Analysis, Design and Implementation of Power-Efficient DC-DC Converters for Biomedical and IoT Applications

Abstract

This thesis focus on improving the power efficiency of DC-DC converters for two different applications. In the field of implantable medical devices, electrical stimulation has been used as an established treatment for several diseases. It aims to deliver a well-defined amount of charge to the tissue in order to build up a specific electric field and generate or block an action potential. To achieve a large spatial resolution, there is the need for an increasing number of independent stimulating channels to accommodate a large number of stimulating sites. This, however, increases the overall size of the stimulator while potentially affecting the power efficiency. In this respect, the first part of this thesis proposes a multi-channel neural stimulator in which the power efficiency does not depend upon the number of channels being operated simultaneously. On the other hand, in the recently established field of IoT, energy harvesters are increasingly used to ensure a perpetual, but heavily duty-cycled, load operation. However, their typically low output voltage would normally require a boost converter cascaded with a buck converter and low drop-out (LDO) linear regulators to generate multiple supplies of _1V VDD nominal voltage to supply nanoscale CMOS circuits and systems. However, LDOs are noisy, bulky and inefficient. Hence, it seems beneficial for IoT devices to be directly connected to the buck converter. However, the lack of (LDO) isolation exposes supply-sensitive blocks such as LC oscillators to the converter output fluctuations that could severely degrade the system performance. In the second part of this thesis (Chapters 4-6), a noise analysis of a switched-capacitor DC-DC converter reveals that those type of voltage regulators can have an output noise level that is much lower than that of LDOs. They are therefore suitable to power up supply-sensitive blocks. This leads to a new scheme in which an SC DC-DC converter directly powers up an LC oscillator, without consuming additional current or requiring any external component.