A Review on Integrated RC Frequency References for IoT Applications

This article presents a technical overview of on-chip RC frequency references, which achieve high energy efficiency and high frequency stability required for internet-of-things applications. Among such references, the frequency locked-loop (FLL) based references have achieved state-of-the-art performances. In the FLL, a large loop gain forces the loop to be locked to the time delay of the RC filter. As a result, the FLL output frequency is dominantly determined by the RC time constant without suffering from artifacts of analog front-end circuits. Furthermore, to compensate high temperature sensitivity of the on-chip resistor, digital FLL architectures based on digital temperature compensation are promising recently by extremely improving temperature inaccuracy. Various structures of RC frequency references are compared, and their performances are summarized.