Power Management Circuits for Miniature Sensor Systems.

Abstract

Miniature sensor nodes have recently opened new markets in VLSI design. In the continuation of Bell's Law, computing systems have been shrunk down to cubic-millimeter scale (e.g. Michigan micro-mote). Especially, since the concept of "Smart Dust" was proposed, rapid advances in low-power wireless sensor nodes are driving the realization of Internet of Things. With unique features such as wireless communication, energy harvesting, hard-to-reach location, and no-invasive and secure placement, the sensor nodes have been developed for a number of applications such as medical, infrastructure, and surveillance.

There are many challenges to realize a miniature sensing system. One of the most critical challenges is the small battery storage capacity. Since the battery size is severely limited, battery storage capacity is also reduced. As a result, low-power circuit design and energy harvesting techniques need to be investigated to allow operation on extremely small power budgets. In miniature sensing systems, there are four different energy flows: 1) energy harvester from a source to a battery, 2) power delivery circuits from a battery to loading circuits, 3) always-on circuits, 4) duty-cycled circuits. Available energy from an energy source changes depending on the environmental condition. Load current also changes considerably due to duty-cycle operation. In this dynamic system, a smart way to optimize an energy harvester and a power converter are required. To minimize average power consumption for longer lifetime, always-on circuits should be designed with lower power than the extremely low system standby power budget. Also, duty-cycled circuits need to be turned off to maintain low standby power.

In this dissertation, to satisfy these requirements, circuits such as a constant energy-per-cycle ring oscillator over wide frequency range and a low power maximum power point tracking circuit are discussed. Also, a low power battery supervisory circuit and a battery health monitoring circuit are covered.