17

LT1786F

the use of an aluminum electrolytic for the transformer

center-tap bypass capacitor with an ESR greater than or

equal to 0.5

Ω. This lowers the peak surge currents to an

acceptable level. In general, the wire and trace inductance

in this path also help reduce the di/dt of the surge current.

This issue only exists with floating lamp circuits as

grounded lamp circuits do not make use of the high-side

sense resistor.

Input Capacitor Type

Caution must be used in selecting the input capacitor type

for switching regulators. Aluminum electrolytics are elec-

trically rugged and the lowest cost, but are physically large

to meet required ripple current ratings, and size con-

straints (especially height) may preclude their use. Ce-

ramic capacitors are now available in larger values and

their high ripple current and voltage rating make them

ideal for input bypassing.

Solid tantalum capacitors would be a good choice except

for a history of occasional failure when subjected to large

current surges during start-up. The input bypass capaci-

tor of regulators can see these high surges when a battery

or high capacitance source is connected. Some manufac-

turers have developed tantalum capacitor lines specially

tested for surge capability (AVX TPS series for instance),

but even these units may fail if the input voltage surge

approaches the capacitor’s maximum voltage rating. AVX

recommends derating the capacitor voltage by 2:1 for high

surge applications.

Applications Support

Linear Technology invests an enormous amount of time,

resources and technical expertise in understanding,

designing and evaluating backlight/LCD contrast solu-

tions 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, efficient 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.

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 Measurement. Linear Tech-

nology Corporation, Design Note 101.

5. LT1182/LT1183/LT1184/LT1184F Data Sheet.

CCFL/

LCD Contrast Switching Regulators. April 1995. Linear

Technology Corporation.

6. Williams, Jim. November 1995.

A Fourth Generation of

LCD Backlight Technology. Linear Technology Corpora-

tion, Application Note 65.