The USB port on most personal computers can be a handy source of power. It is not allowed to deliver the maximum specified current (500 mA) without software negotiation, but in practice the PC has no direct control over current drawn from the USB port. PCs often limit the USB current only through the use of passive automatically resettable fuses.
Fig. 1 shows a simple USB-powered ±12-V supply capable of meeting most demands. Such bipolar supplies — popular for use in audio, instrumentation and other industrial applications — should feature low noise, current limiting and enough output-current capability to drive several amplifiers drawing 5 mA to 10 mA each.
This circuit includes a constant-frequency, current-mode stepup converter (U1) with suitable current limiting. The stepup converter creates the +12-V output, and an external-component charge pump consisting of D1, D2, C1 and C2 generates the negative rail. The values of these diodes and capacitors are determined using basic charge-pump principles, which are outlined in numerous charge-pump datasheets.
The circuit can source or sink about 50 mA, and its current-mode topology (internal to U1) limits the switch current on a cycle-by-cycle basis. Note that U1's internal switch is not in series with the current path, so it cannot limit current when the output voltage is forced below the input supply voltage. Thus, a dead short is limited only by the USB current-limit fuse, which is usually rated much higher than 500 mA. Load regulation for the +12-V output is shown in the MAX1896 datasheet (available at www.MAXIM-IC.com). For the — 12-V output, load regulation is shown in Fig. 2.
U1's constant-frequency operation allows simple filtering and produces less noise than a frequency-modulation scheme. The high switching frequency is easily filtered with passive LC filters or with external transistor-buffered RC filters as shown. As always for switching converters, a good printed circuit board layout is essential for low-noise operation. Further advice on these matters is available in the MAX1896 datasheet.