Beagleboard damaged by improper voltage

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1

Hi,

it seems as if I have damaged a Beagleboard xM by improper DC input
supply. Not sure what happens exactly, but U16, a TL1963A linear
regultor, becomes drastically hot while its output is not the expected
3.3V. The board draws around 2A on 5V but linux boots up properly. I
haven't tracked down exaclty whatfor this voltage is used, as far as I
went it is external USB and network and so it explains that the
internals are still working.

What has happend?

In my application I need an external power suppy that is not safely 5V
and so I decided to design it for 8V-12V input and use a LM2940CT
regultor in order to get internal 5V stable.

Several reasons may caused the issue:

1) I am using a quite expensive Agilent E3645 power suppy which has a
on/off pushbutton. After the issue I checked how it ramps up power and
found that it does not do it in a single step but with an intermediate
voltage. During this time the Beagleboard triggers voltage error,
turning the red light on

2) It also may be that initially the LM2940CT while working without
blocking capacitors went into oscillation (I saw that on the
oscilloscope) and caused the internal supply to break.

It all started because I got some LM2940-5 that were more off from
5.0V than the internal voltage supervision was capable off, so the
board was continuously showing red light. And that was the time when I
started experimenting and finally damaged the board.

I am not sure if the voltage regulator itself is broken (less likely)
but more likely the expander IC U15 became broken since it gets hot,
too. If someone could tell me where to get one (or maybe one has one
left over) I can try to replace it. Soldering this case is easy for
me.

Consequence of that: Take care that your 5V are stable and ramping up
without any weird behaviour.

Cheers, Günter (dl4mea)

2

Meanwhile found that the damaged chip is offered by Digikey, ordered a
few and will report if I was lucky getting it repaired.

3

Hi,

I got the replacement SMSC LAN9514 from Digikey, unsoldered the defect
one with a SMD hot air gun and put the new ones on, and the
Beagleboard xM is back working properly. Resoldering was pretty easy
with a 50W solder tap since the solder resist of the board works very
good.

If you want to see a photo of the repaired chip, look at
http://www.qsl.net/dl4mea/Beagleboard/Beagleboard-Repair01.jpg

There surely is an issue with my LM2940 external power regulator,
because I have even damaged another Beagleboard while taking care on
possible ground loops, checking the raise of power and all that stuff.
Will head out for a simple 7805 after the holidays.

All I noticed is that the power for the SMSC chip is on for a short
moment during board power on, so it might be that the regulation at
that time is not properly perfect and so the overswing of input
voltage or anything else in this matter reaches the chip, causing it
to go into a kind of latch up.

Cheers, Günter (dl4mea)