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 ●