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LT1942
(Rev.:RevA)

Quad DC/DC Converter for Triple Output TFT Supply Plus LED Driver

Linear Technology 

LT1942

PI FU CTIO S

PGND23 (Pin 1): Power Ground for VOFF and VON Switcher.

Tie directly to local ground plane.

SW3 (Pin 2): Switch Pin for VON Switcher. This is the

collector of an internal NPN power switch. Minimize the

metal trace area connected to this pin to minimize EMI.

D3 (Pin 3): Output of VON Switcher. Place output decoup-

ling capacitor from this pin to local ground plane.

VOUT3 (Pin 4): Collector of Internal Output Disconnect

PNP Transistor for the VON Switcher. Place a capacitor

from this pin to local ground plane of at least 0.1µF.

FB3 (Pin 5): Feedback Pin for VON Switcher. Reference

voltage is 1.25V. Connect resistive divider tap here.

Minimize trace area at FB3. Set VOUT3 according to VOUT3

= 1.25(1 + R5/R6).

NFB2 (Pin 6): Feedback Pin for VOFF Switcher. Reference

voltage is –150mV. Connect resistive divider tap here.

Minimize trace area at NFB2. Set VOUT2 according to

VOUT2 = (– 0.15)(1 + R3/R4).

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

VCTRL4/10. This allows an 8:1 dimming range for the LEDs.

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 VON supply is

turned on. The delay time is initiated when AVDD reaches

97% of its final value.

AGND (Pin 10): Analog Ground. Tie directly to local

ground plane.

SS1 (Pin 11): Soft-Start Pin for AVDD 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.

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 AVDD Switcher.

Open-collector NPN. Stays open until the VFB1 voltage

reaches 97% of target value at which point the pin will pull

down.

FB1 (Pin 14): Feedback Pin for AVDD Switcher. Connect

resistive divider tap here. Minimize trace area at FB1. Set

VOUT1 according to VOUT1 = 1.25(1 + R1/R2).

FB4 (Pin 15): Feedback Pin for LED Switcher. Place a

resistor from this node to ground. Choose R7 to set LED

current (ILED) according to the following formula:

if VCTRL4 ≥ 2.5V, R7 = 0.2/ILED

In this case, ILED is the total LED current. If two strings of

LEDs are used, ILED is the sum of the current in the two

strings. If only a single string of LEDs is used, ILED is

simply the single string current. For dimming LED current,

drive VCTRL4 to a lower voltage. See the Applications

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.

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