LT1184CS datasheet(16/24 Pages) LINER

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LT1182/LT1183/LT1184/LT1184F

change in quiescent current. An active low shutdown pin

typically reduces total supply current to 35

µA by shutting

off the 2.4V regulator and locking out switching action for

standby operation. The ICs incorporate undervoltage lock-

out by sensing regulator dropout and locking out switch-

ing below about 2.5V. The regulators also provide thermal

shutdown protection that locks out switching in the pres-

ence of excessive junction temperatures.

A 200kHz oscillator is the basic clock for all internal timing.

The oscillator turns on an output via its own logic and

driver circuitry. Adaptive anti-sat circuitry detects the

onset of saturation in a power switch and adjusts base

drive current instantaneously to limit switch saturation.

This minimizes driver dissipation and provides rapid turn-

off of the switch. The CCFL power switch is guaranteed to

provide a minimum of 1.25A in the LT1182/LT1183/

LT1184/LT1184F and the LCD power switch is guaranteed

to provide a minimum of 0.625A in the LT1182/LT1183.

The anti-sat circuitry provides a ratio of switch current to

driver current of about 50:1.

Simplified Lamp Current Programming

A programming block in the LT1182/LT1183/LT1184/

LT1184F controls lamp current, permitting either grounded-

lamp or floating-lamp configurations. Grounded configu-

rations control lamp current by directly controlling one-

half of actual lamp current and converting it to a feedback

signal to close a control loop. Floating configurations

control lamp current by directly controlling the Royer’s

primary side converter current and generating a feedback

signal to close a control loop.

Previous backlighting solutions have used a traditional

error amplifier in the control loop to regulate lamp current.

This approach converted an RMS current into a DC voltage

for the input of the error amplifier. This approach used

several time constants in order to provide stable loop

frequency compensation. This compensation scheme

meant that the loop had to be fairly slow and that output

overshoot with startup or overload conditions had to be

carefully evaluated in terms of transformer stress and

breakdown voltage requirements.

The LT1182/LT1183/LT1184/LT1184F eliminate the error

amplifier concept entirely and replace it with a lamp

current programming block. This block provides an easy-

to-use interface to program lamp current. The program-

mer circuit also reduces the number of time constants in

the control loop by combining the error signal conversion

scheme and frequency compensation into a single capaci-

tor. The control loop thus exhibits the response of a single

pole system, allows for faster loop transient response and

virtually eliminates overshoot under startup or overload

conditions.

Lamp current is programmed at the input of the program-

mer block, the ICCFL pin. This pin is the input of a shunt

regulator and accepts a DC input current signal of 0

µA to

100

µA. This input signal is converted to a 0µA to 500uA

source current at the CCFL VC pin. The programmer circuit

is simply a current-to-current converter with a gain of five.

By regulating the ICCFL pin, the input programming current

can be set with DAC, PWM or potentiometer control. The

typical input current programming range for 0mA to 6mA

lamp current is 0

µA to 50µA.

The ICCFL pin is sensitive to capacitive loading and will

oscillate with capacitance greater than 10pF. For example,

loading the ICCFL pin with a 1× or 10× scope probe causes

oscillation and erratic CCFL regulator operation because

of the probe’s respective input capacitance. A current

meter in series with the ICCFL pin will also produce oscil-

lation due to its shunt capacitance. Use a decoupling

resistor of several kilo-ohms between the ICCFL pin and the

control circuitry if excessive stray capacitance exists. This

is basically free with potentiometer or PWM control as

these control schemes use resistors. A current output

DAC should use an isolating resistor as the DAC can have

significant output capacitance that changes as a function

of input code.

Grounded-Lamp Configuration

In a grounded-lamp configuration, the low voltage side of

the lamp connects directly to the LT1182/LT1183/LT1184/

LT1184F DIO pin. This pin is the common connection

between the cathode and anode of two internal diodes. In

previous grounded-lamp solutions, these diodes were

discrete units and are now integrated onto the IC, saving

cost and board space. Bi-directional lamp current flows in

the DIO pin and thus, the diodes conduct alternately on half

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