Yao Shi

A 110 nW Resistive Frequency Locked On-Chip Oscillator with 34.3 ppm/°C Temperature Stability for System-on-Chip Designs

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.

Energy-Autonomous Wireless Communication for Millimeter-Scale Internet-of-Things Sensor Nodes

This paper presents an energy-autonomous wireless communication system for ultra-small Internet-of-Things (IoT) platforms. In the proposed system, all necessary components, including the battery, energy-harvesting solar cells, and the RF antenna, are fully integrated within a millimeter-scale form factor. Designing an energy-optimized wireless communication system for such a miniaturized platform is challenging because of unique system constraints imposed by the ultra-small system dimension. The proposed system targets orders of magnitude improvement in wireless communication energy efficiency through a comprehensive system-level analysis that jointly optimizes various system parameters, such as node dimension, modulation scheme, synchronization protocol, RF/analog/digital circuit specifications, carrier frequency, and a miniaturized 3-D antenna. We propose a new protocol and modulation schemes that are specifically designed for energy-scarce ultra-small IoT nodes. These new schemes exploit abundant signal processing resources on gateway devices to simplify design for energy-scarce ultra-small sensor nodes. The proposed dynamic link adaptation guarantees that the ultra-small IoT node always operates in the most energy efficient mode for a given operating scenario. The outcome is a truly energy-optimized wireless communication system to enable various classes of new applications, such as implanted smart-dust devices.

A 10 mm 3 Inductive Coupling Radio for Syringe-Implantable Smart Sensor Nodes

We present a near-field radio system for a millimeter-scale wireless smart sensor node that is implantable through a 14-gauge syringe needle. The proposed system integrates a radio system on chip and a magnetic antenna on a glass substrate within a total dimension of 1 × 1 × 10 mm3. We demonstrate energy-efficient active near-field wireless communication between the millimeter-scale sensor node and a base station device through an RF energy-absorbing tissue. The wireless transceiver, digital baseband controller, wakeup controller, on-chip baseband timer, sleep timer, and MBUS controller are all integrated on the SoC to form a millimeter-scale sensor node, together with a 1 × 8 mm2 magnetic antenna fabricated with a 1.5-μm-thickness gold on a 100 μm-thickness glass substrate. An asymmetric link is established pairing the sensor antenna with a codesigned 11 × 11 cm2 base station antenna to achieve a link distance of up to 50 cm for sensor transmission and 20 cm for sensor reception. The transmitter consumes a 43.5 μW average power at 2 kb/s, while the receiver power consumption is 36 μW with a -54 dBm sensitivity at 100 kb/s. When powered by a 1×2.2 mm2 thin-film battery (2 μAh, 4.1 V), the designed system has a two week expected lifetime without battery recharging when the system wakes up and transmits and receives 16 b data every 10 min.

7.4 A 915MHz asymmetric radio using Q-enhanced amplifier for a fully integrated 3×3×3mm 3 wireless sensor node with 20m non-line-of-sight communication

Enabling long range (>10m) wireless communication in non-line-of sight (NLOS) scenarios would dramatically expand the application space and usability of mm-scale wireless sensor nodes. The major technical challenges posed by a mm-scale form-factor are poor antenna efficiency and the small instantaneous current limit (∼10s of μA) of thin-film batteries. We address these challenges in several ways: 1) We found that a magnetic dipole antenna achieves better efficiency at an electrically-small size than an electric dipole, when the antennas are resonated with off-chip lumped components. In addition, the high impedance of electrically-small electric dipoles (∼4kΩ compared to 10Ω for the magnetic antenna) requires an impractically large off-chip inductor to resonate. 2) By simultaneously considering the magnetic dipole efficiency, frequency-dependent path-loss, and wall penetration loss, we found that a 915MHz carrier frequency is optimal for a 3×3×3mm3 sensor node in NLOS asymmetric communication with a gateway. This is despite the resulting low antenna efficiency (0.21%) which typically drives mm-scale radios to operate at ≫1GHz frequency [1]. 3) In transmit (TX) mode, instead of using a PA and PLL, we utilize a cross-coupled driver to resonate the magnetic antenna at 915MHz with a quality factor (Q) of 110 in order to reduce overall power consumption. 4) In receive (RX) mode, we propose an approach of reusing the cross-coupled driver in a non-oscillating mode to raise the Q of the resonant tank to 300, resulting in 49dB voltage gain at 43µW, thereby replacing a power-hungry LNA and bulky off-chip filter. 5) A sparse pulse-position modulation (PPM) combined with a sensor-initiation communication protocol [2] shifts the power-hungry calibration of frequency offset to the gateway, enabling crystal-free radio design. The complete radio, including the transceiver IC, a 3D antenna, off-chip capacitors, a processor, a power management unit (PMU) and memory, is integrated within a 3×3×3mm3 sensor node, demonstrating stand-alone bi-directional 20m NLOS wireless communication with variable data rates of 30b/s to 30.3kb/s for TX and 7.8kb/s to 62.5kb/s for RX. The transmitter generates −26.9 dBm equivalent isotropically radiated power (EIRP) consuming 2mW power and the receiver has a sensitivity of −93dBm consuming 1.85mW.

22.6 A fully integrated counter-flow energy reservoir for 70%-efficient peak-power delivery in ultra-low-power systems

Recent advances in circuits have enabled significant reduction in the size of wireless systems such as implantable biomedical devices. As a consequence, the battery integrated in these systems has also shrunk, resulting in high internal resistances (∼10kΩ). However, the peak-current requirement of power-hungry components such as radios remains in the mW range, and hence cannot be directly supplied from the battery. Therefore, duty-cycled architectures such as pulsed-based radios have been proposed that transmit a short burst (∼1µs) of high power (∼10mW) supplied by an internal energy storage capacitor [1–3]. The capacitor is then recharged using a current limiter to protect the battery from excessive droop. This paradigm raises two challenges: 1) to supply sufficient energy, very large capacitance (>50nF) is often needed (200mV droop, for 10mW and 5µs), leading to large die area or bulky off-chip discrete components; 2) only a small fraction (∼5%) of energy stored in the capacitor is actually delivered to the high power components since the capacitor can only be discharged by a few 100s of mV while maintaining proper circuit operation (Fig. 22.6.1).

A 66pW discontinuous switch-capacitor energy harvester for self-sustaining sensor applications

We present a discontinuous harvesting approach for switch capacitor DC-DC converters that enables ultra-low power energy harvesting. By slowly accumulating charge on an input capacitor and then transferring it to a battery in burst-mode, switching and leakage losses in the DC-DC converter can be optimally traded-off with the loss due to non-ideal MPPT operation. The harvester uses a 15pW mode controller, an automatic conversion ratio modulator, and a moving sum charge pump for low startup energy upon a mode switch. In 180nm CMOS, the harvester achieves >40% end-to-end efficiency from 113pW to 1.5μW with 66pW minimum input power, marking a >10× improvement over prior ultra-low power harvesters.

A 20-pW Discontinuous Switched-Capacitor Energy Harvester for Smart Sensor Applications

We present a discontinuous harvesting approach for switch capacitor dc–dc converters that enables ultralow-power energy harvesting. Smart sensor applications rely on ultralow-power energy harvesters to scavenge energy across a wide range of ambient power levels and charge the battery. Based on the key observation that energy source efficiency is higher than charge pump efficiency, we present a discontinuous harvesting technique that decouples the two efficiencies for a better tradeoff. By slowly accumulating charge on an input capacitor and then transferring it to a battery in burst mode, dc–dc converter switching and leakage losses can be optimally traded off with the loss incurred by nonideal maximum power point tracking operation. Harvester duty cycle is automatically modulated instead of charge pump operating frequency to match with the energy source input power level. The harvester uses a hybrid structure called a moving-sum charge pump for low startup energy upon a mode switch, an automatic conversion ratio modulator based on conduction loss optimization for fast conversion ratio increment, and a <15-pW asynchronous mode controller for ultralow-power operation. In 180-nm CMOS, the harvester achieves >40% end-to-end efficiency from 113 pW to 1.5

26.7 A 10mm3 syringe-implantable near-field radio system on glass substrate

\mu \text{W}

Millimeter-scale computing platform for next generation of Internet of Things

with 20-pW minimum harvestable input power.