Phase-locked loop circuit

A phase-locked loop (PLL) circuit generating an output signal with an output frequency, comprising:

• a voltage-to-current conversion circuit configured to generate a first control current according to a control voltage, wherein the control voltage is generated according to a reference signal and a feedback signal derived from the output signal;
• a frequency compensation circuit configured to generate a second control current according to the control voltage;
• and
• a current-controlled oscillator (ICO) configured to generate the output signal according to an oscillation current, wherein the oscillation current is a sum of the first control current and the second control current,
• wherein in response to the frequency compensation circuit detecting that the control voltage is out of a predetermined range, the second control current is controlled to be negatively correlated with the output frequency;
• wherein the frequency compensation circuit comprises:
• a voltage monitor configured to generate a selection signal for indicating whether the control voltage is out of the predetermined range;
• a frequency selector configured to select one of the reference signal and the feedback signal according to the selection signal to serve as a control signal;
• a phase generation circuit configured to generate a first switching signal and a second switching signal according to the control signal, wherein the first and second switching signals have the same switching frequency;
• a frequency-to-current conversion circuit configured to receive the first and second switching signals and generate a first bias current according to the switching frequency, wherein the first bias current is positively correlated with the switching frequency;
• and
• a current bias circuit coupled to the frequency-to-current conversion circuit and configured to generate the second control current according to the first bias current, wherein the second control current is negatively correlated with the first bias current.