This paper presents an ultra-lower-power (ULP) digital-to-time-converter (DTC)-assisted fractional-N all-digital phase-locked loop (ADPLL) suitable for IoT applications. A proposed hybrid time-to-digital converter (TDC) extends the vernier-TDC input range with little power overhead in order to overcome the stability issue in the conventional architectures. The hybrid TDC also facilitates a background gain calibration to achieve a stable in-band phase noise insensitive to process, voltage, and temperature (PVT) variations. The implementation of a buffer-cascaded DTC simplifies the design complexity of the fractional-N operation. The ADPLL also features a 200 μW low-phase-noise inverse-class-F (class-F⁻¹) digitally controlled oscillator (DCO) without the need of two-dimensional (2-D) capacitor tuning for frequency alignment of the fundamental and 2nd-harmonic. Fabricated in 65-nm CMOS, the ULP ADPLL prototype achieves 868 fs $ _{rms}$ jitter in a fractional-N channel when consuming only 529 μW, corresponding to a figure-of-merit (FoM) of -244 dB.