Expansion header safe current limits

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1

Hi all,

I was originally looking to just control a 3W Luxeon LED from the
BeagleBoard GPIOs until I saw the +5V rail on the expansion header.
The only documentation that I've been able to find on it suggested
that the current output of that rail is limited only by the AC adapter
that's running the board. The LED power supply would be drawing
somewhere in the neighborhood of 600-800mA off the 5V rail and I just
wanted to make sure that its safe to draw so much current off of the
board directly.

The Digikey AC adapter that's powering the board should be able to
handle the extra load (3W + 2W < 6W), but if pulling that much power
directly through the board has a risk of damaging it (melting a trace
or such), I'd rather pull out a new 5V rail before the adapter goes
into the board.

Best,
Pavak

2

The connectors should handle the current and the board as well, but it would be on the upper limit of things. I would suggest that you bring 5V in separately to power your LED. Not for current reasons per se, but LEDs are famous for making power rails noisy.

Gerald

3

I second Gerald. I would also use a separate supply with good by-pass
(100 uF low ESR in parallel with 100 nF) near the LED. If you are
planning to use the PWM control, you will probably see rather large
current spikes. The rails are not the main problem but you are very
close to what TPS65950 is capable of supplying for the external load.
Be sure not to have too much load connected to the USB port!

Please let us know your results and (possible) problems with this project.

I am doing RGB LED control myself but in the 50 + 2 x 30 mA range.

Good luck,
siñ

4

Thanks for the feedback. I think I'll likely just use two separate
power supplies for now and use a GPIO pin just to turn the LED's
current supply on and off.
I'm driving one of these: http://www.luxeonstar.com/luxeon-iii-star-led-blue-lambertian-23-lm-700ma-p-280.php
(soon to be deprecated and replaced with a new model)
With one of these: http://www.luxeonstar.com/buckpuck-1000ma-dc-led-driver-pcb-mount-p-7.php

Out of curiosity are there any specs as to what level of noise is
acceptable for the Beagleboard to run stably?

At the moment everything runs off of AC power supplies, but I'd like
to move to battery packs- probably a separate pack for the LED and the
board. Sparkfun has these convenient little 5V packs that run off of
AA cells but see 100mV ripples at 135kHz. I'm hoping to do something
similar with 4 or 6 cells in parallel to drive enough current to the
board. Do you think this would be too noisy? What is a typical
approach for conditioning a battery source to drive a Beagleboard?

Best,
Pavak

5

There are no specs as to noise level per se. It can some handle noise, but voltage spike are a different story. That is what you need to watch for. A ripple of 100mV should not be an issue on the 5V rail. As to a battery configuration, similar things to what you have identified as been used for this purpose. It should work fine for powering the board. If you need USB or EDID, then you need a good 5V in. If you are not using 5V, then a battery configuration providing as low as 3.7V-4.0V should work as well.

Gerald