Liquid Crystal Displays - eitidaten.fh-pforzheim.deeitidaten.fh-pforzheim.de/daten/.../vorlesungen/displays/ed_lcds.pdfET/IT & TI Liquid Crystal Displays 5 AM LCD - Panel with Digital

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

[email protected]

www.displaylabor.de

Liquid Crystal Displays

(LCDs, Flüssigkristall-Anzeigen)

Grundlagen - Eigenschaften - Ausführungen



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)



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



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



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


Panel Electronics



6


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



7

Panel Electronics for Mid Resolution AM LCDs

Row (gate)

driver

Column (data) driver



8

Example of Row and Column Drivers

Row (gate)

driver

Column (data) driver



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



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)



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



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)



13

Beyond Industrial : Avionics & Automotive

9.5“ WVGA

Harsh environment :

Aircraft (PHILIPS), automotive (SHARP)



14

Beyond Commodity : High Resolution Displays 5 MPixel

• Only LCDs

• Applications : Medical, CAD, Simulation, …

PLANAR 21” 5 MPixel



15

Liquid Crystal fundamentals

rinciple of operation


4 Active Matrix

1 Introduction

2 LCD - Basics

5 Backlights


Overview



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



17

LCD History

• LCD TV



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



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.



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



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.)



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



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



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



30

Polarization Filter



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



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”



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



35

STN (180°- 270°) : Birefringence Principle

180° twist

Index ellipsoid

• Passive Matrix

• Contrast: difference of luminance + color



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



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)



41


4 Active Matrix

1 Introduction

2 LCD - Basics

5 Backlights


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



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 !



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%



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 !



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



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




50

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)



52

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



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)



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



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)



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.



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



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 !)




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



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!



66


4 Active Matrix

1 Introduction

2 LCD - Basics

5 Backlights


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, ...)



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.



68

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



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



70

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



72

Active Matrix Cell : Elements



73

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)



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



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




81

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?



82

Basics

LED

(CCFL vanishing)


4 Active Matrix

1 Introduction

2 LCD - Basics

5 Backlights


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.



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 !



8484

AMLCD Module

Focus: Backlight unit



85

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%



86

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“



8888

AMLCD Backlight Tasks : Homogeneous Light Output

Refraction

Reflection

Diffusing

L

E

D



90

Increasing Luminance by Brightness Enhancement Films

Enhances perpendicular luminance

at the cost of viewing angle degradations



94

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



95

White LED - Backlight & Colour Filter



96

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)



9999

LED - Backlight Point sourceMonitor, TV

Mobile LCDs, high end TVs

• Direct Type

• Side light type



100

LED Backlight LCD Examples



103

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



106

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



109

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%



110

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



111

Trends for LCD TV

Local LED backlight dimming & motion blur reduction

~ 50% power saving

also larger gamut as CCFL … see § Video on FPDs



112

LED Backlight Dimming Examples

Without with dimming

Same image quality

but 60% less power



113

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!



114

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



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



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



117

- 5x

Backlights : CCFL vs. LED (III)



118

LCD Power Saving by Improved Backlight

Bachelor thesis of Jan Jarosch @ Display Lab 2013

presented at:

The following slides are part of this presentation



119

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



120

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”



121

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



122

Hardware

LED-Treiber-ICs µC

Ausschnitt BacklightLED Backlight mit

48 weißen LEDs

und Lichtleiter

Wärmebild-Aufnahme

Display Display Controller Board

USB (PC)

VGA (PC)



123

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



125

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



127

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!



128

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



130

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



131

LED Backlight Driver ICs

PWM

dimming



132


IC needs no coils, I²C input for dimming



133


Single string design not recommended,

no light if a single LED fails (open).



134

LED vs. CCFL : 5.7” AM LCD samedifferent



135

CCFL vs. LED : 10.4” AM LCD samedifferent



136

CCFL vs. LED : 15” AM LCD samedifferent



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



139

Questions

• What are the main requirements for backlights?

• Which parameters are more relevant for automotive backlights?

• What are benefits of LEDs compared to CCFLs?



140

Viewing angle

White pixel

Response time (overdrive)


4 Active Matrix

1 Introduction

2 LCD - Basics

Overview

5 Backlights




141

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



142

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 !



143

TN Viewing Angle

Bright State

Symmetric brightness

Medium Grey Level

Asymmetric brightness

Dark State

Light leakage



144

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



148

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



152

Improvement of Viewing Angle : Multi Domain Vertical Alignment

E-field

E-field between protrusions



154

Improvement of Viewing Angle : Multi Domain Vertical Alignment



159

Further Improvements on (AM) LCDs : Colour Filters

6

primaries

for

larger

colour

gamut

RGBW for

higher luminance



160

AR + BEFAG

Reflection reductionAutomotive Improvements

Further Improvements on (AM) LCD : Dedicated Filters



161

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



162

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



163

AMLCD Response Time between Grey Levels

Without - with Overdrive

Grey shade

dependent !



169

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!

Documents

Liquid Crystal Displays - eitidaten.fh-pforzheim.deeitidaten.fh-pforzheim.de/daten/.../vorlesungen/displays/ed_lcds.pdfET/IT & TI Liquid Crystal Displays 5 AM LCD - Panel with Digital