LT1183CS datasheet(15/24 Pages) LINER

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LT1182/LT1183/LT1184/LT1184F

APPLICATIONS INFORMATION

Manufacturers offer a wide array of monochrome and

color displays. LCD display types include passive matrix

and active matrix. These displays differ in operating volt-

age polarity (positive and negative contrast voltage dis-

plays), operating voltage range, contrast adjust range, and

power consumption. LCD contrast supplies must regu-

late, provide output adjustment over a significant range,

operate over a wide input voltage range, and provide load

currents from milliamps to tens of milliamps.

The small size and battery-powered operation associated

with LCD equipped apparatus dictate low component

count and high efficiency for these circuits. Size con-

straints place severe limitations on circuit architecture and

long battery life is a priority. Laptop and handheld portable

computers offer an excellent example. The CCFL and its

power supply are responsible for almost 50% of the

battery drain. Displays found in newer color machines can

have a contrast power supply battery drain as high as 20%.

Additionally, all components including PC board and hard-

ware, usually must fit within the LCD enclosure with a

height restriction of 5mm to 10mm.

The CCFL switching regulator in the LT1182/LT1183/

LT1184/LT1184F typically drives an inductor that acts as

a switched mode current source for a current driven Royer

class converter with efficiencies as high as 90%. The

control loop forces the regulator to pulse-width modulate

the inductor’s average current to maintain constant cur-

rent in the lamp. The constant current’s value, and thus

lamp intensity is programmable. This drive technique

provides a wide range of intensity control. A unique lamp

current programming block permits either grounded-

lamp or floating-lamp configurations. Grounded-lamp cir-

cuits directly control one-half of actual lamp current.

Floating-lamp circuits directly control the Royer’s primary

side converter current. Floating-lamp circuits provide

differential drive to the lamp and reduce the loss from stray

lamp-to-frame capacitance, extending illumination range.

The LCD contrast switching regulator in the LT1182/

LT1183 is typically configured as a flyback converter and

generates a bias supply for contrast control. Other topol-

ogy choices for generating the bias supply include a boost

converter or a boost/charge pump converter. The supply’s

variable output permits adjustment of contrast for the

majority of available displays. Some newer types of dis-

plays require a fairly constant supply voltage and provide

contrast adjustment through a digital control pin. A unique,

dual polarity, error amplifier and the selection of a flyback

converter topology allow either positive or negative LCD

contrast voltages to be generated with minor circuit

changes. The difference between the LT1182 and LT1183

is found in the pinout for the inputs of the LCD contrast

error amplifier. The LT1182 brings out the error amplifier

inputs individually for setting up positive and negative

polarity contrast capability. This feature allows an output

connector to determine the choice of contrast operating

polarity by a ground connection. The LT1183 ties the error

amplifier inputs together and brings out an internal refer-

ence. The reference may be used in generating negative

contrast voltages or in programming lamp current.

Block Diagram Operation

The LT1182/LT1183/LT1184/LT1184F are fixed frequency,

current mode switching regulators. Fixed frequency, cur-

rent mode switchers control switch duty cycle directly by

switch current rather than by output voltage. Referring to

the block diagram for the LT1182/LT1183, the switch for

each regulator turns ON at the start of each oscillator cycle.

The switches turn OFF when switch current reaches a

predetermined level. The operation of the CCFL regulator

in the LT1184/LT1184F is identical to that in the LT1182/

LT1183. The control of output lamp current is obtained by

using the output of a unique programming block to set

current trip level. The contrast voltage is controlled by the

output of a dual-input-stage error amplifier, which sets

current trip level. The current mode switching technique

has several advantages. First, it provides excellent rejec-

tion of input voltage variations. Second, it reduces the 90

phase shift at mid-frequencies in the energy storage

inductor. This simplifies closed-loop frequency compen-

sation under widely varying input voltage or output load

conditions. Finally, it allows simple pulse-by-pulse cur-

rent limiting to provide maximum switch protection under

output overload or short-circuit conditions.

The LT1182/LT1183/LT1184/LT1184F incorporate a low

dropout internal regulator that provides a 2.4V supply for

most of the internal circuitry. This low dropout design

allows input voltage to vary from 3V to 30V with little

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