Low-power temperature-to-frequency converter consisting of subthreshold CMOS circuits for integrated smart temperature sensors

A low-power temperature-sensing oscillator was developed using a 0.35-μm standard CMOS process. This oscillator generates a clock pulse whose frequency is proportional to absolute temperature (PTAT frequency). It consists of a PTAT current generator controlled with an external reference clock and a frequency-locked loop biased with an external reference voltage. The PTAT current generator makes use of the exponential current characteristic of MOSFETs operated in the subthreshold region. Theoretical analyses and experimental results showed that the circuit can be used as a temperature sensor with a low-power consumption of 10 μW or less. The temperature coefficient of the output frequency was insensitive to variation in device parameters, so the sensor circuit can be used only with one-point calibration. Its temperature-sensing error was from −1.8−1.8 to +1° C in a temperature range of 10–80 °°C. This temperature sensor would be suitable for use in subthreshold-operated, power-aware LSIs.