LT1942 LINER [Linear Technology], LT1942 Datasheet - Page 11

• Manufacturer Part Number: LT1942
• Description: Quad DCDC Converter for Triple Outputs TFT Supply Plus LED Driver
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Setting the Output Voltages
Unlike charge-pump-based TFT supplies, the LT1942 TFT
outputs can all be independently set. The AV
V
As such, the output voltages are set by choosing the right
feedback divider ratio. The AV
the reference voltage is 1.25V. The V
NFB2 and the reference voltage is –150mV. The V
feedback pin is FB3 and the reference voltage is 1.25V. The
LED driver is also an inductively based DC/DC converter.
Output Disconnect
The V
nect. When the V
will charge up the D3 node. With the output disconnect
PNP (Q5) turned off, the voltage on V
Once the power sequencing circuitry turns on the PNP,
V
The V
disconnect due to the switching topology (inverting con-
figuration). When the LT1942 is in shutdown, V
to zero volts.
The AV
used for an external output disconnect function. This pin
can drive the gate of a PMOS device. When the LT1942 is
first enabled, the AV
charge up the output capacitor. Since PGOOD is still an
open collector at this point, the PMOS device is turned off,
leaving the AV
capacitor charges to 97% of the final value, PGOOD
ON
OUT3
outputs are all inductively based DC/DC converters.
ON
OFF
will go to nearly the same voltage as D3.
DD
switching regulator has built in output discon-
switching regulator provides a PGOOD pin to be
switching regulator naturally achieves output
DD
ON
output at zero volts. Once the output
U
supply is enabled, the boost converter
DD
U
switching regulator begins to
DD
feedback pin is FB1 and
W
OFF
OUT3
feedback pin is
DD
will be zero.
U
, V
OFF
OFF
will go
and
ON
latches low turning on the PMOS device and bringing the
AV
Soft-Start
The higher current regulators, AV
programmable soft-start functions built into the part.
Simply place a capacitor from the SS1 pin to GND to
achieve soft-start for the AV
switcher, place a capacitor from the SS4 pin to GND. A
good value for both soft-start capacitors is 0.1µF. The
soft-start time is dependent on the load characteristics
which will vary depending on the type and size of the TFT
display. It is a good practice to experiment to determine
the best value for the soft-start capacitors.
Power Sequencing
The TFT supply has integrated supply sequencing built-in
to ensure proper initiation of the TFT display. Figure 1
shows a state diagram of the power sequencing circuitry.
The power sequencing circuitry insures that the AV
supply comes up first, then the negative V
lastly the V
a particular delay, use the following equation:
Figure 2 shows the power sequencing during start-up of
the TFT power supply for the circuit shown in the figure on
the first page of this data sheet. All bias supplies start up
in a well controlled and well timed manner. The LED
backlight driver is independently controlled via the CTRL4
pin.
DD
C
T
output quickly to the desired voltage.
=
10
ON
µ
A t
positive supply. To pick the CT capacitor for
0 7
.
•
V
DELAY
DD
switcher. For the LED
DD
and LED, have user
OFF
LT1942
supply and
11
1942fa
DD