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LT1786F

APPLICATIONS INFORMATION

lamp current programmer circuit. The compensation ca-

pacitor on the CCFL VC 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 2.65M

Ω. With an operating

lamp voltage of 400V and an operating lamp current of

6mA, the parasitic current is 150

µA. 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 0.1

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

CCFL VC pin to null against the source current provided by

the lamp current programmer circuit. The compensation

capacitor on the CCFL VC 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 0.1

Ω high-sidesenseresistorontheLT1786F

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

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