Design of Switched-Capacitor DC-DC Voltage-Down Converters Driven by Highly Resistive Energy Transducer

Abstract

Electrostatic vibration energy transducers have a relatively high output impedance (RET) and open-circuit voltage (VIN), so that voltage-down conversion is required for sensor/RF ICs. Switched-capacitor converters are the best candidate to create small-form-factor technology and are a low-cost solution because of their capability to fully integrate into sensor/RF ICs. To design switched-capacitor voltage-down converters (SC-VDCs) with a minimum circuit area for electrostatic vibration energy transducers, two steps are required. The first step requires an optimum design of DC-DC SC-VDCs driven by high RET with a minimum circuit area, and the second step requires an optimum design of AC-DC SC-VDCs based on the first step, to minimize the converter circuit area. This paper discusses circuit analysis and design optimization aimed at the first step. Switching frequency, the number of stages and the capacitance per stage were determined as a function of RET, VIN and the output voltage (Vo) and current (Io) to the load, to achieve a minimum circuit area. The relationship between Io and the power conversion efficiency was studied as well. The performance was validated by SPICE simulation in 250 nm BCD technology. An optimum design flow was proposed to design DC-DC SC-VDCs driven by high RET with a minimum circuit area under conditions where RET, VIN, Vo and Io were given. The second design step remains as future work.