LINEAR TECHNOLOGY LT1943 Technical data

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Quad Output Switching Converter Provides Power
for Large TFT LCD Panels

– Design Note 349

Dongyan Zhou

Introduction

The LT®1943 is a highly integrated, 4-output regulator
designed to power large TFT LCD panels. The LT1943
employs switching regulators—instead of linear regula­tors—to minimize power dissipation and accommodate a
wide input voltage range. The wide input range, 4.5V to
22V, allows it to accept a variety of power sources,
including the commonly used 5V, 12V and 19V AC adap­tors. The first buck regulator provides a logic voltage with
up to 2A of current. The other three switching regulators
provide the three bias voltages, AV
required by LCDs.

CMDSH-3

0.47µF

L1

4.7µH

0.22µF

V

OFF

–10V

50mA

V

LOGIC

3.3V

1.5A

2.2µF
10V

ZHCS400

ZHCS400

80.6k

10.0k

16.5k

10.0k

22µF

6.3V

DD

10pF

7.5k

100pF

, VON and V

4.5V TO 8V

10µF

10V

SW4

NFB4

FB4

BIAS

BOOST

SW1

FB1

VC1

100pF

V

4.7nF2.7nF

L3

10µH

B230A

,

OFF

IN

V

SW3

IN

LT1943

VC2 VC3 VC4

4.7k

All four regulators are synchronized to a 1.2MHz internal
clock, allowing the use of small, low cost inductors and
ceramic capacitors. Since different types of panels may
require different bias voltages, all output voltages are
adjustable for maximum flexibility. Programmable soft­start capability is included in all outputs to limit inrush
current. The LT1943 has a built-in start-up sequence and
panel protection feature. The LT1943 is available in a low­profile 28-pin TSSOP package.

, LTC and LT are registered trademarks of Linear Technology Corporation.

6.8µH

RUN-SS

SS-234

PGOOD

30k

100pF

1.5nF

L2

4.7µH

SW2

FB2

V

FB3

GND

SGND

C

T

ON

E3

0.015µF

13k
100pF

2.2nF

B240A

0.015µF

0.047µF

10µF

16V

95.3k 100k

10.0k

232k

10.0k

2.2µF
35V

AV

500mA

V
30V
20mA

0.47µF
35V

Si2343DS

DD

13V

ON

10/04/349

L1: TDK RLF7030T-4R7M3R4
L2: TDK SLF6028T-4R7M1R6

L3: TDK RLF5018T-100MR94

DN349 F01

Figure 1. Quad Output TFT LCD Power Supply with 4.5V to 8V Input Voltage Range

4-Output Supply with Soft-Start

Figure 1 shows a 4-output TFT LCD power supply with a

4.5V to 8V input range. The first output provides a 3.3V,
up to 1.5A, logic supply using a buck regulator. The
second output employs a boost converter to generate a
13V, 500mA AV
and an inverter generate V

bias supply. Another boost converter

DD

and V

ON

OFF

.

When power is first applied to the input, the RUN/SS pin
starts charging. When its voltage reaches 0.7V, switcher 1
is enabled. The capacitor at RUN/SS pin controls the
V

ramping rate and inrush current in L1.

LOGIC

Switchers 2, 3 and 4 are controlled by the BIAS pin, which
is usually connected to V

. When the BIAS pin is

LOGIC

higher than 2.8V, the SS-234 pin begins charging to
enable switchers 2, 3 and 4. When AV

reaches approxi-

DD

mately 90% of its programmed voltage, the PGOOD pin is
pulled low. When AV
programmed voltages, the C
current source begins to charge C
reaches 1.1V, an output PNP turns on, enabling V
Since V
V

ON

has to be present to turn on the LCD panel, the

ON

turn-on delay gives the column drivers and digital

DD

, V

and E3 all reach 90% of their

OFF

timer is enabled and a 20µA

T

. When the CT pin

T

ON

.

circuitry in the LCD panel time to get ready, preventing
high currents from flowing into the panel.

Figure 2 illus­trates the start-up sequencing of the 4-output power
supply in Figure 1. Figure 3 gives the overall efficiency for
the circuit in Figure 1.

If one of the regulated voltages, V

LOGIC

, AVDD, V

OFF

or E3
drops more than 10%, the internal PNP turns off to shut
down VON. This action protects the panel in a fault condi­tion. The PGOOD pin is used to drive an optional PMOS
device at the output of the AV
disconnect AV

from the input during shutdown.

DD

boost regulator to

DD

The converter uses all ceramic capacitors. X5R or X7R
type ceramic capacitors are recommended, as these
materials retain their capacitance over a wide temperature
range.

Wide Input Range Supply

If the input voltage may be higher than the AVDD set value,
a SEPIC regulator can be used in place of a boost regulator
to generate the AV

output. This covers the commonly

DD

used 12V and 19V inputs. Details for this are covered in
the LT1943 data sheet.

Conclusion

The LT1943 simplifies and shrinks power supplies for TFT
LCD panels. Its four integrated switching regulators en­able a wide input voltage range and reduce power dissipa­tion. All regulators have a 1.2MHz switching frequency
and allow the exclusive use of ceramic capacitors to
minimize circuit size, cost and output ripple.

100

RUN-SS

2V/DIV
V

LOGIC

5V/DIV

AV

DD

10V/DIV

V

OFF

10V/DIV

V

E3

20V/DIV

V

ON

50V/DIV

I

IN(AVG)

1A/DIV

Figure 2. Start-Up Waveforms of the
Power Supply in Figure 1

Data Sheet Download

http://www.linear.com

5ms/DIV

DN349 F02

90

80

EFFICIENCY (%)

70

60

300

0

LOAD AT V

600

900

LOGIC

(A)

1200

1500

DN349 F03

Figure 3. Total Circuit Efficiency of the Power
Supply in Figure 1 (Load at AVDD: 500mA)

For applications help,

call [phone], Ext. 3531

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone] ● www.linear.com

dn349f LT/TP 1004 305K • PRINTED IN THE USA

© LINEAR TECHNOLOGY CORPORATION 2004