Electronics > Beginners
Linear regulator output capacitors - ceramic, electrolytic, ?
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joeyjoejoe:
Man, electrolytic caps are a pain for hobbyists. Last night I was looking around for the same footprints off Arrow, and realized I'll probably need to change the footprint for a few of them. I thought, man, it'd be great to just use ceramics - 1206 is 1206.

I just finished reading a TI report on regulator stability with ceramics, which ruined that dream a bit. ( http://www.ti.com/lit/an/snva167a/snva167a.pdf ) My circuit does indeed fall to the left of some of those graphs - very low current (16mA) requiring high ESR for stability.

Three thoughts come to mind:

1. I'm not sure if this is specifically targeting LDO's, and if a regular 2V dropout such as a 7805/7812 is equally affected by this, or less so?

2. Or, the articles implies there might be some regulators that are designed with this in mind - how would I find these? Any product series that people can recommend?

3. Finally, would a tantalum cap have enough ESR to prevent loop stability issues and let me have more of a standard footprint on PCB's for these? They are fairly pricey though...
Kleinstein:
The classical linear regulators (e.g. LM317, 78xx) with a drop in the 1-2,5 V range are usually not very sensitive to the output cap. However even if stable, regulation performance gets better with a suitable cap.  However even there a few (e.g. 7905) need care. LM1117 or similar ae a little in between - with a dropout in the 1 V range and still not that picky about the capacitor as true LDOs.

There are some LDOs made to work with very low ESR output capacitor (e.g. ceramic).  I don't know a good way finding them. If you are lucky the manufacturer has a corresponding column in there parametric table or a corresponding subgroup. It is often the newer one that are suitable for ceramic output capacitor. Otherwise it's looking at the descriptions.

There are quite different types of tantalum caps. The more usual ones are typically OK with there ESR for the LDOs. One should still ckeck the data.
Al electrolytic ones are usually OK at room temperature and above, but can be too high in ESR below freezing. They may need an additional ceramic capacitor in parallel.
The alternative could be using a ceramic capacitor with a resistor as a second element.
wraper:
78xx work fine with ceramic capacitors. For LDO it depends. Some work fine with ceramic capacitors. Some require that capacitor ESR should be above certain resistance. The right answer is to read datasheet of particular IC.
wraper:

--- Quote from: Kleinstein on December 08, 2018, 03:47:15 pm ---LM1117 or similar ae a little in between - with a dropout in the 1 V range and still not that picky about the capacitor as true LDOs.

--- End quote ---
They are not in between. Depending on brand of 1117 these could be among the most susceptible. (Edit: particularly LM1117 by TI is something in between). And it has nothing to do with voltage drop as such but how output stage is made and compensation to counter that. Traditional Vregs have emitter follower, hence they are not susceptible by default. There are LDOs with very small voltage drop which are stable with ceramic capacitor.
joeyjoejoe:
Thanks guys!

So I dug this up https://www.st.com/resource/en/datasheet/l78m.pdf, and indeed section 7 (page 24) says an output cap isn't even needed, but it's recommended, and in fact, something with a low ESR is recommended! And, unlike TI's application note which says the caps can be put further from the output, this one says it should be as close as possible to the output of the regulator.

Good news for my hobbyist inventory needs :)
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