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LT1786F

APPLICATIONS INFORMATION

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 kilohms between the ICCFL pin and the

IOUT pin if excessive trace stray capacitance exists. Nor-

mally, this resistor is not required.

In some applications, the maximum programming current

required at the ICCFL pin for a maximum lamp current will be

less than the full-scale output current of the DAC, which is

100

µA. The system designer can either limit the maximum

programming current through software built into the system,

or use a current splitter which shunts a percentage of the full-

scale current from the ICCFL pin. A splitter circuit is illustrated

in Figure 1. A divider string is used from a reference voltage

to set up a voltage level equal to the ICCFL summing voltage,

or 465mV. The main current flowing in the divider string

should be chosen to swamp out the effects of the shunted

current into the divider string.

IOUT FULL-SCALE

100

µA

V(ICCFL)

465mV

VREF

I = 100

µA

0 < X < 1

SELECT V1 WITHIN THE DAC IOUT

COMPLIANCE RANGE

(EX. V1 = 2V FOR VCC = 3.3V OR 5V)

CHOOSE I1 >> (1 – X)I

R1 = (V1 – 0.465)/[(X)(100

µA)]

R2 = (V1 – 0.465)/[(1 – X)(100

µA)]

R3 = (VREF – 0.465)/I1

R4 = 0.465R3/[(1 – X)(100

µA)(R3)

+ (VREF – 0.465)]

1786 F01

V(ICCFL)

(1 – X)I

Figure 1

Grounded Lamp Configuration

In a grounded lamp configuration, the low voltage side of

the lamp connects directly to the LT1786F 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.

Bidirectional lamp current flows in the DIO pin and thus,

the diodes conduct alternately on half cycles. Lamp cur-

rent 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 VC

pin and nulls against the source current provided by the

Simplified Lamp Current Programming

A programming block in the LT1786F controls lamp

current, permitting either grounded lamp or floating lamp

configurations. Grounded configurations 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 start-up or overload conditions had to be

carefully evaluated in terms of transformer stress and

breakdown voltage requirements.

The LT1786F eliminates the error amplifier concept

entirely and replaces it with a lamp current programming

block. This block provides an easy-to-use interface to

program lamp current. The programmer 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 capacitor. The con-

trol loop thus exhibits the response of a single pole

system, allows for faster loop transient response and

virtually eliminates overshoot under start-up 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 from the DAC. This input signal is converted to a

µA to 500µA source current at the CCFL VC pin. The

programmer circuit is simply a current-to-current con-

verter with a gain of five.

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

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