A charge pump includes a first current source unit and a second current source unit. The first current source unit is connected between a first voltage terminal and the control node. The second current source unit is connected between the control node and a second voltage terminal. According to a phase comparing signal, the first current source unit provides a first switching current to the control node. The second current source unit includes a first sub-switching current generator, a second sub-switching current generator and a select circuit. According to a voltage level of the phase comparing signal, the first sub-switching current generator generates a first sub-switching current. According to the voltage level of the phase comparing signal, the second sub-switching current generator generates a second sub-switching current. By the select circuit, the first sub-switching current or the second sub-switching current is provided to the control node.