A Regulated 400-mV CMOS DC-DC Converter with On-the-Fly Equivalent Output Resistance Tuning

Energy harvesting is a technology that can be applied to IoT systems to eliminate the need for batteries. Many types of energy sources are available for energy harvesting, such as light, thermal, vibration, and electromagnetic energy. Indoors, where most IoT devices are located, artificial light, such as from LED lamps, can be used for energy harvesting in circuits with very ultra-low power consumption. Integrated switch-capacitor DC-DC converters are required for this type of system to convert the harvested energy into a constant output voltage suitable for powering an electronic circuit. The idea of this work is to use a hysteretic feedback control consisting of comparators and a logic system to adjust the switching frequency and the voltage conversion ratio (VCR) of the converter. With this, the equivalent output resistance is tuned to a value that results in a constant output voltage. A new method for modeling the equivalent output resistance based on charge flow analysis is proposed, which also considers the effects of source resistance. An integrated energy-harvesting system consisting of a switched-capacitor DC-DC converter is implemented to obtain an output voltage of 400 mV using a small photovoltaic cell for energy harvesting from indoor light. The proposed system can power an ultra-low-power device between 20 μ W and 40 μ W with a minimum input voltage of 230 mV. Electrical simulation results show that the implemented converter can achieve a peak efficiency of 81.24% at an input voltage of 260 mV for a 20 μ W load.