I bought the screen you can see on this wiki : http://wiki.sunfounder.cc/index.php?tit ... spberry_Pi
it have a usb output dedicated to power the pi. it worked fine with the pi 3B, but I wanted to upgrade to pi 3b+ and the power fail. the new pi power on for about 3 seconds ans then bothled shut down. The same pi works fine with the official power plug.
on this picture http://wiki.sunfounder.cc/index.php?tit ... creen5.jpg
, you can see that the output was given for 5V and 2.5A, the same as the official plug, but it doesn't works.
in fact, the little component that does the regulation is the component I highlighted in the attached picture.
this component is a mp1484 (https://www.mouser.com/ds/2/277/MP1484-1186021.pdf
) and is given for 4.75-18V and up to 3A continuous current. So it seems to be enough.
added to that, you can see on this picture : http://wiki.sunfounder.cc/index.php?tit ... creen4.jpg
that there is a 470uF (polarized) capacitor.
I don't know how much current the pi3b+ drag, but it seem to be a big amount for a long time (I tested without any usb peripheral plugged, not even the touchscreen controller, nor the hdmi cable)
I would ask to you electronician, and hardware designer, what would you do to make this power works for the pi 3b+
1/ change capacitor for a higher value? (which value)?
2/ change the mp1484 for a better reference that would not drop it's voltage under load (wich one?)
3/ create my own power from the 12V input of the card ? (once again which regulator I should take as even the mp1484 can't do it)
Thank to all for your hints!