A 0.2 V, 23 nW CMOS Temperature Sensor for Ultra-Low-Power IoT Applications

Divya Akella Kamakshi,Benton Calhoun,Aatmesh Shrivastava

doi:10.3390/jlpea6020010

Abstract

We propose a fully on-chip CMOS temperature sensor in which a sub-threshold (sub-VT) proportional-to-absolute-temperature (PTAT) current element starves a current-controlled oscillator (CCO). Sub-VT design enables ultra-low-power operation of this temperature sensor. However, such circuits are highly sensitive to process variations, thereby causing varying circuit currents from die to die. We propose a bit-weighted current mirror (BWCM) architecture to resist the effect of process-induced variation in the PTAT current. The analog core constituting the PTAT, the CCO, and the BWCM is operational down to 0.2 V supply voltage. A digital block operational at 0.5 V converts the temperature information into a digital code that can be processed and used by other components in a system-on-chip (SoC). The proposed temperature sensor system also supports resolution-power trade-off for Internet-of-things (IoT) applications with different sampling rates and energy needs. The system power consumption is 23 nW and the maximum temperature inaccuracy is +1.5/−1.7 °C from 0 °C to 100 °C with a two-point calibration. The temperature sensor system was designed in a 130 nm CMOS technology and its total area is 250 × 250 μm2.

Highlights

The Internet-of-things (IoT) is a rapidly evolving space in the semiconductor industry
IoT devices focus on a wide range of applications, ranging from personal electronics to sensors for monitoring environmental, physiological, industrial, structural signals, etc
A 400 K poly-silicon resistor with low temperature dependence and operating the PTAT transistors at sub-VT voltages enables generation of currents in the nanoampere range. Such low currents contribute to the low power consumption of the temperature sensor system

Summary

Introduction

The Internet-of-things (IoT) is a rapidly evolving space in the semiconductor industry. In the presence of harvested energy, the circuits are required to consume ultra-low-power (ULP) and operate from the lowest voltage possible to enable longer device lifetimes. A 400 K poly-silicon resistor with low temperature dependence and operating the PTAT transistors at sub-VT voltages enables generation of currents in the nanoampere range. Such low currents contribute to the low power consumption of the temperature sensor system. In the Monte Carlo simulations, both process (inter-die) and mismatch (intra-die) variations were considered It covers the whole range of process variation including typical, sloJw. L,owanPdowefraEsltecctroonr.nAeprpsl..20W16e, 6o, 1b0serve that the mean PTAT current is 39 nA, and the 3σ varia5toiof 1n5 of 25 nA is very high. In the secTthioenP,TaAbTit-cwureriegnhttiesdmcuirrrroernedt musiirnrogra(BBWWCCMM)toissdtaersvceritbheedtrtoandseisctroerassoefsaucChCpOroacsesshso-iwndnuinced powFiegrurdeis5s,iwpahtiicohng. enerates frequencies proportional to temperature

Bit-Weighted Current Mirror

Findings

Comparison with Prior Art