A phase locked loop includes a charge pump, a voltage-current converter, and a current controlled oscillator. The charge pump generates a pump current based on a bias voltage and a phase difference detection signal, in which the pump current is for adjusting a control voltage. The voltage-current converter is self-biased, generates the bias voltage, and converts the control voltage to a converter current. The current controlled oscillator generates an oscillator current based on the bias voltage, and generates an output signal that has a frequency corresponding to the oscillator current. The phase locked loop is insensitive to a division ratio and PVT variations.