Suyoung Bang

A Modular 1 mm

A 1.0 mm3 general-purpose sensor node platform with heterogeneous multi-layer structure is proposed. The sensor platform benefits from modularity by allowing the addition/removal of IC layers. A new low power I2C interface is introduced for energy efficient inter-layer communication with compatibility to commercial I2C protocols. A self-adapting power management unit is proposed for efficient battery voltage down conversion for wide range of battery voltages and load current. The power management unit also adapts itself by monitoring energy harvesting conditions and harvesting sources and is capable of harvesting from solar, thermal and microbial fuel cells. An optical wakeup receiver is proposed for sensor node programming and synchronization with 228 pW standby power. The system also includes two processors, timer, temperature sensor, and low-power imager. Standby power of the system is 11 nW.

^{3}

Wireless sensor nodes have many compelling applications such as smart buildings, medical implants, and surveillance systems. However, existing devices are bulky, measuring >;1cm3, and they are hampered by short lifetimes and fail to realize the “smart dust” vision of [1]. Smart dust requires a mm3-scale, wireless sensor node with perpetual energy harvesting. Recently two application-specific implantable microsystems [2][3] demonstrated the potential of a mm3-scale system in medical applications. However, [3] is not programmable and [2] lacks a method for re-programming or re-synchronizing once encapsulated. Other practical issues remain unaddressed, such as a means to protect the battery during the time period between system assembly and deployment and the need for flexible design to enable use in multiple application domains.

Die-Stacked Sensing Platform With Low Power I

This paper presents a fully integrated energy harvester that maintains >35% end-to-end efficiency when harvesting from a 0.84 mm 2 solar cell in low light condition of 260 lux, converting 7 nW input power from 250 mV to 4 V. Newly proposed self-oscillating switched-capacitor (SC) DC-DC voltage doublers are cascaded to form a complete harvester, with configurable overall conversion ratio from 9× to 23×. In each voltage doubler, the oscillator is completely internalized within the SC network, eliminating clock generation and level shifting power overheads. A single doubler has >70% measured efficiency across 1 nA to 0.35 mA output current ( >10 5 range) with low idle power consumption of 170 pW. In the harvester, each doubler has independent frequency modulation to maintain its optimum conversion efficiency, enabling optimization of harvester overall conversion efficiency. A leakage-based delay element provides energy-efficient frequency control over a wide range, enabling low idle power consumption and a wide load range with optimum conversion efficiency. The harvester delivers 5 nW-5 μW output power with >40% efficiency and has an idle power consumption 3 nW, in test chip fabricated in 0.18 μm CMOS technology.

^{2}

Ultra-low power microsystems are gaining more popularity due to their applicability in critical areas of societal need. Power management in these microsystems is a major challenge as a relatively high battery voltage (e.g., 4V) must be down-converted to several low supplies, such as 0.6V for near-threshold digital circuits and 1.2V for analog circuits [1]. Furthermore, the small form factors of such systems rule out the use of external inductors, making switched-capacitor (SC) DC-DC converters the favored topology [2-4].

C Inter-Die Communication and Multi-Modal Energy Harvesting

Recent advances in low-power circuits have enabled mm-scale wireless systems [1] for wireless sensor networks and implantable devices, among other applications. Energy harvesting is an attractive way to power such systems due to limited energy capacity of batteries at these form factors. However, the same size limitation restricts the amount of harvested power, which can be as low as 10s of nW for mm-scale photovoltaic cells in indoor conditions. Efficient DC-DC up-conversion at such low power levels (for battery charging) is extremely challenging and has not yet been demonstrated.

A modular 1mm 3 die-stacked sensing platform with optical communication and multi-modal energy harvesting

We present a 2×4×4mm3 imaging system complete with optics, wireless communication, battery, power management, solar harvesting, processor and memory. The system features a 160×160 resolution CMOS image sensor with 304nW continuous in-pixel motion detection mode. System components are fabricated in five different IC layers and die-stacked for minimal form factor. Photovoltaic (PV) cells face the opposite direction of the imager for optimal illumination and generate 456nW at 10klux to enable energy autonomous system operation.

An ultra-low power fully integrated energy harvester based on self-oscillating switched-capacitor voltage doubler

The Internet of Things (IoT) is a rapidly emerging application space, poised to become the largest electronics market for the semiconductor industry. IoT devices are focused on sensing and actuating of our physical environment and have a nearly unlimited breadth of uses. In this paper, we explore the IoT application space and then identify two common challenges that exist across this space: ultra-low power operation and system design using modular, composable components. We survey recent low power techniques and discuss a low power bus that enables modular design. Finally, we conclude with three example ultra-low power, millimeter-scale IoT systems.

A fully integrated successive-approximation switched-capacitor DC-DC converter with 31mV output voltage resolution

We present a self-adapting power management unit (PMU) for ultra-low power wireless sensor nodes. The PMU uses 1.03nF of on-chip MIM capacitance in a reconfigurable switched-capacitor network (SCN) that automatically adapts to different battery voltages for down-conversion and different harvesting sources/harvesting conditions for up-conversion. The PMU achieves 63.8% / 60.7% down-conversion efficiency at 17.9μW active mode / 12.8nW sleep mode power loading. With the adaptive down-conversion ratio, load power range is improved by 3.76× and 5.48× in sleep and active mode, respectively. We show how the proposed adaptation method enables harvesting with solar, microbial fuel cell, and thermal energy sources, increases harvesting efficiency by 1.92× and achieves the peak extraction efficiency of 99.8% for solar cell.

23.3 A 3nW fully integrated energy harvester based on self-oscillating switched-capacitor DC-DC converter

We propose an ultra-low power optical wake-up receiver with a novel front-end circuit and communication scheme suitable for miniature wireless sensor node applications. Named “FLOW” for Free-space Low-Power Optical Wake-up, the receiver consumes 695pW in standby mode, which is ~6,000× lower than previously reported RF and ultrasound wake-up radios. In active mode, it consumes 140pJ/bit at 91bps. A pulse width modulated communication encoding scheme is used, and chip-ID masking enables selective batch-programming and synchronization of multiple sensor nodes.

A millimeter-scale wireless imaging system with continuous motion detection and energy harvesting

This work presents a sub-μW on-chip oscillator for fully integrated system-on-chip designs. The proposed oscillator introduces a resistive frequency locked loop topology for accurate clock generation. In this topology, a switched-capacitor circuit is controlled by an internal voltage-controlled oscillator (VCO), and the equivalent resistance of this switched-capacitor is matched to a temperature-compensated on-chip resistor using an ultra-low power amplifier. This design yields a temperature-compensated frequency from the internal VCO. The approach eliminates the traditional comparator from the oscillation loop; this comparator typically consumes a significant portion of the total oscillator power and limits temperature stability in conventional RC relaxation oscillators due to its temperature-dependent delay. A test chip is fabricated in 0.18 μm CMOS that exhibits a temperature coefficient of 34.3 ppm/°C with long-term stability of less than 7 ppm (12 second integration time) while consuming 110 nW at 70.4 kHz. A radio transmitter circuit that uses the proposed oscillator as a baseband timing source is also presented to demonstrate a system-on-chip design using this oscillator.