A BJT-Based Temperature-to-Frequency Converter With +/- 1 degrees C (3 sigma) Inaccuracy From-40 degrees C to 140 degrees C for On-Chip Thermal Monitoring

Abstract

This article presents a bipolar junction transistor (BJT)-based CMOS temperature-to-frequency converter (TFC). A relaxation oscillator (ROSC) consisting of an integrating capacitor, a current source, and a comparator converts the ratio of the complementary-to-absolute-temperature (CTAT) voltage to the proportional-to-absolute-temperature (PTAT) voltage created by the BJT core to frequency. To improve the temperature accuracy, we incorporate a capacitor flipping technique to remove an offset of the comparator and a settling time error when resetting the integrating capacitor. Moreover, bootstrapped NMOS switches with the triple well structure are implemented to suppress leakage current and prevent from forming parasitic BJTs when the negative CTAT is applied to them by the proposed flipping technique. The prototype fabricated in a 0.11-mu m CMOS process consumes 3.1-mu W power and occupies a 0.025-mm(2) active area. Measurements of 18 samples with the digital trimming show an inaccuracy of +/- 1 degrees C (3 sigma) from -40 degrees C to 140 degrees C. The area- and energy-efficient TFC is applicable to monitor the temperature inside integrated chips.