Arduino Micro MOSFETs Questions / Concerns, Input Voltage Range

[twomeyw2]

Hi,

I'm evaluating the Arduino Micro for a project and have a couple of questions / concerns related to the schematic and to interface with the various voltage rails.

As I understand, MOSFET T1 acts as the "5V selector".  If Vin is present, it turns off the MOSFET to isolate the Vin generated 5V rail from the USB 5V rail.  I looked at the datasheet for FDN340P and it specifies a maximum Gate-Source voltage of +/-8V.  When the board is first connected to a power supply the T1 Source voltage will be 0V, so anything over 8V could be stressing the part.  Even after the 5V regulator ramps on, the T1 Source will be 5V, allowing for a maximum Vin of 5+8 or 13V.  The Arduino Micro webpage specifies a maximum input of 20V.  Is this an error or am I misunderstanding something about this circuit?

Also, I don't think I have an understanding for what MOSFET T2 is for.  Can someone explain this? 

Thanks and Best

[ron_sutherland]

Are you looking at the Due, it has T1 and T2 (however Uno and Leonardo only have T1). Anyway on the Due, Uno and Leonardo; T1 looks fine, since the LMV358 is powered by 5V and ground, its output is limited to that range. Given that gate of T1 can be driven between 0V and 5V and the source of T1 is connected to 5V, I think the 8V absolute max gate to source voltage is fine. Now looking at T2 on the Due ... hmm that does not make sense, the gate is tied to ground, it will always be on which connects 5V to the USB programming (to ATMega16U2) power.

Another set of eyes on this Due T2 part would be nice, since I am likely missing something.

[twomeyw2]

Thanks for the reply Ron.  I'm looking at the Arduino Micro, see schematic here:
http://arduino.cc/en/uploads/Main/arduino-micro-schematic.pdf

The T1 Gate is tied directly to Vin.  I'm fairly certain this is an issue.  Is there away to go about reporting hardware issues?  It can be fixed with the addition of a resistor and zener diode.

My guess is that T2 is acting like an ideal diode.  Assuming that a reverse voltage cannot result in +Vin being higher than +5V I think it should work.

I'm thinking the Arduino Micro may still be safely usable if I put a 7V regulator in front of it, I'm just worried about headroom for the 5V supply over temperature / life, etc.

[twomeyw2]

FYI - I created a Bug Report about this on github - #2534

https://github.com/arduino/Arduino/issues/2534

[ron_sutherland]

Interesting how the Arduino "Micro" did not exist in my mind until I looked at the schematic you linked.

Looks like T1 on the MICRO board may have issues, but if I am reading it correct once Vin is over six volts the U2 regulator will drive +5V. So the Gate of T1 (with Vin) could go up to about thirteen volts while the regulator is on (it is a problem if U2 is in current/thermal limit).

Looks like T2 on the MICRO board works like a diode until +VIN is above +5V enough for gate-source turn on voltage.

I am still not sure what is up with T2 on the DUE board.

[twomeyw2]

Good point about current/thermal limiting.  I'm still going to use the Arduino Micro but with a 7V pre-regulator to keep it safe.

I looked at T2 on the DUE and that is a mystery to me as well.  If 2 engineers can't figure it on a schematic it is probably a good candidate for a note or description of some sort  (assuming it has a purpose).