A Compact and Ultra-Low-Power 32-kHz Conduction-Angle-Adaptive Crystal Oscillator

This article presents an ultra-low-power and compact 32-kHz crystal oscillator (XO) with intrinsic start-up capability for real-time clock (RTC) applications. The key idea is to operate a Pierce oscillator in a class-C-like fashion, with a digitally defined conduction angle generated based on the current oscillation amplitude. Hence, the XO can start-up when no oscillation is present and boosts its energy efficiency as the oscillation amplitude grows. Two circuit techniques based on the use of charge pumps are proposed in this work to generate this conduction angle information on-chip and regulate the oscillation amplitude. The proposed circuit topology, named the conduction-angle-adaptive XO (CAAXO), is verified through a fabricated integrated circuit (IC) implemented in 28-nm CMOS technology. Measurement results across ten samples demonstrate that under a 0.38-V supply, the proposed circuit functions well across a temperature range of ${-} 20~{^{\circ } }$ C to $85~{^{\circ } }$ C while only consuming 0.36 nW of power at $25~{^{\circ } }$ C room temperature. Furthermore, the proposed CAAXO only occupies 820 $\mu $ m² of core area, marking it the smallest XO among the state of the art.