A320 (CFM) FADEC
 

Where can I find info (FCOM was no help) regarding when the FADEC is actually powered off. is it as soon as the masterswich is set to off or only when the engine goes bellow a certain N1/n2?

Thanks in advance

Talking Boeing here, but I'd be shocked if Airbus was different:
The FADEC is powered by a dedicated Permanent Magnet Alternator (PMA) on the engine gearbox. The PMA will keep the FADEC powered whenever N2 is above 8% (it's usually better than that - 'stay alive' can be as low as 4-5% N2. Aircraft power is also supplied (via the fuel On?OFF switch), but it's 'backup' in case the PMA fails - a 'get home' mode. Dispatch is not allowed without a functional PMA.

Five minutes after shutdown.. N2 10% is a governing parameter.

I hate to be ignorant but is there a FADEC capacitor which only obtains a charge from a FADEC PMA which gives the FADEC a 5 minute life? (I never really thought about it...)

It would need to be a heck of a capacitor - the power draw of the FADEC is significant - typically well over 100 watts (enough so that heat rejection is a concern under certain conditions).
Educated guess (and please feel free to correct me if my guess is all wet) - but I'm thinking Airbus may have a time delay relay in the aircraft power circuit that keeps the FADEC powered from the aircraft for five minutes after the fuel shutoff command. We did something like that on the 747-8 so that the engine monitoring equipment could finish downloading everything after shutdown before everything is depowered (except it's 15 minutes).
Easy way to tell if the FADEC is powered is to look at EGT - if EGT is being displayed, then the FADEC must be powered since there is no backup (rotor speeds often have analog backup)

It is from aircraft power after shutdown through the EIU. CFM uses 12%N2

After powering up from cold, the whole aircraft is energized for 5 minutes for systems self-test, then many of the features de-power.

FADEC one of them, easily observed on the ECAM E/WD DU with amber crosses instead of the engine instruments after the time outs.

After ENG shut-down, assuming APU or EXT PWR is connected, the FADEC are left powered (EICAS showing what it should) for the same 5 minutes and then the indications go amber crossed again.

5 minutes

19.2. FADECS

Thanks. I sit corrected.