-
Training Manual Principle of LCD
2. Principle of Liquid Crystal
2-1 Liquid Crystal
What is Liquid Crystal?
Liquid Crystal is a material whose state is between a solid and liquid. It has characteristics of both solids
and liquids, and generally is a white turbid liquid. Its molecules are normally arranged comparatively
opaque and change to transparent with the application of voltage or heat.
Almost all the materials consist of an organic compound taking the form of a slender stick or a flat plate.
There are three types of liquid crystal as shown in Fig. 4, and they depend on the construction and
arrangement of molecules.
Generally Nematic liquid crystal is used for the display apparatus.
(a) Smectic
Molecules are in layers and arranged parallel to each other. The center of gravity is arranged at random
in the layer.
(b) Nematic
Molecules are not in layers. They are arranged parallel. The center of gravity is able to move freely to the
major axis.
(c) Cholesteric
Molecules are in layers and arranged parallel. The arranging direction of the major axis for the neighbor-
ing layers is shifted gradually.
In order to use liquid crystal for display, it is necessary to regularly arrange the molecules of Nematic
(Rubbing-process).
2-2 Rubbing-process
After chemicals for arranging are put on the glass plate, they are hardened, and then the surface on the
plate is rubbed with a cloth to fix the direction of the gaps that are made. The arranging direction of mole-
cules is settled in the gaps.
This process is used to change the characteristics so the molecules that touch the rubbed surface are
arranged to the major axis of the rubbed direction.
This thin film on the glass plate is called “Alignment film.”
■ Principle of Liquid Crystal
-7-
Training Manual Principle of LCD
Principle of Liquid Crystal
Alignment Film
Arranging
Rubbing
Direction
Natural Condition
Liquid Crystal Molecule
Fig. 4 Liquid Crystal
(a) Smectic
(c) Cholesteric
(b) Nematic
Fig. 5 Rubbing-Process
-8-
Training Manual Principle of LCD
Principle of Liquid Crystal
2-3 Operation of Liquid Crystal
The chemistry substance required for liquid crystal material is one that reacts so that the arrangement
direction is changed according to an applied electric field.
In the LCD display, a liquid crystal is placed between two electrodes. When the voltage is supplied
between them, an electric field is generated in the liquid crystal, and liquid crystal molecules are moved
and arranged. The Backlight applied to the liquid crystal is either passed or blocked according to the
arrangement of the molecules.
If an electric field from an external source is applied to liquid crystal, electric dipoles will be generated
that will react to the intensity and direction of the electric field. Through the operation of these electric
dipoles and the electric field, the power changing direction of liquid crystal molecules is generated.
Therefore, according to an external electric field, liquid crystal molecules move and change direction
from horizontal to vertical.
Electric
Dipole
Liquid Crystal
Molecule
Liquid Crystal
Electric
Field
Electric
Field
Transparent
Electrode
Fig. 6 Operation of Liquid Crystal
-9-
Training Manual Principle of LCD
3. Principle of LCD
3-1 Operation of Polarized Board for LCD Panel (Shutter)
Light is an electromagnetic wave that is oscillating at right angles to the direction of advance. In fact, the
oscillating directions of all light is mixed. A polarized board can let only the light in the specific direction
pass from the light with which these various oscillating directions were mixed. Therefore, only the light of
the same direction as the polarization direction of a polarized board can be taken out by letting the light
pass through this polarized board. That is, if the oscillating direction of light and the direction of a polar-
ized board are in agreement, the light will pass through a polarized board. Moreover, if the direction of a
polarized board differs from the oscillating direction of light, the light cannot pass through a polarized
board. When the oscillating direction of a polarized board and light are shifted 90º(right-angled), the light
is blocked completely. The light passes and looks bright if the two boards are in the same direction when
looking at two polarized boards in piles, however, if shifted at right-angles, the light is blocked and looks
dark.
■ Principle of LCD
Polarized
Board
White
Light
Light
Black
Fig. 7 Operation of
Polarized Board
Fig. 8 Operation of
Polarized Board
Oscillating direction of light
Oscillating direction of light
The oscillating direction
of light and the direction
of a polarized board are
in agreement.
The direction of a
polarized board differs
from the oscillating
direction of light
Passage Interception
The two boards are
the same directions.
The two boards are
shifted right-angled.
-10-
Training Manual Principle of LCD
3-2 Operation of Alignment Film
Liquid crystal is inserted into alignment films of an upper and lower plate that have the direction of
grooves shifted by 90º on the LCD display. The liquid crystal molecules of upper alignment plate are
arranged along with the upper alignment film. The liquid crystal molecules of lower alignment plate are
arranged along with the lower alignment film. The liquid crystal layer between these alignment films is
twisted little by little and is arranged so that a spiral is formed. Light entering through the first alignment
plate will have its oscillating direction twisted 90º by the liquid crystal layer between the alignment films.
Now the direction of oscillation is aligned with the second alignment plate and the light will pass through.
Principle of Liquid Crystal
Alignment
Film
Alignment
Film
Alignment
Plate
Direction
of Groove
Direction
of Groove
Liquid
Crystal
Molecule
Fig. 9
Operation of
Alignment Film
By the upper-and-lower
alignment films, spirally, a
liquid crystal molecules are
twisted 90º and arranged.
3-3 Operation of LCD Panel
In the LCD panel, a liquid crystal is inserted and enclosed between two glass plates. The polarized
board, transparent electrode, and the alignment film are formed on these glass plates. The light can be
passed or blocked by supplying voltage or not to this LCD panel.
In the condition (Switch-Off) that the voltage is not supplied, the liquid crystal molecules are twisted 90º
sideways and arranged spirally. The oscillating direction of the light that passed the upper polarized
board is changed by the twisted liquid crystal molecule arrangement. Therefore, the direction of a polar-
ized board and the oscillating direction of the light which is shifted 90º and arranged become the same,
and this light can now pass through a polarized board. This is the liquid crystal shutter-on condition and
an LCD panel (LCD shutter) passes the light.
-11-
Training Manual Principle of LCD
Principle of Liquid Crystal
Light
Light
Polarized
Board
Polarized
Board
Polarized
Board
Transparent
Electrode
(Lower)
Transparent
Electrode
(Upper)
Alignment
Film
Liquid
Crystal
Alignment
Film
Polarized
Board
Transparent
Electrode
(Lower)
Transparent
Electrode
(Upper)
Alignment
Film
Liquid
Crystal
Alignment
Film
Fig. 10 Operation of
LCD Panel
Passage
Interception
3-4 Transparent Electrode
In order to generate an electric field in liquid crystal, voltage is supplied to the upper-and-lower elec-
trodes. If metal is used for these electrodes, the light is interrupted by this metal and cannot pass into the
liquid crystal. Therefore, a transparent electrode that passes light is used for the electrode of the LCD
shutter.
On the contrary, in the condition (Switch-On) that voltage is supplied, the liquid crystal molecules are
arranged in a line at right angles to a glass plate. Since vertical liquid crystal molecules do not affect the
oscillating direction of light, the light that passed the upper polarized board passes as it is without chang-
ing the oscillating direction. Since the oscillating direction of this light differs from direction of the lower
polarized board which is shifted 90º and arranged, the light collides with this polarized board and cannot
pass. This is the liquid crystal shutter-off condition and the LCD panel (LCD shutter) blocks the light.
This is the basic structure (On—Off of the light by the LCD shutter) of an LCD panel. It is a sandwich
structure of the upper and lower sides of transparent electrodes, alignment films, and polarized boards,
with an enclosed liquid crystal material between them.
The LCD panel shown in Fig. 10 is a type of panel that changes the light into a passage condition when
voltage is not supplied between the upper-and-lower polarized boards that are arranged at 90º. This type
of panel has the advantage that black contrast is improved, and it usually works well. This mode is called
“Normally White Mode.”
An LCD panel that passes light when voltage is not supplied is referred to as “Normally Black Mode.” In
practice, with this type (when the upper-and-lower polarized boards are arranged in the same direction),
displaying perfect black becomes difficult due to the leakage of light caused by variations in the arrange-
ment of the liquid crystal molecules.
-12-
Training Manual Principle of LCD
■ Type of LCD Display Construction
4 Type of LCD Display Construction
4-1 Twisted Nematic (TN) Type
A Nematic type of LCD Display where the liquid crystal molecules are twisted 90º between upper and
lower boards is called a Twisted Nematic type (TN type) liquid crystal.
Most LCD displays are of this type and feature high contrast (ratio) under low voltage and power.
4-2 Super TN (STN) Type
Super TN type (STN type) LCD Displays are used for LCD televisions, personal computer monitors, cel-
lular phones, etc. A liquid crystal material developed to improve visual characteristics, such as contrast
ratio is used.
In this STN type liquid crystal molecules are twisted 180º to 270º and arranged between upper and lower
electrodes. By supplying voltage to this liquid crystal, the transparent ratio of light changes more steeply.
Therefore, with the STN type as compared to the TN type, contrast and rise characteristic of the voltage
(response of switch On and Off) are improved, and a clearer picture on larger screens becomes possible.
4-3 Triple STN (TSTN) Type / Film STN (FSTN) Type
A fault of the STN type is that the display colors during On and Off of the LCD shutter become yellowish
green and navy blue. (In TN type, they are white and black.) This is because light of a specific wave-
length is reflected and scattered by the thickness of the LCD panel. Therefore, even if a color filter of
RGB is attached to an STN type liquid crystal, bluish green is mixed with the colors from black, gray to
white, and a natural color picture cannot be displayed. The triple STN type (TSTN type) and the film STN
type (FSTN type) have been developed as an advanced type of STN.
In the TSTN type, optically compensated films (high polymer films) which sandwich the upper and lower
LCD panels are used. They compensate for the twist of the light crystal cell, and the display colors of yel-
lowish green and navy blue are changed to the correct white and black. The “FSTN” type uses a single
optically compensated film
-13-
Training Manual Principle of LCD
Type of LCD Display Construction
Module
LCD Layer
Module
Module
LCD Layer
Module
Module
LCD Layer
Optically
Compensated
Film
Module
Optically
Compensated
Film
Fig. 11 Type of LCD Display Construction
Twist of molecule
(90º)
Twist of molecule
(180º - 270º)
TSTN Type
STN Type
TN Type
-14-
Training Manual Principle of LCD
5 System of LCD Display
5-1 Dot-Matrix System
LCD displays have two drive systems, Segment and Dot-Matrix. The Dot-Matrix system is used for LCD
television displays.
The picture elements (pixels) of the display unit are arranged horizontally (X line) and vertically (Y row)
by this Dot-Matrix system, and various characteristics and figures can be displayed.
Fig. 12 shows a matrix of “X x Y = 10 (pixels)” with the character “Y” displayed. In this Dot-Matrix system,
by making the size of a pixel smaller and increasing the whole number of pixels, the big screen with fine
character or picture becomes possible.
With the present liquid crystal manufacture technology, the number of pixels per inch has reached
200ppi*, and very high definition screen display is possible. Moreover, the number of pixels of an LCD
display panel corresponding to bigger screen sizes can be specified and manufactured. For example, the
number of pixels of the SXGA* panel is about 1,300,000 (1,280 x 1,024 = 1,310,720 pixels).
✐ ppi: pixel per inch
✐ SXGA: Super eXtended Graphics Array
■ System of LCD Display
Y
X
RGB
Fig. 12 Dot-Matrix System
In colorization of LCD panel,
one pixel consists of 3 RGB
dots (sub-pixels).
A character or a figure is
displayed by making the
pixel of each X and Y inter-
section turn on (or off).
-15-
Training Manual Principle of LCD
5-2 Colorization
Since an LCD shutter only passes or blocks light, in itself it cannot display a color picture. The color pic-
ture is made by mixing the three colors of RGB (three primary colors of light) respectively, like the CRT
color television. The color LCD panel has a color filter of RGB attached to the monochrome panel. See
Fig. 13. In this color LCD panel, by controlling the voltages and the waveforms that are supplied at each
RGB pixel, the transparent ratio is controlled and hue and brightness are adjusted. Therefore, smaller
pixels and more numbers of pixels are required for the color LCD Display. For example, although the
SXGA panel described before has about 1,300,000 pixels, in colorization, there are about 4 million dots
(sub-pixels).
System of LCD Display
Fig. 13 Colorization of LCD Display
R
G
B
LCD Shutter
Color Filter
Backlight
White
Color
LCD Shutter
Backlight
White
Monochrome
Color Panel Monochrome Panel
-16-
Training Manual Principle of LCD
5-3 Drive System
The drive systems for LCD display are divided into the following classifications:
The Static Drive System, which is seldom used;
The Passive Matrix System, which is used for still pictures, such as calculators and notebook PCs;
The Active Matrix System, which is suitable for high definition and the high-speed response needed for
big screen LCD television.
System of LCD Display
Drive System Static Drive System
Dynamic Drive System Passive Matrix System
Active Matrix System
Classification of LCD Drive System
5-4 Passive Matrix System
In the structure of a passive matrix system, Y electrodes of the vertical direction (Y-direction) are formed
in upper glass plate, and X electrodes of the horizontal direction (X direction) are formed in lower glass
plate as a matrix. The liquid crystal molecules are sandwiched between these electrodes. By supplying
voltage between the Y electrode and the X electrode in sequence, at a certain time, an electric field is
generated in the liquid crystal where the selected Y electrode and X electrode cross. Therefore, the liquid
crystal molecules of this pixel address (X, Y electrode intersection) change arrangement and an LCD
shutter is turned On or Off.
Glass
Plate
Y Electrode
X Electrode
Y
0 Y1 Y2 Y3 Y4
Glass
Plate
Liquid
Crystal
Layer
X0
X1
X2
X3
X4
Fig. 14 Passive Matrix System
These electrodes are
transparent electrodes.
-17-
Training Manual Principle of LCD
System of LCD Display
Y1
Y0 Y1 Y2
Liquid
Crystal
X2
X0
X1
X2
X3
Fig. 15
Passive Matrix System
LCD shutter is turned on
or turned off in this
address (X2, Y1).
In the dynamic drive system, since the electric signal (voltage) is supplied to the Y electrode and the X
electrode in sequence, the number of pixels which makes all pixels (the total number of pixels are “X x
Y”) turn on or off becomes “X+Y”. Therefore, compared with the static drive system that has an indepen-
dent electrode for each pixel, the number of electrodes of the dynamic drive system is very few.
However, with this dynamic drive system, since the electrode itself is the wiring, it has resistance that
cannot be disregarded in the big screens. This resistance causes the speed of the shutter to become
slower. Therefore, when displaying moving pictures etc., an afterimage is generated.
This passive matrix system is not suitable for LCD televisions with big screens that require moving pic-
tures and high resolution.
The active matrix system was developed in order to overcome these faults.
-18-
Training Manual Principle of LCD
5-5 Active Matrix System
In the active matrix system, a switch element is attached for every pixel at the intersection of the X and Y-
electrodes of a passive matrix system. Each pixel is now controlled by the switch element (active ele-
ment). Since the switch for each pixel is turned On and Off independently, the response speed is
increased. Thin Film Transistor (TFT) is used for the switch element and is attached on the glass board.
The LCD display using this TFT is called “TFT LCD display”.
The upper electrode for the whole pattern is formed on the upper glass plate and is called the “Common
Electrode”. A pixel electrode (pixel pattern), TFT (switch element) which drives a pixel electrode, and X
electrode for gate input and Y electrode for source input of TFT are formed on the lower glass plate. In
this structure, the electric field is generated in the area between the pixel electrode and the common
electrode, and the LCD shutter for 1 pixel is operated.
When an electric signal (voltage) is supplied to the Y and X electrode of TFT, TFT is turned On, and the
liquid crystal molecules are operated as a light switch. Refer to Fig. 17 (Address X1 and Y0).
System of LCD Display
Glass Plate
(Upper)
Y Electrode
X Electrode
Pixel Electrode
(Pixel Pattern)
Glass Plate
(Lower)
Liquid
Crystal
Layer
COMMON
Electrode
TFT
(Switch Element)
Y Electrode
Equivalent Circuit
(TFT)
Y
0
Y0
Y1
X1
X1
Y
0
X1
X2
X Electrode
Pixel Electrode
Pixel
Electrode
Liquid
Crystal
Liquid
Crystal
COMMON
Electrode
COMMON
COMMON
Electrode
Drain
Gate
Source
TFT
TFT
Equivalent Circuit
(Switch)
COMMON
Drain
Gate
Source
Switch
(On / Off)
Fig. 16 Structure of Active
Matrix System
Fig. 17 Equivalent Circuit of
Active Matrix System
By TFT, the shutter of a pixel at the
address (X1, Y0) is turned On or Off.
-19-
Training Manual Principle of LCD
System of LCD Display
Y0 Y1 Y2 Y3
X0
X1
X2
X3
Liquid Crystal
COMMON Electrode
TFT (Switch)
Fig. 18 Structure of TFT Matrix
The LCD shutter is operated by
TFT at the address (X1, Y0).
The amplification operation of a transistor is used for the TFT switch in the active matrix system. In this
system, switching speed is unified over the whole display, increasing drive response speed as compared
with the passive matrix system. Therefore, TFT LCD display (active matrix system) is adopted for the
highly efficient display, which can provide the response speed required for big screens or quickly moving
pictures. However, further response speed is needed for high definition LCD television. This will be
described later.
5-6 Drive of Active Matrix System
The TFT LCD display consists of a matrix of n lines of X direction (X0 - Xn-1) and of n rows of Y direction
(Y0 - Yn-1). The line of X direction is called the “gate line” and the line (row) of Y direction is called the
“data line.”
First, the scan is started from the pixel address
(X0, Y0), and when the address (X0, Yn-1) is
selected the scan of X0 line is completed. Next, all
the pixels from X1 line to Xn-1 line are scanned in
sequence, and the final address is (Xn-1, Yn-1).
The operation of selected pixel address (X1, Y2)
is explained below.
First, (signal) voltage is supplied to X1 line (gate
of TFT), next voltage is supplied to Y2 row (source
of TFT), and the address of the intersection of X1
line and Y2 row is selected and its TFT is turned
On or Off. However, just switching the TFT on and
off will not change the brightness of the screen.
The brightness of a screen is changed by control-
ling the voltage of a data line (Y row). Fig. 19
shows the voltage characteristic of the matrix sys-
tem.
Time
Active Matrix System
Voltage to liquid crystal
Passive Matrix System
Fig. 19 Voltage Characteristic of Matrix System
Since the time for the drive voltage to reach its
required value is shorter in the active matrix system,
the response time of the display becomes quicker.
-20-
Training Manual Principle of LCD
In Fig. 20, the voltage of the data line (Y2) is supplied in the positive direction to a common electrode
(DC drive). In practice a uniform AC voltage is supplied to the common electrode (AC drive) to prolong
the life of the liquid crystal.
Y0
Data Line Drive Circuit (Y row)
Gate Line Drive Circuit (X line)
Y1 Y2 Y3
• • •
• • •
Yn-1
X0
X1
X2
X3
Xn-1
COMMON
COMMON
COMMON
Brightness
of Screen
Power
Circuit
X Direction
Variable
Voltage
TFT: On (X1, Y2)
TFT: On
TFT: Off
Video Data
Processor
Timming Controller
(Scan Converter)
Y Direction
X1
Y2
Y2
Y2
Y2
Y2
Liquid
Crystal
Pixel
Electrode
Glass Plate
(Common)
Glass Plate
(TFT)
TFT
Fig. 20 LCD Drive Circuit (Normally White Type)
In practice, driven by AC signal
to COMMON. (AC Drive)
System of LCD Display
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