LT1184F_15 Datasheet, PDF (18/24 Pages) Linear Technology – CCFL/LCD Contrast Switching Regulators

LT1182/LT1183/LT1184/LT1184F
APPLICATIONS INFORMATION
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 0.5Ω. 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.
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