A low temperature drift coefficient crystal-less frequency reference clock compensated by temperature sensor for microsystem

Quartz oscillator has been widely used as reference clock source in the microsystems due to its good performance. But a good crystal oscillator costs too much and its bulky size is not desired. This paper aims at designing an alternative integrated oscillator to replace the external quartz oscillator. The proposed circuit used maneatis delay cell to construct a ring oscillator for its superior linear I-V characteristic. As for a frequency reference clock, its frequency stability over temperature is required at first. After detailed mathematical deducing and careful analysis, a formula is proposed to describe the relationship between desired control voltage and temperature by assuming the frequency as constant. This paper utilized bipolar transistor as the temperature sensor, combining it with CTAT current source and resistor to create a first-order temperature compensation control voltage. The chip with typical frequency of 10 MHz was fabricated in a 0.35 μm CMOS technology and occupied 0.45 mm². The measured results show that the frequency variation is ±0.2% for supply changes from 4.8 V to 5 V, and frequency variation is 48 ppm when the temperature change is from -40°C to 85°C, while the average current of the tested chip consumes 50 μA from 5 V.