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How to consider the stability issue in an classical LDO?

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Member level 1
Dec 7, 2004
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ldo heavy load

hi guys:
As an classical LDO,comprised by three stage:an high gain stage,a buffer stage and a output stage.A big cap is at the output and the esr is essential to the stability.
There are 2 poles and 1 zero in the loop.My question is when the loading is changing and the 2 poles will be changing and the dominant pole will be exchanged between the 2 poles,how to analysis the stability issue?
Can anyone help,thanks~~:D

you need refer to Rincon Mora's papers

pole exchange under different load condition maybe a potential problem, try to make the dominant pole stable under all load condition.

if the ldo use the external capacitor, the dominant pole will vary under different load condition.

so, how can we make the dominant pole stable under all load condition?

thanks guys,I think maybe I should get several paper for reading

In most of LDOs, the dominant pole will change as load. For light load, the dominant pole is at the output node. For heavy load, the dominant pole might locate inside. However, you can design a LDO that dominant pole is at outout node for your max loading if all internal poles are pushed to very high frequency.

Added after 1 minutes:

or internal poles are compensated by internal zeroes. Many commercial LDO has internal zeroes to form pole-zero pairs.

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