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Caps for USB powering the cerb40


#1

While waiting for my Cerb40s to arrive, I thought I’ll get the parts needed for USB power.

The regulator is no issue but I find the info on the 47uF capacitors a bit confusing. Yes, the product page mentions these in plural but I’m not sure where on the PCB to solder them.

Furthermore, SMD capacitors of the implied size tend to be ceramic but there is a plus “+” sign next to the solder pads which would indicate polarity.

Please have a look at the attached image too see what I mean.

Does anyone have info to clarify matters?


#2

Through holes are for the RTC crystal on the other side.
The capacitor is similar to the ones that are used on USB Client DP module.

Check the image


#3

Ah, yes for the lying down cylinder shaped 32.768 kHz RTC crystal…I saw it’s footprint on the top side but didn’t understand those were them coming out on the bottom side.

Ok, so it’s a single capacitor then - what kind you think, tantalum?


#4

Yes - tantalum.


#5

Correct. We/community will be updating the wiki with part numbers and pictures.


#6

Thanks, that would be great!


#7

Architect, I think you are wrong. The through holes for the RTC crystal are located near PB8/VBAT on the other side of the PCB.


#8

@ Eric

You are right! I guess the other two through holes are for a different type of USB connector.Which are not used right now.


#9

Datasheet (for the LM1117-3.3) says tantalum is to be used, but general consensus among very smart people is that tantalum is massive overkill.

However, for a single cap, the cost difference is inconsequential, so I’d probably use tantalum :slight_smile:

What size is it? 1206?


#10

What would be the recommendation from the very smart people to use instead? ;D


#11

Read Russell’s answer here: http://electronics.stackexchange.com/questions/18384/why-does-the-lm1117-data-sheet-specifically-specify-tantalum-capacitors

He says you should be just fine with “wet aluminum”, a variety I wasn’t familiar with before.


#12

I would tend to think of it this way: Why would the company that designed and manufactured the component go to all the trouble of specifying a certain type of part if it was not really necessary? All the specifications in a data sheet are there for a reason and I would rather error on the side of believing the experts who made the component than the opinion of some random person on the Internet.

Of course this is just the opinion of some random guy on the Internet :slight_smile:


#13

Russell gives the reason -why- right here:

It also makes it easier to meet the required ESR numbers. You have to be a little more careful with wet aluminum, but it can be done.


#14

That is just his opinion. Without knowing what led the manufacturer to their recommendation or without ‘Russell’ doing some long term MTBF studies there is no way to justify that claim.

To be fair, for the hobbyist it will likely make little difference, and I’m not suggesting the world will end either way. For a company making thousands of boards it can make a very big difference. The reflow profiles, MTBF, and package size are all important factors. Electrolytic capacitors are bigger which might create more challenges with board layout and assembly equipment.

Mainly I just wanted to stress that manufactures go to all the trouble of listing such specifications for a reason. They are the experts on the components they make.


#15

Sorry, double post…


#16

@ Jeff_Birt - post again post again. I agree I agree… too much coffee too much coffee :slight_smile:

I just want a cap that’s easy to order in singles or tens…