If there are more than one feedback loop in any circuit ( voltage regulator); to check the stability should one break all the loops and check for stability OR the loop having highest loop gain (A*beta)
will have potential stability threat, if the circuit is safe there, other feedback loops may not be as significant.

In other words, having more than feedback loop is bad design, because optimising the circuit for all the three loops (say) to be stable is difficult task.

Regards

this one also might be useful http://www.designers-guide.org/Forum/YaBB.pl?num=1198131494

Best
Tosei

Thanks to Frank, Tosei and Lutz for the pointers and comments.
Looks like much has been discussed on the topic ..

I have gathered that most of the posts were devoted to where to break the loop vs advantages of not breaking the loop. I agree 'stb' analysis is convenient over classical breaking the loop method.
I would have been agreed before seeing the interesting behavior ..
I am working on voltage regulator which has traditional 2 stage op-amp followed by CS PMOS third stage.
I have 3 potential place where I can break the loop. At O/P of 1st stage, at O/P of 2nd stage and I/P of 1st stage diff pair.
Theoretically I should get the same DC loop gain but I am seeing different loop gains. The obvious reason is that DC op points are not same while doing 'stb' analysis.
Frank, will you comment.

Thanks

My recommendation is to open the loop at X3 and to place an AC voltage source of 1V BETWEEN both nodes (left and right of the break point).
Thus, the dc loop and, hence, the operating point remains unchainged.
The ac loop gain then is simply the ratio of both node voltages.
This is a modified MIDLLEBROOK method and it works always - as long as there is no severe load change at the point chosen to insert the voltage source.

Hi BHARAT,

As far as I can see, you only have one loop and in principle it does not matter at which point (X1, X2 or X3) you are going to measure/simulate the loop gain.
However, when you don´t intent to simulate the load separately which is disconnected by introducing the ac source , you have to select a point where a small output resistance looks into a high resistance - and that´s X3.
I know, that there will remain a small error (because of non-ideal load conditions), but I think it will be acceptable.
But I must confess that I am not familiar with the STB analysis (I use PSpice and similar packages), but I am sure that the PM and GM at X2 and X3 (if properly determined !!) will be identical to the values at X3.
Because it is the same loop !
Another problem may be: The concept of PM and GM is applicable not to all feedback systems but only to those systems which allow the simplified Nyquist criterion (stable open loop system and no zeros in the RHP and only one loop gain crossing of the 0 dB-line).
Did you check this?
Regards

bharat wrote on Oct 24^th, 2009, 9:51am:


Hi Bharat,

Concerning your first question,: the stb is accurate as long as the hypothesis for applying such analysis are fullfilled. -> in order for the stb analysis to be valid your ckt should be able to be analyzed as a single feedback system This is valid only when:
a) all local feedback loops inside the main loop are individually stable
b) all the invovled loops have a common break point

Your circuit has to fulfill at least one of these two requirements in order to be analyzed as a single loop system (and therefore use stb analysis).

This idea goes along with the lines Frank pointed out in 1) in his post.

As for Frank's second bullet, he is referring to a heavy feedforward component which - in usual feedback theory - is not considered. This might affect the overall stability of your system too.

One more comment: it is usually difficult to mimic the ckt behavior with a zero source and infinite load impendace AC source that breaks the loop, since usually stability is set by the voltage and current feedback loop. Usually with your AC voltage source you only analyise the stability of your voltage feedback loop.
As buddypoor pointed out, the approximation is valid only when the loading effects are negligible, but this method is error-prone. so it is always recommended NOT to break the loop, but rather include your probe inside the loop. Actually the stb analysis is supposed to be more accurate since it consider both loops: voltage and current.

Sorry for the long post

Best
Tosei

HI-
generally 60-65 is very good number irrespective of number of loops. But you should consider gain margin also other wise there could be small ripple on top of exponential settling. typically 12dB is good gain margin. So I would suggest you to Quote both always. Check fig-35 in the following paper.

http://web.mit.edu/klund/www/papers/ACC04_opcomp.pdf

regarding multi-loops: All the faster loop poles will be non dominate pole for the slow loop, so as long as they are very far from the slow loop UGB system is stable. I didn't under stand http://web.mit.edu/klund/www/papers/ACC04_opcomp.pdf

In generally it's bit hard to find single loop, take miller compensation, through Cgd 2nd stage will have inner loop. take Ahuja compensation, this will have inner loop through current buffer.

Best regards,
Raj.

Dear thetan-

Does this mean that the inner loops should also have a PM of 60degrees and GM of 12dB?
Unfortunately I am not expert in the stability theory, but I have designed several working multi loop feedback systems and general belief is make sure all loops are having stable nature (60deg PM and 12dB GM) and diff bandwidths means at least 4-5X variation among stages (of course it depends on your power and area budget).

From the transfer function (TF) derivation, can we not say that analyzing slow loop (red cross point) is a good point for LDO design (as the denominator should not be 0)
Yes slow loop determines the overall UGB, But to find overall loop stability you need to break all loops at time, hence you need to break only in the green cross. red cross is the rite one.

The above theory works pretty well in the practice, best example is PLL.
Intersting theory: Control system theory says any composite feedback network with unstable inner loops can be compensated with gain (aka Conditionally stable systems). Example: take a system with RHP poles say 1/(s-10) now keep this insde a simple loop with gain >10 say 20. Overall loop transfer function is 20/(s+10), stable. I have never tried in analogue this, would like to know if any one have experience on this. One problem to implement the above theory in circuits is once a system is unstable bias point will disturb and can't use linear theory any more, but not sure.

Hope this helps,
Raj.