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I'm designing a PCB to hold an array of WS2812B LEDs. The LEDs are in groups of 4, with roughly 1cm gap between them, 60 total LEDs per PCB.

As per the documentation recommends I add a 0.1uF decoupling capacitor to each LED. Which is what I've done in the attached image.

I'm wondering, as I understand it, the capacitors are there to supply current when the LEDs are turning on really fast, to prevent flickering as the power supply cannot provide the current fast enough. Is there a way I can just use a larger capacitor per group of 4, rather than 4 individual capacitors?

To save PCB space and ease of assembly.

Happy to hear any insights, recommendations or corrections if my assumptions are incorrect, thanks!

LED PCB Design

Explorex
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  • If your capacitor has low enough ESR and ESL, and your connection from the capacitor to each LED has low enough inductance compared to the one capacitor per LED, then yes. You can simulate it. – winny Oct 13 '20 at 06:28
  • The formula U = I x R is the culprit. Long and thin leads = higher R and when there is sudden changes of , it leads to increases voltage drop. And this is a challenge with LEDs that often uses PWM for dimming the LEDs. So without capacitors you need thick and short leads. – Mats Karlsson Oct 13 '20 at 06:45
  • Possible duplicate of https://electronics.stackexchange.com/questions/392925/can-i-replace-these-many-capacitors-with-just-one?rq=1 – MarkU Oct 13 '20 at 06:53
  • @MatsKarlsson Of more importance for such short distances (where R is probably insignificant) and digital electronics with very short rise and fall times is U = L * dI/dt. Where nanosecond dt can still produce large voltage drops over a small trace inductance. – Unimportant Oct 13 '20 at 07:24
  • @Unimportant please look at the PCB and the width of the leads and the number of LEDs. And I agree that the L is also has a impact. – Mats Karlsson Oct 13 '20 at 07:39

1 Answers1

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Can I use a single large capacitor rather than multiple small?

Yes, you can BUT there is a very high chance that using only one supply decoupling capacitor will generate lots of issues, especially if you use only one capacitor for all 60 LEDs.

So yes you can but it is "asking for trouble" so I would strongly recommend against using only one capacitor when you have 60 LEDs.

What kind of issues?

Issues like LEDs not doing what you program then to do because the data gets corrupted. LEDs not passing the data correctly on to the next LED in the string. Any other unexpected behavior. Also, you might introduce EMI and EMC issues due to the long wires between the LEDs and the supply decoupling capacitor.

I would simply avoid this and make sure that there is at least one supply decoupling capacitor close to every LED. Instead of having a capacitor for each LED, in my view you could get away with using one capacitor per 4 LEDs so one capacitor on each PCB. Place the capacitor in the middle between all 4 LEDs and make the traces between capacitors and LEDs as short as possible. It is also good practice to make the ground connection not as separate traces but as a ground plane. Look at how this is done on other PCBs.

Bimpelrekkie
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  • Perfect answer I was looking for, thanks for clarifying and explaining why, it is really much appreciated! – Explorex Oct 14 '20 at 01:40