> I am working on a fairly complicated instrument which includes SBC, DSP,
> PLD, various digital circuits, 24 bit ADC, various analogue circuits. My
> problem is that I would like to be able to turn "off" all the circuitry
> but the SBC, using a 3.3V GPIO on the device.
>
> My first thought was this is simple, logic lever N type FETS used as low
> side switches to completely cut the power to the various circuits. Then
> I thought that this could be a problem with the analogue circuitry and
> ADC as these need a good connection to the analogue ground plane.
>
> Would it be better to use a high side switch? If so how? I know that you
> use P type FETS, but have never done it.
>
> My other problems/thoughts were that P types are generally more
> expensive, and this thing costs enough already! And that I can only
> afford to use a single GPIO (not many left).


A PNP high side transistor would work very well.

A FET will take more effort.
P Channel FETs tend to have higher Rdson than equivalent N channel FETs and
logic level versions are harder to come by. Here you want extremely low
logic level - say around 2v Vth. Even FETs which nominally work at such low
gate voltages tend to have poor Rdson until gate voltage gets up to more
like 5 volts "which you have not got" (Naming of Parts).

Don't know what current you need to support, but for up to about 1 A a small
E-line Zetex (like TO92 but better heat dissipation) will do the job
beautifully and a few of these will do several amps OK.  Start with ZTX749.
Members in this family typically have Betas in the several 100's range at 1A
(!!!).

A very cheap alternative at lower currents is eg BC327. Depending on current
a number of small TO92's are available with reasonable Betas.

A PNP can be driven with a single resistor directly from a PIC pin or with 2
resistors (pull off or on as desired) if the pin is to go high impedance
(such as when the PIC is off :-) ).

.

        RM

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