Application Note September 2005 AN1208.0 screens don't flicker th

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Application Note September 2005 AN1208.0
screens don't flicker they?
When comparing screens, most popular differences issue flicker. common assumption that screens flicker while screens not. truth, both screens have some amount flicker. mechanisms different methods correction have varying amounts success. This appnote presents cause flicker screens offers solution avoiding flicker using ISL45041/2 Module Calibrator panels. screens have array pixels constantly backlight. constancy light removes type flicker found screens (phosphors pulsing with each refresh cycle). Instead, pixel upper lower plates with grooves perpendicular each other Figure These grooves align crystals form channels backlight pass through front panel. amount light emitted depends upon orientation liquid crystals proportional applied voltage.
VCOM
FIGURE CIRCUIT DIAGRAM SINGLE PIXEL
While this set-up functional, reduces panel lifetime. Assuming Vcom voltage ground, voltage across pixel varies from 10V. Assuming average there substantial voltage across each pixel. This voltage causes charge storage, memory. long term, form aging, degrading pixels electroplating impurities onto electrodes pixel. This contributes image retention, commonly known sticking image. construction panel symmetrical (Figure either positive negative voltage used align crystals. capitalize this aspect moving common voltage (Figure midpoint video signal, video signal swings above below common voltage (Vcom), creating zero effect pixel. This zero effect liquid crystal eliminates aging image retention issues. tradeoff this technique resolution, since video signal travels full brightness instead 10V.
FIGURE SINGLE PIXEL WITH GROOVED PLATES LIQUID CRYSTAL MOLECULES
circuit driving single pixel shown Figure gate voltage acts switch commonly amplified become 20V. video source, typically ranging from 10V, provides intensity information that appears across pixel. bottom pixel commonly connected backplane panel. voltage this node Vcom.
Vcom voltage needs placed exactly midpoint video signal avoid flicker. illustrate panel will flicker, let's assume that manufacturing panel Vcom 5.5V. video signal swings between 10V, full-scale voltage will different each field. field, full-scale voltage will 4.5V other, full-scale voltage will 5.5V. This difference full-scale voltage translates difference intensity, experienced flicker. variations construction each panel, optimal Vcom voltage differ from panel panel across single panel. Original Equipment Manufactures must therefore adjust each panels coming
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Application Note 1208
factory eliminate flicker. small screens where backplane considered low-impedance ground, single potentiometer added common voltage adjustment. Traditionally, this achieved using mechanical potentiometers that required additional manhours (Figure This acceptable small panels, even though size, precision, could easily break during assembly-requiring replacement whole module. corners center. this case, Digitally Controlled Potentiometers (DCPs) allow manufacturer automate process, necessity larger panels where manual adjustment practical. system implementation conversion from mechanical simple. Figure illustrates system application implementation buffered Vcom driver. ISL45041/2 current output, non-volatile that operate with AVDD 20V. ISL45041/2 uses two-wire, down interface. extremely accurate 7-bit device with resolution steps. desired Vcom value stored on-board EEPROM. digital circuit voltage range from 2.25V 3.6V, this enables interface with many controllers used today. analog voltage supply, running analog resistor ladder device, operate from 4.5V 20V. This important characteristic small panels that typically require less than analog supply, well large panels that require supply greater than 15V. output voltage buffered Vcom through EL5111 (180mA output current) amplifier. Contrary popular belief, panels exhibit flicker. Simple potentiometer adjustments made minimize effect since flicker arises from offset common voltage, refresh signal. LCDs grow popularity size, manual adjustment single point backplane longer possible. Using ISL45041/2 EL5411 Vcom buffer allow automatic correction Vcom offsets multiple sites backplane.
Driver
FIGURE ADJUSTMENT POTENTIOMETER REDUCE FLICKER
panels exceeding 19", backplane cannot considered single low-impedance node. Multiple corrections needed various locations screen. There localized compensation networks, four
ISL45042
column Driver
Software Control Interface
FIGURE SOFTWARE PROGRAMMABLE VCOM IMPLEMENTATION
Intersil Corporation reserves right make changes circuit design, software and/or specifications time without notice. Accordingly, reader cautioned verify that Application Note Technical Brief current before proceeding.
information regarding Intersil Corporation products, www.intersil.com
AN1208.0 September 2005

Application Note September 2005 AN1208.0 screens don't flicker th

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