This paper describes a wideband ultra-fast-settling fractional-N bang-bang digital phase-locked loop (DPLL) for multi-ISM-band ultra-low-power (ULP) radios. We propose a mismatch-free digital-to-time-converter (DTC) gain calibration scheme to effectively shorten the calibration time, while the split coarse-fine PLL loops with different loop bandwidths accelerate the loop settling speed. The employed ring VCO (RVCO) aids to extend the frequency tuning range and generate multi-phase outputs. Prototyped in 65-nm CMOS, the DPLL consumes 1.2-2.4 mW over a wide frequency locking range of 68.3% (1.3-2.65 GHz) and occupies a die area of 0.12 mm. The settling time measures 8μs at an 82-MHz initial frequency error.