A 265- $\mu$ W Fractional- ${N}$ Digital PLL With Seamless Automatic Switching Sub-Sampling/Sampling Feedback Path and Duty-Cycled Frequency-Locked Loop in 65-nm CMOS

This article proposes a fractional-N digital phase-locked loop (DPLL) that achieves a 265-μW ultra-lowpower operation. The proposed switching feedback can seamlessly change the DPLL from sampling operation to sub-sampling operation without disturbing the phase-locked state of the DPLL to reduce the number of building blocks that works at the oscillator frequency, leading to significant power reduction. With the reduced number of high-frequency circuits, scaling the reference frequency is fully used to reduce the power consumption of the DPLL. Together with an out-of-dead-zone detector and a duty-cycled frequency-locked loop running in the background, the switching feedback achieves robust frequency and phase acquisition at start-up and helps the sub-sampling PLL recover when large phase and frequency disturbances occur. A transformer-based stacked-gm oscillator is proposed to minimize the power consumption while providing the sufficient swing to drive the subsequent stages. A truncated constant-slope digital-to-time converter is proposed to improve the power efficiency while retaining good linearity. The proposed fractional-N DPLL consumes only 265 μW while achieving an integrated jitter of 2.8 ps and a worst case fractional spur of -52 dBc, which corresponds to a figure of merit (FOM) of -237 dB.