Feeding an external board.

Hi there.

I wondered if anybody has feeded an external board from the onboard 3.3V
power. My question is regarding if it is safe, and how much current can be
drain.

Thanks for your help and best regards.

It would all depend on the power supply used on your board and the internal
power requirements. The onboard regulator may not have any headroom for
your external supply unless you only need a few mA. On the Mini210 this is
a switched mode IC capable of about 2A but that does not mean the design
will support 2A as the heat dissipation may not be sufficient or the
components may only support a lower current.

If I was you and to avoid damage to the onboard regulator I would use an
external regulator and power this from the VDD_5V output from the board or
from your external supply.

If your requirements are very small you can use a simple linear regulator
such as the NCP1117LPST33T3G, which I have used with great success.

How much current does you external hardware require?

Dave...

Hi Dave. Thanks for your answer.

The external board will drain 50mA when idle, and 150mA at certain times.
The reason for what I want to use the onboard supply, is to avoid the use
of an external PCB for the regulator (more than the regulator itself). 

So I want to find out if it is possible before I have to add...another PCB.

ferite

Which board are you using?

Maybe knowing this someone else might be able to answer your query or we
can make a guess from the onboard power supply design of your board
(FriendlyArm one that is)

Dave...

It is a RFID module. It is supposed that I can give deeper details as the
module reference etc... :-(

What i know by analizing that board, is that it does not have an on board
regulator. Because of that, the feeding 3.3v should be regulated. I am
afraid I should use an external board :-(

Hi Ferite,

I mean, which FriendlyArm board are you using?

Dave...

Hi David. Sorry for the misunderstanding. My board is a Mini2440 whith W35
LCD.

Hi Ferite,

I downloaded the schematic and the 3.3V regulator is a low dropout linear
one capable of providing 800mA.

As most of the components on the board are running at 3.3V (processor has
additional supplies) you could measure the current drawn at 5V and kind of
work out what sort of current is being drawn on the 3.3V

I won't be accurate but it might be enough for you to determine if you can
draw power but to be honest and also to reduce the likelyhood of spikes on
the 3.3V supply because you are using an RFID device which does not draw a
constant current, I would use an external 3.3V regulator to power your RFID
module. There are quite a few in through hole packages that should be easy
to wire up if you don't want to use SMD.

Also have a look at Recom as they do nice switched mode supplies that use
the same footprint as TO220 style 78xx packages.

Dave...