A Linearity-Enhanced Time-Domain CMOS Thermostat with Process-Variation Calibration

This study proposes a linearity-enhanced time-domain complementary metal-oxide semiconductor (CMOS) thermostat with process-variation calibration for improving the accuracy, expanding the operating temperature range, and reducing test costs. For sensing temperatures in the time domain, the large characteristic curve of a CMOS inverter markedly affects the accuracy, particularly when the operating temperature range is increased. To enhance the on-chip linearity, this study proposes a novel temperature-sensing cell comprising a simple buffer and a buffer with a thermal-compensation circuit to achieve a linearised delay. Thus, a linearity-enhanced oscillator consisting of these cells can generate an oscillation period with high linearity. To achieve one-point calibration support, an adjustable-gain time stretcher and calibration circuit were adopted for the process-variation calibration. The programmable temperature set point was determined using a reference clock and a second (identical) adjustable-gain time stretcher. A delay-time comparator with a built-in customised hysteresis circuit was used to perform a time comparison to obtain an appropriate response. Based on the proposed design, a thermostat with a small area of 0.067 mm2 was fabricated using a TSMC 0.35-μm 2P4M CMOS process, and a robust resolution of 0.05 °C and dissipation of 25 μW were achieved at a sample rate of 10 samples/s. An inaccuracy of -0.35 °C to 1.35 °C was achieved after one-point calibration at temperatures ranging from -40 °C to 120 °C. Compared with existing thermostats, the proposed thermostat substantially improves the circuit area, accuracy, operating temperature range, and test costs.