Provided are a phase locked loop (PLL) circuit, a lock detector employable with a PLL circuit, a system including such a PLL circuit and/or lock detector, and a method of detecting a lock/unlock state of a PLL circuit. The PLL circuit may include a clock generating circuit configured to generate an output clock signal having a predetermined frequency in synchronization with a reference clock signal. The lock detector may be configured to determine that the PLL circuit is in a lock state when a phase difference between the reference clock signal and the output clock signal is equal to or less than a first reference value, determine that the PLL circuit is in an unlock state when the phase difference between the reference clock signal and the output clock signal is greater than a second reference value, and generate a lock detection signal.