GekkePrutser
Posts: 36
Joined: Sat Mar 23, 2013 5:09 pm

50mA limit from 3.3V bus also applies to model A w/o LAN IC?

Thu Nov 28, 2013 2:47 pm

I'm just working on a little project which involves a small OLED display which needs about 100-150mA of 3.3V (depending on how much is on the screen of course as it's OLED)

I looked up the specs and I saw that the Raspberry can only provide 50mA of 3.3V. Which is fair enough, I know the DC-DC converter is already operating near its limits. I could add a voltage regulator powered by the 5V bus but I'd like to keep things as simple as possible.

So I was thinking: The pi I will use for this project is a model A. That lacks the LAN9512 ethernet chip but I assume it still has the NCP1117-3v3 converter which should provide up to 1A (I don't have with me right now so I can't check). I'm just wondering whether that limit of 50mA still applies, I would think that I can pull a bit more current from the 3.3V bus considering the whole ethernet chip is missing? I read the LAN9512 uses about 200mA, would that mean I could safely use 250mA? Or is there another reason why I shouldn't use more current from the 3.3V bus.

Ravenous
Posts: 1956
Joined: Fri Feb 24, 2012 1:01 pm
Location: UK

Re: 50mA limit from 3.3V bus also applies to model A w/o LAN

Thu Nov 28, 2013 2:58 pm

Good question (I have several model A s and might want to know this too at some point).

You could try looking at the schematic:
http://www.raspberrypi.org/wp-content/u ... .2_027.pdf

...the board is the same for both models, so the schematic would still apply. To be sure I think we'd also need to check if the regulator is identical on both models. Not sure how we'd establish that (Getting exact part numbers is tricky sometimes - maybe someone has one of both and can see a part number?)

To be honest, if I ever need to find out I'd just run one to destruction, with an ammeter on the 3.3V load, and check the regulator for excess temperature with a surplus finger.

Ravenous
Posts: 1956
Joined: Fri Feb 24, 2012 1:01 pm
Location: UK

Re: 50mA limit from 3.3V bus also applies to model A w/o LAN

Thu Nov 28, 2013 3:19 pm

To answer the first part of the question, according to the schematic the LAN chip is running off the 3V3 line. So, as long as both board models have the same regulator, you are correct and there is extra capacity.

I doubt they would bother to specify a different regulator for the Model A, but that is just a guess, we don't know what the actual manufacturers are doing...

GekkePrutser
Posts: 36
Joined: Sat Mar 23, 2013 5:09 pm

Re: 50mA limit from 3.3V bus also applies to model A w/o LAN

Thu Nov 28, 2013 3:52 pm

Thanks for the quick reply - I did indeed check the schematic but I'm not sure what's different between the two models (for example the USB ports are both wired through the 9512 on the model B but the single port on the model A is wired directly to the BCM chip on the model A). So there must at least be some zero-ohm resistors or something on the board to reroute the USB port. I wasn't sure what other changes have been made.

But good point about simply comparing the boards, I have actually both a rev2 model B and a model A at home so I can check it out.

GekkePrutser
Posts: 36
Joined: Sat Mar 23, 2013 5:09 pm

Re: 50mA limit from 3.3V bus also applies to model A w/o LAN

Sat Nov 30, 2013 12:05 am

Checked it out, they both have the same RG2 chip by the looks of it but the markings are different.

The first line is 17-33G in both cases. But the second line is different. The Model A one is marked RQB44, the Model B one is RQC18. Strange enough I can't find any reference to these markings in the datasheet. So I don't know what they mean, perhaps it's just a different batch or something?

Edit: Oops, I spoke too soon. It is actually in the datasheet, on page 14. The second line is indeed the batch/facility/work week. So they are the same part, just different batches.

So that probably makes it possible to pull more current out of the 3.3v line on the model A right? I will try it (perhaps over the weekend).

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