The proposed temperature sensor is based on CMOS ring oscillators and a frequency-to-digital converter capable of simple and efficient temperature conversion to digital value. The proposed temperature sensor consumes 400uW at a conversion rate of 366kS/s and performs the fastest temperature-to-digital conversion among those introduced in previous work. The whole block occupies 0.0066 mm2 (0.0013 mm2 for temperature sensor). Four multiphase clocks were utilized to enhance the resolution of the sensor 8 times better. After one point calibration, the chip-to-chip measurement spread was +2.748°C ∼ -2.899°C over the temperature range of -40°C to 110°C.