Abstract: A fully differential charge pump for a frequency synthesizer is proposed.The pump eliminates the effect of channel-length modulation and makes the charging and discharging currents match perfectly in a wide output range to minimize the ripple of the control voltage.A common-mode feedback circuit with rail-to-rail input range is employed to ensure a large unrestricted swing of the charge pump.The proposed fully differential charge pump is designed and fabricated in a 1.8V SMIC 0.18μm CMOS mixed-signal process;the chip area is about 450μm×280μm;and the power dissipation is 1mW.A reference spur level of -73dBc is achieved in the worst case,satisfying the performance of the frequency synthesizer.

Key words: frequency synthesizer, fully differential charge pump, current matching, replica, ripple, spur