NCP349 curious activity on BB-xM rev C

Hi,

I've used a Beagleboard xM for a month or so with a random 5V wall
cube switcher with around 5.4V output with no trouble, getting on with
my development, sometimes unplugging the DC supply for the night and
so on, until one time the board would no longer boot on powering but
rather light up the red led for overvoltage, same every time I plug it
in. D14 is also lit, implying the USB hub is getting power normally,
but nothing is disabling it.

Now, the output of a cheap power supply wandering a bit towards
overvoltage levels would be quite understandable, so I plugged in
another, with somewhat lower voltage. With the same result. However,
connecting the mini-USB boots the board up normally, while the
overvoltage led again stays lit.

So out comes the lab PSU. Connecting a cable with exactly 5V to the
board always shows overvoltage, as expected. Lowering the voltage to
4.5V however boots the board most of the time, where it gets a bit
weird. If I keep the cable connected and cut the power from the power
supply's output enable, I can raise the voltage to 5V and the board
boots up every time I press the enable button, after a short red flash
from the led.

So, NCP349 power-on timer fried or something else entirely? Any bright
ideas, anyone?
I might go around this by feeding the 5V with an external slow-
starting SMPS, but that might solve it. Time for RMA then? It would be
faster and easier if there was a simple fix I could do myself. Also,
there's a small supercapacitor soldered as the RTC battery, will this
void warranty?

Thanks,

-Tommi

NPC349 is an over voltage circuit that prevents voltage over a certain threshold from passing through. The initial flash of the LED is normal because when you plug in a supply, 5V makes first followed by the ground. Without a good ground the circuit trips and activates. There is no timer involved.

Gerald

Well, actually there IS a timer inside the chip, it delays the power
switching by about 10ms from the initial input voltage rise so that
the input voltage has stabilized before connecting the load. Might not
have anything to do with the issue, of course.

The board just suddenly seems to be very sensitive to input voltage
rise time or something similar. Grounding is not likely to be a reason
here, as with the lab PSU I kept the ground connected to the board's
through plated bolt hole and connected the 5V wire separately (during
the testing, not during use of course). Also, the other NCP349 driving
the USB hub supply seems to be working normally (D14 is on). A display
has not been connected during fault search.

Tommi

I was referring to external timers, not the internals of the device. A timer carries the connotation that it can be changed. We have seen cases where that device if hit a few times by over voltage can become a little strange. Best solution would be to get it replaced.

Gerald

Ok, that's about what I was hoping to hear, weird things happening
before with it. To the hospital then, poor little board.

Thanks,

Tommi

They will take care of it! They are a cage free facility so it will get plenty of exercise!

Gerald