Design Idea:
Triple Output TFT-LCD Bias Supply
Uses All Ceramic Capacitors

by Gary Shockey

Current power supply requirements for TFT-LCD panels call for an 8V or 10V main supply plus two or more auxiliary outputs. The overall layout must be small and meet tight height requirements of under 2mm. Bulky inductors and capacitors must be eliminated if the design is to meet space requirements. The circuit described in this design idea features the new LT1949 and delivers 8V at 200mA from 3.3V while generating auxiliary 24V and –8V.

The LT1949 is a boost switching regulator that comes in the MSOP-8 package and has an integrated 1.1A NPN power transistor. The LT1949's switching frequency is fixed at 600kHz, allowing compact inductors to be used. Its external compensation pin allows for loop characteristics to be tuned so that tiny ceramic output capacitors can be used. These charateristics result in a compact, low profile circuit.

Figure 1. 3.3V to 8V/200mA DC/DC converter with auxiliary 24V and –8V outputs

To understand circuit operation, refer to Figure 1. The LT1949 generates the 8V output in the normal boost mode configuration, while using charge pumps for the 24V and –8V outputs. During boost operation, the SW pin is switching between V_OUT and ground. When at V_OUT, capacitor C6 is charged to V_OUT through D5. When the SW pin flies to ground, C6 holds its charge, causing D6 to be forward biased, charging C5 to –8V. The positive 24V output is developed in a similar fashion except that V_OUT is tripled. Figure 2 details the transient response of V_OUT to an 80mA to 200mA load step.