Self-calibrated oscillator using fractional current steering for automotive application

Abstract

This paper addresses a self-calibrated oscillator using fractional current steering in high-safety automotive applications. As automobiles become more electrified, the number and importance of automotive integrated circuits (ICs) are increasing. In particular, circuits that perform essential functions for driving and safety require high safety through complementary circuit placement and constant accuracy in harsh temperature environments. This paper proposes a frequency-locked loop structure that uses a complementary circuit as a temporary reference oscillator during self-calibration. Once calibration is complete, the complementary circuit is disconnected to satisfy high-safety requirements. To achieve high resolution in a small area even after disconnection, we utilize a digital-to-analog converter (DAC) with fractional current steering and a search algorithm based on the Farey sequence. The Farey sequence can generate exponentially more combinations than a conventional DAC with the same area. The proposed self-calibrated oscillator is fabricated using DBH 180 nm technology and has a size of 0.1 mm$^2$. It achieves a 3σ inaccuracy of ±0.62% over 267 sample measurements and a temperature coefficient of 25.6 ppm/℃ over the temperature range of -40~150℃ over 50 sample measurements.