LT1184CS datasheet(17/24 Pages) LINER

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LT1182/LT1183/LT1184/LT1184F

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 VC 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 2.65M

Ω. With an operating

lamp voltage of 400V and an operating lamp current of

6mA, the parasitic current is 150

µA. 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 0

µA to 500uA 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 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 0.1

Ω 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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