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REDUCING ELECTRONIC NOISE IN A LIQUID CRYSTAL DISPLAY

IP.com Disclosure Number: IPCOM000009475D
Original Publication Date: 1999-Sep-01
Included in the Prior Art Database: 2002-Aug-27
Document File: 2 page(s) / 119K

Publishing Venue

Motorola

Related People

Jonathan Carr: AUTHOR [+3]

Abstract

There is a trend towards reducing the system voltage in portable RF communications products from 5 volts to 3.3, 2.7 and now 1.8 volts. One advantage of this trend for RF products is that as the system voltage has been reduced, the overall system noise floor has been reduced. Unfortunately, liquid crystal display (LCD) drive voltages have not been reduced as quickly as system voltages. This is because a reduction in LCD drive voltage results is unacceptable response times below 0 degrees centi- grade. Therefore, other more inventive means are required to reduce the noise level in liquid crystal displays. Several methods are reviewed for reduc- ing the EMI, RF1 and crosstalk associated with a display. The methods keep the driver supply volt- ages and duty cycle unchanged, and therefore do not impact the electro-optic performance of the display.

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Technical MOTOROLA @ Developments

REDUCING ELECTRONIC NOISE IN A LIQUID CRYSTAL DISPLAY

by Jonathan Carr, John Burgan and Carlos Sanchez

l BACKGROUND

  l There is a trend towards reducing the system voltage in portable RF communications products from 5 volts to 3.3, 2.7 and now 1.8 volts. One advantage of this trend for RF products is that as the system voltage has been reduced, the overall system noise floor has been reduced. Unfortunately, liquid crystal display (LCD) drive voltages have not been reduced as quickly as system voltages. This is because a reduction in LCD drive voltage results is unacceptable response times below 0 degrees centi- grade. Therefore, other more inventive means are required to reduce the noise level in liquid crystal displays. Several methods are reviewed for reduc- ing the EMI, RF1 and crosstalk associated with a display. The methods keep the driver supply volt- ages and duty cycle unchanged, and therefore do not impact the electro-optic performance of the display.

APPLICATION

  There are three methods used to reduce the sys- tem noise:

  1. Varying the frequency of the front plane and back plane scanning, as well as the charge pump fre- quency.

  2. Software control of the drive impedance of the front plane and back plane drivers.

  3. Software control of the charge pump scan rate, and hardware control of the charge pump impedance.

  The application of these methods can be used individually or together. For illustration purposes, we divide the noise in a display module architecture into three sections:

Passive LCD display panel

   Driver circuitry supplying voltage waveforms to the LCD panel

  l Charge pump circuitry used to generate the higher voltages needed by the display

NOISE FROM THE LCD PANEL

  Two sources of noise from the panel have been identified: crosstalk on the panel, and EMI occur- ring between a LCD panel and any circuitry sur- rounding the display panel. Crosstalk in an LCD panel is an undesirable effect that occurs because of capacitively coupled interference between a pixel that is ON and a pixel that is OFF. Display panels are acutely susceptible to crosstalk because of the long parallel conductors that occur on the panel. The interference on the panel increases when the frequency of the driving waveform increases, and conversely, interference decreases when the wave- form switching frequency decreases. A second source of EMI and RFI comes from the combination of two waveforms. For each back plane waveform there are n combinatorial BPx-FPy waveforms, where n is the product of the number of front planes times the number of back planes. These combinato- rial waveforms benefit from a reduction in the num- ber of switching transitions, and act to further reduce EM1 and RFI.

NOISE EMITTED BY THE DISPLAY...