An ultra-low power voltage regulator system for wireless sensor networks powered by energy harvesting
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A DC-DC converter is an important power management module as it converts one DC voltage level to another suitable for powering a desired electronic system. It also stabilizes the output voltage when fluctuations appear in the power supplies. For those wireless sensor networks (WSNs) powered by energy harvesting, the DC-DC converter is usually a linear regulator and it resides at the last stage of the whole energy harvesting system just before the empowering sensor node. Due to the low power densities of energy sources, one may have to limit the quiescent current of the linear regulator in the sub-uA regime. This severe restriction on quiescent current could greatly compromise other performance aspects, especially the transient response. This dissertation reports a voltage regulator system topology which utilizes the sensor node state information to achieve ultra-low power consumption. The regulator system is composed of two regulators with different current driving abilities and quiescent current consumptions. The key idea is to switch between the two regulators depending on the sensor state. Since the "right" regulator is used at the "right" time, the average quiescent current of the regulator system is minimized, and the trade-off between low quiescent current and fast transient response has been eliminated. In order to minimize the average quiescent current of the system, nano-ampere reference current design is studied, and the proposed reference current circuit is shown (theoretically and experimentally) to reduce the supply voltage dependence by 5X. The regulator system has been fabricated and tested using an ON Semiconductor 0.5 μm process. It has been verified through experiments that the proposed system reduces the quiescent current by 3X over the state-of-the-art in the literature; and, more importantly, it achieves low quiescent current, low dropout voltage, and fast transient response with small output voltage variation all at the same time. The thesis further presents data on the application of energy harvesting system deriving energies from various RF signals to power a commercial off-shelf wireless sensor node.