Startup Circuit for Relaxation Oscillators with Low Functional Current and Minimal Area

This paper presents a novel start up technique for low current, low area, closed loop relaxation oscillator. In the proposed implementation, start-up time is improved by keeping the circuit initially in open loop and then in closed loop when the frequency is settled. In open loop mode, current is injected in the ring oscillator to charge the internal nodes. This results in approximately 3x improvement in start-up time (from 466us to 150us) across process, voltage, and temperature variations. This is achieved without increasing the functional current of the oscillator which consumes 52 nanoamperes of current. The start-up circuit is implemented by using current mirrors and digital circuits which do not require large area and offer very less leakage current. Proposed circuit is implemented in 16FFC technology for a 32KHz, 94nW relaxation oscillator circuit. The circuit operates on 1.8V (+/−10%) supply voltage and over a temperature range of-40°C to 125°C. The oscillator circuit shows +/− 5% post trim frequency variation across voltage and temperature.