A 32-mV Supply Ring Oscillator Composed of Modified Schmitt Trigger Delay Cells for Integrated Start-Up Circuits in DC Energy Harvesting Systems

Implementing fully integrated start-up energy harvesting (EH) systems is very challenging. The most popular start-up circuit implementations use a low-voltage ring oscillator (RO). The characteristics of this RO rapidly deteriorate at VDD values below the transistor’s threshold voltage. A novel delay cell based on a Schmitt trigger (ST) inverter leveraging dynamic body biasing (DBB) is proposed to mitigate this deterioration. The RO of the proposed cell, implemented in a 22 nm FD-SOI CMOS technology, is compared against conventional and state-of-the-art circuits implemented in the same technology. Simulated results show the proposed RO achieves the highest output swing and DC gain within the 40 to 100 mV supply voltage range. A prototype chip comprising a 27-stage RO was fabricated and tested in a 22 nm FD-SOI CMOS technology. When fed by a 50 mV supply voltage, this RO produces an output swing of
$\mathbf {80\%}$
of that voltage while oscillating at a 0.8 kHz clock frequency and consuming 300 pW. In reported experiments, oscillations started with VDD as low as 32 mV. To the authors’ knowledge, the proposed CMOS RO operates with the second-lowest ever reported supply voltage. Combining its low power and low area, this RO is most suitable for EH systems with restricted power and area budget.