LT1184CS#TRPBF Datasheet LT1184FIS#PBF, LT1182IS#PBF, LT1184CS#TRPBF | P16-P18

LT1182/LT1183/LT1184/LT1184F

change in quiescent current. An active low shutdown pin

typically reduces total supply current to 35µA 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 I

CCFL

pin. This pin is the input of a shunt

regulator and accepts a DC input current signal of 0µA to

100µA. This input signal is converted to a 0µA to 500uA

source current at the CCFL V

pin. The programmer circuit

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

By regulating the I

CCFL

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 0µA to 50µA.

The I

CCFL

pin is sensitive to capacitive loading and will

oscillate with capacitance greater than 10pF. For example,

loading the I

CCFL

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 I

CCFL

pin will also produce oscil-

lation due to its shunt capacitance. Use a decoupling

resistor of several kilo-ohms between the I

CCFL

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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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 V

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 2.65MΩ. With an operating

lamp voltage of 400V and an operating lamp current of

6mA, the parasitic current is 150µA. 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 0µA to 500uA sink

current at the CCFL V

pin to null against the source

current provided by the lamp current programmer circuit.

The compensation capacitor on the CCFL V

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 0.1Ω 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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very large if the total impedance in this path is small and

the voltage source has high current capability. Linear

Technology recommends the use of an aluminum electro-

lytic for the transformer center tap bypass capacitor with

an ESR greater than or equal to 0.5Ω. This lowers the peak

surge currents to an acceptable level. In general, the wire

and trace inductance in this path also help reduce the di/

dt of the surge current. This issue only exists with floating

lamp circuits as grounded-lamp circuits do not make use

of the high-side sense resistor.

Optimizing Optical Efficiency vs Electrical Efficiency

Evaluating the performance of an LCD backlight requires

the measurement of both electrical and photometric effi-

ciencies. The best optical efficiency operating point does

not necessarily correspond to the best electrical effi-

ciency. However, these two operating points are generally

close. The desired goal is to maximize the amount of light

out for the least amount of input power. It is possible to

construct backlight circuits that operate with over 90%

electrical efficiency, but produce significantly less light

output than circuits that operate at 80% electrical effi-

ciency.

The best electrical efficiency typically occur’s just as the

CCFL’s transformer drive waveforms begin to exhibit

artifacts of higher order harmonics reflected back from the

Royer transformer secondary. Maximizing electrical effi-

ciency equates to smaller values for the Royer primary

side, resonating capacitor and larger values for the Royer

secondary side ballast capacitor. The best optical effi-

ciency occurs with nearly ideal sinusoidal drive to the

lamp. Maximizing optical efficiency equates to larger

values for the Royer primary side resonating capacitor and

smaller values for the Royer secondary side ballast capaci-

tor. The preferred operating point for the CCFL converter

is somewhere in between the best electrical efficiency and

the best optical efficiency. This operating point maximizes

photometric output per watt of input power.

Making accurate and repeatable measurements of electri-

cal and optical efficiency is difficult under the best circum-

stances. Requirements include high voltage measure-

ments and equipment specified for this operation, special-

ized calibrated voltage and current probes, wideband RMS

voltmeters, a photometer, and a calorimeter (for the

backlight enthusiast). Linear Technology’s Application

Note 55 and Design Note 101 contain detailed information

regarding equipment needs.

Input Supply Voltage Operating Range

The backlight/LCD contrast control circuits must operate

over a wide range of input supply voltage and provide

excellent line regulation for the lamp current and the

contrast output voltage. This range includes the normal

range of the battery pack itself as well as the AC wall

adapter voltage, which is normally much higher than the

maximum battery voltage. A typical input supply is 7V to

28V; a 4 to 1 supply range.

Operation of the CCFL control circuitry from the AC wall

adapter generates the worst-case stress for the CCFL

transformer. Evaluations of loop compensation for over-

shoot on startup transients and overload conditions are

essential to avoid destructive arcing, overheating, and

transformer failure. Open-lamp conditions force the Royer

converter to operate open-loop. Component stress is

again worst-case with maximum input voltage conditions.

The LT1182/LT1183/LT1184/LT1184F open-lamp pro-

tection clamps the maximum transformer secondary volt-

age to safe levels and transfers the regulator loop from

current mode operation into voltage mode operation.

Other fault conditions include board shorts and compo-

nent failures. These fault conditions can increase primary

side currents to very high levels, especially at maximum

input voltage conditions. Solutions to these fault condi-

tions include electrical and thermal fuses in the supply

voltage trace.

Improvements in battery technology are increasing bat-

tery lifetimes and decreasing battery voltages required by

the portable systems. However, operation at reduced

battery voltages requires higher, turns-ratio transformers

for the CCFL to generate equivalent output drive capability.

The penalty incurred with high ratio transformers is higher,

circulating currents acting on the same primary side

components. Loss terms increase and electrical efficiency

often decreases.

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Description:

Switching Voltage Regulators Grounded Bulb CCFL Regulator

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