ECE264D_Lecture4

ECE264D_Lecture4 - Design of CMOS RF Frequency Synthesizers...

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Design of CMOS RF Frequency Synthesizers ecture 4: PLL blocks arge pump PFD etc Lecture 4: PLL blocks - charge pump, PFD etc. GANG ZHANG September 2009 © 2009 G. Zhang ECE264D Fall 2009
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! charge injection; Drain-switched charge pump From Rhee Page 2 ! Switch inject charge through Cgd ! Charge sharing: ! M1 and M2 drain parasitic cap share charge with output loop filter; ! Nmos M1: drain voltage is 0V when DN switch is off, when DN switch is on, drain parasitic cap is first charged up (charge sharing); ! Similar case for Nmos M2; ! Speed relatively slow: ! M1, M2 needs to come out of triode region before establish current; ! Charge up high impedance M1, M2 drain (RC time constant is large); ECE264D RF Frequency Synthesizers © 2009 G. Zhang
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Drain-switched charge pump: current steering From Rhee A B Page 3 ! To address charge sharing ! Drains of current mirror are kept at Vout, no parasitic cap charging; ! When UP is on, DWB is on, unity gain buffer set node A to be at same voltage as OUT; ! High speed ! Current mirrors are always on, no need to re-establish current; ! Buffer needs to have enough slew rate and output current; ! Static current consumption ! Current mirrors are always on; ! Buffer static current; ECE264D RF Frequency Synthesizers © 2009 G. Zhang
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Gate-switched charge pump From Rhee Page 4 ! M1, M2 drains are always connected to OUT, no charge sharing ! Speed: need to charge up gates Cgs, ! For M1: time constant: Cgs/Gm3; for M2: Cgs/Gm2; ! Cgs1, Cgs2 can be very large, charge pump usually has large devices to improve matching;
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ECE264D_Lecture4 - Design of CMOS RF Frequency Synthesizers...

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