A voltage converter for use within small implantable electrical devices, such as a microstimulator, uses a coil, instead of capacitors, to provide a voltage step up and step down function. The output voltage is controlled, or adjusted, through duty-cycle modulation. Good efficiencies are achieved for virtually any voltage within the compliance range of the converter. In accordance with one aspect of the invention, applicable to implantable devices having an existing RF coil through which primary or charging power is provided, the existing RF coil is used in a time-multiplexing scheme to provide both the receipt of the RF signal and the voltage conversion function. This minimizes the number of components needed within the device, and thus allows the device to be packaged in a smaller housing, or frees up additional space within an existing housing for other circuit components. In accordance with another aspect of the invention, the voltage up/down converter circuit is controlled by a pulse width modulation (PWM) low power control circuit. Such operation allows high efficiencies over a wide range of output voltages and current loads. The invention thus provides a voltage converter circuit for use within an implantable device that is compact, efficient, and provides a wide range of output voltages and currents.