TFT LCD Voltages

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TFT-LCD Modules " I mproving VideoPerformance/ Reducing Cost

Michael Jennings, Director of Marketing, Intersil Corp., Milpitas,

CA1/3/2005 3:45 PM EST

The popularity of LCD display s has grow n rapidly over the past 5 y ears. The technology which enabled thegrowth of the notebook m arket has n ow taken over the CRT in m any desktop applications, where they of f er very sharp tex t reproduction and large f ormat display s in a f orm f actor that doesn't take over the whole desk.

Now, w ith th e merging of the PC an d the multimedia experience, video reproduction is becom ing a keyrequirem ent f or th is tech nology. Som e of the historical disadvantages of this technology hav e been w eak color

representation , the sm earing of moving im ages and th e inability to reproduce detail, especially in the very brightor very dark picture regions.

In addition to the technological issues, the other h istorical draw back of the LCD display has been cost. If spaceis not a concern, the incum bent CRT provides a very cost ef f ective solution . Panel prices hav e been droppingrapidly, especially over the later h alf of this year w here supply has outstripped dem and. This h as led to m anymanuf acturers dropping th eir price, althoug h som e have also cut back on production to bring the supply/dem and curve back in line.

In this article we will take a look at som e of the new technologies w hich enable better v ideo reproduction usin g

TFT-LCDs, and how th e pressure to m ove to lower cost contradicts th is drive.LCD Module Components

Figure 1: In addition to row and column d river s, the LCD display system also depends on gam ma

cor rection and multiple supply voltage s.

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Figure 1 sh ows th e ty pical circuitry of an LCD m odule. Wh ether th e pan el is designed f or notebook, desktop


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monitor or TV application s, all of these f unctions are required, but the perf ormance of each product section doesvary.

The tim ing controller is a digital IC w hich sits at the heart of the panel. It is responsible f or con trollin g the timingof the scanning m echanism used to control th e writing of data in to the LCD display. Th is device resets th e rowand colum n driv ers to start writing data in at th e top of the display and scan ning one row at a tim e to the bottomof the display. The row drivers are pow er drives used to select which row is having it's data written at a given

time. The colum n driver con verts th e digital video data in put to th e display in to an analog voltage to be stored across each in dividual pixel cell.

Every LCD pan el obviously requires a pow er supply. Th e pan els carry 4 prim ary voltage supplies. The m ainsupply (Avdd) provides a high voltage that is used to power m any of the analog ICs w ithin th e LCD display ,includin g the colum n drivers that are used to driv e image con tent in to the display itself . Although m any peoplethink of TFT-LCD displays as a digital display , the brightness of each of the pixels is actually determ ined by ananalog voltag e level stored across th at pixel. RGB f ilters in f ront of the pixels are used to enable color reproduction. Because of the analog nature of the display, th e Av dd supply rail has to of f er good load regulationand be capable of supply ing enough curren t to ch arge an d discharg e all of the pixels w ithin the display very

quickly. Other supplies within the display include th e logic supply f or the digital IC products, as w ell as a hig hVon voltage and a negativ e VOFF voltage which are used to pow er the row driv ers w ithin th e display. For h igh

perf ormance video displays, it is very im portant to use a hig h perf ormance TFT-LCD regulator to reduce im agedistortion and sm earing. Devices like the EL7585 (Figure 2, below ) include all 4 of these supplies in a sin gle IC,and also includes supply sequencing which is im portant to av oid dam age to the LCD display

Figur e. 2. EL7585 gener ates four LCD pow er supply voltages.

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The V COM am plif ier is used to supply a very stable ref erence voltag e f or all of the pix els with in the display . Thissupply is ty pically about half the v alue of AVDD and the brightness at every pixel is determ ined by the dif f erence

between the voltage driven by th e colum n drivers an d this V COM voltag e.

The relationsh ip betw een the light em itted f rom a pixel and the voltage applied to it is a n on-linear relationship.


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This so called 'Gam ma Curve' (Figure 3) is actually an S-curve in nature and can be either positive or n egativeref erenced to V COM . In f act, m ost panels switch altern ating pixels betw een one polarity and the oth er. This giv esan average DC voltage across the display of 0V, avoidin g the burn in af f ect associated w ith such of f sets. As each

panel has a dif f erent gam ma curve response, the colum n driv ers need a ref erence curve so that they can drive theright voltage to each pixel to get th e brig htness required. These curves are ty pically supplied using the gam ma

buf f ers and a strin g of resistors w hich can be used to m imic the curv e.

Figu re 3. The S-shaped 'Gam ma Curve' show s a non-linear r elation ship between the light emitted

from a pixel and the voltage applied to it.

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The f inal device in our LCD block diag ram is the backlig ht driver. Nearly all LCD panels today use a CCFL(Cold Cath ode Fluorescent Lam p) backlig ht. These are required as the LCD panel itself does n ot emit light, butactually gates on or of f the light f rom the lig ht source behind it. These CCFL devices require a v ery high voltageAC wav ef orm to driv e them. Typical notebook an d monitors can use just one CCFL controller per panel.

However large panel, TV ty pe display s require m uch brigh ter backlights, requirin g more CCFLs an d theref oremore drivers. The light f rom each of these CCFLS also has to be m atched, oth erwise the im age brigh tness willvary across th e screen .

Reducing Motion Blur

One of the biggest issues with view ing video on LCD displays is m otion blur. This ef f ect com es f rom the veryslow respon se of the liquid crystal within th e display, leading to the appearance of a smear behind any m ovingimage. Today's liquid crystal techn ologies are yield response tim es in the 12m s to 16m s range. Although m uchf aster than the 20m s to 30m s of a couple of years ago, this still isn't f ast enough to rem ove this artif act altogether.

Of course, th is has been a major area of f ocus f or LCD developm ent and there are now a num ber of technologies w hich can be used to elim inate the problem .

The m ost obvious solution f rom the LCD panel f ront is to speed up th e response of the liquid crystal m aterial.The respon se of current m aterials can be increased by overdriv ing each pixel. Howev er, this still does not

produce the quick response that is required. Som e com panies though are now dem onstrating new materials withresponse tim es of 5ms (gray to gray), and with gray to black tim es of around 1m s. The f ast gray to black tim ealso enables the very ef f ective use of black insertion (See Figure 4). This technique inserts a black f rame

between each of the im age f rames. Th is creates a f ast pulsing ef f ect sim ilar to a CRT. The hum an brain f ilters outthis f lickering and autom atically creates the interm ediate im ages. Dem onstration s of this technique at recent


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exhibition s has proved v ery ef f ective.

Figure 4. Black insertion puts a black frame betw een each of the image f rames. This cr eates a f astpulsing ef f ect similar to a CRT w hich the h uman br ain integrates to effect motion.

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If f ast liquid crystal is not available, this ef f ect can also be sim ulated by pulsing the backlight. This technique hasalso been demonstrated very ef f ectively. By scann ing the backlight f rom top to bottom , f urther motion artif actscan be rem oved resulting in very sharp m oving images.

Reducing Flicker

Due to of f set with in the LCD panel, th e required V COM voltage can dif f er slightly f rom the ideal of one-half AVDD. This in turn can cause the appearan ce of f licker with in the display . To elim inate this ef f ect, th e VCOM isusually adjusted on a pan el-by-panel basis until the appearance of f licker is rem oved. This m echanical pot is n ow

being replaced by digital poten tiometers which enable autom atic adjustm ent of these of f sets, elim inating the possibility of of f sets through hum an error. Figure 5a show s the In tersil ISL45041 driv ing an EL5111 h igh pow er VCOM am plif ier in such an application.

Figur e 5. To elim inate flicker, th e V COM is usually ad justed on a panel-by-panel. The IntersilISL45041 can drive an EL5111 high pow er V COM am plif ier in such an application (a). New er panels

use the actual V COM value inside the panel is used to close a control loop (b).

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As each line is latch ed in to the LCD pan el, ch arge in jection in to the VCOM plane can also cause of f sets whichare seen as f licker. To elim inate this issue, new er pan els are n ow using a techn ique where the actual VCOMvalue in side th e panel is used to close a control loop which minimizes these of f sets. This in turn reduces f lickeringartif acts. Figure 5b show s such a circuit.

A third approach is to try to reduce th e switching currents at the en d of each line. A Von-slice circuit is used to


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discharge the Von voltage at the end of each line, reducing the sw itching charge in jection in to the V COM line.This can be seen in Figure 6.

Figur e 6. To elim inate f licker, the V COM can also be d ischarged by r educing the sw itching cur rents at

the end of each line.

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Removing Gamma Mism atch

Due to variation s in m anuf acturing and process param eters, each LCD that com es f rom a production line ex hibitsslightly dif f erent gam ma response. Typically a single gam ma curve is used f or all panels w hich means that w henwe place m ultiple panels n ext to each other, each will h ave a slig htly dif f erent color response. For m anyapplications, th is is extrem ely undesirable. To ov ercom e this issue, the gam ma curv es ideally need to be set on a

part by part basis as part of a closed loop sy stem. Program mable gam ma generators such as the EL5325 (Figure7) f rom Intersil en able the gam ma curve to be controlled via a m icro-processor and theref ore reprogram med as

part of the manuf acturing process.

Figur e 7. Programmable gamma generators such as th e EL5325 enable the gamma curve to be

controlled via a micro-processor.

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Im proving Contrast f or Dar k and Bright Images

Another issue with LCD displays is that they are n ot very good at disting uishin g dif f erent contrast lev els when producing very bright or v ery dark im ages. To overcom e this issue, dyn amic gamma can be used. This is wherethe gamma curve is adjusted on a f rame by f rame basis depending on the im age content. This can be done in thedigital dom ain as part of the scaling process, but lim itation in bit depths of these devices leads to a n um ber of undesirable artif acts. To overcom e these artif acts, new system s will adjust the gam ma ref erence curve using

program mable gam ma generators such as the EL5325 described above.


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Im proving Color Reproduction

The use of CCFL technology as a backlig ht source seriously im pacts the color spectrum we can produce in anLCD display. Th e ty pical CCFL backlight w ill enable an LCD display to reproduce 100% of the color spectrum of NTSC. In addition, these n ew LED

backlights elim inate the m ercury f ound in CCFL backlig hts, m aking them suitable f or green application s.

The LED backlight also has a num ber of other advan tages. For larger panels, separate Red, Green and BlueLEDs are used. This enables the color tem perature to easily be adjusted in a display. The f ast response of thesedevices also m akes them very suitable f or strobing backlight application s.

Although m any manuf acturers are w orking with these n ew techn ologies to im prove im age perf ormance in their displays, th ey are also com ing under intense pressure to reduce the cost of these dev ices to w in market sh are.With out cost reduction, th e growth of the market will be lim ited. This h as led to m any m anuf acturers rem ovingf unctionality f rom the displays instead of im provin g the perf ormance. It is v ery com mon now in mainstreammonitors to n ot use gam ma buf f ers. The curv e is typically generated using a sim ple string of resistors and the

perf ormance degradation is accepted.

As the f iercely com petitive desktop m onitor m arket con tinues to g row, w e will see lim ited adoption of these n ewtechnologies f or better v ideo perf ormance introduced in to these displays. Man y of these desktop m onitors areused in corporate en vironm ents where the em phasis is on text and graphics usage. But f or the rapidly grow ingLCD-TV m arket, and f or hom e PC m ulti-m edia stations, im age quality is going to becom e a very im portan tf actor. Intersil has products targeted at both th ese m arket. For LCD-TV, Intersil is th e only com pany to of f er DC:DC, VCOM, DCP, Gam ma and Backligh t products. For the desktop m arket, h igh lev els of integrationenable a reduction in cost. Figure 8 sh ows th e EL7642 w hich includes DC:DC, VCOM, gam ma and Von -slicingf unctions in a single package.

Figure 8. The EL7642 w hich includes DC-DC, VCOM, gamm a and Von-slicing functions in a single


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package.

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TFT LCD Voltages