Liquid Crystal Displays

Transcription

1 1 (LCDs, Flüssigkristall-Anzeigen) Grundlagen - Eigenschaften - Ausführungen Karlheinz Blankenbach HS Pforzheim, Tiefenbronner Str. 65, Pforzheim Tel.: / , Fax : kb@displaylabor.de

2 2 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!?"

3 3 LCD from Segmented to E-Signage Size, price, complexity, C D Embedded system approach A Segment 8 B Character Monochrome graphics B Color graphics Direct drive MUX Passive Matrix Active Matrix Resolution

4 4 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

5 AM LCD - Panel with Digital RGB - Input Electronics Point of View Digital input signals EO Transfer Fct. Gamma Corr. R G B Sync Controls Panel Electronics Timing Controller (TCON) V com Row Driver Bank Driver 1 Driver 2 Column Driver Bank Driver 1 Driver 2 Driver 3 Backlight driver LCD Module Power Supply Sync 5

6 6 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 Row (line, scan) Display

7 7 Panel Electronics for Mid Resolution AM LCDs Column (data) driver Row (gate) driver

8 8 Example of Row and Column Drivers Column (data) driver Row (gate) driver

9 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 Front plane eo - layer Substrate (glass, plastic) Color filter (option) This is the very basic difference of displays Back plane (not to scale) Matrix drive, x-si, electronics Substrate, additional backlight for LCDs 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) 10

11 Power Consumption and Cost Timing Controller Row Driver Backlight Panel electronics, front and backplane have similar impact on module price: ~ 25% Column Driver Backlight draws about 75% of total power Typical values for 10.4 VGA Color AM LCD Polarizer etc. Backlight Misc. TFT / back plane Controller Driver ICs Front plane color filter 11

12 12 LCD Display World Resolution QXGA HDTV SXGA XGA SDTV VGA QVGA P r o j e c t i o n Car p-si PCmonitor Smartphone, tablet 150 ppi a-si Industrial See Active Matrix Notebook LCD TV Further reading (not relevant for exam) Projection system PDP (for comparison) Low information content displays Display Size /

13 13 Beyond Industrial : Avionics & Automotive 9.5 WVGA Harsh environment : Aircraft (PHILIPS), automotive (SHARP)

14 14 Beyond Commodity : High Resolution Displays 5 MPixel Only LCDs Applications : Medical, CAD, Simulation, PLANAR 21 5 MPixel

15 15 Overview 1 Introduction 2 LCD - Basics 3 Direct Drive & Passive Matrix Liquid Crystal fundamentals rinciple of operation 4 Active Matrix 5 Backlights 6 LCD - Optimization

16 16 TN Direct Twisted Nematic Guest Host Dynamic Scattering Polymer dispersed STN Multiplex, Passive Matrix Standard TN Supertwisted ECB OMI Nematic 3 Terminal TN, VA, IPS, Active Matrix Silicon Amorphous Si poly Si Bulk (LCOS) Non-Silicon 2 Terminal Diode Threshold enhanced (MIM, Varistor,...) Smectic A Thermal, electric Bi-stable Smectic C Ferroelectric, Guest Host LC-class Driving Mainstream

17 17 LCD History LCD TV

18 18 LCD Cross Section Principle : Voltage driven switching of light U (Back-) Light Polarizer Glass 1 mm ITO 50 nm LC 10 µm Alignment layer 50 nm Colour TFT TFT plane back plane Spacer Analyzer CF plane front 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. 19

20 Characteristics of Liquid Crystals Chemistry Examples ZLI ZLI Mechanics Physics T C / C (1kHz, 20 C) n = n o n = n e n (589nm, 20 C) Effects in electric field LC molecule align to electric field: - Mechanical orientation within ms - Induced charge adapt within ns 20

21 21 Temperature Characteristics of Liquid Crystals Degree of order Useful range for LCDs: 0 40 C (except automotive etc.) High Low Solid (crystal) Liquid crystal phase Liquid (isotropic) 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 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.

22 22 Properties of Liquid Crystals Anisotropic properties - Electric permittivity ( = - ) - Refractive index ( n = n - n ) - Elastic constants (k ii ) Alignment - On surfaces Alignment layer (see next slide) - By electric fields orientation to E-field from pixel voltage U

23 Interaction LC Alignment Layer Side view (example TN 90 ) 10 µm 90 twist Alignment layer Alignment layer Top view (example TN 90 ) Tilt angle ~ 2 Tilt angle Relevant for - STN multiplexing - Domain free orientation - Switching time (T Rise ) 27

24 29 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

25 30 Polarization Filter

26 32 ET/IT & TI TN (90 ) : Light guide principle Other LC principles like IPS and xva see Optimizations Light Polarizer Glass ITO Alignment layer Positive Mode Alignment direction Orientation of polarizer U on E Orientation of polarizer Lower polarizer orientation Direct Drive & Active Matrix Contrast: Difference of luminance

27 TN 90 : Light guide principle 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 Light Polarizer Glass ITO Alignment layer Alignment direction Orientation of 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 Positive Mode U on E Orientation of polarizer 33

28 34 TN (90 ) : Light guide principle Negative Mode Light Polarizer Glass ITO Alignment layer 10 µm Alignment direction U on E Lower polarizer orientation Orientation of polarizer Direct Drive & Active Matrix Contrast: Difference of luminance

29 STN ( ) : Birefringence Principle Index ellipsoid 180 twist Passive Matrix Contrast: difference of luminance + color 35

30 38 Electro - optic Curve of LC Pixel Transmission eo curve 90 % 10 % Positive mode Slope and Shape: - Viewing Angle - Twist - Pre-tilt - T - LC Type -... STN TN 0 Driving Voltage U off U on

31 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)

32 41 Overview 1 Introduction Fundamental rule of LC driving: No DC offset is allowed for LCD driving thus applying DC-free pulse waveforms 2 LCD - Basics 3 Direct Drive & Passive Matrix 4 Active Matrix Driving signals 8 - Segment Multiplex Passive Matrix 5 Backlights 6 LCD - Optimization

33 42 Direct Drive Principle: Each pixel (Dot) is driven by one dedicated signal Plate, electrode f = Hz Front (pixel, dot, segment) U S 0 Back, common U S 0 U Pixel = U Front - U Back U S 0 DC-free! -U S OFF ON OFF

34 43 Transmission and Driving Voltage Transmission For U off (10%) and U on (90%) C R = 9 : 1 (10-90 definition in electronics) but larger for 0 U drive 90% 10% U off U on U Direct drive Driving voltage For direct drive the driving pixel voltage is not limited!

35 44 Direct Drive for Segment 8 (I) A V DD V SS Control input to Exclusive OR gate A B XOR C D Segment Common B C V DD V SS V DD V SS Osc input (clock) to Exclusive OR gate 180 phase shifted output of Exclusive OR gate B - C = D D V B-C 0 Resultant display waveform measured segment to common plate DC-free! Pixel OFF Pixel ON Pixel OFF Simple driving with XOR because of automatic inversion - just set pixel!

36 45 Direct Drive for Segment 8 (II) EXOR Driving principles and display controller see Embedded Systems a b c d e f g Clock µc a f b g e c d Common crossover Segment plate Common plate

37 49 Basics of Low Res LCD - Production (I) ITO deposition Photoresist printing Mask align Exposition Developing, Etching Alignment layer printing Further reading (not relevant for exam) For frontand backplane Rubbing (LC orientation) Seal-printing Spacer AMLCD: + TFT and CF

38 50 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

39 52 LC Driving Method Direct drive is limited to 40 segments (pixel, MUX 400) Solution: Matrix drive by rows and columns Static Direct: Every pixel has a dedicated driver output Passive (PM) Matrix Active (AM, TFT) LC - Types TN STN TN Principle X X

40 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 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. 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 Row (scan) Column (data) ITO (not to scale)

41 54 Matrix Driving Parameter Activation time for a pixel T on 1 N f frame where N : number of rows (e.g. 480) f frame : frame frequency (e.g. 60 Hz) Row Scan signal waveform Frame time T frame ~ 1 / 60s Pulse width (T on ~ 34.7 µs) Example for 60 Hz : - VGA T on 35 µs - SXGA T on 16 µs m VGA - Panel, 60 Hz frame rate, 480 lines ( rows )

42 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) Driving Voltage Column (data) 0 - U ITO + U ITO Row (scan) U 2U Glass 0 Scan and data on 1 Pixel 0 U different planes Effective pixel voltage (Scheme for reference only) U is not intended resulting in an unwanted grey level Ghosting for PM! 56

43 Driving Waveforms for 2 x 2 Passive Matrix U D Waveforms for Pixel 'a' and 'b' 0 U D Data a SXGA: Scan b t Data U S 0 U S 0 One line addressing T Inverted (no DC) Scan (simplified example) b a a 2U on = U off U off 0 PM is reasonable for QVGA and possible up to SVGA. U 57

44 Contrast Adjustment for High Multiplex Ratios Transmission 90 % Trans. Shifted by U contrast U select (mux) See copy machine Trans. U select (static) 10 % twist 270 Driving Voltage U non-select U non-select Alt & Pleshko Issue : Ghosting Electrical black is not optical black (same for white) 58

45 60 PM: Calculation of Driving Voltages Alt & Pleshko - formula Further reading (not relevant for exam) Driving Voltage vs. Multiplex Rate R U U select non select N N 1 1 R = U select / U non-select 2.2 N : # of lines = Multiplex ratio 1.8 Example for Low Resolution Graphics 1.4 N = 64 R = U 13 % e.g. U sel = 10 V U 1.3 V 20 mv per gray level for 6 Bit Number of rows = multiplex rate (U select temperature dependent!) Reduced contrast ratio compared to direct drive : 1 direct CR CR N 1

46 61 Contrast Voltage - Contrast Ratio (I) Not readable OK Ghosting Contrast Ratio T Optimum contrast voltage depends on temperature and must be controlled during operation at different temp. Contrast (Operating) Voltage

47 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!

48 66 Overview 1 Introduction 2 LCD - Basics The challenge for AM is yield (Ausbeute) in mass production AM is pushed by high resolution multimedia trend with great image quality (contrast, color,...) 3 Direct Drive & Passive Matrix 4 Active Matrix 5 Backlights AM Module a-si TFTs vs. LTPS p-si TFTs 6 LCD - Optimization

49 AM LCD Side View Black Matrix Glas Polarizer Color Filter Substrate Contact Color Filter Seal TFT LC Spacer Align. ITO TFT Array Substrate Diffusors Light guide 67

50 68 Active Matrix Subpixel Data Scan Typical pixel shape MOSFET Storage capacitor LC Frontplane - Scan and data signal on one plate - 1 TFT per pixel (AM OLED 2) - Capacitor stores pixel voltage (data) during frame time - 1 pixel = 3 RGB subpixel - Aperture ratio 60% - SXGA : 1280 x 1024 x 3 TFTs 4 Mio. TFTs

51 Basic Driving Waveforms for 2 x 2 Active Matrix U D U G U D 0 U FP 0 U G Frontplane (V com ) Scan / row (gate) T Frame t Data / column b a (simplified example) a ON Rel. L Waveforms for Pixel 'a' and 'b' U t Scan Data - FP = U b OFF AM pixel voltage not limited high contrast ratio. No ghosting as TFT isolates pixel t 69

52 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, C LC When the pixel is charged the TFT switch is opened Charge remains on pixel AM pixel voltage is not limited compared to PM drive 70

53 72 Active Matrix Cell : Elements

54 Active Matrix Cell : Layers & Equivalent Circuit Front plane TN 90 Same price Back plane Scan Data 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) 73

55 75 ET/IT & TI 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)

56 80 ET/IT & TI TFT Technologies p-si, CG (SHARP) Further reading a-si (not relevant for exam) e - Mobility / cm²/vs Process Temperature / C > LTPS 300 C Substrate Single crystal Quartz / glass Glass Substrate size x x 72 Display application µdisplays Small size Large area

57 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?

58 82 Overview 1 Introduction 2 LCD - Basics 3 Direct Drive & Passive Matrix 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. 4 Active Matrix 5 Backlights Basics LED (CCFL vanishing) 6 LCD - Optimization

59 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! 83

60 84 AMLCD Module Focus: Backlight unit

61 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% Only light with same polarization as polarizer pass 85

62 AM LCD Optimizations : Selected topics see below = 5% : Backlight L: Transmission No CF for sequential color backlight Intelligent dimming Light CF or RGBW (gamut ) Rise aperture Increase LED Edgelight 100% Directional films 86

63 88 AMLCD Backlight Tasks : Homogeneous Light Output Diffusing Refraction L E D Reflection

64 90 Increasing Luminance by Brightness Enhancement Films Enhances perpendicular luminance at the cost of viewing angle degradations

65 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 94

66 White LED - Backlight & Colour Filter 95

67 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 96

68 99 LED - Backlight Monitor, TV Point source Direct Type Side light type Mobile LCDs, high end TVs

69 LED Backlight LCD Examples 100

70 LED - Backlight Characteristics rel. L C rel. L Spec 25 C Reduction by temperature, 0 20,000 40,000 60,000 80, , Operating time /h LED Junction Temperature / C LED backlights degrade mostly by high junction temperature! 103

71 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 106

72 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% 109

73 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% 20% 110

74 111 Trends for LCD TV ~ 50% power saving Local LED backlight dimming & motion blur reduction also larger gamut as CCFL see Video on FPDs

75 LED Backlight Dimming Examples Without with dimming Same image quality but 60% less power 112

76 LED Backlight : Adaptive Global Dimming x 100% = Spread GL Grey levels not used + reduce backlight for same luminance x 50% = Power Saving strongly depends on image: 50% power saving! dark images large savings vs. bright image virtual no saving 113

77 114 LED Backlight : Adaptive Local Dimming High Contrast L max = 630 cd/m 2 L min = 0.03 cd/m 2 Power Saving 50% avg. (image dependant) C R = 630 / 0.03 = 21,000

78 LED Backlight : Adaptive Light Output (I) Dimming of white LEDs or RGB LEDs (but no color dimming) When LEDs are dimmed, grey levels of pixels have to be increased Contrast ratio Dimming No 0D 1D 2D Power consumption Principle 100 % 80% 70% 50% All LEDs 100 % ON All LEDs dimmed LED in lines, # of lines drvs LED matrix, h x v drivers Visualization (white yellow) 115

79 LED Backlight : Adaptive Light Output (II) Dimming of white LEDs vs. color dimming of RGB LEDs When LEDs are dimmed, grey levels of pixels have to be increased Dimming No 2D white 2D color Power consumption Principle 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) 116

80 Backlights : CCFL vs. LED (III) - 5x 117

81 118 LCD Power Saving by Improved Backlight Bachelor thesis of Jan Display Lab 2013 presented at: The following slides are part of this presentation

82 Prinzip des Local Dimmings / Unsere Methode Standard-LCD Alle LEDs immer aktiviert Meist einseitig angebracht Nur Global-Dimming Energieverbrauch 16 LEDs an: 100% Local Edgelight Dimming LEDs meist an einer Seite Individuelles Dimming 8 von 16 LEDs an: 50% aber entfernte Elemente dunkel Unsere Methode LEDs an allen Seiten Individuelles Dimming 7 von 16 LEDs an: 44% und helle Elemente Beispiele vereinfacht 119

83 120 Software - Blockdiagramm Implementierung in MATLAB Für alle LEDs gemessen Aktueller Bildinhalt LED-Lichtverteilung Berechnung der LEDs, die am meisten zum Bildinhalt beitragen Graustufen anpassen Inhomogenes Backlight GS anpassen Uniformity Individuelle LED-PWMs nur LEDs an, die zur Bildhelligkeit wesentlich beitragen

84 121 Blockdiagramm des Versuchsaufbaus zur Evaluierung PC steuert Bildinhalte und Backlight- USB SPI Microcontroller LED- Treiber Dimming PC LED-Backlight (selbst erstellt inkl. Lightguide) 7 LCD* 800 x 480 Display VGA, DVI Controller Board* LVDS *: Mit freundlicher Unterstützung von

85 Hardware LED Backlight mit 48 weißen LEDs und Lichtleiter Display Ausschnitt Backlight Display Controller Board VGA (PC) Wärmebild-Aufnahme USB (PC) LED-Treiber-ICs µc 122

86 Ergebnisse Beispiel Balken unten ähnliche Ergebnisse für andere Testbilder Schwarz Weiss Topic Backlight 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, , ,25 Kontrastverhältnis 440 : : : 1 LED Leistungsaufnahme (W) 4,90 1,55 2,58 50% *: linear angepasst 123

87 Sequential Colour LCD with RGB LED Backlight + + = 1 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 125

88 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 127

89 Motion Blur Reduction Techniques Frequency Doubling + No luminance loss + Fit for LCD and PDP Commercial (1x0 & 2x0 Hz) - Fast & advanced signal processing Motion Blur reduction like FRC incl. motion compensation & 24p + Fits for LCD and PDP Data - Luminance loss - Flicker may occur Impulsive Drive (like CRT) - Only for LED LCDs Backlight - Luminance loss - Flicker may occur Professional 128

90 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 130

91 LED Backlight Driver ICs PWM dimming 131

92 LED Backlight Driver ICs IC needs no coils, I²C input for dimming 132

93 LED Backlight Driver ICs Single string design not recommended, no light if a single LED fails (open). 133

94 134 LED vs. CCFL : 5.7 AM LCD different same

95 CCFL vs. LED : 10.4 AM LCD different same 135

96 CCFL vs. LED : 15 AM LCD different same 136

97 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

98 139 Questions What are the main requirements for backlights? Which parameters are more relevant for automotive backlights? What are benefits of LEDs compared to CCFLs?

99 140 Overview 1 Introduction 2 LCD - Basics 3 Direct Drive & Passive Matrix 4 Active Matrix 5 Backlights 6 LCD - Optimization Viewing angle White pixel Response time (overdrive)

100 Improvement of Viewing Angle (I) TN (Twisted Nematic) (Vertical Alignement) IPS VA (In Plane Switching) (special LC) Blue Phase PC LCD TV LCD TV LCD TV prototypes 141

101 142 Improvement 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 C R or ΔE is useless!

102 143 TN Viewing Angle Bright State Symmetric brightness Medium Grey Level Asymmetric brightness Dark State Light leakage

103 144 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.

104 148 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

105 152 Improvement of Viewing Angle : Multi Domain Vertical Alignment E-field E-field between protrusions

106 154 Improvement of Viewing Angle : Multi Domain Vertical Alignment

107 159 Further Improvements on (AM) LCDs : Colour Filters RGBW for 6 higher luminance primaries for larger colour gamut

108 160 Further Improvements on (AM) LCD : Dedicated Filters Automotive Improvements Reflection reduction AG AR + BEF

109 161 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 Rel. L t Ideal response 1 frame 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

110 162 Overdrive Principle Definitions Grey level 1 frame Boost grey level Target grey level Previous grey level Rel. L Ideal response t Boost luminance t Hardware block diagram Previous grey Frame level Buffer Look FIFO up table Boost grey level Target grey level

111 163 AMLCD Response Time between Grey Levels Without - with Overdrive Grey shade dependent!

112 169 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!