An AC-DC step-up converter circuit architecture for generating multiple output voltages, both positive and negative, in an implantable biomedical device is disclosed. Switches and active rectifiers are used inside the converter for charging capacitors from the AC source and delivering currents to the loads. Regulated output voltages with high power efficiency are obtained by controlling the on/off times of the switches using feedback loops that include integrator circuits configured to provide control parameters related to the various output voltages and their associated predetermined reference voltages.