Simplest is a charge pump converter, RS 232 serial chips use them to supply - volts, they have a relatively high output impedance. Microchip TC7662 for example. See http://en.wikipedia.org/wiki/Charge_pump If you need better regulation, then an inverting switch mode power supply. Solutions from most companies that make SMPS chips, and some may have a demo board that you can purchase. Cheers, Richard
