Ultra low power on-chip hybrid start-up for wireless sensor networks

This paper presents ultra low power hybrid energy harvesting start-up circuit for 1V , implemented in a standard 0.18 μm CMOS technology. It consists of solar and RF energy harvesters, capable to harvest ambient solar and RF sources respectively. The start-up circuit enables operation of the system even in the absence of any one of these ambient energy sources. The integration of solar and RF on a common platform provides improved charging time in crucial start-up phase. In general, to charge the super capacitor (Cstartup=100pF) to 1V , the solar energy harvester alone (with minimum input voltage of 317mV ) will take a charging time of 542.3μs. With the addition of RF interface, the charging time is drastically reduced to 86.8μs, for -26dBm input power level, at 953MHz providing a 83.9% reduction in start-up time. The start-up circuit delivers output power of 7.4μW with an efficiency of 68.5% for the combined solar and RF input at 317mV and -20dBm respectively.