Hello all, I just found a flaw in a design due to me not paying 
attention to an important spec.

I'm trying to high-side switch an input from 3 AA batteries wired in 
series, and I'd like it to work down to 2.5V or lower if possible.

The pfet I'm using doesn't have solid specs below 4.5V for Vgs.  Others 
will have a spec down to 2.5V, but are high Ron (up to 30 ohms) and low 
current capability (300mA).  These are marginally useful.

While I'm probably going to change the design to avoid this issue 
entirely, I'm curious how this situation can be handled.

It seems that the possible ways to do this are:

1. use a negative voltage for the gate on the pfet, to achieve say -5V 
relative to the voltage being switched, so that it goes fully on.

2. use an nfet instead, and use a voltage above the battery voltage (a 
few volts - whatever the Vgs for the nfet is) to switch it.

I'm not sure which is uglier to do, neither are convenient.

Ideally, I'd like to be able to switch up to 300mA (with inrush of say 
800mA) down to 2V, and have the current draw in either the off or on 
state be in the 50uA or less range.  So if some sort of voltage boost / 
invert circuit is needed, it has to be real low quiescent.

Thoughts?
Thanks!
J
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