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LT1182/LT1183/LT1184/LT1184F
APPLICATIONS INFORMATION
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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
