Re: gEDA-user: Need help repairing a damaged FPGA board (GR-PCI-XC2V)

[Peter Clifton <pcjc2@xxxxxxxxx>]

On Mon, 2010-03-01 at 17:00 -0500, Timothy Normand Miller wrote:

> Any suggestions and help would be most appreciated!

Just as a reference.. I presume you have the user manual for the board.
It mentions the power supply stages, and shows a diagram..

5V in -> 3.3V -> 1.5V core voltage, via 2 LDO regulators (the LM1085
chips on the board). Looking up their pin-outs, noting pin 1 is the one
with the dot next to it.. you should have:

1: Adj/Ground (Might connect to GND, might be a divided version of the
output.
2: Output (also connected to the metal tab at the back)
3: Input.

One regulator ought to show 5V in, 3.3V out, the other should be 3.3V
in, 5V out.

Certainly change out the damaged components first. The tantalum
capacitor might have blown due to over-volts - or reverse polarity. (Was
that what happned?)

The diode might be the suggested reverse input -> output connection
diode for the regulator - OR, be a reversed protection diode to short
the input in case it is the wrong polarity - OR, a series pass diode for
the input. (Or none of the above).


I would suggest initial power-up via a current limited lab PSU. Don't
set it too low though, or the board won't boot. I's suggest trying at
0.5A to start with, giving it up to 1A as it wants..

That ought to help prevent further collateral damage if the regulators
are blown.

Having just thought of this.. ramp up the voltage slowly.. these are LDO
regulators, not switchers - so you "ought" to be able to persuade them
to come up slowly. At 2 or 3V input, the 1.5V output "might" just start
to regulate, make sure it doesn't exceed 1.5V. Similarly - watch the
3.3V output, and make sure it doesn't rise above 3.35, 3.4 (say), as you
power up.

In any case - if there was damage to be done, it has probably already
been done - so you're unlikely to screw things up further.

You might even try powering up without the diode - perhaps even without
the capacitor - or solder a similar value capacitor in its place for
testing.

Best wishes,

-- 
Peter Clifton

Electrical Engineering Division,
Engineering Department,
University of Cambridge,
9, JJ Thomson Avenue,
Cambridge
CB3 0FA

Tel: +44 (0)7729 980173 - (No signal in the lab!)



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