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Browse Prior Art Database

LIQUID CRYSTAL DISPLAY DIGITAL CONTRAST ADJUST

IP.com Disclosure Number: IPCOM000008173D
Original Publication Date: 1997-Jun-01
Included in the Prior Art Database: 2002-May-23
Document File: 4 page(s) / 210K

Publishing Venue

Motorola

Related People

John Burgan: AUTHOR [+2]

Abstract

A Liquid Crystal Displays operate on a zero biased AC waveform. The waveform is always present regardless of whether the pixels on the display are on or off. The Rh4S voltage of the wave- form controls the pixels on the display. The displays have a specified RMS "on" voltage range and an RMS "off' voltage range. These ranges are much smaller and much more critical on larger displays than on smaller ones. For example, on a small one line numeric display, a fixed LCD voltage works with all displays over a fairly wide range of temper- atures, but a larger four line alphanumeric display has an adjustable drive voltage that is compen- sated for temperature variations. On these larger displays, the voltage must be adjusted to optimize the contrast and viewing angle of the display. If the voltage varies, the contrast of the display varies. In addition, the contrast of the display varies with temperature, and the voltage must be adjusted with temperature to compensate and maintain a constant contrast.

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Page 1 of 4

MO7VROLA Technical Developments

@

LIQUID CRYSTAL DISPLAY DIGITAL CONTRAST ADJUST

by John Burgan and Fernando Hidalgo

  A Liquid Crystal Displays operate on a zero biased AC waveform. The waveform is always present regardless of whether the pixels on the display are on or off. The Rh4S voltage of the wave- form controls the pixels on the display. The displays have a specified RMS "on" voltage range and an RMS "off' voltage range. These ranges are much smaller and much more critical on larger displays than on smaller ones. For example, on a small one line numeric display, a fixed LCD voltage works with all displays over a fairly wide range of temper- atures, but a larger four line alphanumeric display has an adjustable drive voltage that is compen- sated for temperature variations. On these larger displays, the voltage must be adjusted to optimize the contrast and viewing angle of the display. If the voltage varies, the contrast of the display varies. In addition, the contrast of the display varies with temperature, and the voltage must be adjusted with temperature to compensate and maintain a constant contrast.

  With current display driver technology, the D.C. magnitude of the supply voltage is changed to adjust the RMS value of the AC waveform and hence the contrast of the display. This requires complicated analog circuitry to generate and control an adjustable voltage.

  Figure 1 shows some typical waveforms that would be used to drive a small display with four back planes and six front planes. The front plane and back plane drivers can all output one of four voltage levels, and the signals follow an eight step sequence. The eight steps (called a frame) are divided into two groups of four steps (called a field), the first field being applied the opposite volt- age of the second field to maintain zero bias. Each step in the field represents one of the four back planes being active.

  The back plane (BPl-BP4) waveforms are always the same regardless of how many pixels are on or off. For the fast half of the cycle, the back plane voltage is at VLL3 when the particular back plane is being driven, and at VLLl when it is not. For the second half of the waveform, it is at VLLO when selected and VLL2 when not selected.

  The front plane (FPl-FP6) waveforms change depending on how many pixels are on. You can see that the FP4 sample waveform is shown with the BP2 and BP4 pixel on. For the first half of this waveform, the front plane voltage is VLLO if the pixel is on, and VLL2 if the pixel is off. For the sec- ond half of the waveform it is VLL3 is the pixel is on and VLLl if the pixel is off.

0 Mofomlq Inc. 1997 80 June 1997

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m MozoRoLA Technical Developments

Fig. 1 The current state of the art

  Although different displays operate from diierent is in the off state (BP2-FP2) is at a +I or -1 volt voltage ranges, and the voltage is often adjusted over potential all the time, so it'...