Switched Capacitor DC-DC Converter for Miniaturised Wearable Systems

Motivated by the demands of the integrated power system in the modern wearable electronics, this paper presents a new method of inductor-less switched-capacitor (SC) based DC-DC converter designed to produce two simultaneous boost and buck outputs by using a 4-phases logic switch mode regulation. While the existing SC converters missing their reconfigurability during needed spontaneous multi-outputs at the load ends, this work overcomes this limitation by being able to reconfigure higher gain mode at dual outputs. From an input voltage of 2.5 V, the proposed converter achieves step-up and step-down voltage conversions of 3.74 V and 1.233 V for Normal mode, and 4.872 V and 2.48 V for High mode, with the ripple variation of 20–60 mV. The proposed converter has been designed in a standard 0.35 μm CMOS technology and with conversion efficiencies up to 97–98% is in agreement with state-of-the-art SC converter designs. It produces the maximum load currents of 0.21 mA and 0.37 mA for Normal and High modes respectively. Due to the flexible gain accessibility and fast response time with only two clock cycles required for steady state outputs, this converter can be applicable for multi-function wearable devices, comprised of various integrated electronic modules

Power management using photovoltaic cells for implantable devices

This paper presents a novel inductor-less switched capacitor (SC) DC-DC converter, which generates simultaneous dual-output voltages for implantable electronic devices. Present dual output converters are limited to fixed ratio gain, which degrade conversion efficiency when the input voltage changes. The proposed power converter offers both step-up and step-down conversion with 4-phase reconfigurable logic. With an input voltage of 1 V provided by photovoltaic (PV) cells, the proposed converter achieves step-up, step-down and synchronised voltage conversions in four gain modes. These are 1.5 V and 0.5 V for Normal mode, 2 V and 1 V for High mode, 2 V for Double Boost mode, as well as 3 V and 2 V for Super Boost mode with the ripple variation of 14-59 mV. The converter circuit has been simulated in standard 0.18 μm CMOS technology and the results agree with state-of-the-art SC converters. However, our proposed monolithically integrated PV powered circuit achieves a conversion efficiency of 85.26% and provides extra flexibility in terms of gain, which is advantageous for future implantable applications that have a range of inputs. This research is therefore an important step in achieving truly autonomous implantable electronic devices

Energy-Efficient Start-up Power Management for Batteryless Biomedical Implant Devices

This paper presents a solar energy harvesting power management using the high-efficiency switched capacitor DC-DC converter for biomedical implant applications. By employing an on-chip start-up circuit with parallel connected Photovoltaic (PV) cells, a small efficiency improvement can be obtained when compared with the traditional stacked photodiode methodology to boost the harvested voltage while preserving a single-chip solution. The PV cells have been optimised in the PC1D software and the optimal parameters modelled in the Cadence environment. A cross-coupled circuit with level shifter loop is also proposed to improve the overall step up voltage output and hybrid converter increases the start-up speed by 23.5%. The proposed system is implemented in a standard 0.18-μm CMOS technology. Simulation results show that the 4-phase start-up and cross coupled with level-shifter can achieve a maximum efficiency of 60%

Energy-Efficient Start-up Power Management for Batteryless Biomedical Implant Devices

This paper presents a solar energy harvesting power management using the high-efficiency switched capacitor DC-DC converter for biomedical implant applications. By employing an on-chip start-up circuit with parallel connected Photovoltaic (PV) cells, a small efficiency improvement can be obtained when compared with the traditional stacked photodiode methodology to boost the harvested voltage while preserving a single-chip solution. The PV cells have been optimised in the PC1D software and the optimal parameters modelled in the Cadence environment. A cross-coupled circuit with level shifter loop is also proposed to improve the overall step up voltage output and hybrid converter increases the start-up speed by 23.5%. The proposed system is implemented in a standard 0.18-μm CMOS technology. Simulation results show that the 4-phase start-up and cross coupled with level-shifter can achieve a maximum efficiency of 60%

Spintronic Nanodevices for Neuromorphic Sensing Chips

Recent developments in spintronics materials and physics are promising to develop a new type of magnetic sensors which can be embedded into the silicon chips. These neuromorphic sensing chips will be designed to capture the biomagnetic signals from active biological tissue exploited as brain-machine interface. They lead to machines that are able to sense and interact with the world in humanlike ways and able to accelerate years of fitful advance in artificial intelligence. To detect the weak biomagnetic signals, this work aims to develop a CMOS-compatible spintronic sensor based on the magnetoresistive (MR) effect. As an alternative to bulky superconducting quantum interference device (SQUID) systems, the miniaturised spintronic devices can be integrated with standard CMOS technologies makes it possible to detect weak biomagnetic signals with micron-sized, non-cooled and low-cost. Fig. 1 shows the finite element method (FEM)-based simulation results of a Tunnelling-Magnetoresistive (TMR) sensor with an optimal structure in COMSOL Multiphysics. The finest geometry and material are demonstrated and compared with the state-of-the-art. The proposed TMR sensor achieves a linear response with a high TMR ratio of 172% and sensitivity of 223 μV/Oe. The results are promising for utilizing the TMR sensors in future miniaturized brain-machine interface, such as Magnetoencephalography (MEG) systems for neuromorphic sensing

Power management systems based on switched-capacitor DC-DC converter for low-power wearable applications

The highly efficient ultra-low-power management unit is essential in powering low-power wearable electronics. Such devices are powered by a single input source, either by a battery or with the help of a renewable energy source. Thus, there is a demand for an energy conversion unit, in this case, a DC-DC converter, which can perform either step-up or step-down conversions to provide the required voltage at the load. Energy scavenging with a boost converter is an intriguing choice since it removes the necessity of bulky batteries and considerably extends the battery life. Wearable devices are typically powered by a monolithic battery. The commonly available battery such as Alkaline or Lithium-ion, degrade over time due to their life spans as it is limited by the number of charge cycles- which depend highly on the environmental and loading condition. Thus, once it reaches the maximum number of life cycles, the battery needs to be replaced. The operation of the wearable devices is limited by usable duration, which depends on the energy density of the battery. Once the stored energy is depleted, the operation of wearable devices is also affected, and hence it needs to be recharged. The energy harvesters- which gather the available energy from the surroundings, however, have no limitation on operating life. The application can become battery-less given that harvestable energy is sufficiently powering the low-power devices. Although the energy harvester may not completely replace the battery source, it ensures the maximum duration of use and assists to become autonomous and self-sustain devices. The photovoltaic (PV) cell is a promising candidate as a hypothetical input supply source among the energy harvesters due to its smaller area and high power density over other harvesters. Solar energy use PV harvester can convert ambient light energy into electrical energy and keep it in the storage device. The harvested output of PV cannot directly connect to wearable loads for two main reasons. Depending on the incoming light, the harvested current result in varying open-circuit voltage. It requires the power management circuit to deal with unregulated input variation. Second, depending on the PV cell's material type and an effective area, the I-V characteristic's performance varies, resulting in a variation of the output power. There are several works of maximum power point tracking (MPPT) methods that allow the solar energy harvester to achieve optimal harvested power. Therefore, the harvested power depends on the size and usually small area cell is sufficient for micro-watt loads low-powered applications. The available harvested voltage, however, is generally very low-voltage range between 0.4-0.6 V. The voltage ratings of electronics in standard wearable applications operate in 1.8-3 V voltages as described in introduction’s application example section. It is higher than the supply source can offer. The overcome the mismatch voltage between source and supply circuit, a DC-DC boost converter is necessary. The switch-mode converters are favoured over the linear converters due to their highly efficient and small area overhead. The inductive converter in the switch-mode converter is common due to its high-efficiency performance. However, the integration of the inductor in the miniaturised integrated on-chip design tends to be bulky. Therefore, the switched-capacitor approach DC-DC converters will be explored in this research. In the switched-capacitor converter universe, there is plenty of work for single-output designs for various topologies. Most converters are reconfigurable to the different DC voltage levels apart from Dickson and cross-coupled charge pump topologies due to their boosting power stage architecture through a number of stages. However, existing multi-output converters are limited to the fixed gain ratio. This work explores the reconfigurable dual-output converter with adjustable gain to compromise the research gap. The thesis's primary focus is to present the inductor-less, switched-capacitor-based DC-DC converter power management system (PMS) supplied by a varying input of PV energy harvester input source. The PMS should deliver highly efficient regulated voltage conversion ratio (VCR) outputs to low-power wearable electronic devices that constitute multi-function building blocks

A CMOS Analog Front-End for Tunnelling Magnetoresistive Spintronic Sensing Systems

This paper presents a CMOS readout circuit for an integrated and highly-sensitive tunnel-magnetoresistive (TMR) sensor. Based on the characterization of the TMR sensor in the finite-element simulation, using COMSOL Multiphysics, the circuit including a Wheatstone bridge and an analogue front-end (AFE) circuit, were designed to achieve low-noise and low-power sensing. We present a transimpedance amplifier (TIA) that biases and amplifies a TMR sensor array using switched-capacitors external noise filtering and allows the integration of TMR sensors on CMOS without decreasing the measurement resolution. Designed using TSMC 0.18 μm 1V technology, the amplifier consumes 160 nA at 1.8 V supply to achieve a dc gain of 118 dB and a bandwidth of 3.8 MHz. The results confirm that the full system is able to detect the magnetic field in the pico-Tesla range with low circuit noise (2.297 pA/√Hz) and low power consumption (86 μW). A concurrent reduction in the power consumption and attenuation of noise in TMR sensors makes them suitable for long-term deployment in spintronic sensing systems

Spintronic Nanodevices for Neuromorphic Sensing Chips

Recent developments in spintronics materials and physics are promising to develop a new type of magnetic sensors which can be embedded into the silicon chips. These neuromorphic sensing chips will be designed to capture the biomagnetic signals from active biological tissue exploited as brain-machine interface. They lead to machines that are able to sense and interact with the world in humanlike ways and able to accelerate years of fitful advance in artificial intelligence. To detect the weak biomagnetic signals, this work aims to develop a CMOS-compatible spintronic sensor based on the magnetoresistive (MR) effect. As an alternative to bulky superconducting quantum interference device (SQUID) systems, the miniaturised spintronic devices can be integrated with standard CMOS technologies makes it possible to detect weak biomagnetic signals with micron-sized, non-cooled and low-cost. Fig. 1 shows the finite element method (FEM)-based simulation results of a Tunnelling-Magnetoresistive (TMR) sensor with an optimal structure in COMSOL Multiphysics. The finest geometry and material are demonstrated and compared with the state-of-the-art. The proposed TMR sensor achieves a linear response with a high TMR ratio of 172% and sensitivity of 223 μV/Oe. The results are promising for utilizing the TMR sensors in future miniaturized brain-machine interface, such as Magnetoencephalography (MEG) systems for neuromorphic sensing

Integrated circuit & system design for concurrent amperometric and potentiometric wireless electrochemical sensing

Complementary Metal-Oxide-Semiconductor (CMOS) biosensor platforms have steadily grown in healthcare and commerial applications. This technology has shown potential in the field of commercial wearable technology, where CMOS sensors aid the development of miniaturised sensors for an improved cost of production and response time. The possibility of utilising wireless power and data transmission techniques for CMOS also allows for the monolithic integration of the communication, power and sensing onto a single chip, which greatly simplifies the post-processing and improves the efficiency of data collection. The ability to concurrently utilise potentiometry and amperometry as an electrochemical technique is explored in this thesis. Potentiometry and amperometry are two of the most common transduction mechanisms for electrochemistry, with their own advantages and disadvantages. Concurrently applying both techniques will allow for real-time calibration of background pH and for improved accuracy of readings. To date, developing circuits for concurrently sensing potentiometry and amperometry has not been explored in the literature. This thesis investigates the possibility of utilising CMOS sensors for wireless potentiometric and amperometric electrochemical sensing. To start with, a review of potentiometry and amperometry is evaluated to understand the key factors behind their operation. A new configuration is proposed whereby the reference electrode for both electrochemistry techniques are shared. This configuration is then compared to both the original configurations to determine any differences in the sensing accuracy through a novel experiment that utilises hydrogen peroxide as a measurement analyte. The feasibility of the configuration with the shared reference electrode is proven and utilised as the basis of the electrochemical configuration for the front end circuits. A unique front-end circuit named DAPPER is developed for the shared reference electrode topology. A review of existing architectures for potentiometry and amperometry is evaluated, with a specific focus on low power consumption for wireless applications. In addition, both the electrochemical sensing outputs are mixed into a single output data channel for use with a near-field communication (NFC). This mixing technique is also further analysed in this thesis to understand the errors arising due to various factors. The system is fabricated on TSMC 180nm technology and consumes 28µW. It measures a linear input current range from 250pA - 0.1µW, and an input voltage range of 0.4V - 1V. This circuit is tested and verified for both electrical and electrochemical tests to showcase its feasibility for concurrent measurements. This thesis then provides the integration of wireless blocks into the system for wireless powering and data transmission. This is done through the design of a circuit named SPACEMAN that consists of the concurrent sensing front-end, wireless power blocks, data transmission, as well as a state machine that allows for the circuit to switch between modes: potentiometry only, amperometry only, concurrent sensing and none. The states are switched through re-booting the circuit. The core size of the electronics is 0.41mm² without the coil. The circuit’s wireless powering and data transmission is tested and verified through the use of an external transmitter and a connected printed circuit board (PCB) coil. Finally, the future direction for ongoing work to proceed towards a fully monolithic electrochemical technique is discussed through the next development of a fully integrated coil-on-CMOS system, on-chip electrodes with the electroplating and microfludics, the development of an external transmitter for powering the device and a test platform. The contributions of this thesis aim to formulate a use for wireless electrochemical sensors capable of concurrent measurements for use in wearable devices.Open Acces

A miniaturised autonomous sensor based on nanowire materials platform: the SiNAPS mote

A micro-power energy harvesting system based on core(crystalline Si)-shell(amorphous Si) nanowire solar cells together with a nanowire-modified CMOS sensing platform have been developed to be used in a dust-sized autonomous chemical sensor node. The mote (SiNAPS) is augmented by low-power electronics for power management and sensor interfacing, on a chip area of 0.25mm2. Direct charging of the target battery (e.g., NiMH microbattery) is achieved with end-to-end efficiencies up to 90% at AM1.5 illumination and 80% under 100 times reduced intensity. This requires matching the voltages of the photovoltaic module and the battery circumventing maximum power point tracking. Single solar cells show efficiencies up to 10% under AM1.5 illumination and open circuit voltages, Voc, of 450-500mV. To match the battery’s voltage the miniaturised solar cells (~1mm2 area) are connected in series via wire bonding. The chemical sensor platform (mm2 area) is set up to detect hydrogen gas concentration in the low ppm range and over a broad temperature range using a low power sensing interface circuit. Using Telran TZ1053 radio to send one sample measurement of both temperature and H2 concentration every 15 seconds, the average and active power consumption for the SiNAPS mote are less than 350nW and 2.1 μW respectively. Low-power miniaturised chemical sensors of liquid analytes through microfluidic delivery to silicon nanowires are also presented. These components demonstrate the potential of further miniaturization and application of sensor nodes beyond the typical physical sensors, and are enabled by the nanowire materials platform