Comment by Aurornis | Hacker News Reader

Comment by Aurornis | Hacker News Reader

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The advice to use 1uF or 2.2uF for typical projects is good. It's common to see higher value capacitors in modern reference designs because this is well known among people who actually engineer circuits instead of copy and paste from 20 year old wisdom.

Don't go to a larger package to get more capacitance, though. Capacitors that are physically larger will have worse higher frequency performance. The physical package is part of the limiting factor. Very high speed designs will prefer 0201 capacitors.

Also keep in mind the distribution of your decoupling capacitors. Putting a single 2.2uF in a board in place of multiple 100nF caps distributed around the PCB would be a mistake. The decoupling capacitor needs to be physically close to what you're trying to decouple.

For hobby projects and microcontrollers most of this just doesn't matter. Pick a capacitor and put it on the board. For real high speed work you have to consider the layout. Number, size, and location of vias around the capacitor has a big impact. Loop area is also a big factor. Don't use narrow traces or locate capacitors far away.

On your note about capacitor sizes — at my first EE job, my boss taught me about capacitance-voltage derating[0] for ceramic capacitors and it was quite the revelation. There is a significant inverse relationship between the two, which no one tells you about in college!

I'm now very careful to pick ceramic capacitors with enough headroom on their rated voltage as you lose a lot if you're close to the rated value. This curve is dependent on the different ceramic types as well (C0G, X7R, etc). Cheaper ceramics have a steeper rolloff.

For personal projects I am very careful to pick higher quality ceramics (X7R if I can) and use caps rated to 2-3x my operating voltage. Likely overkill, but I'm not optimizing for cost at volume.

[0] https://resources.altium.com/p/voltage-derating-ceramic-capa...

It is not actually true that MLCC DC bias derating scales with voltage rating. The voltage rating itself actually has nothing to do with it. The correlation is with package size. (Package size and voltage rating are often loosely correlated (and were strongly correlated back in the day), which is where the misconception comes from.) The physical origin of the effect is electric field strength in the dielectric material; thicker dielectrics reduce the field strength, so you don't come as close to hitting the polarizability limit of the piezoelectric materials, at a given applied voltage. Voltage rating doesn't really show up in that analysis.

If you don't believe me, poke around a bit in SimSurfing or similar. You should also notice that most capacitors are actually binned by voltage rating these days: a 16V part and a 50V part might be identically specified, but one's curves just cut off at 16V. I don't know if that's strictly binning or just testing, but it's pretty clear they're the same parts under the hood.

> It is not actually true that MLCC DC bias derating scales with voltage rating. The voltage rating itself actually has nothing to do with it.

This statement used to be false (I used to design boards where I would bump the voltage rating to get better DC bias behavior), but it looks like the engineering behind these capacitors has changed "recently" (as in the last 10 years), and it is now mostly true.

Looking at Murata caps, for example:

1.0uF--uniformly 50% derating from 6-16V:

https://www.murata.com/en-us/products/productdetail?partno=G...

https://www.murata.com/en-us/products/productdetail?partno=G...

https://www.murata.com/en-us/products/productdetail?partno=G...

100nf--uniformly 2% derating from 6-16V:

https://www.murata.com/en-us/products/productdetail?partno=G...

https://www.murata.com/en-us/products/productdetail?partno=G...

https://www.murata.com/en-us/products/productdetail?partno=G...

Interesting. TIL.

Thanks for pointing that out.

Thank you (and the GP) for the correction! I'll admit this lesson came to me a decade ago and I am speaking to a rule of thumb I developed as a result. Time to update my knowledge banks.

The other thing worth mentioning is that there are multiple formulations, and they're not all equal. Just to pick on Murata, the last character of their part number is a reeling code (something so boring it's often omitted or wildcarded), and then the three characters before that represent the specific dielectric material in use. (Or something like that. It's a private use field and I'm reverse engineering it here.) For the examples above, that's "E01", "A01", or "A88". Each of those will behave differently in SimSurfing, but all parts with the same dielectric code will have the same DC bias behavior. (At least, they will if they have the same size and value, etc. When those change too, behavior still follows the usual trends.)

Parts with different codes can have vastly different behavior under DC bias. You'll find that one of them is the clear winner in most cases. Unfortunately, Murata knows this too, and that one is invariably more expensive in distribution.

But at least you can specify it!

Other vendors do this too, but it's easiest to see with Murata's setup and tools.

I think 100nF is arguably still a safer option for people that are just going to copy the datasheet. As mentioned at the end of the post, lots of low inductance capacitance can reduce the phase margin of your linear regulators, and you can very quickly end up with a hundred decoupling caps on a small board. This is of course a solvable problem and sometimes it's just not a problem at all, but it's horrendously difficult to determine when it's going to start being a problem in many cases with big, low inductance power planes and caps littered across them various distances away from each other and the regulator.

The solution is to additionally stuff a bigass electrolytic capacitor on the rail, either tantalum or aluminum. The ESR of the electrolytic will damp out the naughty high-Q tendencies of the ceramics and everything will work out wonderfully in typical cases. (Of course there are pathological cases out there. There are always pathological cases. And if there ever stop being pathological cases, I'll be out of a job!)

At this point—thank you!—a Zachtronics game was born in my head. I’d like to play it!

(Maybe it’s a good secret level in that Zachtronics game about nondeterministic infinitesimals portrayed as getting things done in a corporate environment;… what was the name of that one again?)

Having more decoupling capacitance on the board will increase inrush current as well, as all the capacitors have to be charged up once power is connected. Using larger decoupling capacitors than necessary might mean you'll have to add measures to decrease inrush current where you'd otherwise not need to.

Softstart is often good design though and shouldn't be too hard anymore.

How many circuits cannot afford 100us of softstart?

I guess to your point though: 100uF of capacitance (because of a lot of 10uF caps) to 3.3V requires a 3.3Amp softstart over those 100us startup time. So you still can't go crazy spammy.

While 100x 100 nF caps is only 10uF all together.

For hobby projects, paying attention to the datasheet model / reference application diagram, layout, PCB and other notes is likely to matter and still be the right thing when you can't tell any better - even when the datasheet does not mention the optimal decoupling capacitor. And that's because NOT doing that can lead to problems too difficult for the hobbyist to troubleshoot. At least start with the model layout and only then increase the decoupling capacitance. So many hobbyists seem to totally ignore the reference layout.

In particular adding capacitance in random places or seat-of-the-pants-ing a layout is not helpful.

Or just open the simulator/charts on the capacitor manufacturer website and look at which capacitor filters which frequencies at which temperatures?

Apparently most EE's don't do this.. I've seen decoupling caps in designs that basically do nothing.

Do you believe they do nothing or know they do nothing? The number of times a manufacturers website told me one thing and the actual hardware told me something different is quite high.

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