This work proposes a new charge pump design suitable for energy harvesting on small human wearable devices. By using only one system for both cold start and high efficiency harvesting, the required silicon area is heavily reduced. The most fitting energy transducer is determined to be a photovoltaic cell, based on aspects of voltage, power and size. Both indoor and outdoor lighting conditions are suitable. The literature on fully integrated voltage boosting with special attention to cold start shows that the capacitive charge pump is the best solution for high efficiency, low power and small size voltage boosting. For cold start applications, a Dickson charge pump with dynamic charge transfer switches is the best solution. For achieving high efficiency conversion, theMakowski charge pump is the best.
The proposed design combines the advantages of the Dickson charge pump for cold start and the high efficiency aspects of aMakowski charge pump for normal operation in one system with minimal silicon area. Using capacitor splitting, the conversion ratio of the cold start charge pump can be orthogonally chosen from
the conversion ratio of normal operation. Dynamic charge transfer switches are used for the cold start charge pump to increase the performance. An implementation of the proposed design is made in Cadence Virtuoso and is tested for cold start capability and efficiency across five process corners and three temperatures. The results prove that the proposed design can achieve high efficiency energy harvesting that is on average 5% below the efficiency of a normal Makowski charge pump. Cold start was achieved from 100mV that is boosted to a battery voltage of 1.8 V within the micro-watt power range. With these results, the proposed charge pump is suitable for photovoltaic energy harvesting in small devices. The required silicon area is reduced by 50%, compared to systems where cold start and high efficiency harvesting are provided by separate systems.
The proposed design is made into a lay-out, which is currently being tested and will be manufactured in the coming months.