Analysis and implementation of a power management unit with a multiratio switched capacitor DC–DC converter for a supercapacitor power supply

An energy-harvesting system requires an energy-storing device to store the energy retrieved from the surrounding environment. Rechargeable batteries are commonly used to store this energy; however, because of the limited number of charge/discharge cycles, they need to be periodically replaced. A supercapacitor, which has, ideally, a limitless number of charge/discharge cycles, avoids this problem. In this case, it is required for the power management unit to produce a constant output voltage as the supercapacitor discharges. This paper presents a system with a multiratio switched capacitor DC-DC converter, in a 130-nm technology, with a maximum output power of 2mW, a maximum efficiency of 79.63% and a maximum output ripple, in the steady state, of 23mV for an input voltage range of 2.3-0.87V. The proposed converter has four operation states, to maximize its efficiency, that correspond to the conversion ratios of 1/2, 2/3, 1/1 and 3/2. Its clock frequency is automatically adjusted to produce a stable output voltage of 1V. These features are implemented through two distinct controller circuits that use two asynchronous time machines to dynamically adjust the clock frequency and to select the active state of the converter.