lonemy wrote on Nov 17th, 2011, 5:03am:Dear All:
I have a question about the stability of charge pump in PLL.
the ckt is like attachment fig.1. if the charge pump sink current to the LPF, there two loops in ckt, like fig.2. we can see that loop1 is a negative feedback loop and loop2 is positive. my question is: at low frequency, the impedance of LPF is larger than ro2, so the positive feedback is stronger than negative feedback, then the ckt is not stable, but in real case, this ckt works normally, could anyone tell me where my mistake is ?
thank you~
The loops do not look like they interact to me - just make sure that loop1 is stable to start with. Loop 2 is positive, but don't look at it's DC gain or small signal critera - pure positive feedback won't make an oscillator - it will just force the output to a rail. Don't worry about it -- the system should be designed around this circuit to prevent it from sticking at a rail.
You need to look at the stability of the *system*. The stability of the system is determined by how long the switch is on and the time constant of the LPF - look at the voltage change at the LFP when the switch is turned on for "dT" seconds. It will change dV = Idc/C*dT. Yes, Idc has a small dependance on the current output (i.e. positive feedback additional current from ro2) but the contribution of ro2 will be small. Even if it is not, you are only worried about the amount of dV, not how you got it. You can do a discrete time analysis and check the stability of the *system* but it doesn't have much to do with the positive feedback in loop2.
The instability you want to avoid is having the LPF jump rail to rail with each update. You prevent that by limiting the maximum charge time and/or making the cap large enough to prevent a large jump. I doubt you will have to worry about this as I'm positive that the settling requirements for the PLL itself will impose far more restrictive conditions to ensure stability.