LTC3524

9

3524f

OPERATION

The LTC3524 is a highly integrated power converter

intended for small to medium-sized TFT LCD display

modules. The part generates the required bias voltages

for the LCD panel as well as regulated current for one or

two white LED backlight strings. The LCD bias and white

LED boost converters are powered from a common input

voltage between 2.5V and 6V and share a 1.5MHz oscil-

lator, allowing tiny inductors and capacitors to be used.

The LCD bias supply and each white LED string can be

independently enabled and a low current shutdown mode

(<2μA) is activated when all outputs are disabled.

The LCD bias includes a synchronous PWM boost con-

verter that can be programmed between 3.0V and 6.0V.

power three charge pump converters. The charge pump

circuits operate at one-sixteenth the boost frequency

(about 94kHz). The generated output voltages are internally

sequenced to insure proper initialization of the LCD panel.

A digital shutdown input (ELCD) rapidly discharges each

generated output voltage to provide a near instantaneous

turn-off of the LCD display.

The white LED driver circuitry consists of a PWM boost

converter with an internal low loss NPN power switch and

external Schottky diode. The LED boost output (VLED) can

power as many as ten white LEDs at up to 25mA. LED cur-

rent is programmable and current in each string matched

with an internal loop. PWM dimming can be implemented

through the enable pins (ELED1 and ELED2) to extend the

dimming range of the application.

LCD Bias Boost Converter

A synchronous boost converter is used to generate the

main analog LCD bias supply for the TFT display. The

converter utilizes current mode control and includes

internally set control loop and slope compensation for

optimized performance and a simple design. Only an

tors at FBVO are required to complete the design of the

25mA boost. The 1.5MHz operating frequency produces

very low output ripple and allows the use of small low

profile inductors and tiny external ceramic capacitors.

during shutdown to avoid loading the input power source.

Soft-start produces a controlled ramp of the converter

input current during start-up, greatly reducing the burden

on the input power source. Very low operating quiescent

current and synchronous operation allow for greater than

90% conversion efficiency.

Soft-start operation provides a gradual increase in the

current drawn from the input power source during initial

start-up of the LCD bias boost converter. The rate at which

the input current will increase is set by two external com-

2). Upon initial application of power the voltage on ELCD

one time constant, ELCD will rise to approximately 63.2%

ELCD, no switching will occur because the threshold is

0.65V (typ). From 0.65V to 1V the maximum switch pin

current capability of the LTC3524 will gradually increase

from near 0A to the maximum current limit.

LCD Bias Charge Pumps

The LTC3524 uses three internal charge pump circuits to

generate low current, high voltage outputs typically used

to bias the LCD gate drive. The three charge pumps include

a doubler, quadrupler, and inverting configuration. Each

charge pump requires two small external capacitors, one

to transfer charge, and one for filtering. The charge pumps

feature fixed frequency operation for high efficiency and

lowest noise performance. The charge pump converters

operate at one-sixteenth the boost converter frequency.

3524 F01

ELCD

1M

6.8nF