A 7-MHz process, temperature and supply compensated clock oscillator in 0.25 μm CMOS

This paper reports on the design and implementation of a process, temperature and supply compensated 7-MHz clock oscillator in a 0.25 μm, double poly, 5-metal CMOS process. The clock generator is based on a 3-stage differential ring oscillator. The compensation technique incorporates a unique combination of a process corner sensing scheme and a temperature compensating network to appropriately change the control voltage of the differential ring oscillator. Measurements made across a temperature range of -40°C to 125°C and 64 samples collected over 3 runs indicate an average variation of ±0.82% (±46 ppm/°C) in the clock frequency with temperature, ±2.1% with process across chips at room temperature, and a worst-case combined variation of ±2.6% (with process, temperature and supply). The variation of frequency with power supply was ±0.31% for a supply voltage range of 2.4-2.75 V. The measurement results are in good agreement with the simulation results. The oscillator is intended to serve as a start-up clock for microcontroller applications.