LTC3524_15 Datasheet, PDF (9/16 Pages) Linear Technology – Adjustable TFT Bias Supply with WLED Driver

LTC3524
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.
This output (V_OUT) is used as the main LCD supply and to
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
inductor, output capacitor and V_OUT programming resis-
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.
The boost converter also disconnects its output from V_IN
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.
V_IN
R_SS
1M
ELCD
C_SS
6.8nF
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Figure 1. 1ms Soft-Start with 3.6V V_IN
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-
ponents (R_SS and C_SS) connected to ELCD (refer to Figure
2). Upon initial application of power the voltage on ELCD
will increase relative to the time constant R_SS × C_SS. After
one time constant, ELCD will rise to approximately 63.2%
of the voltage on V_IN. From 0V to approximately 0.65V on
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.
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