A Digitally-Assisted Electrothermal Frequency-Locked Loop

A digitally-assisted electrothermal frequency-locked loop (FLL) is presented, whose output frequency is determined by the temperature-dependent thermal diffusivity of bulk silicon. In contrast to previous work, its noise bandwidth is defined by a digital, rather than an analog, filter. This obviates the need for external capacitors, thus enabling full CMOS integration. Without trimming, an implementation in a 0.7 mu m CMOS process achieves an output frequency spread of about +/- 0.3% (3 sigma) from -55 degrees C to 125 degrees C. This corresponds to a temperature sensing inaccuracy of about +/- 0.7 degrees C (3 sigma).