Delivered by Ingenta to: Minghung Chang IP : 140.113.169.139 Wed, 25 Apr 2012 05:04:10 Copyright © 2012 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Low Power Electronics Vol. 8, 63–72, 2012 A 0.4 V 520 nW 990 m 2 Fully Integrated Frequency-Domain Smart Temperature Sensor in 65 nm CMOS Ming-Hung Chang ∗ , Shang-Yuan Lin, and Wei Hwang ∗ Department of Electronics Engineering and Institute of Electronics, National Chiao-Tung University, Hsin-Chu, 300, Taiwan (Received: 15 June 2011; Accepted: 10 October 2011) This work presents an ultra-low voltage fully integrated frequency-domain smart temperature sensor. Two temperature sensitive ring oscillators (TSROs) are used to build a temperature-to-frequency- ratio generator capable of operating at 0.4 V supply voltage. One is operated in near-threshold region, while the other is operated in sub-threshold region. The ratio of their output frequencies is then a monotonic function of temperature that is reasonably insensitive to process variation. With one-point calibration, a -1.81  C∼+1.52  C inaccuracy over a 0  C∼100  C temperature operation range has been measured for 12 test chips. At a conversion rate of 45 k samples/s, the proposed temperature sensor consumes an average power of 520 nW and achieves 0.49  C/LSB at 11-bit output resolution. It occupies only 990 m 2 in a TSMC 65-nm general purpose bulk CMOS process. Keywords: Near-Threshold Circuits, Sub-Threshold Circuits, Smart Temperature Sensor, Extreme Ultra Low Power, Variation-Aware Design. 1. INTRODUCTION Thermal and power management are major challenges in emerging energy-constrained applications target to extreme long lifetime. A fully integrated high-resolution, small- size, and ultra-low power temperature sensor is the key to providing vital environmental data for management units efficiency enhancement. On the other hand, pursu- ing longer operational lifetimes of portable platforms has driven the integrated circuit design into ultra-low volt- age regime where process, voltage, and temperature (PVT) variations are much more severe than the conventional super-threshold design. 1–3 In this regime, threshold voltage shifts caused by local variation exponentially exacerbate the weak I ON - I OFF -ratio. In order to ensure the function- ality in the presence of PVT variations, it motivates the design of variation-aware near-/sub-threshold circuits. 4 In some energy-limited miniature devices, they are powered by energy harvesting from the environment to increase the lifetime. The supply voltage it generated is usually not ∗ Authors to whom correspondence should be addressed. Emails: hwang@mail.nctu.edu.tw, tako.ee88g@nctu.edu.tw larger than 0.5 V. Therefore, a temperature sensor capa- ble of ultra-low voltage operation is essential. Moreover, a new class of package technologies, three-dimensional integrated circuit (3D-IC), 5 6 for achieving multi-function integration, improving system speed, and reducing power consumption makes on-die hot-spot problem even worse because of increasing power density and unbalanced ther- mal stresses distribution. Temperature variations over time induced by those stacking structures in 3D-IC require a fast and area-efficient temperature sensor to enable real- time multiple-location hot-spot detection. Most high-accuracy and high-resolution temperature sensors are based on the temperature characteristics of parasitic bipolar transistors. The inaccuracy of the state- of-the-art smart voltage-domain temperature sensors were ±0.1  C (3 with resolution of 25 mK 7 and 10 mK. 8 Those were achieved by using dynamic element matching, a combination of correlated double-sampling and system- level chopping for offset cancellation, precision mismatch- elimination layout, and individual trimming at room temperature after packaging. In Ref. [9], energy-efficient “zoom-ADC” architecture was presented to maintain the resolution and accuracy of -ADCs. An inaccuracy of ±0.2  C (3 with resolution 15 mK at conversion rate J. Low Power Electron. 2012, Vol. 8, No. 1 1546-1998/2012/8/063/010 doi:10.1166/jolpe.2012.1177 63