1. A charge pump circuit, comprising: an input node for connection to an input voltage; flying capacitor nodes for connecting to flying capacitors; a first pair of output nodes and a second pair of output nodes; a switch network for interconnecting said nodes; and a controller operable to control the switch network to provide first bipolar output voltages at the first pair of output nodes and second bipolar output voltages at the second pair of output nodes; wherein the switch network comprises at least a first NMOS switch formed on a first region of a semiconductor substrate wherein the first region of the semiconductor substrate is isolated from the rest of the substrate by one or more n-well regions.