LT1184F_15 Datasheet, PDF (17/24 Pages) Linear Technology – CCFL/LCD Contrast Switching Regulators

LT1182/LT1183/LT1184/LT1184F
APPLICATIONS INFORMATION
cycles. Lamp current is controlled by monitoring one-half
of the average lamp current. The diode conducting on
negative half cycles has one-tenth of its current diverted to
the CCFL pin and nulls against the source current provided
by the lamp current programmer circuit. The compensa-
tion capacitor on the CCFL V_C pin provides stable loop
compensation and an averaging function to the rectified
sinusoidal lamp current. Therefore, input programming
current relates to one-half of average lamp current.
The transfer function between lamp current and input
programming current must be empirically determined and
is dependent on the particular lamp/display housing com-
bination used. The lamp and display housing are a distrib-
uted loss structure due to parasitic lamp-to-frame capaci-
tance. This means that the current flowing at the high
voltage side of the lamp is higher than what is flowing at
the DIO pin side of the lamp. The input programming
current is set to control lamp current at the high voltage
side of the lamp, even though the feedback signal is the
lamp current at the bottom of the lamp. This insures that
the lamp is not overdriven which can degrade the lamp’s
operating lifetime.
Floating-Lamp Configuration
In a floating-lamp configuration, the lamp is fully floating
with no galvanic connection to ground. This allows the
transformer to provide symmetric, differential drive to the
lamp. Balanced drive eliminates the field imbalance asso-
ciated with parasitic lamp-to-frame capacitance and re-
duces “thermometering” (uneven lamp intensity along the
lamp length) at low lamp currents.
Carefully evaluate display designs in relation to the physi-
cal layout of the lamp, it leads and the construction of the
display housing. Parasitic capacitance from any high
voltage point to DC or AC ground creates paths for
unwanted current flow. This parasitic current flow de-
grades electrical efficiency and losses up to 25% have
been observed in practice. As an example, at a Royer
operating frequency of 60kHz, 1pF of stray capacitance
represents an impedance of 2.65MΩ. With an operating
lamp voltage of 400V and an operating lamp current of
6mA, the parasitic current is 150µA. The efficiency loss is
2.5%. Layout techniques that increase parasitic capaci-
tance include long high voltage lamp leads, reflective
metal foil around the lamp, and displays supplied in metal
enclosures. Losses for a good display are under 5%
whereas losses for a bad display range from 5% to 25%.
Lossy displays are the primary reason to use a floating-
lamp configuration. Providing symmetric, differential drive
to the lamp reduces the total parasitic loss by one-half.
Maintaining closed-loop control of lamp current in a
floating lamp configuration now necessitates deriving a
feedback 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 LT1182/
LT1183/LT1184F for simplicity of design and ease of use.
An internal 0.1W resistor monitors the Royer converter
current and connects between the input terminals of a
high-side sense amplifier. A 0A to 1A Royer primary side,
center tap current is translated to a 0µA to 500uA sink
current at the CCFL V_C pin to null against the source
current provided by the lamp current programmer circuit.
The compensation capacitor on the CCFL V_C pin provides
stable loop compensation 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 primary side converter
current and input programming current must be empiri-
cally determined and is dependent upon a myriad of
factors including lamp characteristics, display construc-
tion, 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 insure that the lamp
is not overdriven.
The internal 0.1Ω high-side sense resistor on the LT1182/
LT1183/LT1184F is rated for a maximum DC current of 1A.
However, 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 powered by the battery pack or an AC wall
adapter. The amount of current delivered at start-up can be
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