Sub-microAmp Energy Harvesting and Power Management Units for Self-Powered IoT SoCs: Analog vs. Digital Implementations

The power consumption of ultra-low-power (ULP) Internet-of-Things (IoT) SoCs and components has been scaling down from µW to pW levels over the past ten years. Designing energy harvesting and power management units (EH-PMUs) that consume sub-µA quiescent current to efficiently provide such low load current is challenging. This paper reviews the trends and techniques for sub-µA EH-PMUs with a specific focus on the choice between analog and digital implementations. We first discuss ULP EH-PMU design trends based on recent published results and then analyze three design examples. The first example reviews a popular multiple-input multiple-output (MIMO) EH-PMU architecture with ultra-low quiescent current and compares tradeoffs for analog vs. digital zero-current detectors. The second example discusses the design of analog and digital low-dropout regulators (LDOs) with a performance comparison from silicon measurement results. The digital LDO can achieve faster settling time for step response than the analog structure, but the analog LDO has no ripple, making it ideal for noise-sensitive blocks like RF. Finally, an analog power monitor for maximum-power-point tracking (MPPT) in a piezoelectric energy harvester utilizes subthreshold transistor characteristics to simply a complex algorithm and to maintain low power consumption.