A method of calibrating a phase-locked loop (PLL) while maintaining lock includes detecting that a control signal to an oscillator in a PLL has exceeded a threshold value while the PLL is locked to an input signal. In response, an operating current of the oscillator is adjusted to return the control signal below the threshold value while maintaining lock of the PLL to the input signal. Adjusting the operating current includes slowly varying an output current of a calibration circuit coupled to the PLL, enabling the PLL to maintain lock to the input signal during adjustment of the operating current.