How to design/specify smoothing capacitors?

Hardware
1

Hi All,

I’ve been looking at the Relay Shield power schematic which shows a number of smoothing capacitors on both the input and output of the Richtek RT8259:

The Richtek datasheet suggests a much simpler approach:

Can anyone provide any design guidance or reasoning behind the Relay Shield implementation?

Thanks,
Sam

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2

There are many considerations to make in capacitor usage. The manufacturer’s recommendations are a good start plus the other devices it’s powering. Then taming the signals with an oscilloscope and different types of power supplies with your application’s different loads, while using what capacitors are laying around, plus ordering new ones when you can’t find what you need, then realize you messed up and order the right ones only to find they don’t smooth the noise you’re seeing, so you order new ones that fix some signals that makes another really bad one much clearer, so you order more. Next is cosmetic design. Sometimes 2 can be much cleaner than 1 huge one, and there are different packages that look better.

Is that what you meant? :wink:

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3

Thanks, to be honest I was hoping for some kind of design guide or cheat sheet - but perhaps I was being too optimistic!
I hadn’t considered aesthetics or the physical layout so thanks for pointing that out!

4

It’s really not that bad. There are really 2 main things to look out for. Ripple and consistent current. I’m not going to get into protection. If you see dips in the voltage, it’ll cause brown-outs that make it do funky things. The problem with observing them is that they could be on the nanosecond scale, so 10uF, 1uF, 0.1, 0.01 can take care of most of what can’t be easily seen. After that is current required over time. The more current to keep a circuit fully powered, the bigger the cap for the same duration.

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5

@samjenkins It’s an interesting observation and question!

The datasheet almost always provides the bare minimum to get an operational device and should only be used as a design reference. There were several reasons to choose multiple smaller caps over one larger capacitor.

  1. You achieve higher capacitance with cheaper ceramic capacitor that provide lower ESR compared to aluminum electrolytic caps.
  2. You can achieve lower ESR by having multiple caps in parallel
  3. Its easier to place them compared to one large capacitor

Here is some more indepth info on this topic:

I actually had selected the values of the capacitors poorly in the previous revision of the Relay Shield that sent the regulator in self-destructive oscillations when the user powered it up with noisy power supplies or when the power cable was too long.

Hope this helps!

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