LT1942 LINER [Linear Technology], LT1942 Datasheet - Page 7

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

PI FU CTIO S
PGND23 (Pin 1): Power Ground for V
Tie directly to local ground plane.
SW3 (Pin 2): Switch Pin for V
collector of an internal NPN power switch. Minimize the
metal trace area connected to this pin to minimize EMI.
D3 (Pin 3): Output of V
ling capacitor from this pin to local ground plane.
V
PNP Transistor for the V
from this pin to local ground plane of at least 0.1µF.
FB3 (Pin 5): Feedback Pin for V
voltage is 1.25V. Connect resistive divider tap here.
Minimize trace area at FB3. Set V
= 1.25(1 + R5/R6).
NFB2 (Pin 6): Feedback Pin for V
voltage is –150mV. Connect resistive divider tap here.
Minimize trace area at NFB2. Set V
V
SHDN (Pin 7): Shutdown. Tie to 1V or greater to enable the
part. Tie to 0.4V or lower to disable. This pin is the global
shutdown pin for all four switching regulators.
CTRL4 (Pin 8): Shutdown for the LED Switcher. Tie to
0.25V or greater to enable the LED switcher. Tie to 0.1V or
lower to disable. CTRL4 can also override the default 200mV
reference for the LED switcher. If CTRL4 is tied to 2.5V or
greater, the reference voltage is 200mV. If CTRL4 is less
than 2.5V, the LED reference voltage is approximately
V
Refer to the Applications Section for more information.
CT (Pin 9): Programmable Delay for the TFT Supplies.
Place a capacitor from this pin to local ground plane. After
the delay, the output disconnect PNP for the V
turned on. The delay time is initiated when AV
97% of its final value.
OUT3
OUT2
CTRL4
U
= (– 0.15)(1 + R3/R4).
(Pin 4): Collector of Internal Output Disconnect
/10. This allows an 8:1 dimming range for the LEDs.
U
U
ON
ON
Switcher. Place output decoup-
Switcher. Place a capacitor
ON
OUT3
OFF
ON
Switcher. This is the
OFF
Switcher. Reference
Switcher. Reference
according to V
OUT2
and V
according to
ON
ON
DD
Switcher.
supply is
reaches
OUT3
AGND (Pin 10): Analog Ground. Tie directly to local
ground plane.
SS1 (Pin 11): Soft-Start Pin for AV
soft-start capacitor from this pin to ground. Upon start-up,
3µA of current charges the capacitor to 1.25V. Use larger
capacitor for slower start-up. Leave floating if not in use.
SS4 (Pin 12): Soft-Start for LED Switcher. Place a soft-
start capacitor from this pin to ground. Upon start-up, 3µA
of current charges the capacitor to 1.25V. Use larger
capacitor for slower start-up. Leave floating if not in use.
PGOOD (Pin 13): Power Good Indication for AV
Open-collector NPN. Stays open until the V
reaches 97% of target value at which point the pin will pull
down.
FB1 (Pin 14): Feedback Pin for AV
resistive divider tap here. Minimize trace area at FB1. Set
V
FB4 (Pin 15): Feedback Pin for LED Switcher. Place a
resistor from this node to ground. Choose R7 to set LED
current (I
In this case, I
LEDs are used, I
strings. If only a single string of LEDs is used, I
simply the single string current. For dimming LED current,
drive V
Section for more information.
D4 (Pin 16): Output of LED Switcher. Place output de-
coupling capacitor from this pin to local ground plane.
LED2 (Pin 17): Second Input of Current Mirror for an LED
String. Connect the negative end of second string of LEDs
to this pin. If not in use, leave floating.
OUT1
if V
CTRL4
according to V
CTRL4
LED
≥ 2.5V, R7 = 0.2/I
) according to the following formula:
LED
to a lower voltage. See the Applications
LED
is the total LED current. If two strings of
is the sum of the current in the two
OUT1
= 1.25(1 + R1/R2).
LED
DD
DD
Switcher. Connect
Switcher. Place a
LT1942
DD
FB1
Switcher.
voltage
LED
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is