Integrated High-Side Current Sensors

This thesis describes the design and implementation of integrated high-side current sensors for IoT applications. As explained in chapter 1, the main challenges are the need to achieve low power, low cost and low area while maintaining a reasonably low gain error. To meet them, the focus of this thesis is on (1) the design of precision HV interface circuitry that does not need a HV supply, and (2) the design of energy-efficient temperature compensation schemes that enable the integration of shunt resistors with CMOS circuitry. Several new techniques at both system-level and circuit-level have been proposed and their effectiveness is verified in two prototypes. An integrated shunt-based current sensor consists of an interface circuit, a shunt resistor and a temperature compensation scheme. Chapter 2 gives an overview of these three elements. It first describes two sensing configurations: low-side sensing and high-side sensing, followed by a discussion of their pros and cons. High-side sensing is favored because of its ability to avoid the ground disturbance and detect the high load current caused by accidental shorts. However, it makes the design of the HV interface circuitry more challenging, as this must accurately and safely extract weak differential signals in the presence of large CM voltages. Several existing solutions are reviewed. However, these either consume too much power or occupy large silicon area. This observation leads to the first challenge addressed by this thesis: the design of power-efficient and compact HV circuitry for high-side current sensing. In the two prototypes described in this thesis, low-cost shunt resistors based on the metal layers of a CMOS process, or the lead-frame of a standard plastic package were used. However, both of them suffer from a large temperature coefficient of resistance (TCR) (>0.3%/°C), and so a temperature compensation scheme is necessary to achieve reasonable low inaccuracy. Two types (analog and digital) of temperature compensation schemes are reviewed. Analog ones achieve poor (>1%) inaccuracy while digital ones need a dedicated temperature sensor and a rather complex calibration process. This leads to the second challenge that this thesis addresses: the design of temperature compensation schemes that are low power and easy to use, while still achieving reasonable low inaccuracy.