16.1 A 265μW Fractional-N Digital PLL with Seamless Automatic Switching Subsampling/Sampling Feedback Path and Duty-Cycled Frequency-Locked Loop in 65nm CMOS

The demand for highly energy-efficient circuits and systems has exponentially increased for Systems on Chip (SoC). A fractional-N phase-locked loop (PLL) is one of the most important building blocks in SoCs for a variety of applications, such as frequency synthesis for wireless transceivers and system clock generation for processors, memories, and I/O interfaces. Recent developments in fractional-N digital PLLs (DPLLs) [1]–[3] have shown great potential for achieving low-power operation and small chip area. However, none of these works have achieved power consumption below $500 \mu \mathrm {W}$ due to the number of building blocks operating at the oscillator frequency. Furthermore, the digitally controlled oscillators (DCOs) in [1]–[3] consume more than $250 \mu \mathrm {W}$ of power to achieve a good phase noise and a high-enough amplitude for DPLL locking. A digital sub-sampling architecture [1], [2], [4] can potentially reduce the overall power consumption by bypassing these high-frequency building blocks. Unfortunately, the absence of frequency acquisition makes such architecture vulnerable to sudden or large frequency disturbances. Even though a background frequency-locked loop (FLL) [1], [4] can be applied, it consumes large power due to the counter working at the DCO frequency. The typical solution to save power consumption is to turn off the FLL [2] after the PLL has been stabilized. Despite the benefit of the power reduction, a sub-sampling PLL has multiple frequency lock-in ranges near the integer multiple of the reference frequency, which could cause false locking if the frequency disturbances are within those ranges. To address the above issues, this work presents a fractional-N DPLL achieving a $265 \mu \mathrm {W}$ power consumption with robust phase and frequency acquisition with negligible power overhead in a 65nm CMOS technology. It also achieves an rms jitter of 2.8ps, which corresponds to an FoM of-236.8dB.