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An ultra-low-power temperature sensor circuit has been developed using a 0.35-mum standard CMOS process. The circuit consists of a proportional-to-absolute-temperature (PTAT) current generator and a frequency-locked loop, and generates a PTAT clock frequency. The PTAT current generator is the key component of the sensor and was constructed by using the characteristics of a MOSFET in the subthreshold region. Theoretical analyses and experimental results showed that the circuit can be used as a temperature sensor with ultra-low-power consumption of 10 muW or less. The accuracy of the sensor output was within plusmn1.8degC in a temperature range from 10degC to 80degC. Our sensor would be suitable for use in subthreshold-operated, power-aware LSIs.