Simple power supply fits into small spaces
Edited by Bill Travis
Raymond Zheng, Linear Technology, Milpitas, CA -- EDN, November 13, 2003
The demand for negative-voltage power supplies is increasing with the popularity of applications for portable devices. It can be expensive and relatively complicated to generate a negative supply from a positive input source, especially when the design requires both positive and negative outputs. Figure 1 shows a simple and cost-effective solution that combines a voltage inverter and a voltage doubler in a single charge-pump circuit. It produces a regulated –5V output and an unregulated 10V output from a 5 to 6V input. The circuit requires only five small, ceramic, surface-mount capacitors and two diodes in addition to the charge-pump IC in an SOT-23 package.
From a 6V input, the inductorless dc/dc inverter can deliver 100 mA at a regulated –5V (±5%), and the voltage doubler can provide 50 mA at 10.5V with ±7% variation. The inverter's output-voltage regulation follows the relationship (VIN–5)>(IOUT×ROUT); you can determine the values of ROUT and IOUT for VIN=5V from the graph in Figure 2. (The ROUT and IOUT values for other VIN values are available in the LTC1983's data sheet.) If the variables don't meet this inequality condition, the part runs in open-loop mode and acts as a low-output-impedance inverter in which the output voltage is VOUT1= –[VIN–(IOUT×ROUT)]. You can define the output voltage of the voltage doubler as VOUT2=2VIN–2VD, where VD is the forward voltage drop across the diodes. Figure 3 shows the efficiency of the circuit, which exceeds 81% and peaks at approximately 85%. Figure 4 shows the inverter's output-voltage regulation versus inverter output current. The IC includes short-circuit and thermal protection.
