Power management circuits for ultra-low power systems

Abstract

Power management circuits perform a wide range of vital tasks for electronic systems including DC-DC conversion, energy harvesting, battery charging and protection as well as dynamic voltage scaling. The impact of the efficiency of the power management circuits is highly profound for ultra-low power systems such as implantable, ingestible or wearable devices. Typically the size of the system for such applications does not allow the integration of a large energy storage device. Therefore, extreme energy efficiency of the power management circuits is critical for extended operation time. In addition, flexibility and small form factor are desirable to conform to the human body and reduce the system's over all size. Thus, this thesis presents highly efficient and miniature power converters for multiple applications using architecture and circuit level optimization as well as emerging technologies. The first part presents a power management IC (PMIC) featuring an integrated reconfigurable switched capacitor DC-DC converter using on-chip ferroelectric caps in 130 nm CMOS process. Digital pulse frequency modulation and gain selection circuits allow for efficient output voltage regulation. The converter utilizes four gain settings (1, 2/3, 1/2, 1/3) to support an output voltage of 0.4 V to 1.1 V from 1.5 V input while delivering load current of 20 [mu]A to 1 mA. The PMIC occupies 0.366 mm² and achieves a peak efficiency of 93% including the control circuit overhead at a load current of 500 [mu]A. The second part presents a solar energy harvesting system with 3.2 nW overall quiescent power. The chip integrates self-startup, battery management, supplies 1 V regulated rail with a single inductor and supports power range of 10 nW to 1 [mu]W. The control circuit is designed in an asynchronous fashion that scales the effective switching frequency of the converter with the level of the power transferred. The ontime of the converter switches adapts dynamically to the input and output voltages for peak-current control and zero-current switching. The system has been implemented in 180 nm CMOS process. For input power of 500 nW, the proposed system achieves an efficiency of 82%, including the control circuit overhead, while charging a battery at 3 V from 0.5 V input. The third part focuses on developing an energy harvesting system for an ingestible device using gastric acid. An integrated switched capacitor DC-DC converter is designed to efficiently power sensors and RF transmitter with a 2.5 V regulated voltage rail. A reconfigurable Dickson topology with four gain settings (3, 4, 6, 10) is used to support a wide input voltage range from 0.3 V to 1.1 V. The converter is designed in 65 nm CMOS process and achieves a peak efficiency of 80% in simulation for output power of 2 [mu]W. The last part focuses on flexible circuit design using Molybdenum Disulfide (MoS₂), one of the emerging 2D materials. A computer-aided design flow is developed for MoS₂-based circuits supporting device modeling, circuit simulation and parametric cell-based layout - which paves the road for the realization of large-scale flexible MoS₂ systems.