Charge pump with near zero offset current

Abstract

A charge pump of a phase-locked loop includes a first P channel transistor and a first N channel transistor coupled to mirror a current in a constant current source. The P channel transistor and N channel transistor are formed with dimensions that match transient responses of currents through the N and P channel transistors during switching rather than matching the gains of the N and P channel transistors. In one embodiment, the channel length of the N channel transistor is twice a channel length of the P channel transistor. A second P channel transistor and a second N channel transistor connected in series with the first P and N channel transistors switch the current through the first P channel transistor and the first N channel transistor respectively. The second P channel transistor and the second N channel transistor have matched gate-drain capacitances so that they have the same switching speed. A first capacitor coupled between the gate of the first P channel transistor and a supply voltage and a second capacitor coupled between the gate of the first N channel transistor and a reference voltage reduce the effect that jitter in the supply and reference voltages has on the charge pump.