Hello all! Regarding local decoupling capacitors @ each pixel, all of my WS2811/WS2812 strips have one decoupling capacitor immediately adjacent to each pixel. This is one of those old-school “rules of thumb” when doing PCB design to provide local reservoirs of charge during switching transients and to stabilize the supply voltage @ each pixel. I am noticing that the new APA102 strips that I have seen do not have these capacitors. Does anyone have any insight or comments as to whether they are actually needed for the APA102? Are manufacturers just trying to save money at the expense of performance or doesnt it matter because the APA102 are “more stable” or are they built-in (I doubt it)? Thx for any insights!
-frenchy (Steve French)
http://www.voltvision.com
Very good observation !
If the APA102 do not have built-in caps then I would say that the strips have a flaw in their design.
JP,
I just checked the 5 different datasheets that I have for the APA102 pixels and none of them mention built-in capacitance. Furthermore, I have seen APA102 strips from at least 3 different non-related manufacturers and none of them use the local decoupling caps…hmmm…
I think I will go to fab with the decoupling caps on the PCB, but then not assemble them in production…then do an elaborate blog post with transient oscilloscope captures as I start adding the caps one by one on a test board. In theory these transients are the same independent of the SPI frequency… so I am curious about how a related failure mode would reveal itself in the real world? Limit your data rate (not sure why/how)? Limit your Vin range tolerance? Hmmm… good thing adding them to the PCB doesnt cost more $!
I agree, I have also noticed the lack of decoupling caps in the Greeled and APA Electronics data sheets.
Well, in my humble opinion, there is no reason at all to deal with the APA102 differently than the WS2812 from a decoupling capacitors requirements standpoint for strips. They both have similar if not identical current switching characteristics.
JP, I agree about the identical current switching characteristics, but perhaps the internal IC is “more tolerant”? I asked my factory about it and this is their response:
“APA has two signal curcuit, but ws2812 only Dat. APA signal stable than ws2812. Don’t need capacitor.”
Typical Chinese tech (more likely sales) support answer. Well, what can I say… it is their product and you have to accept the statement but… it is still not playing right in my mind !!
“it is their product and you have to accept the statement”
But you said it yourself: it’s not their product, it’s their engineers’ product. They’re just selling it.
@Steve_French @Lucas_Morgan I think this discussion relates to the boards shown by Lucas in this post: https://plus.google.com/+LucasMorgan42/posts/6retDq4xaio
From the photos it seems that you didn’t use caps. I’d like to design a PCB with multiple APA102 LEDs as well and I’m wondering what you found out regarding to using or not using caps.