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LT1942
(Rev.:RevA)

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

Linear Technology 

LT1942

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 AVDD, VOFF and

VON outputs are all inductively based DC/DC converters.

As such, the output voltages are set by choosing the right

feedback divider ratio. The AVDD feedback pin is FB1 and

the reference voltage is 1.25V. The VOFF feedback pin is

NFB2 and the reference voltage is –150mV. The VON

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 VON switching regulator has built in output discon-

nect. When the VON supply is enabled, the boost converter

will charge up the D3 node. With the output disconnect

PNP (Q5) turned off, the voltage on VOUT3 will be zero.

Once the power sequencing circuitry turns on the PNP,

VOUT3 will go to nearly the same voltage as D3.

The VOFF switching regulator naturally achieves output

disconnect due to the switching topology (inverting con-

figuration). When the LT1942 is in shutdown, VOFF will go

to zero volts.

The AVDD switching regulator provides a PGOOD pin to be

used for an external output disconnect function. This pin

can drive the gate of a PMOS device. When the LT1942 is

first enabled, the AVDD switching regulator begins to

charge up the output capacitor. Since PGOOD is still an

open collector at this point, the PMOS device is turned off,

leaving the AVDD output at zero volts. Once the output

capacitor charges to 97% of the final value, PGOOD

latches low turning on the PMOS device and bringing the

AVDD output quickly to the desired voltage.

Soft-Start

The higher current regulators, AVDD and LED, have user

programmable soft-start functions built into the part.

Simply place a capacitor from the SS1 pin to GND to

achieve soft-start for the AVDD switcher. For the LED

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 AVDD

supply comes up first, then the negative VOFF supply and

lastly the VON positive supply. To pick the CT capacitor for

a particular delay, use the following equation:

CT

=

10µA • tDELAY

0.7V

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.

1942fa

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