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A 220-nA 32-kHz Crystal Oscillator with Wide Voltage Range (1.0–5.5 V) for Battery-Operated Microcontrollers

Most microcontrollers (MCUs) have a real-time clock function driven by a 32-kHz crystal oscillator. A low-power 32-kHz crystal oscillator for operation in battery-operated MCUs over a wide voltage range (1.0–5.5 V) is described. The circuit features a proportional to absolute temperature (PTAT) bias-current generator to ensure an oscillation margin and an adaptive comparator for detecting small oscillation signals with little effect from by process and temperature variations. Experimental results indicate that the circuit achieves a small operating current of only 220 nA while ensuring a 10-times oscillation margin with a low-CL (3.1 pF) quartz crystal, providing sufficient noise tolerance and quick start-up. The duty cycle is 40–60% and start-up time is less than 0.6 s. This circuit is expected to realize the smallest MCU standby current of 420 nA.