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LT1182/LT1183/LT1184/LT1184F

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 electro-

lytic 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.

Optimizing Optical Efficiency vs Electrical Efficiency

Evaluating the performance of an LCD backlight requires

the measurement of both electrical and photometric effi-

ciencies. The best optical efficiency operating point does

not necessarily correspond to the best electrical effi-

ciency. However, these two operating points are generally

close. The desired goal is to maximize the amount of light

out for the least amount of input power. It is possible to

construct backlight circuits that operate with over 90%

electrical efficiency, but produce significantly less light

output than circuits that operate at 80% electrical effi-

ciency.

The best electrical efficiency typically occur’s just as the

CCFL’s transformer drive waveforms begin to exhibit

artifacts of higher order harmonics reflected back from the

Royer transformer secondary. Maximizing electrical effi-

ciency equates to smaller values for the Royer primary

side, resonating capacitor and larger values for the Royer

secondary side ballast capacitor. The best optical effi-

ciency occurs with nearly ideal sinusoidal drive to the

lamp. Maximizing optical efficiency equates to larger

values for the Royer primary side resonating capacitor and

smaller values for the Royer secondary side ballast capaci-

tor. The preferred operating point for the CCFL converter

is somewhere in between the best electrical efficiency and

the best optical efficiency. This operating point maximizes

photometric output per watt of input power.

Making accurate and repeatable measurements of electri-

cal and optical efficiency is difficult under the best circum-

stances. Requirements include high voltage measure-

ments and equipment specified for this operation, special-

ized calibrated voltage and current probes, wideband RMS

voltmeters, a photometer, and a calorimeter (for the

backlight enthusiast). Linear Technology’s Application

Note 55 and Design Note 101 contain detailed information

regarding equipment needs.

Input Supply Voltage Operating Range

The backlight/LCD contrast control circuits must operate

over a wide range of input supply voltage and provide

excellent line regulation for the lamp current and the

contrast output voltage. This range includes the normal

range of the battery pack itself as well as the AC wall

adapter voltage, which is normally much higher than the

maximum battery voltage. A typical input supply is 7V to

28V; a 4 to 1 supply range.

Operation of the CCFL control circuitry from the AC wall

adapter generates the worst-case stress for the CCFL

transformer. Evaluations of loop compensation for over-

shoot on startup transients and overload conditions are

essential to avoid destructive arcing, overheating, and

transformer failure. Open-lamp conditions force the Royer

converter to operate open-loop. Component stress is

again worst-case with maximum input voltage conditions.

The LT1182/LT1183/LT1184/LT1184F open-lamp pro-

tection clamps the maximum transformer secondary volt-

age to safe levels and transfers the regulator loop from

current mode operation into voltage mode operation.

Other fault conditions include board shorts and compo-

nent failures. These fault conditions can increase primary

side currents to very high levels, especially at maximum

input voltage conditions. Solutions to these fault condi-

tions include electrical and thermal fuses in the supply

voltage trace.

Improvements in battery technology are increasing bat-

tery lifetimes and decreasing battery voltages required by

the portable systems. However, operation at reduced

battery voltages requires higher, turns-ratio transformers

for the CCFL to generate equivalent output drive capability.

The penalty incurred with high ratio transformers is higher,

circulating currents acting on the same primary side

components. Loss terms increase and electrical efficiency

often decreases.

APPLICATIONS INFORMATION

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