A High-Precision Reference-Free Relaxation Oscillator With Supply-Tracking Switching Threshold

Abstract

Conventional relaxation oscillators typically rely on reference circuits to generate switching threshold voltages or charging currents, resulting in increased chip area and power consumption. This article presents a high-precision relaxation oscillator that eliminates the need for reference circuits. A holding capacitor is charged through a timing resistor, and the switching threshold voltage is directly derived from the power supply using a resistor divider. Since both the charging speed and the switching threshold voltage scale with $V_{\!D\!D}$ , their dependencies cancel each other out, resulting in an oscillation period determined solely by the time constant of the holding capacitor and timing resistor. Moreover, the tracking behavior of the switching threshold with respect to $V_{\!D\!D}$ further reduces the sensitivity of the comparator delay to supply voltage variations. A dual-path charge–discharge scheme is also employed to eliminate frequency deviations caused by digital logic delays and capacitor discharge time. Fabricated in a 110-nm CMOS process, the oscillator operates at 24.5 MHz with an active area of 0.026 mm². Measurement results indicate a power consumption of $160~\mu $ W and a maximum frequency error of 1.88% across a temperature range from −55 °C to 125 °C, corresponding to a temperature coefficient of 140 ppm/°C. When $V_{\!D\!D}$ increases from 3.0 to 3.6 V, the output frequency variation is 0.4%.