Low-power voltage to a frequency-based smart temperature sensor with +0.8/-0.75 $^{\circ}$C accuracy for -55 $^{\circ}$C to 125 $^{\circ}$C


Abstract: The high power densities in system-on-chips demand accurate, low power, and compact smart temperatures for thermal monitoring. In this paper, a low-power CMOS-based smart temperature sensor, operating in the subthreshold region, for military applications (-55 $^{\circ}$C to 125 $^{\circ}$C) is presented. The sensor exploits the thermal dependency of the threshold voltage of MOS transistors to generate a voltage proportional to absolute temperature ($V_{PTAT})$. A frequency locked loop technique is employed to generate frequency from $V_{PTAT}$. The frequency variation due to temperature is measured by counting the rising edge with an asynchronous 12-bit counter. The large die area and requirement of external voltage regulator and reference clock hinder the integration of the on-chip sensors in SoCs. Therefore, a compact smart temperature is introduced with embedded temperature insensitive reference signal generators. The sensor is designed in CMOS 65 nm standard process and its operation is validated through postlayout results, considering a worst-case scenario. The sensor consumes power and area of 8.8 $\mu $W and 0.009 mm$^{2}$, respectively, at a power supply of 0.5-1 V. After one-point calibration, the sensor has an accuracy of +0.8/-0.75 $^{\circ}$C and a resolution of 0.26 $^{\circ}$C for -55 $^{\circ}$C to 125 $^{\circ}$C. The sensor consumes energy of 4.8 nJ and has a figure of merit of 0.12 nJK$^{2}$.

Keywords: Smart temperature sensor, low power, frequency locked loop, temperature insensitive ring oscillator, switched capacitors, calibration

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