DN417 - Drive Large TFT-LCD Displays with a Space-Saving Triple-Output Regulator

Drive Large TFT-LCD Displays with a Space-Saving Triple-Output
Regulator – Design Note 417
Jesus Rosales
Introduction
The power appetite of large TFT-LCDs appears to be
insatiable. Power supplies must feed increasing numbers
of transistors and improved display resolutions, and do
so without taking much space.
The triple output supply shown in Figure 1 shows a compact design based on the LT3489, which is optimized for
driving large TFT panel displays. The main output provides
8V at 600mA while the 23V and –8V outputs provide 10mA
and 20mA respectively, all from a 3.3V input. Even though
TFT converters generally run from a regulated 3.3V or
5V source, this converter can operate seamlessly from
a Lithium-Ion battery, delivering 5W when the battery is
drained to 3.3V and 8W when it is at 4.2V.
The LT3489 squeezes a 2.5A, 0.12Ω, 40V switch into a
tiny 3mm × 3mm MS8E footprint. It offers external or
internal compensation, an internal soft-start, a 2MHz
switching frequency, and it is also pin compatible with
the popular LT1946. The design process with the LT3489
is easy and predictable.
The circuit in Figure 2 operates as a SEPIC converter,
allowing the output to be higher or lower than the input.
Pulling the ⎯S⎯H⎯D⎯N pin to ground sets the output at 0V even
while the voltage source is connected to the input.
Both applications take full advantage of the soft-start
feature in which a single capacitor programs the voltage
ramp rate of the output at startup. The 2MHz switching
frequency makes possible the use of small surface mount
inductors and ceramic capacitors, which reduce the total
footprint of the design. Figure 3 shows how small the
triple-output TFT circuit in Figure 1 can be.
, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
D1
BAT54S
D2
BAT54S
C6
1μF
10V
VIN
3.3V TO 5V
VIN
C5
1μF
10V
C4
1μF
10V
L1
2.2μH
SW
D3
DFLS220L
SHDN
LT3489
SS
COMP
VC
C1
4.7μF
6.3V
C2
100nF
10V
R2
47k
C3
1nF
C7
1μF x2
25V
C9
10μF x2
10V
R3
28.7k
1%
C14
1μF
10V
L1: COOPER ELECTRONICS; SD25-2R2
R4
5.23k
1%
D5
BAT54S
C12
1μF x2
10V
DN417 F01
Figure 1. A 3.3V Input, Triple-Output —8V, 23V and –8V—2MHz, TFT Converter
05/07/417
AVDD
8V
600mA
VOFF
–8V
20mA
FB
GND
VON
23V
10mA
Conclusion
For large TFT-LCD panel displays, local bias supplies,
DSL modems or portable devices, the LT3489 delivers
big power from a small 3mm × 3mm MS8E package.
C1
1μF
L1
2μH
VIN
3V TO 10V
VIN
Its rugged 2.5A, 0.12Ω, 40V internal switch, soft-start
feature, fixed frequency and flexible compensation simplify the design and improve the performance of many
applications.
D1
DFLS220L
L2
2μH
SW
SHDN
SS
LT3489
COMP
C4
100nF
10V
C2
10μF x2
10V
8V
400mA
FB
VC
GND
C3
4.7μF
10V
R1
28.7k
1%
R2
5.23k
1%
R3
47k
C5
1nF
L1, L2: SUMIDA CDRH4D28-2R0
DN417 F02
Figure 2. A 2MHz, 3V to 10V Input to 8V at 400mA to 900mA with Output Disconnect Converter
Figure 3. LT3489 Demo Circuit with Layout for the Figure 1 Schematic
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2759
www.linear.com
Linear Technology Corporation
dn417f LT/TP 0507 305K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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