Driving Large TFT-LCD Displays with Space-Saving Triple-Output Regulator

The triple output power supply in Figure 1 shows a compact LT3489-based design 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, from a 3.3V input. Although TFT converters typically operate from a regulated 3.3V or 5V supply, this converter can also seamlessly switch to a Li-Ion battery during operation, delivering 5W when the battery voltage drops to 3.3V and 8W when the battery voltage is at 4.2V.

The LT3489 integrates a 2.5A, 0.12Ω, 40V switch into a 3mm x 3mm MS8E package. It offers external or internal compensation, internal soft-start, 2MHz switching frequency, and is pin compatible with the popular LT1946. The design process using the LT3489 is easy and predictable.

The circuit in Figure 2 functions as a SEPIC converter, allowing the output voltage to be higher or lower than the input voltage. When the SHDN pin is pulled to ground, the output is set to 0V, even when a voltage supply is connected to the input.

Both applications take advantage of the soft-start feature, which uses a single capacitor to set the output voltage ramp rate at startup. The 2MHz switching frequency enables the use of very small surface mount inductors and ceramic capacitors, reducing the overall footprint of the design. Figure 3 shows how small this triple output TFT circuit can be.

For large TFT-LCD panel displays, local bias supplies, DSL modems or portable devices, the LT3489 delivers high power from a small 3mm x 3mm MS8E package. Its rugged 2.5A, 0.12Ω, 40V internal switch, soft-start function, fixed frequency and flexible compensation simplify design and improve performance in many applications.