Browse Prior Art Database

Method to Measure and Reduce Unbalanced DC Voltage in Active Matrix LCDs

IP.com Disclosure Number: IPCOM000109248D
Original Publication Date: 1992-Aug-01
Included in the Prior Art Database: 2005-Mar-23
Document File: 3 page(s) / 125K

Publishing Venue

IBM

Related People

Howard, WE: AUTHOR [+4]

Abstract

It is well known that active matrix LCDs (AMLCDs) can achieve better display performance than conventional simple-matrix addressed LCDs. In an AMLCD, each active matrix element serves as a switch which, in each frame time, is closed to charge a pixel to a desired voltage level for a given period of time and then opened to hold the charge in the pixel for another given period of time. However, since most active matrix elements used in the AMLCDs, such as thin film transistors (TFTs), metal insulator metal (MIM) non-linear elements, and diodes etc., are non-ideal switches with unsymmetrical characteristics, an unbalanced DC voltage will exist at each display pixel. Without proper design to minimize this unbalanced DC voltage, there are high risks of poor display performance and short panel lifetime.

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Method to Measure and Reduce Unbalanced DC Voltage in Active Matrix LCDs

       It is well known that active matrix LCDs (AMLCDs) can
achieve better display performance than conventional simple-matrix
addressed LCDs.  In an AMLCD, each  active matrix  element serves as
a switch which, in each frame time, is closed to charge a pixel to a
desired voltage level for a given period of time and then opened to
hold the charge in the pixel for another given period of time.
However, since most active matrix elements used in the AMLCDs, such
as thin film transistors (TFTs), metal insulator metal (MIM)
non-linear elements, and diodes etc., are non-ideal switches with
unsymmetrical characteristics, an unbalanced DC voltage will exist at
each display pixel.  Without proper design to minimize this
unbalanced DC voltage, there are high risks of poor display
performance and short panel lifetime.  The existence of this
unbalance DC voltage is known from numerical simulation of the AMLCD
system.  In this disclosure, we describe a method to sample and
monitor the unbalanced DC voltage in the panel, and manually or
automatically adjust it nearly to null.

      The figure shows a schematic block diagram of a circuit which
will sample and monitor the unbalanced DC voltage of an AMLCD panel.
The n TFTs (n=100, for instance), each of which has the identical
design to those TFTs used in the display area, are parallel to each
other.  Their n source/drain lines are tied together and connected to
any of the data signal lines of the panel.  Their n gate lines are
also tied together and connected to any of the gate signal lines of
the panel.  These n TFTs drive a big pixel having an area equal to n
times that of the pixel area in the display area of the panel.  The
purpose of doing this is to multiply the pixel capacitance n times,
making the measurement easier and more accurate.  However, the ratio
of the pixel capacitance to the device capacitance remains the same
as in the display area.  This big active pixel (TFTs + pixel) can be
located outside the display area of the panel by an appropriate
design.  Two unity gain, high input impedance amplifiers, A1 and A2,
are used to isol...