Venkata Abhishek Pullela received his MS Dual Degree in Electronics and Communication Engineering (ECE). His research work was supervised by Dr. Zia Abbas. Here’s a summary of his research work on Design techniques and architectural solutions for low-power analog circuits targeting IoT applications:
Abstract
Decades of relentless efforts in manufacturing integrated circuits and reducing their cost has enabled the design of almost every system which was thought impossible before. As the semiconductor industry thrives to expand its market every year, a question keeps popping up every decade or so asking what kind of technology will continue expanding the market. Previous technological waves include laptops and smart-phones, however they are tending towards market saturation. The convergence of social and technological trends suggest that there is a growing need to efficiently utilize the resources on earth and create smart environments. This is possible when every object on earth, ex. cars, drones, buildings, etc. share the appropriate data and provide suggestions for optimal usage of the resources and things around us. It is evident that these objects need to be connected to the Internet to perform such tasks and hence the term Internet-of-Things (IoT) rose into picture in 1999. The IoT is still in its technological infancy and the growing IoT wave will soon dominate the semi-conductor market in the upcoming years.
The IoT application space is vast and includes wide variety of applications like biomedical, surveillance, environmental, etc. These applications have stringent power and area constraints and hence the circuits present in the IoT nodes need to comply with these conditions. The average power consumption is a critical factor in deciding the system’s lifetime, which can be reduced to an extent by applying the concept of duty-cycling. However, blocks which are always-on, are required for the application of duty-cycling and they contribute to the sleep mode power consumption inevitably. Hence, they need to be designed with extremely low power consumption. Apart from the power constraint, they are desired to be process and supply invariant, and have low area. This thesis discusses the design of always-on blocks that include voltage reference, current reference and analog temperature sensor.
Chapter 1 introduces the concept and evolution of IoT over the past few years, its applications, challenges that will be faced by it to dominate the semi-conductor market and its future. After understanding IoT applications, we introduce voltage reference, current reference, analog temperature sensor and explain their functionalities in Chapter 2. With this background, we move on to the design of the always-on blocks in the successive chapters.
The references and sensor incorporate thin-oxide devices to achieve pico-watt power consumption while consuming low-area. Resistor based architectures are eliminated as their values should be in the order of Gs to achieve pico-watt power consumption. Other resistor-less architectures suffer from the drawbacks as mentioned in the corresponding chapters. The thin-oxide devices or gate-leakage transistors serve as effective replacements for resistors. However, they have non-linear characteristics with various parameters and hence cannot be directly visualized as resistors. The temperature dependencies of gate-leakage current in inversion and accumulation region are studied in Chapter 3 and then exploited in the designs of always-on blocks.
Chapter 4 discusses the design of pico-watt current reference using gate-leakage transistors in inversion region while the design in Chapter 5 operates them in accumulation region to achieve a bandgap reference. It is beneficial if the functionality of both voltage and current reference are included in a single circuit. Chapter 6 deals with such an architecture by again exploiting accumulation-mode gate-leakage current characteristics. Chapter 7 deals with the design of a pico-watt process-invariant temperature sensor using gate-leakage transistors in accumulation region.
PVT variations are inevitable in any kind of circuit and most of the circuits try to achieve PVT invariant designs. Design techniques for achieving process-invariant voltage reference and temperature sensor are discussed in Chapters 5 and 7. Current references cannot usually be made process-invariant and hence process-invariant current reference is out of scope of this thesis. Techniques for achieving high supply noise invariance are discussed in Chapters 8 and 9. Chapter 8 proposes high PSRR voltage reference, current reference and temperature sensor in a single circuit while Chapter 9 deals with achieving high PSRR in composite-pair based temperature sensors. Chapter 10 finally concludes the thesis and mentions the scope of improvement for this work.