16
LT1786F
APPLICATIONS INFORMATION
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lamp current programmer circuit. The compensation ca-
pacitor on the CCFL V
C
pin provides stable loop compen-
sation and an averaging function to the rectified sinusoidal
lamp current. Therefore, input programming current re-
lates 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 ensures that
the lamp is not overdriven which can degrade the lamp’s
operating lifetime. Therefore, the full scale current of the
DAC does not necessarily correspond to the current
required to set maximum lamp current.
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, its 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
degrades 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. This additional cur-
rent must be supplied by the transformer secondary.
Layout techniques that increase parasitic capacitance
include long high voltage lamp leads, reflective metal foil
around the lamp and displays supplied in metal enclo-
sures. 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 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 LT1786F for
simplicity of design and ease of use. An internal 0.1Ω
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 0µA to 500µA 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 com-
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 0.1Ω high-side sense resistor on the LT1786F
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
