# Why should small capacitors be placed far away from the 'output' pin on a medium-sized LDO?

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!

• 0.1uF in parallel with a larger cap could form a resonance. The 5mm spacing would add some inductance that might reduce the resonance. This is really a guess. – EE_socal Mar 19 '18 at 20:08

There is no obvious reason but moving the low ESR Ceramic away from the regulator lowers the self resonance frequency from trace inductance.

It will make significant improvements with 2nd order LPF effect for attenuating noise and emanations but I am guessing will also improving loop phase margin with better transient feedback.

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.

• Interesting, thank you! It might be a little while before I really understand that explanation, but it definitely gives me some more topics to look into. – Will Mar 19 '18 at 21:39
• The SIM allows you change values and see the switch effects – Tony Stewart Sunnyskyguy EE75 Mar 20 '18 at 1:19