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'[ee]: High or low side switches?'
2002\02\21@053550 by Graham North

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Hi all,

Would just like a quick bit of advise.

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).

Hope someone can comment/help,

Kind regards

Graham North

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2002\02\21@081020 by Russell McMahon

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> 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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2002\02\21@083449 by Rick C.

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Not exactly sure what you want but I think this part will work for you.
www.chipdocs.com/pndecoder/datasheets/SMNS/BTS409L1.html?ReR=GG
btw, this site, chipsdoc.com is the best site for locating any hard to find
docs on devices.
Rick




Graham North wrote:

{Quote hidden}

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2002\02\21@090811 by Olin Lathrop

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> Would it be better to use a high side switch?

Yes.

> If so how? I know that you
> use P type FETS, but have never done it.

Yes, a P channel FET is probably a good solution.  You need to do the rest
of the homework on exactly how to use one.

If you do this, you need to consider the voltage drop accross the FET when
on.  Unless your currents are high, this shouldn't be much of an issue
however.  If the parts you want to selectively turn off have their own
regulator, then you can put the FET at the input of the regulator.  The FET
won't effect the on voltage at all then, as long as the unregulated supply
has the additional headroom to account for the voltage drop accross the FET.
Again, this won't likely be an issue in your application, but you should do
the math to be sure.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, spam_OUTolinTakeThisOuTspamembedinc.com, http://www.embedinc.com

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