LINEAR TECHNOLOGY LT3489 Technical data

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 T FT panel displays. The main ou tput provides 8V at 600 mA while the 23V and –8V ou tputs 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
1μF 10V
SW
FB
GND
V
3.3V TO 5V
4.7μF
6.3V
L1
IN
C1
C2
100nF
10V
2.2μH
V
IN
SHDN
LT3489
SS
COMP
V
C
R2 47k
C3 1nF
L1: COOPER ELECTRONICS; SD25-2R2
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 p in t o g r o u n d s e t s t h e o u t p u t a t 0 V e v e n
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.
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D1
BAT54S
C4
D3
DFLS220L
C14 1μF 10V
C5 1μF 10V
R3
28.7k 1%
R4
5.23k 1%
D2
BAT54S
C6 1μF 10V
D5 BAT54S
DN417 F01
C7 1μF x2 25V
C9 10μF x2 10V
C12 1μF x2 10V
V
ON
23V 10mA
AV
DD
8V 600mA
V
OFF
–8V 20mA
05/07/417
Figure 1. A 3.3V Input, Triple-Output8V, 23V and –8V2MHz, TFT Converter
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.
Its rugged 2.5A, 0.12Ω, 40V internal switch, soft-start feature, fi xed frequency and fl exible compensation sim­plify the design and improve the performance of many applications.
V
3V TO 10V
4.7μF
IN
10V
SW
C1
1μF
FB
V
C
R3 47k
C5 1nF
L1
2μH
V
IN
SHDN
LT3489
SS
COMP
GND
C3
C4
100nF
10V
D1
DFLS220L
L2 2μH
DN417 F02
8V
R1
28.7k 1%
R2
5.23k 1%
L1, L2: SUMIDA CDRH4D28-2R0
C2 10μF x2 10V
400mA
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
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