Hi Raja,

Thanks for your reply. I did not understand your second answer "hence you need to break only in the green cross. red cross is the rite one."

Can you please make this more clear?

Thanks!

Hi Raja,

Thanks for your replies and sharing your experience. I agree with you that red cross is a good point indeed as that's the place which shows real closed loop behavior. I saw in few more trials that there can be situations when inner loop (green cross point ) shows bad Phase margins even less than 40degrees. But the fast behavior seen is still the same. Saying that, I mean even if PM > 60 or PM < 40 degrees for the green cross point I don't see much change in transient response.

In the paper you attached, I liked the way gain margin explains the ringing in figure 35. From experiments I see if I increase the gain margin of red cross loop analysis to > 20dB then the transient response becomes smoother. So, may be in this design higher gain margin is a better specification. like >20dB?

Also, one another query was about your explanation of DC biasing getting disturbed with an unstable inner loop. But if this happens should it not appear in the transient response? Arent the models which we use for designing in cadence suppose to show this problem of DC biasing getting disturbed in case of an instablity? or is it something which can only be tested on-chip?

Thanks in advance!