National Semiconductors seminar on LCD column drivers.docx
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National Semiconductors seminar on LCD column drivers.docx
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NationalSemiconductorsseminaronLCDcolumndrivers
Introduction
I'dliketothankyouforyourinterestinNationalSemiconductor'sseminaronLCDcolumndrivers.
Slide1
IamDickMcCartneywiththeDisplaysDivisionatNational,andwe'vegotagoodbitofmaterialtocover,especiallywhentalkingaboutcolumndrivers,sowe'regoingtogetstartedrightaway.
Slide2
Theoutlineoftoday'spresentationisanoverviewoftheTFT/LCDsystem,justtogetusallonthesamepage,andthenwe'llhavetotalkabitaboutLCdisplayoperationinordertofullyunderstandthefunctionsthatthecolumndriverprovides.Thenwe'lldealwiththecolumndriveritself,thefunctionsitprovides.We'llwanttolistthoseout.Theseareafewofthemhere,withD-Aconversionandgammacorrection,V(com)compensationandpolarityinversion.That'lltakeaboutthefirsthalfofthesession.ThesecondhalfwilldealwithRSDSanddifferentialbuses,whichisaparticularlyimportantpartofgettingcolumndirectorstobehaveright,andthenofcoursewe'llsummarizefromthere.
Slide3
Thecolumndriverhasanumberofimportantfunctions,asurprisingnumber,actually.Itnotonlyhasalltheanalogdrivecapabilitythatyou'dexpectwithriseandfalltimesandvoltagedynamicrange,offsetcontrol.Thosearethingswe'renotgoingtotalkabout.Thethingsthatyouseeonyourscreeninblueincludesenergyrecovery,somethingwecallchargesharing,somethingthatweinventedhereatNational,andalsoD-to-Aconversion,thegammacorrection,thedynamicV(com)compensation.Itservesasasinglelinebufferforthedigitaldataandalsopolarityinversion,sowe'lldealwitheachoneofthosethingsinthisseminar.
Slide4
Solet'sbeginwithanoverviewoftheLCDoperation.FirstitshouldbepointedoutthatLCD'sfallintotwocategories,generally,andoneisthenormallyblackandtheotheristhenormallywhite.Yearsago,we'donlyhavetohavetalkedaboutnormallywhite,becausevirtuallyalldisplays,notebookPCsarenormallywhite.Butwiththelargewideviewingangledisplays,twootherimportantmodes,IPSandMVAhavecomeintoplayandthosearetypicallynormallyblackoperations.Butbasicallythedifferenceisthatwithincreasingvoltagethedisplaygetsblackinanormallywhitedisplayandgetswhiteinanormallyblackdisplay,sovoltage,increasingvoltage,createstheblackinanormallywhitedisplay.OneoftheimportantthingsthatanLCdriverhastodoistooperatetheLCDinanACmodeandsothereasonforthisisifyougetaDCcontenttotheliquidcrystal,you'llgetchargetomigrateacrosstheliquidcrystalanditwillsticktothesurfacesandthatcausesimageretentionissues,astickingimage,andsothetechniqueistoreversethepolarityperiodically,typicallyeveryframe.
Slide5
Iwanttotalkalittlebitaboutoneoftheproblemswiththeactivematrix.Sowetalkedalittlebitabouttheliquidcrystalasanentityitself,thecellitself,andthenyouputitinanactivematrix,theactivematrixintroducesanewvariableandthatischargeinjection.HereyouseeaTFTindicatedhere.ThisistheTFTswitchthatoperatesinsidetheactivematrix,soagatebuslineisdrivenbyagatebusdriverandthesourcebuscolumndriversdrivetheinputtoaninputterminalinthisTFT.Whenthatcharge,whenthegaterisesfromlowtohigh,asitdoesoverhereonthispartofthewaveform,itturnsthetransistoronandatthesametimeitinjectsthecharge,butthatchargeisinjecteditisdisregardedbecauseduringthisregionoftimethepixelisactuallychargingtotherightvalue.
Slide6
However,whenyoushutthepixeloffwiththisfallingwaveform,yousuckchargeoutofthedisplay,outofthepixel,andsowhatyoucanseeisthisnextfoilisyouendupwithadelta-V,adropthatoccurs,sotheorangetracetheregoingfromhereonourleft,startingatalowvoltageandoncethegatelineturnsonitchargestowardsthisfinalvalue,butwhenthegatelineturnsoff,thefinalvalueleftonthepixelisnottheonethatyouprogrammeditwith,andthathappens,bytheway,inbothdirections,andsothenetresultisthatyouendupwithadelta-Vdropthere,whichisanoffset.
Slide7
Hereyoucanseeinthisillustrationthat,youknow,inthisregion,youchargeittothevoltagesthatyouintendedandinthisregionit'ssettleddowntodifferentvoltages,andsoyouneedtocompensateforthatoffset.WedothatwithsomethingcalledV(com)andV(com)is,ingeneral,astaticvoltagethatyouadjust.However,thereisadynamiccomponenttoit.We'lltalkabitaboutthat.Thisequivalentcircuit,hereintheright-handcornerofyourimage,illustratestheeffect.IfwehadaperfectTFTthenwewouldn'thavethischargeinjectionbutwewouldhavethislittledelta-Vvoltagehereasawayofmodelingthatconstantdelta-V,andsothefunctionofV(com)istocompensateforthatdelta-V,andsoit'sanadjustablevalue,adjustableparameterfromdisplaytodisplay.
Slide8
Asadescriptionofdelta-V,youcanwriteanequationthattakesthatcircuitandanalyzesit,andthedelta-Vvoltageisdescribedhere.Itisafunctionofthedelta-Vgateswing,ofcourse,andsomecoefficientwhichisalltheseterms,C(gd)overC(gd)plusC(lc)plusC(s),andthatequationdescribeswhatisbehindthedelta-V,andthatclassicequationappearshereatthetopofthispage.
Slide9
ButIliketorewritethatequationintotwocomponents,acomponentthatisastaticcomponentshownhere,andadynamiccomponentoverhere.Now,whatmakesthedifferencebetweenthedynamicandstaticcomponentisthiscomponentoverhereoftheequationhasC(liquidcrystal)asaparameter,soC(liquidcrystal)isavariablecapacitance,it'snotacapacitancethat'sconstantwithvoltage,andthatmeansthatdelta-Vwillchangewithgraylevel,andsoit'simportanttorealizethatthedelta-V,thatwehaveabasicsenseforwhereitcomesfrom.It'sreallyafunctionofgraylevelanditmoveswithgraylevelandsoweneedsomewayofcompensatingforthatbecausewecan'thaveasingleV(com)adjustmentthatwillcompensateallgraylevelssimultaneously,soitbecomesthejobofthecolumndrivertocompensateforthatdelta-V,thatdynamiccomponentandsothisbottomequationdownhereshowsdelta-Vasafunctionofboththestaticterm,whichissomethingthatyouwouldinanactualdisplayuseatrim-pottoset,andthedynamicterm,whichissomethingthatthecolumndriverinducesandwe'regoingtotalkabouthowthatis.
Slide10
Allright,thestaticterm,hereyoucanseeI'veseparatedthemintobothV(com)asafunctionofgraylevelandV(com)static.
Slide11
TheclassicwaytodothisisjusttomovethatfunctionofthatV(com)asafunctionofgraylevelupintothesourcedriver,sothatthesourcedriveractuallyoutputsthenanoffsetasafunctionofgraylevel.
Slide12
Thewaythatthat'sdone,we'lltalkaboutinjustaminute,butoneoftheotherimportantpiecesofthecolumndriveristocompensateorcorrectoutthegammacharacteristicsofthedisplay.Thisgraphshowstwotypesofliquidcrystaldisplays.TheoneontheleftisanormallywhitetwistednematicandtheoneontherightisanormallyblackIPSmode,forexample,orMVA,andsothefunctionthatthecolumndriverperformsistoputoutuniformstepsofluminanceasafunctionofincomingvoltage,sothedisplayitselftakesinvoltageandproducesluminanceandsothecolumndriverhastotakeinadigitalvaluewhichcommandsacertainuniformstepofbrightnessandtranslatesthatintoavoltagethatproducesthatluminanceonthedisplay.
Slide13
Sothefunctionofthecolumndriveristhenshownhere.It'stheinversetransferofthosecurves,andI'mjusttakingtheTNasanexampleofthosecurves.Italsohastoswingbothpositiveandnegative,andthat'swhyyouseethissymmetricalcurvearoundsomemid-point,sothattypically,forinstance,ifthemaximumvoltageoutofthecolumndriverweretenvolts,themid-voltageisgoingtobearoundfivevoltsandthelowestonewillbearoundzerovolts,sothatwillbethefullswingofthecolumndriver.Halfofthatrangeisgoingtobetodrivethepositivesignals,halfofthatrangeisgoingtobetodrivethenegativesignalsandthecenterpointisgoingtobethereferenceforthepixelsandthatiswhereV(com)wouldbe.ThatwillbethereferenceofV(com).
Slide14
Now,whenwelookataddingthedynamicV(com)tothewaveformsfromthecolumndriver,whatwedoiswetakethecurvesthatyou'dseethereontheleftandwe'daddthisoffset,andwhatyoucanseeisthattheoffsetisalwaysinthesamedirection.Hereit'spositiveandhereit'spositiveandwhileit'salwaysinthesamedirection,it'sofadifferentmagnitudeasafunctionofgraylevel.Hereyoucanseeit'slargeandhereit'sbecomingincreasinglysmallerasitgetsfromoneendofthegrayscalerangetotheother.Thatisthedynamiccomponenttoit.Thatistheoffsetasafunctionofgraylevel.
Slide15
Now,whatthatdashedcurveshowsyouthenisanasymmetriccurve,andthecurveisnowdrawninblackontherighthereandwhatitillustratesisthateventhoughthesymmetryoftheliquidcrystalisperfect,thatis,apositivetwovoltsproducesthesameluminanceasanegativetwovolts,thefactisthatbecauseofthisdynamicV(com)youwouldwanttohavesomeoffsettothat,someasymmetryandthat'swhatcausesthesecurvestobeasymmetric.
Slide16
Anotherthingweshoulddiscussisthethree-levelgatedrive,andwe'renotgoingtodiscussitinanykindofdetail,butwhatIdowanttosayaboutitisthatthereisameanstogetridofV(com)altogethe
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