Browse Prior Art Database

Improved Backlighting for Tablet/Laptop Computer Displays

IP.com Disclosure Number: IPCOM000113529D
Original Publication Date: 1994-Sep-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 4 page(s) / 115K

Publishing Venue

IBM

Related People

Allard, SJ: AUTHOR [+2]

Abstract

Described is a hardware implementation to provide an improved method of controlling the backlighting of tablet/laptop computer displays. The technique is designed to increase the brightness range of backlighting devices and to allow the power usage to drop to as low as ten percent of the maximum brightness level. This provides the user with up to fifty percent more battery life in certain conditions.

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Improved Backlighting for Tablet/Laptop Computer Displays

      Described is a hardware implementation to provide an improved
method of controlling the backlighting of tablet/laptop computer
displays.  The technique is designed to increase the brightness range
of backlighting devices and to allow the power usage to drop to as
low as ten percent of the maximum brightness level.  This provides
the user with up to fifty percent more battery life in certain
conditions.

      Typically, transflective and transmissive Liquid Crystal Diode
(LCD) displays require a backlight for normal operations.  The
backlight is usually a Cold Cathode Florescent Lamp (CCFL) that is
powered by Direct Current (DC) to Alternating Current (AC) invertor
circuitry.  The initial starting voltage required to start the CCFL
is known to change with age and temperature.  To ensure that all
lamps will fire throughout the life of the system, it is preferred
that CCFL invertor start at its maximum power level.  Since CCFLs
begin to flicker at low current levels, the current levels will also
change with the age of the CCFL and with temperature.  To prevent
flicker, the range of brightness must be limited.  Prior art methods
of preventing flicker have forced the range of brightness to be
reduced from 100% to 40%, or approximately a two to one brightness
change.

      Fig. 1 shows a graph of the efficiency of the backlighting
brightness as voltage increases at the CCFL invertor.  It can be seen
that there is a loss in efficiency at lower voltages.  In an effort
to conserve the drain on the battery, the user will turn down the
backlight controls, thereby running the invertor at it's least
efficient point.  Fig. 2 shows a graph of the relative brightness as
related to the input power.  It can be seen that the CCFL brightness
is directly proportional to the input power, but not always
proportional to the control voltage as shown in Fig. 1.

      The concept described herein is designed to control the
invertor by means of Pulse Width Modulation (PWM) instead of the
prior art method of varying the control voltage.  The amplitude of
the pulse is set at the point where the invertor is most efficient
and the power output is at the maximum.  The control voltage can now
be switched and its duty cycle varied.  This design has distinct
advantage, as follows:
  o  The On time of the CCFL will always fire at the maximum power
    ...