1. A circuit comprising: a supply rail to receive a supply voltage; an open-loop voltage regulator coupled to the supply rail and including a transistor, the open-loop voltage regulator to provide a regulated voltage to a digital-controlled oscillator (DCO); a calibration circuit coupled to the open-loop voltage regulator, the calibration circuit to, during a calibration mode of the circuit, adjust a calibration voltage provided to the open-loop voltage regulator to provide the regulated voltage with a target value; wherein the calibration circuit is further to: receive a digital tuning code that is used to tune an output frequency of the DCO; determine if the tuning code is outside a pre-determined range of tuning codes; and if the tuning code is outside the pre-determined range of tuning codes, adjust the calibration voltage so that the tuning code is inside the pre-determined range of tuning codes.