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I have a general question regarding the PSR of the LDO. What type of pass element (NMOS or PMOS) gives better PSR if all the other circuit elements are the same?
We could say we have a high impedance between output and ground in PMOS pass element case which couples a large portion of supply noise in contrast to common drain which has a low output impedance in NMOS pass element case?Look for impedances for both cases
Could you please elaborate why it has a supply zero? which RC case this zero?The problem with the PMOS (and PNP) pass element
is that its Miller capacitance tends to magnify supply
perturbations with close-in high (and variable) gain,
and the error amplifier has to defeat this (but cannot,
at higher frequencies, if it is designed with limited
power and slew rate).
Especially where lazy designers have chosen to make
the PMOS pass device "forward pole" the main
compensation capacitor, neglecting that it also adds
a "supply zero".