Analysis and Design of the Three-Inverter Schmitt Trigger for Supply Voltages Down to 50 mV

This brief analyzes the operation of the three-inverter Schmitt trigger (TI-ST) at ultra-low supply voltages. Included in the analysis is a model to predict the supply voltage at which the TI-ST changes from the amplifier mode to the hysteresis mode. We show theoretically that the TI-ST can exhibit hysteresis for a supply voltage as low as the fundamental limit of unity gain of the CMOS inverter. We also demonstrate experimentally that, with a suitable design, hysteresis starts to occur from a supply voltage as low as 48.5 mV. A relaxation oscillator built with a TI-ST and standard inverters is experimentally demonstrated to start to oscillate from supply voltages as low as 62 mV.