18.4 A 0.55nW/0.5V 32kHz Crystal Oscillator Based on a DC-Only Sustaining Amplifier for IoT

An always-ON, stable, 32kHz crystal oscillator (XO) provides key time-keeping, synchronization, and sleep-timer functions in most electronic systems. Ultra-low power (ULP) consumption of the XO is critical in highly duty-cycled, energyconstrained systems such as Internet-of-things (IoT). Conventional XOs, typically implemented in the Pierce configuration, consume 10 to 100nW: a large enough transconductance, g
_m
> ω
_O
²
·C
_L
²
·R
_m
, is needed to compensate for loss in the crystal's motional resistance, Rm, and sustain oscillations at frequency ωO, across load capacitors (C
_L
> 15pF in Fig. 18.4.1) that are inevitably large to ensure frequency stability over process, voltage, and temperature (PVT) variations. Recent 32kHz XO designs report sub-10nW power consumption [1-4] by either duty-cycling the sustaining amplifier [4] or by providing energy to compensate the crystal losses only via narrow pulses at the peaks of the XO waveform [1-3]. However, they require high-power components [2], complicated designs with multiple power domains [1-2] to properly time the pulses, or significant calibration and off-chip components [4] to reduce PVT sensitivity.