Seokhyeon Jeong

A Fully-Integrated 71 nW CMOS Temperature Sensor for Low Power Wireless Sensor Nodes

We propose a fully-integrated temperature sensor for battery-operated, ultra-low power microsystems. Sensor operation is based on temperature independent/dependent current sources that are used with oscillators and counters to generate a digital temperature code. A conventional approach to generate these currents is to drop a temperature sensitive voltage across a resistor. Since a large resistance is required to achieve nWs of power consumption with typical voltage levels (100 s of mV to 1 V), we introduce a new sensing element that outputs only 75 mV to save both power and area. The sensor is implemented in 0.18 μm CMOS and occupies 0.09 mm 2 while consuming 71 nW. After 2-point calibration, an inaccuracy of + 1.5°C/-1.4°C is achieved across 0 °C to 100 °C. With a conversion time of 30 ms, 0.3 °C (rms) resolution is achieved. The sensor does not require any external references and consumes 2.2 nJ per conversion. The sensor is integrated into a wireless sensor node to demonstrate its operation at a system level.

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

IoT design space challenges: Circuits and systems

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.

27.6 A 0.7pF-to-10nF fully digital capacitance-to-digital converter using iterative delay-chain discharge

Capacitance sensors are widely used to measure various physical quantities, including position, pressure, and concentration of certain chemicals [1-6]. Integrating capacitive sensors into a small wireless sensor system is challenging due to their large power consumption relative to the total system power/energy budget, which can be as low as a few nW [4]. Typical capacitance-to-digital converters (CDCs) use charge sharing or charge transfer between capacitors to convert the sampled capacitance to voltage, which is then measured with an ADC [1-6]. This approach requires complex analog circuits, such as amplifiers and ADCs, increasing design complexity and often increasing power consumption. Moreover, the initial capacitance to voltage conversion essentially limits the input capacitance range because of output voltage saturation. This paper presents a fully digital CDC that is based on the observation that when a ring oscillator (RO) is powered from a charged capacitance, the number of RO cycles required to discharge the capacitance to a fixed voltage is naturally linear with the capacitance value. This observation enables a simple, fully digital conversion scheme that is inherently linear. As a result, the proposed CDC performs conversion across a very wide capacitance range from 0.7pF to over 10nF with <; 0.06% linearity error. The CDC senses 11.3pF input capacitance with 35.1 pJ conversion energy and 141fJ/c-s FoM.

A 5.8 nW CMOS Wake-Up Timer for Ultra-Low-Power Wireless Applications

This work presents an ultra-low-power oscillator designed for wake-up timers in compact wireless sensors. In a conventional relaxation oscillator, a capacitor periodically resets to a fixed voltage using a continuous comparator, thereby generating an output clock. The reset is triggered by a continuous comparator and thus the clock period is dependent on the delay of the continuous comparator which therefore needs to be fast compared to the period, making this approach power hungry. To avoid the power penalty of a fast continuous comparator, a constant charge subtraction scheme is proposed in this paper. As a constant amount of charge is subtracted for each cycle, rather than discharging/charging the capacitor to a fixed voltage, the clock period becomes independent of comparator delay. Therefore, the high power continuous comparator can be replaced with a coarse clocked comparator, facilitating low-power time tracking. For precise wake-up signal generation, an accurate continuous comparator is only enabled for one clock period at the end of the specified wakeup time. A wake-up timer using the proposed scheme is fabricated in a 0.18 μm CMOS process. The timer consumes 5.8 nW at room temperature with temperature stability of 45 ppm/°C (-10 °C to 90 °C) and line sensitivity of 1%/V (1.2 V to 2.2 V) .

21.5 A current-mode wireless power receiver with optimal resonant cycle tracking for implantable systems

Continuous health monitoring has become feasible, largely due to miniature implantable sensor systems such as [1]. To recharge batteries of such systems, wireless power transfer is a popular option since it is non-invasive. However, there are two main challenges: 1) strict safety regulations of incident power on human tissue; 2) small coil size for better biocompatibility. These issues reduce the received power at the coil, make it difficult to obtain sufficient power for implanted devices, and call for high power-efficiency (ηP)-transfer techniques, especially at very low received power levels.

System-On-Mud: Ultra-Low Power Oceanic Sensing Platform Powered by Small-Scale Benthic Microbial Fuel Cells

A self-sustainable sensing platform powered entirely by small-scale benthic microbial fuel cells (MFCs) for oceanic sensing applications is presented. An ultra-low power chip featuring an ARM Cortex-M0 processor, 3 kB of SRAM, and power management unit (PMU) is designed to consume 11 nW in sleep mode for perpetual sensing operation. The PMU includes a switched-capacitor DC/DC converter designed for efficient energy harvesting and step-down conversion for a wide range of input and output power. A small-scale MFC with 21.3 cm2 anode surface area was connected to the PMU to charge a thin-film battery of 1 mAh capacity. A 49.3-hour long-term experiment with 8-min sleep interval and 1-s wake-up time demonstrated the sustainability of system-on-mud concept. During sleep mode operation, the system charges the 4 V battery at 380 nA from the micro-MFC generating 5.4 μW of power, which allows up to 20 mA of active mode current with net energy neutrality.

5.8 A 4.7nW 13.8ppm/°C self-biased wakeup timer using a switched-resistor scheme

Miniaturized computing platforms typically operate under restricted battery capacity due to their size [1]. Due to low duty cycles in many sensing applications, sleep-mode power can dominate the total energy budget. Wakeup timers are a key always-on component in such sleep modes and must therefore be designed with aggressive power consumption targets (e.g., <;10nW). Also, accurate timing generation is critical for peer-to-peer communication between sensor platforms [1]. Although a 32kHz crystal oscillator can provide low power [2] and accurate long-term stability, the requirement of an off-chip component complicates system integration for small wireless sensor nodes (WSNs).

A 120nW 8b sub-ranging SAR ADC with signal-dependent charge recycling for biomedical applications

We present an 8-bit sub-ranging SAR ADC designed for bursty signals having long time periods with small code spread. A modified capacitive-DAC (CDAC) saves previous samples MSB voltage and reuses it throughout subsequent conversions. This prevents unnecessary switching of large MSB capacitors as well as conversion cycles, reducing energy consumed in the comparator and digital logic and yielding total energy savings of 2.6×. In 0.18μm CMOS, the ADC consumes 120nW at 0.6V and 100kS/s with 46.9dB SNDR.

8.5 A 60%-efficiency 20nW-500µW tri-output fully integrated power management unit with environmental adaptation and load-proportional biasing for IoT systems

As Internet-of-Things (IoT) systems proliferate, there is a greater demand for small and efficient power management units. Fully integrated switched-capacitor (SC) DC-DC converters are promising candidates due to their small form factor and low quiescent power, aided by dynamic activity scaling [1-3]. However, they offer a limited number of conversion ratios, making them challenging to use in actual systems since they often require multiple output voltages (to reduce power consumption) and use various input power sources (to maximize flexibility). In addition, maintaining both high efficiency and fast load response is difficult at low output current levels, which is critical for IoT devices as they often target low standby power to preserve battery charge. This paper presents a fully integrated power management system that converts an input voltage within a 0.9-to-4V range to 3 fixed output voltages: 0.6V, 1.2V, and 3.3V. A 7-stage binary-reconfigurable DC-DC converter [1-2] enables the wide input voltage range. Three-way dynamic frequency control maintains converter operation at near-optimum conversion efficiency under widely varying load conditions from 5nW to 500μW. A proposed load-proportional bias scheme helps maintain high efficiency at low output power, fast response time at high output power and retains stability across the entire operating range. Analog drop detectors improve load response time even at low output power, allowing the converter to avoid the need for external sleep/wakeup control signals. Within a range of 1-to-4V input voltage and 20nW-500μW output power, the converter shows >60% conversion efficiency, while maintaining responsiveness to a 100× sudden current increase.