LT1183CS datasheet(19/24 Pages) LINER

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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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