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Energy harvesting has emerged as a feasible and attractive option to improve battery lifetime in micro-scale electronic systems such as biomedical implants and wireless sensor nodes. A key challenge in designing micro-scale energy harvesting systems is that miniature energy transducers (e.g., photovoltaic cells, thermo-electric generators, and fuel cells) output very low voltages (0-0.4V). Therefore, a fully on-chip power converter (usually based on a charge pump) is used to boost the output voltage of the energy transducer and transfer charge into an energy buffer for storage. However, the charge transfer capability of widely used linear charge pump based power converters degrades when used with ultra-low voltage energy transducers. This paper presents the design of a new tree topology charge pump that has a reduced charge sharing time, leading to an improved charge transfer capability. The proposed design has been implemented using 65nm technology and circuit simulations demonstrate that the proposed design results in an increase of up to 30% in harvested power compared to existing linear charge pumps.