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LT1182/LT1183/LT1184/LT1184F
Size Constraints
Tighter length, width, and height constraints for CCFL and
LCD contrast control circuitry are the result of LCD display
enclosure sizes remaining fairly constant while display
screen sizes have increased. Space requirements for
connector hardware include the input power supply and
control signal connector, the lamp connector, and the
contrast output voltage connector.
Even though size requirements are shrinking, the high
voltage AC required to drive the lamp has not decreased.
In some cases, the use of longer bulbs for color, portable
equipment has increased the high voltage requirement.
Accommodating the high voltage on the circuit board
dictates certain layout spacings and routings, involves
providing creepages and clearances in the transformer
design, and most importantly, involves routing a hole
underneath the CCFL transformer. Routing this hole mini-
mizes high voltage leakage paths and prevents moisture
buildup that can result in destructive arcing. In addition to
high voltage layout techniques, use appropriate layout
techniques for isolating high current paths from low-
current signal paths.
This leaves the remaining space for control circuitry at a
premium. Minimum component count is required and
minimum size for the components used is required. This
squeeze on component size is often in direct conflict with
the goals of maximizing battery life and efficiency. Com-
promise is often the only remaining choice.
LCD Contrast Circuits
The LCD contrast switching regulator on the LT1182/
LT1183 operates in many standard switching configura-
tions and is used as a classic DC/DC converter. The dual-
input-stage error amplifier easily regulates either positive
or negative contrast voltages. Topology choices for the
converter include single inductor and transformer-based
solutions. The switching regulator operates equally well
either in continuous mode or discontinuous mode. Effi-
ciencies for LCD contrast circuits range from 75% to 85%
and depend on the total power drain of the particular
display. Adjustment control of the LCD contrast voltage is
provided by either potentiometer, PWM, or DAC control.
Applications Support
Linear Technology invests an enormous amount of time,
resources, and technical expertise in understanding, de-
signing and evaluating backlight/LCD contrast solutions
for system designers. The design of an efficient and
compact LCD backlight system is a study of compromise
in a transduced electronic system. Every aspect of the
design is interrelated and any design change requires
complete re-evaluation for all other critical design param-
eters. Linear Technology has engineered one of the most
complete test and evaluation setups for backlight designs
and understands the issues and tradeoffs in achieving a
compact, effficient and economical customer solution.
Linear Technology welcomes the opportunity to discuss,
design, evaluate, and optimize any backlight/LCD contrast
system with a customer. For further information on back-
light/LCD contrast designs, consult the references listed
below.
References
1. Williams, Jim. August 1992.
Illumination Circuitry for
Liquid Crystal Displays
. Linear Technology Corporation,
Application Note 49.
2. Williams, Jim. August 1993.
Techniques for 92% Effi-
cient LCD Illumination
. Linear Technology Corporation,
Application Note 55.
3. Bonte, Anthony. March 1995.
LT1182 Floating CCFL
with Dual Polarity Contrast
. Linear Technology Corpora-
tion, Design Note 99.
4. Williams, Jim. April 1995.
A Precision Wideband Cur-
rent Probe for LCD Backlight Meaasurement
. Linear Tech-
nology Corporation, Design Note 101.
APPLICATIONS INFORMATION
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