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Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
1
Karlheinz Blankenbach
HS Pforzheim, Tiefenbronner Str. 65, 75175 Pforzheim
Tel.: 07231 / 28 – 6658, Fax : - 6060
kb@displaylabor.de
www.displaylabor.de
Liquid Crystal Displays
(LCDs, Flüssigkristall-Anzeigen)
Grundlagen - Eigenschaften - Ausführungen
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
2
3 Direct Drive & Passive Matrix
4 Active Matrix
1 Introduction
2 LCD - Basics
5 Backlights
6 LCD - Optimization
„The LCD Monster takes it all !?"
OverviewFurther reading(not relevant for exam)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
3
LCD from Segmented to E-Signage
Resolution
Size, price, complexity, …
Direct drive MUX Active MatrixPassive Matrix
A
B
C D
Segment 8
Character
Monochrome
graphics Color
graphicsB
Embedded system
approach
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
4
Fundamental Flat Panel Display Principle
From (digital) input to TFT drive
G
TTL-RGB
Row
Column
Electronics Point of View
Pixel converts voltage/current to light
Pixel
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
5
AM LCD - Panel with Digital RGB - Input
EO Transfer Fct.
Gamma Corr.
Power Supply
Backlight driver
Driver 1 Driver 3
Vcom
Column Driver Bank
Row
Driver
Bank
R
G
B
Sync
Sync
Timing
Controller
(TCON)
LCD
Module
Driver 2
Driver
1
Driver
2Digital
input
signals
Controls
Electronics Point of View
Panel Electronics
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
6
Electronics Point of View
Display Driving : Matrix Drive
- Scanning of rows
- Write data to columns
ControllerData from
µC, PC, ..
Column (data)
Display
Row
(line,
scan)
Fixed resolution !
- Select one row
- Write data of all columns at one time
Parallel data : Row-at-a-time addressing
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
7
Panel Electronics for Mid Resolution AM LCDs
Row (gate)
driver
Column (data) driver
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
8
Example of Row and Column Drivers
Row (gate)
driver
Column (data) driver
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
9
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
(not to scale)
Front plane
Back plane
eo - layer
Substrate (glass, plastic)
Color filter (option)
Matrix drive, x-Si, electronics
Substrate, additional
backlight for LCDs
This is the very basic
difference of displays
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
Liquid Crystal DisplaysET/IT & TI
10
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
Liquid Crystal DisplaysET/IT & TI
11
Power Consumption and Cost
Typical values for 10.4“
VGA Color AM LCD
Backlight
Column
Driver
Timing
Controller
Row
Driver
TFT / back plane
Front plane
color filterDriver ICs
Controller
Backlight
Polarizer etc.Misc.
• Panel electronics ,
front and backplane
have similar impact on
module price: ~ 25%
• Backlight draws about
75% of total power
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
12
Projection
system
LCD Display World
1 10 20 40 60 Display Size /“
Resolution
QXGA
HDTV
SXGA
XGA
SDTV
VGA
QVGA Low information content displays
Note-
book
PC-
monitor
LCD
TV
Car
Smart-
phone,
tablet
150 ppi
p-Si a-Si
PDP (for comparison)
P
r
o
j
e
c
t
i
o
n Indus-
trial
See § Active Matrix
Further reading(not relevant for exam)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
13
Beyond Industrial : Avionics & Automotive
9.5“ WVGA
Harsh environment :
Aircraft (PHILIPS), automotive (SHARP)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
14
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
Liquid Crystal DisplaysET/IT & TI
15
Liquid Crystal fundamentals
rinciple of operation
3 Direct Drive & Passive Matrix
4 Active Matrix
1 Introduction
2 LCD - Basics
5 Backlights
6 LCD - Optimization
Overview
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
16
Liquid Crystal Displays
Nematic
Direct
Multiplex, Passive
Matrix
Active Matrix
Bi-stable
Standard TN Supertwisted ECB OMI
Silicon
Amorphous Si poly Si Bulk (LCOS)Non-Silicon
Diode Threshold enhanced
(MIM, Varistor, ...)
Smectic A Thermal, electric
Smectic CFerroelectric,
Guest HostLC-class Driving
Twisted Nematic Guest Host Dynamic Scattering
3 Terminal
2 Terminal
Mainstream
Polymer dispersedTN
TN, VA,
IPS, …
STN
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
17
LCD History
• LCD TV
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
18
U
(Back-) Light
Polarizer
Glass 1 mm
ITO 50 nm
Alignment layer 50 nm
LC 10 µm
Spacer
Analyzer
LCD Cross Section
Principle : Voltage driven ‚switching„ of light
Colour TFT
CF plane
front plane
TFT plane
back plane
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
19
Basic Technologies
• Reflective(low resolution
and monochrome)
L
C
D
+ Power consumption
- Night vision
Mainstream low res.
• Transflective
(good performance
but too expensive)
L
C
D
+ Power consumption
+ Night & day vision
• Transmissive(high resolution
and color)
L
C
D
+ Vivid Colors
- Power consumption
- Daylight vision
Mainstream high res.
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
20
Characteristics of Liquid Crystals
• Chemistry
• Physics
• Effects in electric field
LC molecule align to electric field:
- Mechanical orientation within ms
- Induced charge adapt within ns
• Mechanics
Examples ZLI-
3125
ZLI-
2585
TC /°C 63 70
(1kHz, 20°C) + 2.4 - 4.4
n = no1.467 1.469
n|| = ne1.519 1.506
n (589nm, 20°C) 0.052 0.037
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
21
Temperature Characteristics of Liquid Crystals
Degree of order
T Melting Clearing
Solid Liquid
(crystal)
High
Low
(isotropic)
Liquid crystalphase
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.
Useful range for LCDs: 0 … 40°C (except automotive etc.)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
22
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
Liquid Crystal DisplaysET/IT & TI
27
Interaction LC ↔ Alignment Layer
• Side view
(example TN 90°) 10 µm 90° twist
Alignment layer
• Top view
(example TN 90°)
• Tilt angle
Relevant for
- STN multiplexing
- Domain free orientation
- Switching time (TRise)
Tilt angle~ 2°
Alignment layer
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
29
LC Preinciple: Alignment on Surfaces vs. Electric Field
Fundamental to most LCDs ; Principle by Fredericksz (1929)
0 3V 10 V Pixel voltage V
E
LC aligned to surface
(alignment layer)LC aligned to E-field
To control the transmission of pixel: polarizer needed
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
30
Polarization Filter
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
32
TN (90°) : Light guide principle
Positive Mode
Ori
enta
tio
n
of p
ola
rizerE
Uon
Polarizer
Alignmentlayer
Alignmentdirection
Light
GlassITO
Orientation ofpolarizer
Lower polarizer || orientation
• Direct Drive & Active Matrix
• Contrast: Difference of luminance
Other LC principles like IPS
and xVA see § Optimizations
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
33
TN 90° : Light guide principle
Positive
Mode
Ori
enta
tio
n
of p
ola
rizerE
Uon
Polarizer
Alignmentlayer
Alignmentdirection
Light
GlassITO
Orientation ofpolarizer
TN 90°: Twisted nematic LC with 90° helix (no voltage)
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
pixel is “white”
With “high” driving voltage:
LC orientates to E-field set by voltage
No helix, polarized light orientation
is unchanged, lower polarizer cannot
be “passed” pixel is “black”
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
34
TN (90°) : Light guide principle Negative Mode
Lower polarizer orientation
E
Polarizer
Alignment layer
Alignment direction
Light
GlassITO
Orientation of polarizer
Uon
10 µm
• Direct Drive & Active Matrix
• Contrast: Difference of luminance
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
35
STN (180°- 270°) : Birefringence Principle
180° twist
Index ellipsoid
• Passive Matrix
• Contrast: difference of luminance + color
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
38
STN
TN
Transmission
0 Driving Voltage
10 %
90 %
Uon
U off
eo curve
Slope and Shape:
- Viewing Angle
- Twist
- Pre-tilt- T
- LC Type- ...
Electro - optic Curve of LC
Positive
mode
Pixel
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
40
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
Liquid Crystal DisplaysET/IT & TI
41
3 Direct Drive & Passive Matrix
4 Active Matrix
1 Introduction
2 LCD - Basics
5 Backlights
6 LCD - Optimization
Driving signals
8 - Segment
Multiplex
Passive Matrix
OverviewFundamental rule of LC driving:
No DC offset is allowed for LCD driving
thus applying DC-free pulse waveforms
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
42
ON OFFOFF
US
0
0
Plate, electrode
Front
(pixel, dot, segment)
Back, common
0UPixel = UFront - UBack
f = 30 - 70 Hz
Direct Drive
Principle: Each pixel (Dot) is driven by one dedicated signal
US
US
-USDC-free !
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
43
Transmission and Driving Voltage
For Uoff (10%)
and Uon (90%)
→ CR = 9 : 1
(10-90 definition
in electronics)
but larger for
0 … Udrive
Transmission
Driving voltageUon
Uoff
Direct driveU
For direct drive the driving pixel voltage is not limited !
90%
10%
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
44
Simple driving with XOR because of automatic inversion - just set pixel !
Direct Drive for Segment 8 (I)
B - C = D
XORB
C
Segment
A
Common
D
A
VDD
Exclusive OR gateControl input to
VSS
BOsc input (clock) to
Exclusive OR gate
VDD
VSS
Pixel
OFF
Pixel
OFF
Pixel
ON
D
Resultant display waveform measuredsegment to common plate
0
VB-C
C180° phase shiftedoutput of ExclusiveOR gateVSS
VDD
DC-free !
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
45
Direct Drive for Segment 8 (II)
Common
crossover
Segment plate Common plate
e c
f
g
b
a
d
a
b
c
d
e
f
g
EXOR
Clock
µC
Driving principles and
display controller
see § Embedded Systems
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
49
Basics of Low Res LCD - Production (I)
ITO deposition
Photoresist printing
Mask align
Exposition For
front-
and
back-
plane
Alignment layer
printing
Rubbing (LC orientation)
Seal-printing
Spacer
Developing, Etching
AMLCD:
+ TFT and CF
Further reading(not relevant for exam)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
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Cell alignment
Baking
Scribe & break
LC filling by vacuum
UV-seal
Final assembly:
Polariser
Driver
Bezel
Basics of Low Res LCD - Production (II) Further reading(not relevant for exam)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
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LC Driving
Static
MethodDirect: Every pixel has a
dedicated driver output
LC - Types TN
Principle
X
Matrix
Passive Active
(PM) (AM, TFT)
STN TN
X
Direct drive is limited to 40 segments (pixel, MUX 400)
Solution: Matrix drive by rows and columns
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
53
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?
• Passive Matrix
The pixel is exposed to the voltage of
the other pixel in the column. Thus driving
waveforms are complex and ghosting takes place.
• 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, …)
ITO
ITO
Row (scan)
Column (data)
1 Pixel(not to scale)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
54
Matrix Driving Parameter
Scan signal waveformActivation time for a pixel
where
N : number of rows (e.g. 480)
fframe : frame frequency (e.g. 60 Hz)
frame
onfN
1T
Example for 60 Hz :
- VGA Ton 35 µs
- SXGA Ton 16 µs
VGA - Panel, 60 Hz frame rate, 480 lines ( rows )
Pulse width (Ton ~ 34.7 µs)
Frame time
Tframe ~ 1 / 60s
Row
1
2
3
m
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
56
ITO
ITO
Row (scan)
Column (data)
1 Pixel
Passive Matrix (PM) Addressing
Glass
(Scheme for reference only)
Scan and data on
different planes
Driving
Voltage0 - U
+ U
0
0
|2U|
Effective pixel voltage
|U|
|U|
|U| is not intended resulting in an
unwanted grey level Ghosting for PM !
Passive Matrix LCDs are easy
to manufacture as only ITO line
electrodes have to be manufactured
(lines + 1 TFT per pixel for AM)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
57
Driving Waveforms for 2 x 2 Passive Matrix
0
a0
T
Scan
Data
1 2 1 2U
D
US
b0
US
0
UD
a
Inverted(no DC)
t
One lineaddressing
(simplified example)
off
SXGA: 1 ... 1024
1 2 1 2a
Scan
Data
U
1 2 1 2b
-
=
on2U
U
Waveforms for Pixel 'a' and 'b'
Uoff ≠ 0
PM is reasonable for QVGA
and possible up to SVGA.
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
58
Contrast Adjustment for High Multiplex Ratios
Issue : Ghosting
Electrical black
is not optical black
(same for white)
Transmission
10 %
90 %
Unon-select 1.134 Unon-select
Driving Voltage
Uselect
(static)
twist 270°
Uselect
(mux)
Shiftedby Ucontrast
Alt & Pleshko
Trans.
Trans.
See poti @ copy machine
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
60
PM: Calculation of Driving Voltages
R = Uselect / Unon-select
1.0
1.4
1.8
2.2
1 10 100
Number of rows = multiplex rate
Driving Voltage vs. Multiplex Rate
N : # of lines = Multiplex ratio
Alt & Pleshko - formula
1N
1N
U
UR
selectnon
select
Reduced contrast ratio compared to direct drive : 1CN
1C direct
RR
Example for Low Resolution Graphics
N = 64 R = 1.134 U 13 %
e.g. Usel = 10 V U 1.3 V
20 mV per gray level for 6 Bit
(Uselect temperature dependent !)
Further reading(not relevant for exam)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
61
Contrast Voltage - Contrast Ratio (I)
Contrast (Operating) Voltage
Contrast RatioT
Optimum contrast voltage
depends on temperature and
must be controlled during
operation at different temp.
Not readable GhostingOK
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
65
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
Liquid Crystal DisplaysET/IT & TI
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3 Direct Drive & Passive Matrix
4 Active Matrix
1 Introduction
2 LCD - Basics
5 Backlights
6 LCD - Optimization
AM Module
a-Si TFTs vs. LTPS p-Si TFTs
Overview
• The challenge for AM is yield
(Ausbeute) in mass production
• AM is pushed by high resolution
multimedia trend with great
image quality (contrast, color, ...)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
67
AM LCD Side View
Diffusors
Color Filter
Black Matrix Glas
Polarizer
LC SpacerTFTSeal
Contact
Light guide
ITO
Color Filter
Substrate
TFT Array
Substrate
Align.
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
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Typical pixel shape
- 1 pixel = 3 RGB subpixel
- Aperture ratio 60%
- SXGA : 1280 x 1024 x 3 TFTs
4 Mio. TFTs
- Scan and data signal on one plate
- 1 TFT per pixel (AM OLED 2)
- Capacitor stores pixel voltage
(data) during frame time
Active Matrix Subpixel
MOSFET
StorageLC
Frontplane
capacitor
ScanData
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
69
Basic Driving Waveforms for 2 x 2 Active Matrix
0
TFrame
Scan / row (gate)
1 2 1 2
UG
b0
UG
t
a
(simplified example)
Waveforms for Pixel 'a' and 'b'
ON OFF
1 2 1 2
a
FP
Scan
Data
U
1 2 1 2
b
-
=
U
Rel. L
Frontplane (Vcom)
0
UFP
0
Data / column1 2 1 2U
D
0
UD
t t
AM pixel voltage not limited
high contrast ratio.
No ghosting as TFT “isolates” pixel
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
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Basic Driving Waveforms for 2 x 2 Active Matrix
AM pixel voltage is not limited compared to PM drive
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
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
72
Active Matrix Cell : Elements
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
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Active Matrix Cell : Layers & Equivalent Circuit
‚Same price„
Scan
Data
Front plane
Back plane
TN 90°
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
Liquid Crystal DisplaysET/IT & TI
75
→ p-Si for projection LCDs (small size & high resolution)
0.7”
XGA
a-Si vs. p-SiAll high ppi LCDs like APPLEs RETINA LCD are p-Si
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
80
TFT Technologies a-Sip-Si, CG (SHARP)
LTPS 300°C
e - Mobility
/ cm²/Vs
500 50 0.5
Process
Temperature /°C
> 900 600 - 900 250 - 350
Substrate Single crystal Quartz / glass Glass
Substrate size 4‟‟… 12‟‟ … 25‟‟ x 32‟‟ … 60‟‟ x 72‟‟
Display application µDisplays Small size Large area
Further reading(not relevant for exam)
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?
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Basics
LED
(CCFL vanishing)
3 Direct Drive & Passive Matrix
4 Active Matrix
1 Introduction
2 LCD - Basics
5 Backlights
6 LCD - Optimization
Overview • Backlight is needed for color
LCDs as it‟s light is modulated
by LC grey levels and passes
RGB color filters
• Backlight draws about 75% of
the AM LCDs power consumption.
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8383
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
- High life time
- Low cost
- …
All these requirements
cannot be fulfilled by
a single technology !
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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8484
AMLCD Module
Focus: Backlight unit
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Luminance Loss for AMLCD
5%Light efficiency:
0.6 x 0.4 x 0.7 x 0.3 5%
Only light
with same
polarization as
polarizer pass
White light
is RGB filtered,
only 1/3 of
white intensity
passes.
= 5%
100%
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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AM LCD Optimizations : Selected topics
Increase
LEDDirectional
films
No CF for
sequential
color
backlight
Rise
aperture
Light CF
or RGBW
(gamut )
: Backlight L: Transmission = 5%
100%
Intelligent
dimming
Edge-
light
„see below“
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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8888
AMLCD Backlight Tasks : Homogeneous Light Output
Refraction
Reflection
Diffusing
L
E
D
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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
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
RGB LED backlight only for high end, all other white LEDs
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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White LED - Backlight & Colour Filter
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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
• 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
(not to scale)
: LED (or CCFL)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LED - Backlight Point sourceMonitor, TV
Mobile LCDs, high end TVs
• Direct Type
• Side light type
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LED Backlight LCD Examples
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LED - Backlight Characteristics
50
60
70
80
90
100
0 20,000 40,000 60,000 80,000 100,000
Operating time /h
rel. L
@ 25°C
Reduction by
temperature, …
-60 -40 -20 0 20 40 60 80 100 120 140
50
60
70
80
90
100
LED Junction Temperature /°C
rel. L
LED backlights degrade mostly by high junction temperature !
Spec 25°C
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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
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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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Power Savings by Ambient Light Sensor
To detect the amounts of lights available & adjust display
brightness accordingly to save power.
100% 20%Backlight
power
consumption
LED – Backlight : Adaptive Light Output
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Trends for LCD TV
Local LED backlight dimming & motion blur reduction
~ 50% power saving
also larger gamut as CCFL … see § Video on FPDs
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LED Backlight Dimming Examples
Without with dimming
Same image quality
but 60% less power
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Power Saving strongly depends on image:
dark images – large savings vs. bright image virtual no saving
LED – Backlight : Adaptive Global Dimming
100%
50%
x =
x =
Grey levels
not used
Spread GL+ reduce backlight for same luminance
50% power saving!
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Power Saving 50% avg. (image dependant)
Lmax = 630 cd/m2
Lmin = 0.03 cd/m2
CR = 630 / 0.03
= 21,000
High Contrast
LED – Backlight : Adaptive Local Dimming
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
Liquid Crystal DisplaysET/IT & TI
LED – Backlight : Adaptive Light Output (I)
Dimming No 0D 1D 2D
Power
consumption100 % 80% 70% 50%
PrincipleAll LEDs
100 % ON
All LEDs
dimmed
LED in lines,
# of lines drvs
LED matrix,
h x v drivers
Visualization(white yellow)
Dimming of white
LEDs or RGB LEDs
(but no color dimming)
When LEDs are dimmed,
grey levels of pixels
have to be increased
115
Contrast ratio
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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LED – Backlight : Adaptive Light Output (II)
Dimming No 2D white 2D color
Power
consumption100 % 50% 20%
PrincipleAll LEDs
100 % ON
LED matrix,
h x v drivers
RGB LED matrix,
h x v x 3 drivers
Visualization(white yellow)
Dimming of white LEDs
vs.
color dimming
of RGB LEDs
When LEDs are dimmed,
grey levels of pixels
have to be increased
116
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- 5x
Backlights : CCFL vs. LED (III)
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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
16 LEDs an:
100%
Beispiele vereinfacht
8 von 16 LEDs an:
50%
aber entfernte
Elemente dunkel
7 von 16 LEDs an:
44%
und helle Elemente
Energieverbrauch
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Software - Blockdiagramm
Aktueller Bildinhalt
Graustufen
anpassen
LED-Lichtverteilung
Individuelle
LED-PWMs
Inhomogenes Backlight GS anpassen Uniformity
Für alle LEDs
gemessen
Berechnung der LEDs, die am meisten zum Bildinhalt beitragen
Implementierung in MATLAB
“nur LEDs “an”,
die zur
Bildhelligkeit
wesentlich
beitragen”
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Blockdiagramm des Versuchsaufbaus zur Evaluierung
PC
Micro-
controller
7“ LCD*
800 x 480
Display
Controller
Board*
LED-
Treiber
LED-Backlight
(selbst erstellt
inkl. Lightguide)
PC steuert
Bildinhalte
und Backlight-
Dimming
USB SPI
VGA, DVI LVDS
*: Mit freundlicher Unterstützung von
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Hardware
LED-Treiber-ICs µC
Ausschnitt BacklightLED Backlight mit
48 weißen LEDs
und Lichtleiter
Wärmebild-Aufnahme
Display Display Controller Board
USB (PC)
VGA (PC)
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Ergebnisse
Backlight
Topic
Standard-
Backlight
LED 100%
Intelligent
(„unsere“ Methode)
LEDs normal hell LEDs sehr hell*
Bildinhalt W S W S W S
Leuchtdichte (cd/m²) 200 0,45 120 0,15 200 0,25
Kontrastverhältnis 440 : 1 800 : 1 800 : 1
LED Leistungs-
aufnahme (W)4,90 1,55 2,58
*: linear angepasst
50%
Schwarz
Weiss
Beispiel “Balken unten”
ähnliche Ergebnisse für andere Testbilder
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Sequential Colour LCD with RGB LED Backlight
• 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
1 F @ 60 Hz 16.7 ms
=+ +
Data
write
LED „flash‟
Timing
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
AM
CRT
Visualisation Perceived
‚Flashing„
‚Display & hold„
Motion blur reduction is the
“driver” for high frame rates
( 100 Hz) of modern TV sets!
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Motion Blur Reduction TechniquesMotion Blur Reduction Techniques
Frequency
Doubling
Impulsive
Drive (like CRT)
Data
Backlight
+ No luminance loss
+ Fit for LCD and PDP
- Fast & advanced signal processing
+ Fits for LCD and PDP
- Luminance loss
- Flicker may occur
- Only for LED LCDs
- Luminance loss
- Flicker may occur
Commercial(1x0 & 2x0 Hz)
Professional
Motion
Blur
reduction
… like FRC incl. motion compensation & 24p
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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
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LED Backlight Driver ICs
PWM
dimming
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LED Backlight Driver ICs
IC needs no coils, I²C input for dimming
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LED Backlight Driver ICs
Single string design not recommended,
no light if a single LED fails (open).
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LED vs. CCFL : 5.7” AM LCD samedifferent
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CCFL vs. LED : 10.4” AM LCD samedifferent
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CCFL vs. LED : 15” AM LCD samedifferent
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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)
138
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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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Viewing angle
White pixel
Response time (overdrive)
3 Direct Drive & Passive Matrix
4 Active Matrix
1 Introduction
2 LCD - Basics
Overview
5 Backlights
6 LCD - Optimization
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Improvement of Viewing Angle (I)
TN
(Twisted Nematic)
IPS
(In Plane Switching)
(Vertical Alignement)
VA
(special LC)
„Blue Phase“
LCD TV‚PC„ LCD TV prototypesLCD TV
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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Improvement of Viewing AngleImprovement of Viewing Angle (II)
TN IPS VA, OCB
Low driving voltage
Limited viewing angle
Very wide viewing angle
Slow response speed
Low brightness
High contrast ratio
Wide viewing angle
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
Symmetric brightness
Medium Grey Level
Asymmetric brightness
Dark State
Light leakage
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Photosensitive Alignment - Layer
Improvement of Viewing Angle by 4 - Domain TN 90°
A pixel is divided in 4 ‚subpixel„ (but with1 TFT). 4 different alignment
directions instead of one enhance the viewing cone significantly.
4 domain TN 1 domain TN
Iso - Contrast Plot
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Improvement of Viewing Angle : In Plane Switching
E-field is not between
front- and back plane as for TN.
ITO electrodes are only on back
plane for IPS
E-field
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Improvement of Viewing Angle : Multi Domain Vertical Alignment
E-field
E-field between protrusions
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Improvement of Viewing Angle : Multi Domain Vertical Alignment
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Further Improvements on (AM) LCDs : Colour Filters
6
primaries
for
larger
colour
gamut
RGBW for
higher luminance
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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AR + BEFAG
Reflection reductionAutomotive Improvements
Further Improvements on (AM) LCD : Dedicated Filters
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Improvement of LCD Response Time by HW & SWImprovement of LCD Response Time by HW & SW
Boost grey level (overdrive, …)Standard driveIdeal response
Grey
level
Rel. L
1 frame
t
t
Boost grey level is GS dependent and has to be calculated, e. g. in TCON with frame buffer
Same idea (higher set point for short time) as reducing settling time in automation control
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Overdrive Principle
Boost grey level
Ideal response
Grey
level
Rel. L
1 frame
t
t
Target grey level
Previous grey level
Boost
luminanceFrame
Buffer
FIFOLook
up
table
Target grey level
Boost
grey
level
Previous
grey
level
Definitions
Hardware block diagram
Blankenbach / Pforzheim Univ. / www.displaylabor.de / May 2013
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AMLCD Response Time between Grey Levels
Without - with Overdrive
Grey shade
dependent !
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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!
Recommended