Power Arduino Micro from 5V pin?

Using Arduino General Electronics
1

Hi All,

I've done a fair bit of research on this and I can't seem to find a straight answer on this one.

I have a project with a Yun and a Micro communicating with each other over TWI.

As I already have a regulated 5v supply (using a TL2575 switching regulator) for the Yun, I'd like to use the same supply for the Micro. So far I have a PCB that feeds this 5V direct into the 5V pin on the Micro, is this bad? So far it seems to be working ok...

It appears 5V pin is connected directly to the output of the NCP1117-5 regulator (see here: http://arduino.cc/en/uploads/Main/arduino-micro-schematic.pdf and here: http://www.onsemi.com/pub_link/Collateral/NCP1117-D.PDF).

But I still don't really understand what this means or more specifically if this even matters?

Many thanks in advance!

2

See fig 25, add a diode from 5v to Vin.

3

fig. 25 in the datasheet you linked :wink: It's an extra protection. But as long as you keep Vin disconnected you don't really need it and feeding 5V is just fine.

4

Thanks.

  1. Looking at the image I have attached it seems that if we power from the Vin pin, the +5V produced by the regulator is connected to the gate of the P-transistor and current will not flow from VIN to +VIN. Is this possoble? If yes how arduino can work when it's connected only to the VIN pin?

2)If we put the diode between 5V and VIN no current flow in the 5v power supply also if we connect another power supply to VIN, that's ok. But are there any downside considering that current can go from 5v to vin?

Thanks

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5

  1. I don't really know why it has T2... The Uno for example only has T1 to disable the USB power.

  2. I don't get it. If you want no power to the 5V supply when feeding a voltage via Vin you need a diode (or the same contraption as T1) to do that. Diode from Vin to 5V only protects the onboard regulator. The regulator (like most) doesn't like it if there is a voltage on it's output and it's input is pulled low / current is used there. The diode creates a bypass for that.

6

So with the diode we want to force +5V at the regualtor input right?

7

T2 is basically a 'perfect' reverse protection diode.

The body diode conducts it voltage on R9 is higher than on C1.
The fet takes over (shorts) if voltage on C1 is > (5volt+Vgth).

With T2 and 5volt injected on the 5volt rail, e.g. from an external supply, it should be possible to leave a load connected to V-in without damaging the regulator from backflow.
Leo..