Why does LDO poles change with load capacity?

[jutek]

LDO poles

hello

What can be reason that pole related to opamp's output and pass device's input changes with load capacity?

I know it shouldn't so why it's happening? i check it by .pz analysis in hspice and AC of course

what should i check again?

regards

 

[vhegde]

Re: LDO poles

hello

What can be reason that pole related to opamp's output and pass device's input changes with load capacity?

I know it shouldn't so why it's happening? i check it by .pz analysis in hspice and AC of course

what should i check again?

regards

Hi Jutek,
Its not the problem with that pole. Actually your output pole (due to load cap, esr, and ron of pass device) varies as your load current changes. Please check the poles and zeroes after .pz analysis.

 

[jutek]

Re: LDO poles

Hi Jutek,
Its not the problem with that pole. Actually your output pole (due to load cap, esr, and ron of pass device) varies as your load current changes. Please check the poles and zeroes after .pz analysis.

hi

i know the dominant pole changes with Cl and i can see that. problem is with the second pole i'm sure. i checked it with .pz When Cl is big enough the problem disappears, but for lower Cl second pole changes with Cl (lower Cl-> higher dominant and the second pole)and i don't know why

 

[derekqiao]

LDO poles

Because Cl will effect the second pole. If Cl is big, you could ignore it. You can try to write the transfer function which would show the infulence of Cl.