Current power-supply requirements for TFT-LCD panels call for an 8-V or 10-V main supply plus two or more auxiliary outputs. The overall layout must be small and meet tight height requirements (under 2 mm). Bulky inductors and capacitors must be eliminated if the design is to meet these space requirements. The circuit described here delivers 8 V at 200 mA from a 3.3-V input while generating auxiliary 24-V and −8-V outputs.
The circuit operates as follows: U1, an LT1949 boost-switching regulator, generates the 8-V output in the normal boost-mode configuration, while using charge pumps for the 24-V and −8-V outputs (Fig. 1). U1 operates at a fixed switching frequency of 600 kHz, allowing the use of compact inductors. Its external compensation pin lets loop characteristics be tuned in order to use tiny ceramic output capacitors. These features result in a compact, low-profile circuit.
During boost operation, the SW pin is switching between VOUT and ground. When at VOUT, capacitor C6 is charged to VOUT through diode D5. When the SW pin flies to ground, C6 holds its charge, causing D6 to be forward-biased, charging C5 to −8 V. The positive 24-V output is developed in a similar fashion, with the exception that VOUT is tripled.
Figure 2 details the transient response of VOUT to an 80-mA to 200-mA load step.