Can a PNP work in your circuit?

If not, then finding a PFET with low turn on specs would be number one 
for me at least.

If you must use a charge pump to improve turn on with a FET, do you have 
any signal already present you can use to drive the pump?  Some 
oscillator etc.?

Next choice if a PIC is handy (already in the circuit) might be to 
toggle a spare pin to produce the pump drive.  Then feed that into a 
diode/capacitor network and to the FET.

One other source sometimes overlooked: MAX232 (or similar) interface 
chips already produce both plus and minus voltages from on-board charge 
pumps and these can be used to provide the required voltage to turn on 
your FET.

Good luck!

Jesse Lackey wrote:
> 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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