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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 688
QUAD OUTPUT SWITCHING REGULATORS FOR TFT SUPPLY AND LED DRIVER
1
LT1942
DESCRIPTION
Demonstration Circuit DC688 is a quad output power
supply intended for use in small to medium size TFT
panels. The circuit features the LT1942 quad output
switching regulator and generates an LED driver along
with the triple output supply required for the TFT panels.
With an input voltage range of 3V to 4.2V, two step-up
regulators and one inverting regulator provide AV
DD
, V
ON
and V
OFF
bias voltages for the LCD panels. Another high
power boost is used to drive 12 LEDs (2 strings of 6
each). For 3V minimum input voltage, up to 20mA cur-
rent is available for each LED string. A built-in LED bal-
last circuit is included to precisely match the LED cur-
rents in the two strings. The LED driver has a precise
dimming control. A voltage between 0.25V to 2.5V can
be applied to CTRL4 pin to control the dimming level.
An internal high side PNP provides delayed turn on of
the V
ON
signal for panel protection. A P-channel MOSFET
driven by the output of PGOOD pin is added at the out-
put of AV
DD
to provide output disconnect in shutdown.
All four switchers are synchronized to the internal 1MHz
clock, allowing the use of low profile inductors and ce-
ramic capacitors. Both the AV
DD
boost converter and the
LED driver have soft-start to limit inrush current.
Design files for this circuit board are available. Call
the LTC factory.
Table 1.
Performance Summary
PARAMETER CONDITION VALUE
Output Voltage AV
DD
V
IN
= 3V to 4.5V, I
OUT
= 0mA to 40mA 5V ± 4%
Maximum Output Current at AV
DD
V
IN
= 3V to 4.5V (Note 1) 40mA
Current at LED1 or LED2 V
IN
= 3V to 4.5V, R7=4.99
Ω
20mA ± 6%
Typical Output Ripple AV
DD
V
IN
= 4V, load at AV
DD
= 40mA (20MHz BW) 8mV
P–P
Typical Switching Frequency 1MHz
Logic Low Voltage-Off, T
A
= 25°C 0.4V MAX
SHDN Control
Logic High Voltage-On, T
A
= 25°C 1.0V MIN
Note 1: The AVDD output can supply more current with a larger inductor. For example, it can deliver 50mA load current with
Sumida CLS4D14-330 (33
µ
µµ
µ
H).
QUICK START PROCEDURE
Demonstration circuit 688 is easy to set up to evaluate
the performance of the LT1942. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below:
When measuring the input or output voltage ripple,
care must be taken to avoid a long ground lead on the
oscilloscope probe. Measure the input or output volt-
age ripple by touching the probe tip directly across the
input or one of the outputs and GND terminals. See
Figure 2 for proper scope probe technique.
1.
With power off, connect the input power supply to Vin
and GND.
2.
Place JP1 in RUN position; JP2 in ON position.
3.
Set the input voltage between 3V to 4.5V. Turn on the
power at the input.
4.
Check for the proper output voltages. If any output
voltage is lower than the set value, temporarily dis-
connect the load to make sure that the load is not set
too high.
