An Ultra-Low Power CMOS Integrated Pulse-Width Modulated Temperature Sensor

In this paper an ultra-low power, integrated pulse width modulated (PWM) temperature sensor is presented. The PWM temperature output is generated by the comparing a temperature dependent voltage with another voltage which is temperature independent. The proposed sensor leverages a 2-transistor voltage reference cell that is corrected using feedback through the bulk terminal to provide steady output across temperature. The feedback allows the reference to operate with supply voltages as low as 450 mV. Owing to the low supply voltage and operation in the sub-threshold region, the sensor has a measured power consumption as low as 47.2 nW (@ 27 degrees C) and 17.6 nW (@ -20 degrees C). Because of the low voltage required, it is suitable to be powered by harvested energy sources. To realize a robust temperature measurement, the temperature is measured using a ratio of output pulse-widths. A fabricated prototype in 65 nm CMOS has an area of 305x 220 mu m(2). The measured maximum temperature error is +1/-2.5 degrees C, and requires only a two-point calibration for operation over the temperature range -20-80 degrees C.