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LT1942
(Rev.:RevA)

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

Linear Technology 

LT1942

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OPERATIO

Refer to the figure on the first page of this data sheet and

the Block Diagram when following the description of the

operation of the LT1942. The LT1942 contains four switch-

ing regulators. Three switchers are designed to be used as

the power supply for driving small to medium sized TFT

displays. The fourth switcher is designed to power an LED

backlight. All four switching regulators use a constant

frequency, current mode control scheme to provide excel-

lent line and load regulation.

TFT Supply

The three switching regulators for the TFT supply are

named AVDD, VOFF and VON. Both the AVDD and VON

switchers are boost type, while the VOFF is an inverting

type. Power sequencing circuitry is integrated into the

LT1942 to properly power up the TFT power supply. When

the SHDN pin is driven above 1V initially, only the AVDD

switcher is enabled. After the output of the AVDD switcher

reaches 97% of its final value, the PGOOD pin is driven low

and both the VOFF and VON switchers are enabled. This

event is latched rejecting any later drops on AVDD below

97%. The output-disconnect circuitry (Q5 in the Block

Diagram) for the VON switcher is not activated until the

programmable delay set by the CT pin has elapsed. Once

the output-disconnect circuitry is enabled, the VOUT3 pin is

driven to nearly the same voltage as D3. The VOUT3 pin can

only source current and is limited to 15mA nominally. All

three switching regulators are internally compensated and

operated at a fixed frequency of 1MHz. The AVDD switch-

ing regulator has integrated soft-start. Simply place a

capacitor from the SS1 pin to GND to implement soft-start.

LED Supply

The fourth switching regulator in the LT1942 is designed

to drive up to 20 LEDs (2 strings of 10) to power a

backlight. The LED switcher has an integrated current

ballaster (Q6 and Q7 in the Block Diagram) which enables

two strings of LEDs to have active current matching. The

current into LED1 and LED2 will always be actively matched,

regardless of which LED string has the highest voltage

drop. The LED ballast circuitry can be left unused when

driving a single string of LEDs. Simply leave the LED1 and

LED2 inputs floating and return the LED string directly to

the FB4 pin. The LED switcher has an independent control

pin which serves two functions: shutdown and reference

adjustment. The CTRL4 pin needs to be driven above

0.25V to enable the LED switcher. When CTRL4 is between

0.25V and 2.5V, the internal reference for the LED switcher

is attenuated. If CTRL4 is above 2.5V, the internal refer-

ence is limited to 200mV no matter how high CTRL4 is.

This feature allows an 8:1 dimming range for the LEDs.

The LED switching regulator is internally compensated

and operates at a fixed frequency of 1MHz. The LED

switching regulator has LED open-circuit protection. If

any LED fails open, D4 is not allowed to exceed 42V

(typical). This prevents damage to the power switch. The

LED switching regulator also has built in soft-start. By

placing a capacitor from the SS4 pin to GND, the user can

program the soft-start time.

1942fa

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