ET/IT & TI
Liquid Crystal Displays
Liquid Crystal Displays
(LCDs, Flüssigkristall-Anzeigen)
Grundlagen - Eigenschaften - Ausführungen
Karlheinz Blankenbach
HS Pforzheim, Tiefenbronner Str. 65, 75175 Pforzheim
Tel.: 07231 / 28 – 6658, Fax : - 6060
kb@displaylabor.de
www.displaylabor.de
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
1
ET/IT & TI
Liquid Crystal Displays
Overview
Further reading
(not relevant for exam)
1
Introduction
2
LCD - Basics
3
Direct Drive & Passive Matrix
4
Active Matrix
5
Backlights
6
LCD - Optimization
„The LCD Monster takes it all !?"
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
2
Liquid Crystal Displays
ET/IT & TI
LCD from Segmented to E-Signage
Size, price, complexity, …
C
Embedded system
approach
Monochrome
graphics
B
D
Color
graphics
B
A
Character
Segment 8
Direct drive
MUX
Passive Matrix
Active Matrix
Resolution
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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ET/IT & TI
Liquid Crystal Displays
Fundamental Flat Panel Display Principle
Electronics Point of View
From (digital) input to TFT drive
Column
Row
G
TTL-RGB
Pixel
Pixel converts voltage/current to light
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Liquid Crystal Displays
ET/IT & TI
AM LCD - Panel with Digital RGB - Input
Electronics Point of View
EO Transfer Fct.
Panel Electronics
Column Driver Bank
Gamma Corr.
Driver 1
Driver 2
Driver 3
Row
R
Digital
input
signals
G
B
Driver
1
Driver
Driver
2
Bank
Sync
Controls
Power Supply
LCD
Module
Timing
Controller
(TCON)
Vcom
Backlight driver
Sync
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Liquid Crystal Displays
ET/IT & TI
Display Driving : Matrix Drive
Electronics Point of View
- Scanning of rows
- Write data to columns
- Select one row
- Write data of all columns at one time
 Parallel data : Row-at-a-time addressing
Fixed resolution !
Column (data)
Data from
µC, PC, ..
Controller
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Row
(line,
scan)
Display
6
ET/IT & TI
Liquid Crystal Displays
Panel Electronics for Mid Resolution AM LCDs
Column (data) driver
Row (gate)
driver
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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ET/IT & TI
Liquid Crystal Displays
Example of Row and Column Drivers
Column (data) driver
Row (gate)
driver
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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ET/IT & TI
Liquid Crystal Displays
Fundamental Flat Panel Display Principle
Electro-optics (pixel) Point of View
Pixel converts voltage/current to light
Cross section of a pixel of a typical display
Substrate (glass, plastic)
Color filter (option)
Front plane
This is the very basic
difference of displays
eo - layer
Matrix drive, x-Si, electronics
Back plane
(not to scale)
Substrate, additional
backlight for LCDs
Display input data are converted ot matrix drive data
and converted to appropriated voltage/current for the pixel
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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ET/IT & TI
Liquid Crystal Displays
Fundamental Flat Panel Display Principle Keywords
• Display module: Device which covers all subassemblies from data input
interface over data adaptation for matrix drive (PM, AM), electro-optical
conversion, pixel drive (e.g. TFT) and electro-optical layer (e.g. LC, OLED)
• Electro-optical layer/conversion: Voltage (e.g. LCD) or current (e.g.
OLED) is converted to light (transmission for LCD). Display types differ in
electro-optical layer. Panel electronics is basically identical but must be
adapted to electro-optical characteristics
• Panel electronics: Digital display input data are converted of matrix drive
data and converted to appropriated voltage/current for the pixel. Matrix drive
consists of row and columns (electrodes and drivers)
• Front plane: Part of the display facing the observer. Consists typically of
color filter (LCD), electrode, reflection reduction, …; all mounted on a
substrate (mostly glass)
• Back plane: Part of the display opposite the observer. Consists typically of
TFTs (for AM drive) and pixel electrodes…; all mounted on a substrate
(mostly glass)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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ET/IT & TI
Liquid Crystal Displays
Power Consumption and Cost
Timing
Controller
Row
Driver
• Panel electronics ,
front and backplane
have similar impact on
module price: ~ 25%
Backlight
Column
Driver
Typical values for 10.4“
VGA Color AM LCD
• Backlight draws about
75% of total power
Polarizer etc.
Backlight
Misc.
TFT / back plane
Controller
Driver ICs
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Front plane
color filter
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Liquid Crystal Displays
ET/IT & TI
LCD Display World
Further reading
Resolution
QXGA
HDTV
SXGA
XGA
SDTV
VGA
(not relevant for exam)
See § Active Matrix
150 ppi
P
r
o
j
e
c
t
i
o
n
p-Si
Smartphone,
tablet
LCD
TV
PCmonitor
Projection
system
Notebook
Car
Industrial
QVGA
1
a-Si
10
PDP (for comparison)
Low information content displays
20
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
40
60
Display Size /“
12
ET/IT & TI
Liquid Crystal Displays
Beyond Industrial : Avionics & Automotive
9.5“ WVGA
Harsh environment :
Aircraft (PHILIPS), automotive (SHARP)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
13
ET/IT & TI
Liquid Crystal Displays
Beyond Commodity : High Resolution Displays  5 MPixel
• Only LCDs
• Applications : Medical, CAD, Simulation, …
PLANAR 21” 5 MPixel
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
14
ET/IT & TI
Liquid Crystal Displays
Overview
1
Introduction
Liquid Crystal fundamentals
rinciple of operation
2
LCD - Basics
3
Direct Drive & Passive Matrix
4
Active Matrix
5
Backlights
6
LCD - Optimization
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
15
Liquid Crystal Displays
ET/IT & TI
Liquid Crystal Displays
TN
Direct
STN
Multiplex,
Passive
Matrix
Twisted Nematic
Guest Host
Standard TN
Dynamic Scattering
ECB
Supertwisted
Polymer dispersed
OMI
Nematic
3 Terminal
Silicon
TN, VA,
IPS, …
Non-Silicon
Amorphous Si
Active
Matrix
poly Si
Bulk (LCOS)
2 Terminal
Diode
Smectic A
Thermal, electric
Smectic C
Ferroelectric,
Guest Host
Threshold enhanced
(MIM, Varistor, ...)
Bi-stable
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
LC-class
Driving
Mainstream
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Liquid Crystal Displays
LCD History
• LCD TV
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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ET/IT & TI
Liquid Crystal Displays
LCD Cross Section
Principle : Voltage driven ‚switching„ of light
Colour TFT
(Back-) Light
Polarizer
Glass 1 mm
ITO
U
LC
50 nm
TFT plane
back plane
10 µm
Alignment layer 50 nm
Spacer
Analyzer
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
CF plane
front plane
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ET/IT & TI
Liquid Crystal Displays
Basic Technologies
• Reflective
(low resolution
and monochrome)
• Transflective
(good performance
but too expensive)
• Transmissive
(high resolution
and color)
L
C
D
L
C
+ Power consumption
- Night vision
Mainstream low res.
+ Power consumption
+ Night & day vision
D
L
C
D
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
+ Vivid Colors
- Power consumption
- Daylight vision
Mainstream high res.
19
ET/IT & TI
Liquid Crystal Displays
Characteristics of Liquid Crystals
• Chemistry
ZLI3125
ZLI2585
63
70
 (1kHz, 20°C)
+ 2.4
- 4.4
n = no
1.467
1.469
n|| = ne
1.519
1.506
n (589nm, 20°C)
0.052
0.037
Examples
TC /°C
• Mechanics
• Physics
• Effects in electric field
LC molecule align to electric field:
- Mechanical orientation within ms
- Induced charge adapt within ns
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
20
Liquid Crystal Displays
ET/IT & TI
Temperature Characteristics of Liquid Crystals
Degree of order
Useful range for LCDs: 0 … 40°C (except automotive etc.)
High
Solid
(crystal)
Liquid crystal
phase
Liquid
(isotropic)
Low
Melting
Clearing
T
LC molecules have the orientation properties of crystals (fixed atoms, high
order) and the mobility of a liquid (low order) for a temperature range
between melting and clearing (LC phase). One can regard LCs as a material
with a wide T range for the phase transition from solid to liquid. This unique
material was discovered in 1888. Within the liquid crystal phase, the LC
molecules are “self-aligned” but can orientate to an external electric field.
LC properties incl. optical ones depend on temperature.
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
21
ET/IT & TI
Liquid Crystal Displays
Properties of Liquid Crystals
• Anisotropic properties
- Electric permittivity ( =  - )
- Refractive index (n = n - n)
- Elastic constants (kii)
• Alignment
- On surfaces  Alignment layer (see next slide)
- By electric fields  orientation to E-field from pixel voltage U
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
22
Liquid Crystal Displays
ET/IT & TI
Interaction LC ↔ Alignment Layer
• Side view
(example TN 90°)
90° twist
10 µm
Alignment layer
Alignment layer
• Top view
(example TN 90°)
- STN multiplexing
• Tilt angle
~ 2°
Tilt angle
Relevant for
- Domain free orientation
- Switching time (TRise)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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ET/IT & TI
Liquid Crystal Displays
LC Preinciple: Alignment on Surfaces vs. Electric Field
Fundamental to most LCDs ; Principle by Fredericksz (1929)
LC aligned to surface
(alignment layer)
LC aligned to E-field
E
0
3V
10 V Pixel voltage V
To control the transmission of pixel: polarizer needed
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
29
ET/IT & TI
Liquid Crystal Displays
Polarization Filter
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Liquid Crystal Displays
ET/IT & TI
TN (90°) : Light guide principle
Other LC principles like IPS
and xVA see § Optimizations
Light
Positive Mode
Uon
Alignment
direction
Orientation of
polarizer
E
ation
Orient zer
ri
of pola
Polarizer
Glass
ITO
Alignment
layer
Lower polarizer || orientation
• Direct Drive & Active Matrix
• Contrast: Difference of luminance
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Liquid Crystal Displays
ET/IT & TI
TN 90° : Light guide principle
TN 90°: Twisted nematic LC with 90° helix (no voltage)
Polarizer
Glass
ITO
Alignment
layer
Alignment
direction
Orientation of
polarizer
With “high” driving voltage:
LC orientates to E-field set by voltage U
on
No helix, polarized light orientation
is unchanged, lower polarizer cannot
be “passed”  pixel is “black”
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
E
tion
Orienta er
riz
of pola
Polarizer: Let only polarized part of light pass in its direction
Alignment layer: Set Orientation direction of LC
ITO: Indium Tin Oxide, a transparent conducting material (use in all displays)
LC: Forms helix without voltage, polarized light “follows” LC orientation
Light orientation is the same as polarizer orientation
at the “bottom” of the pixel  light passes polarizer
Positive
Mode
 pixel is “white”
Light
33
ET/IT & TI
Liquid Crystal Displays
TN (90°) : Light guide principle
Negative Mode
Light
Polarizer
Glass
ITO
Alignment layer
10 µm
Uon
Alignment direction
E
Orientation of polarizer
Lower polarizer  orientation
• Direct Drive & Active Matrix
• Contrast: Difference of luminance
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
34
ET/IT & TI
Liquid Crystal Displays
STN (180°- 270°) : Birefringence Principle
Index ellipsoid
180° twist
• Passive Matrix
• Contrast: difference of luminance + color
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
35
Liquid Crystal Displays
ET/IT & TI
Electro - optic Curve of LC
Transmission
Pixel
eo curve
90 %
Slope and Shape:
- Viewing Angle
- Twist
- Pre-tilt
-T
- LC Type
- ...
Positive
mode
TN
STN
10 %
0
U off
U on Driving Voltage
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
38
ET/IT & TI
Liquid Crystal Displays
Summary & Questions
• Why is a twist of 90° used for direct and AM drive and > 180° for PM ?
• Why need LCDs a DC-free driving signal ?
• Explain the principle of TN 90° LCDs
• Discuss resolution and OFF state color for (S)TN LCDs (no AM)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
40
ET/IT & TI
Liquid Crystal Displays
Overview
1
Introduction
2
LCD - Basics
Fundamental rule of LC driving:
No DC offset is allowed for LCD driving
thus applying DC-free pulse waveforms
3
Direct Drive & Passive Matrix
4
Active Matrix
5
Backlights
6
LCD - Optimization
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Driving signals
8 - Segment
Multiplex
Passive Matrix
41
ET/IT & TI
Liquid Crystal Displays
Direct Drive
Principle: Each pixel (Dot) is driven by one dedicated signal
Plate, electrode
f = 30 - 70 Hz
US
Front
(pixel, dot, segment) 0
Back, common
UPixel = UFront - UBack
DC-free !
US
0
US
0
-US
OFF
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
ON
OFF
42
ET/IT & TI
Liquid Crystal Displays
Transmission and Driving Voltage
Transmission
For Uoff (10%)
90%
and Uon (90%)
→ CR = 9 : 1
(10-90 definition
UDirect drive
in electronics)
but larger for
10%
0 … Udrive
Uoff
U
on
Driving voltage
For direct drive the driving pixel voltage is not limited !
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
43
Liquid Crystal Displays
ET/IT & TI
Direct Drive for Segment 8 (I)
VDD
A
Control input to
Exclusive OR gate
VSS
VDD
A
B
XOR
B
C
Segment
D
Common
Osc input (clock) to
Exclusive OR gate
VSS
VDD
C
180° phase shifted
output of Exclusive
OR gate
VSS
VB-C
B-C=D
D
DC-free !
Resultant display
waveform measured
segment to common
plate
0
Pixel
OFF
Pixel
ON
Pixel
OFF
 Simple driving with XOR because of automatic inversion - just set pixel !
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
44
Liquid Crystal Displays
ET/IT & TI
Direct Drive for Segment 8 (II)
Driving principles and
display controller
see § Embedded Systems
EXOR
a
b
c
a
f
d
e
f
b
g
e
c
d
g
Common
crossover
Clock
µC
Segment plate
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Common plate
45
ET/IT & TI
Liquid Crystal Displays
Basics of Low Res LCD - Production (I)
Further reading
(not relevant for exam)
ITO deposition
Photoresist printing
Mask align
Exposition
For
Developing, Etching
front-
Alignment layer
printing
and
backplane
Rubbing (LC orientation)
Seal-printing
Spacer
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
AMLCD:
+ TFT and CF
49
ET/IT & TI
Liquid Crystal Displays
Basics of Low Res LCD - Production (II)
Further reading
(not relevant for exam)
Cell alignment
Baking
Scribe & break
LC filling by vacuum
UV-seal
Final assembly:
Polariser
Driver
Bezel
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
50
ET/IT & TI
LC Driving
Liquid Crystal Displays
Direct drive is limited to 40 segments (pixel, MUX 400)
Solution: Matrix drive by rows and columns
Static
Method
LC - Types
Matrix
Direct: Every pixel has a
dedicated driver output
Passive
(PM)
TN
STN
Active
(AM, TFT)
TN
Principle
X
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
X
52
ET/IT & TI
Liquid Crystal Displays
Challenges of LCD Matrix Driving
• Matrix drive:
- Rows are scanned subsequentially
- Columns provide grey level for each
pixel in the activated row (line)
- Each pixel is activated only by a
short time (16.7 ms / number of rows
for 60 Hz frame frequency)
- What happen for the rest of the time
until this pixel is activated again?
Column (data)
ITO
ITO
Row (scan)
• Passive Matrix
1 Pixel
The pixel is exposed to the voltage of
the other pixel in the column. Thus driving
waveforms are complex and ghosting takes place.
(not to scale)
• Active Matrix
The pixel is isolated electrically by a TFT (MOS FET) from the rest of the
column (and line). This results in best quality (contrast, viewing angle, …)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
53
Liquid Crystal Displays
ET/IT & TI
Matrix Driving Parameter
Scan signal waveform
Activation time for a pixel
1
Ton 
N  fframe
Frame time
Tframe ~ 1 / 60s
Row
Pulse width (Ton ~ 34.7 µs)
1
where
N : number of rows (e.g. 480)
2
fframe : frame frequency (e.g. 60 Hz)
3
Example for 60 Hz :
- VGA
Ton  35 µs
- SXGA Ton  16 µs
m
VGA - Panel, 60 Hz frame rate, 480 lines ( rows )
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
54
Liquid Crystal Displays
ET/IT & TI
Passive Matrix (PM) Addressing
Passive Matrix LCDs are easy
to manufacture as only ITO line
electrodes have to be manufactured
(lines + 1 TFT per pixel for AM)
Column (data)
-U
0
Driving
Voltage
ITO
+U
ITO
Row (scan)
Glass
|2U|
0
|U|
0
Scan and data on
1 Pixel
different planes
Effective pixel voltage
(Scheme for reference only)
|U|
|U| is not intended resulting in an
unwanted grey level Ghosting for PM !
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
56
Liquid Crystal Displays
ET/IT & TI
Driving Waveforms for 2 x 2 Passive Matrix
UD
1 2 1 2
Waveforms for Pixel 'a' and 'b'
0
a
Data
UD
SXGA: 1 ... 1024
1
b
2 1 2
1
2 1 2
Scan
-
0
Data
t
US
a
0
US
T
a
2U
=
on
U
off
U
b
0
One line
addressing
Scan
Inverted
(no DC)
(simplified example)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Uoff ≠ 0
PM is reasonable for  QVGA
and possible up to SVGA.
57
ET/IT & TI
Liquid Crystal Displays
Contrast Adjustment for High Multiplex Ratios
See poti @ copy machine
Transmission
Shifted
Trans. by Ucontrast
90 %
(mux)
select
U
Trans.
10 %
(static)
select
U
twist 270°
Driving Voltage
Unon-select 1.134 Unon-select
Alt & Pleshko
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Issue : Ghosting
Electrical black
is not optical black
(same for white)
58
ET/IT & TI
Liquid Crystal Displays
Further reading
PM: Calculation of Driving Voltages
Driving Voltage vs. Multiplex Rate
Alt & Pleshko - formula
Uselect
R

Unonselect
N 1
N 1
(not relevant for exam)
R = Uselect / Unon-select
2.2
N : # of lines = Multiplex ratio
1.8
Example for Low Resolution Graphics
1.4
N = 64  R = 1.134  U  13 %
e.g. Usel = 10 V  U  1.3 V
 20 mV per gray level for 6 Bit
1.0
1
10
Number of rows = multiplex rate
(Uselect temperature dependent !)
Reduced contrast ratio compared to direct drive : CR 
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
100
1 direct
CR
 1
N
60
ET/IT & TI
Liquid Crystal Displays
Contrast Voltage - Contrast Ratio (I)
Not readable
Contrast Ratio
OK
T
Ghosting
Optimum contrast voltage
depends on temperature and
must be controlled during
operation at different temp.
Contrast (Operating) Voltage
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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ET/IT & TI
Liquid Crystal Displays
Summary & Questions
• What is the basic operation principle of an TN LCD?
What are the functions of each layer, part, ...?
• DC-free driving required (results in Image Sticking, similar to Burn-In)
• How can a simple direct drive for 8-Seg. LCDs be achieved?
• Matrix drive increases the number of display pixels as they are
driven by sharing rows and columns
• What are limitations for PM drive ?
• The only benefit for PM drive is cost but AM LCDs become more and
more cheaper as of smartphone mass production and yield improvements!
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
65
ET/IT & TI
Liquid Crystal Displays
Overview
• The challenge for AM is yield
(Ausbeute) in mass production
1
Introduction
2
LCD - Basics
3
Direct Drive & Passive Matrix
4
Active Matrix
5
Backlights
6
LCD - Optimization
• AM is pushed by high resolution
multimedia trend with great
image quality (contrast, color, ...)
AM Module
a-Si TFTs vs. LTPS p-Si TFTs
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
66
ET/IT & TI
Liquid Crystal Displays
AM LCD Side View
Polarizer
Glas
Black Matrix
Color Filter
Substrate
Color Filter
Contact
Seal
TFT
LC
Spacer
Align.
ITO
TFT Array
Substrate
Diffusors
Light guide
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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ET/IT & TI
Liquid Crystal Displays
Active Matrix Subpixel
Data
Typical pixel shape
Scan
MOSFET
LC
Storage
capacitor
Frontplane
- Scan and data signal on one plate
- 1 pixel = 3 RGB subpixel
- 1 TFT per pixel (AM OLED  2)
- Aperture ratio  60%
- Capacitor stores pixel voltage
- SXGA : 1280 x 1024 x 3 TFTs
(data) during frame time
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
 4 Mio. TFTs
68
Liquid Crystal Displays
ET/IT & TI
Basic Driving Waveforms for 2 x 2 Active Matrix
UD
Data / column
1 2 1 2
a
0
UD
b
1 2 1 2
0
UFP
Waveforms for Pixel 'a' and 'b'
Scan
Frontplane (Vcom)
Data
FP
0
UG
a
0
UG
1
2 1 2
b
0
=
U
ON
Scan / row (gate)
Rel. L
TFrame
1 2 1 2
t
(simplified example)
t
OFF
t
AM pixel voltage not limited
 high contrast ratio.
No ghosting as TFT “isolates” pixel
U
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
69
ET/IT & TI
Liquid Crystal Displays
Basic Driving Waveforms for 2 x 2 Active Matrix
Row by Row Addressing
• Each row of pixels is addressed in sequence
– Achieved by applying a positive pulse to the row electrode
– Closes the switches (turns the TFTs on)
• Pixel data is then applied to the column electrodes
– Charges up the pixel capacitance, CLC
• When the pixel is charged the TFT switch is opened
– Charge remains on pixel
AM pixel voltage is not limited compared to PM drive
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Active Matrix Cell : Elements
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Active Matrix Cell : Layers & Equivalent Circuit
Front plane
TN 90°
‚Same price„
Back plane
Data
Scan
TFTs are made of
- amorphous Si (a-Si) for monitors
- polycristalline Si (p-Si) for mobile
p-Si is manufactured as
Low Temperature Poly Silicon (LTPS)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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a-Si vs. p-Si
All high ppi LCDs like APPLEs RETINA LCD are p-Si
XGA
0.7”
→ p-Si for projection LCDs (small size & high resolution)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Further reading
TFT Technologies
e - Mobility
/ cm²/Vs
Process
Temperature /°C
Substrate
p-Si, CG (SHARP)
(not relevant for exam)
a-Si
500
50
0.5
> 900
600 - 900
250 - 350
LTPS  300°C
Single crystal
Quartz / glass
Glass
Substrate size
4‟‟… 12‟‟
… 25‟‟ x 32‟‟
… 60‟‟ x 72‟‟
Display application
µDisplays
Small size
Large area
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Summary & Questions
• Explain how AM driving works.
• The challenge of AM LCDs is TFT manufacturing.
• The benefit of AM driving is image quality and higher resolution as for PM.
• What is the function of major AM LCD subassemblies?
• Which limit of PM driving is not applicable for AM ?
• What are the benefits of LTPS p-Si?
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Overview
Introduction
• Backlight is needed for color
LCDs as it‟s light is modulated
by LC grey levels and passes
RGB color filters
2
LCD - Basics
• Backlight draws about 75% of
the AM LCDs power consumption.
3
Direct Drive & Passive Matrix
4
Active Matrix
1
5
Backlights
6
LCD - Optimization
Basics
LED
(CCFL vanishing)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Backlight Requirements
- High luminance
- Uniformity of luminance
- High dimming ratio specifically for automotive applications
- No flicker
- Low power consumption
- Low profile
- Low weight
- Low heating
- Choice of color
- Extended temperature range
All these requirements
cannot be fulfilled by
a single technology !
- High life time
- Low cost
-…
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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AMLCD Module
Focus: Backlight unit
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Luminance Loss for AMLCD
5%
 = 5%
Light efficiency:
0.6 x 0.4 x 0.7 x 0.3  5%
White light
is RGB filtered,
only 1/3 of
white intensity
passes.
100%
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Only light
with same
polarization as
polarizer pass
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AM LCD Optimizations : Selected topics „see below“
: Backlight
 = 5%
No CF for
sequential
color
backlight
Intelligent
dimming
Increase
LED
Edgelight
L: Transmission
Light CF
or RGBW
(gamut )
Rise
aperture
Directional
films
100%
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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AMLCD Backlight Tasks : Homogeneous Light Output
Diffusing
Refraction
L
E
D
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Reflection
88
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Liquid Crystal Displays
Increasing Luminance by Brightness Enhancement Films
Enhances perpendicular luminance
at the cost of viewing angle degradations
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Backlight & Colour Filter Fundamentals
RGB LED backlight only for high end, all other white LEDs
Backlight spectrum x colour filter spectra =
Light output spectra
Match backlight spectra and colour filter
transmission spectra for maximum light output
with respect to colour management
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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White LED - Backlight & Colour Filter
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Basic Backlight Configurations
• Direct type for point and line light sources
- Typical applications: Monitor, TV sets
- Light guide (design) relative simple
- Many light sources necessary
- Local (and global) dimming easy
: LED (or CCFL)
(not to scale)
• Side light type for point and line light sources
- Typical applications: Mobile LCDs, slim line monitors and TV sets
- Light guide (design) complex
- Few light sources necessary
- Global dimming easy, local complex
• Direct type for area light sources
- For all applications
- Only OLEDs (and EL)
- Global dimming easy, local complex
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LED - Backlight
Monitor, TV
•
Direct Type
•
Side light type
Point source
Mobile LCDs, high end TVs
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LED Backlight LCD Examples
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LED - Backlight Characteristics
rel. L
rel. L
100
100
@ 25°C
90
90
80
80
70
70
60
Reduction by
temperature, …
50
0
20,000 40,000 60,000 80,000 100,000
Operating time /h
Spec 25°C
60
50
-60 -40 -20 0
20 40 60 80 100 120 140
LED Junction Temperature /°C
LED backlights degrade mostly by high junction temperature !
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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RGB LED - Backlight
Prototype of 82” LED backlight, draws 1,000 W for 1120 LEDs
• Colour management for each LED by current & PWM
• Wider colour gamut than white LED
• Too costly and high power consumption
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LED – Backlight Power Saving Methods
• Adaptive light output by ambient light sensor (also applicable for CCFL)
• Local dimming (image content)
• Sequential color (no color filter, 1/3 of pixel [TFTs, …], higher aperture, …)
• Power savings up to 90%
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LED – Backlight : Adaptive Light Output
Power Savings by Ambient Light Sensor
To detect the amounts of lights available & adjust display
brightness accordingly to save power.
Backlight
power
consumption
100%
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
20%
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Trends for LCD TV
~ 50% power saving
Local LED backlight dimming & motion blur reduction
also larger gamut as CCFL
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
… see § Video on FPDs
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LED Backlight Dimming Examples
Without
with dimming
Same image quality
but 60% less power
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LED – Backlight : Adaptive Global Dimming
x
100%
=
Grey levels
not used
+ reduce backlight for same luminance
Spread GL
x
50%
=
50% power saving!
Power Saving strongly depends on image:
dark images – large savings vs. bright image virtual no saving
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LED – Backlight : Adaptive Local Dimming
High Contrast
Lmax = 630 cd/m2
Lmin = 0.03 cd/m2
Power Saving 50% avg. (image dependant)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
 CR = 630 / 0.03
= 21,000
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LED – Backlight : Adaptive Light Output (I)
When LEDs are dimmed,
grey levels of pixels
have to be increased
Dimming of white
LEDs or RGB LEDs
(but no color dimming)
Contrast ratio 
Dimming
Power
consumption
Principle
No
0D
1D
2D
100 %
80%
70%
50%
All LEDs
100 % ON
All LEDs
dimmed
LED in lines,
LED matrix,
# of lines drvs h x v drivers
Visualization
(white  yellow)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LED – Backlight : Adaptive Light Output (II)
Dimming of white LEDs
vs.
color dimming
of RGB LEDs
Dimming
Power
consumption
Principle
When LEDs are dimmed,
grey levels of pixels
have to be increased
No
2D white
2D color
100 %
50%
20%
All LEDs
100 % ON
LED matrix,
h x v drivers
RGB LED matrix,
h x v x 3 drivers
Visualization
(white  yellow)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Backlights : CCFL vs. LED (III)
- 5x
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LCD Power Saving by Improved Backlight
Bachelor thesis of Jan Jarosch @ Display Lab 2013
presented at:
The following slides are part of this presentation
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Prinzip des Local Dimmings / “Unsere” Methode
„Standard-LCD“
 Alle LEDs immer aktiviert
 Meist einseitig angebracht
 Nur Global-Dimming
„Local Edgelight Dimming“
 LEDs meist an einer Seite
 Individuelles Dimming
„Unsere“ Methode
 LEDs an allen Seiten
 Individuelles Dimming
Energieverbrauch

16 LEDs an:
100%

8 von 16 LEDs an:
50%
aber entfernte
Elemente dunkel

7 von 16 LEDs an:
44%
und helle Elemente
Beispiele vereinfacht
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Software - Blockdiagramm
Implementierung in MATLAB
Für alle LEDs
gemessen
Aktueller Bildinhalt
Berechnung der LEDs, die am
Graustufen
anpassen
Inhomogenes Backlight  GS anpassen  Uniformity
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
LED-Lichtverteilung
meisten zum Bildinhalt beitragen
Individuelle
LED-PWMs
“nur LEDs “an”,
die zur
Bildhelligkeit
wesentlich
beitragen”
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Blockdiagramm des Versuchsaufbaus zur Evaluierung
PC steuert
Bildinhalte
und BacklightDimming
USB
PC
VGA, DVI
Microcontroller
SPI
LED-Backlight
(selbst erstellt
inkl. Lightguide)
Display
Controller
Board*
LEDTreiber
7“ LCD*
800 x 480
LVDS
*: Mit freundlicher Unterstützung von
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Hardware
LED Backlight mit
48 weißen LEDs
und Lichtleiter
Ausschnitt Backlight
Display
Display Controller Board
VGA
(PC)
Wärmebild-Aufnahme
USB (PC)
LED-Treiber-ICs
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
µC
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Ergebnisse
Schwarz
Beispiel “Balken unten”
ähnliche Ergebnisse für andere Testbilder
Backlight
Topic
Bildinhalt
Leuchtdichte (cd/m²)
Kontrastverhältnis
LED Leistungsaufnahme (W)
StandardBacklight
LED 100%
Weiss
Intelligent
(„unsere“ Methode)
LEDs normal hell
LEDs sehr hell*
W
S
W
S
W
S
200
0,45
120
0,15
200
0,25
440 : 1
800 : 1
800 : 1
4,90
1,55
2,58
 50% 
*: linear angepasst
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Sequential Colour LCD with RGB LED Backlight
+
+
=
1 F @ 60 Hz  16.7 ms
Timing
Data
write
LED „flash‟
• Fast LC required (6 x 60 Hz = 360 Hz) or scanned backlight (180 Hz)
• No colour filter  high aperture ratio, lower pixel pitch possible
• Loss of luminance  high power LED backlight required
In total power saving because of no color filter loss & high aperture !
• Colour break-up can occur
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Motion Blur Basics
• Motion blur is caused by AM techniques due to lack of ‚auto-tracking„
by human vision (PDP has similar problems)
• Impulsive displays like CRTs don‟t suffer of motion blur
Motion blur reduction is the
“driver” for high frame rates
( 100 Hz) of modern TV sets!
Visualisation
Perceived
AM
‚Display & hold„
CRT
‚Flashing„
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Motion Blur Reduction Techniques
Commercial
(1x0 & 2x0 Hz)
Frequency
Doubling
+ No luminance loss
+ Fit for LCD and PDP
- Fast & advanced signal processing
… like FRC incl. motion compensation & 24p
Motion
Blur
reduction
Data
Impulsive
Drive (like CRT)
Backlight
Professional
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
+ Fits for LCD and PDP
- Luminance loss
- Flicker may occur
- Only for LED LCDs
- Luminance loss
- Flicker may occur
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LED Backlight Driving
… seems to be „simple‟
but
- apply failure reduction methods like clustering LEDs in two
or separate chains instead of one (if a single LED fails, the
backlight is then dark)
- Thermal management of point like sources is more complex
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LED Backlight Driver ICs
PWM
dimming
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LED Backlight Driver ICs
IC needs no coils, I²C input for dimming
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LED Backlight Driver ICs
Single string design not recommended,
no light if a single LED fails (open).
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LED vs. CCFL : 5.7” AM LCD
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
different
same
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CCFL vs. LED : 10.4” AM LCD
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
different
same
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CCFL vs. LED : 15” AM LCD
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
different
same
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Summary
• Overall efficiency of LCDs is very low (~ 5%)
• Nearly all LCDs are equipped with LED backlights, mostly white LEDs
• Slim design by edge light
• LED backlights offer unique advantages of power saving methods
like local dimming
• Driving LEDs is simple compared to old fashioned CCFL
• However there are some pitfalls of LED driving like single string
• Other challenges of LED backlights refer to uniformity and color shifts
(see WS, display measurements)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Questions
• What are the main requirements for backlights?
• Which parameters are more relevant for automotive backlights?
• What are benefits of LEDs compared to CCFLs?
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Overview
1
Introduction
2
LCD - Basics
3
Direct Drive & Passive Matrix
4
Active Matrix
5
Backlights
Viewing angle
White pixel
6
LCD - Optimization
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Response time (overdrive)
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Improvement of Viewing Angle (I)
TN
(Vertical Alignement)
IPS
(Twisted Nematic)
VA
(In Plane Switching)
‚PC„
LCD TV
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
LCD TV
(special LC)
„Blue Phase“
LCD TV prototypes
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Improvement of Viewing Angle (II)
TN
IPS
VA, OCB
Low driving voltage
Very wide viewing angle
High contrast ratio
Limited viewing angle
Slow response speed
Wide viewing angle
Low brightness
Fast response speed
Viewing angle value without minimum CR or ΔE is useless !
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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TN Viewing Angle
Bright State
Medium Grey Level
Dark State
Symmetric brightness
Asymmetric brightness
Light leakage
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Improvement of Viewing Angle by 4 - Domain TN 90°
Photosensitive Alignment - Layer
Iso - Contrast Plot
4 domain TN
1 domain TN
A pixel is divided in 4 ‚subpixel„ (but with1 TFT). 4 different alignment
directions instead of one enhance the viewing cone significantly.
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Liquid Crystal Displays
Improvement of Viewing Angle : In Plane Switching
E-field
E-field is not between
front- and back plane as for TN.
ITO electrodes are only on back
plane for IPS
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Liquid Crystal Displays
Improvement of Viewing Angle : Multi Domain Vertical Alignment
E-field
E-field between protrusions
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Liquid Crystal Displays
Improvement of Viewing Angle : Multi Domain Vertical Alignment
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Liquid Crystal Displays
Further Improvements on (AM) LCDs : Colour Filters
RGBW for
6
higher luminance
primaries
for
larger
colour
gamut
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Further Improvements on (AM) LCD : Dedicated Filters
Automotive Improvements
Reflection reduction
AG
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
AR + BEF
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Liquid Crystal Displays
Improvement of LCD Response Time by HW & SW
Same idea (higher set point for short time) as reducing settling time in automation control
Grey
level
t
Rel. L
1 frame
Ideal response
Standard drive
t
Boost grey level (overdrive, …)
Boost grey level is GS dependent and has to be calculated, e. g. in TCON with frame buffer
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Overdrive Principle
Grey
level
Definitions
1 frame
Boost grey level
Target grey level
Previous grey level
Hardware block diagram
t
Rel. L
Previous
grey
Frame
level
Buffer
Look
FIFO
up
table
Boost
luminance
Ideal response
t
Boost
grey
level
Target grey level
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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AMLCD Response Time between Grey Levels
Without -
with Overdrive
Grey shade
dependent !
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Summary & Questions
• What makes LCDs so unique and universal for display applications?
• What are the main issues to solve for LCDs?
• What are today's hot topics when promoting LCDs?
• Issues of LCDs where other display technologies are superior:
- Ambient light performance: e-paper but no color
(for low res: reflective LCDs)
- Response time & viewing angle, depth: OLED but higher cost
 LCDs are the most universal technology today and is available
from 8 Segment (< 0.5”) to Quad HD, large size (up to 108”)
at best price and optimized for special requirements like automotive!
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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