Hell, at 1 amp use an AO3401
Would the NDP6020P, with maximum GS voltage of 8V, have been a good choice to switch 9V? Wouldn't you want 12V or at least 10V?
If you use a buck converter with enable pin, then you might not need a P-channel fet.
Why 9volt. Doesn't the ItsyBitsy have a Lipo battery connector, and sleep modes...
Leo..
Why is this so difficult?
The AO4409 is listed by Digikey as "Obsolete".
The AO3401 is listed by Digikey as "Not recommended for new designs".
Is the AO4419 suitable for switching with 3.3v logic? Am I correct to assume it is, from the datasheet statement "Vgs(th) (Max) @ Id = 2.7v"?
Yes I would be happier to see a Gss rated at > 9v.
I am building 2 versions. One runs of the LiPo backpack, which can be powered on and off from the same circuit plugging into 2 connections on the backpack.
The other one needs to be powered from batteries, and I don't have room for AA batteries. The 9v option seems to be a reasonable one. Can it be modified to use the Enable input on the Buck Converter? Maybe. Am I capable of designing it? That's a different question ....
It's not difficult, you are making it that way.
There are 10s of millions of the AO3401 available now.
If the AO3401 went out of stock and you were to order 1,000,000 more, they would would start up production just for you !
Besides, if you made 1,000,000 of these boards and you could not get any more AO3401s, you simply redesign a new board around a different MOSFET.
I think you need to get a grip on things. 
You can get 3000+ here for $43.00 USD
An UNO/Mega uses the FDN340P to switch USB power.
Plenty of stock on ebay.
Leo..
And it has the same pinout as the AO3401 
As with the NDP/B 6020P - is the 8v Gss a problem in this circuit with the 9v battery? The V Dss is -20v ...
Trying to find a source for a small qty of the AO3401 that is less than a month delivery ...
Most low-logic smd fets have a low Vgs(max).
If that's is an issue, then use a voltage divider on the gate.
The diagram is post#15 seems over-complicated.
Just use 10k between source and gate and 10k between gate and collector.
No C1,D1,D2
And the button across collector/emitter of the NPN transistor.
Leo..
January 28th ? 
100/$9.00 ( + $9.00 shipping)
Here on the Canadian Amazon site, that translates to any date between Feb 10 and March 3rd.
I wish it were otherwise.
Located where in Canada ?
Toronto.
Calgary here.
Message me with address and I will send you 5.
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