Reading the datasheet for an AZ1117E adjustable LDO voltage regulator capable of supplying 1A, I noticed the following recommendation, which I don't understand:
"Close to the OUTPUT pin, it is not recommended to use a capacitor
smaller than 0.68μF in parallel with output capacitor. When the output
capacitor parallels 0.1μF capacitor, the 0.1μF capacitor must be away
from the OUTPUT pin, the distance is no less than 5mm."
What is the reasoning behind that recommendation? Are they concerned about inrush current through a small ceramic capacitor with very low ESR or something like that? I can see why larger 1μF+ capacitors are needed on the input/output, but I don't get why smaller ones should be kept far away from the part.
I'm not sure if the current draw is actually relevant to this question or I'm simply unobservant, but I haven't noticed similar recommendations in other voltage regulators' datasheets.
Thanks!
Best Answer
There is no obvious reason but moving the low ESR Ceramic away from the regulator lowers the self resonance frequency from trace inductance.
Given Load Regulation: 0.2%/A @I OUT = 1A
What is Zout?
If Vout=3.3V @ 1A then 0.002V/1A= 3 millohms. But this is steady state and not transient error, due to DC gain in feedback error and especially low RdOn in the regulator output.
So what is the dynamic output impedance (f,I)? in scattering parameters called s22.
This can be estimated from step response but is typically measured using an Impedance Analyzer.
Two switches in Falstad SIM to examine input and output. Change any values you want.