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Kepler
Kepler
Associate
November 15, 2019
Question

Supply external analog circuitry with AVDD?

  • November 15, 2019
  • 2 replies
  • 1473 views

So, I'm trying to integrate on a Test JIG a NUCLEO-144 with STM32F746ZG on board.

It should be possible to draw some current since AVDD (CN10 pin1) is connected through a ferrite bead to the +3.3V (and in the user manual is written that the +3.3V can be used as supply if the Nucleo is powered by VIN or E5V, the second one is my case).

So the limit of doing this should be the 500mA max of the LD39050PU33R mounted on the board.

I would need not more than 10mA, I guess.

This topic has been closed for replies.

2 replies

Danish1
Danish1
Lead III
November 18, 2019

I think it is fine to draw 10 mA - 50 mA from the same supply as feeds the AVdd on your nucleo board. You should to be careful not to inject any noise onto that line (as you're making an external connection which might couple to sources of noise).

But I also think the 500 mA spec of the regulator that feeds both Vdd and AVdd is probably not the limiting factor.

For a start, 500 mA is only achievable if the voltage drop across the regulator means that the power dissipation limit is not exceeded (power = volts x amps; more power means the regulator gets hotter as it can only lose heat at a certain rate).

And then any power-supplies involved in feeding that regulator must not be overloaded. They probably also feed other part of the board. If you're feeding power in from e.g. a 9V regulator, there's a regulator to drop the 9V down to 5V which might overheat if you draw too much current.

If you're taking power from the USB of your PC, the USB can only deliver 500 mA (and officially the 500 mA must be negotiated; many laptops can't deliver even that much).

Final comment: I think the title of your question is slightly confusing (in that you're not drawing current from the VddA pin, just from the same supply as also feeds VddA), and that might be why no-one else has replied to it.

Hope this helps,

Danish

Kepler
KeplerAuthor
Associate
November 18, 2019

Thank you Danish for the answer.

The point on the noise is right, I will be careful. I shouldn't have problem.

It would be good to have general consideration like this on integrating the Nucleo in bigger custom ecosystem but, I guess, I'm asking too much.

Consideration on external system all refers to Arduino Shield only, for what I found.

(Well, for the title I think is matter of perspective, if I have to to supply something It's obvious that I need to draw current).

Thank you again.

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