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

Abstract

This article presents an ultra-low-power and com- pact 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 e fficiency 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 ◦C to 85 ◦C while only consuming 0.36 nW of power at 25 ◦C room temperature. Furthermore, the proposed CAAXO only occupies 820 µm2 of core area, marking it the smallest XO among the state of the art.