15

LT1186F

Maintaining closed-loop control of lamp current in a

floating lamp configuration necessitates deriving a feed-

back signal from the primary side of the Royer trans-

former. Previous solutions have used an external preci-

sion shunt and high-side sense amplifier configuration.

This approach has been integrated onto the LT1186F for

simplicity of design and ease of use. An internal 0.1

Ω

resistor monitors the Royer converter current and con-

nects between the input terminals of a high-side sense

amplifier. A 0 – 1 Amp Royer primary-side, center-tap

current is translated to a 0

µA to 500µA sink current at the

the lamp current programmer circuit. The compensation

pensation and an averaging function to the error sink

current. Therefore, input programming current is related

to average Royer converter current. Floating lamp circuits

operate similarly to grounded lamp circuits except for the

derivation of the feedback signal.

The transfer function between lamp current and input

programming current must be empirically determined and

is dependent upon a myriad of factors including lamp

characteristics, display construction, transformer turns

ratio and the tuning of the Royer oscillator. Once again,

lamp current will be slightly higher at one end of the lamp

and input programming current should be set for this

higher level to ensure that the lamp is not overdriven.

The internal 0.1

Ω high-side sense resistor on the LT1186F

is rated for a maximum DC current of 1A. This resistor can

be damaged by extremely high surge currents at start-up.

The Royer converter typically uses a few microfarads of

bypass capacitance at the center tap of the transformer.

This capacitor charges up when the system is first pow-

ered by the battery pack or an AC wall adapter. The amount

of current delivered at start-up can be very large if the total

impedance in this path is small and the voltage source has

high current capability. Linear Technology recommends

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. Cost is fairly high and footprint

can be large.

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, 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, 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.

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.