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'[EE]: low current operation and switching a chip o'
2002\05\18@222414 by Peter Grey

picon face
I have an application where I am running a PIC in SLEEP mode and want to
turn the power on to other chips when the PIC wakes up then when finished I
turn the power off. I am trying to get zero current draw from these other
chips. I also want to have minimum size of the switching element and
minimum weight. Cost is important but it is secondary to the weight and
size configurations. I am running at 3 volts and the remainder of the chips
run at 3 volts. I can achieve this with a PNP & NPN transistor but is there
a way with 1 transistor??

TIA


Peter

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2002\05\18@223450 by M. Adam Davis

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face
Is there a reason why you couldn't simply power these other chips from
the pic output pins themselves?  They can source 20mA per pin, and you
could even parallel some pins if needed (but be very careful if
paralleling them!).  If not, then you can use one transister or mosfet
to switch those other devices on and off.  The output will be inverted
(ie, you'd put a low on a pin to have the transister allow current
through), but there are lots of projects on the internet that show this
mode of operation for a transister.  Look into driving LED displays in
particular.

-Adam

-Adam

Peter Grey wrote:

{Quote hidden}

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2002\05\19@001410 by Bob Ammerman

picon face
As mentioned before: if the other chips draw little current then they can be
driven directly by a PIC output.

Otherwise you can do the following:

PNP transistor

Emitter to Vdd

Collector to power in of other chips

Base, via resistor (say 2K) to a PIC pin.

Optional: for additiional assurance that transistor stays in cutoff when
powered down insert another resistor (say 20K) from the base of the
transistor to Vdd.

When PIC wakes up have it drive pin low. This will turn on transistor and
connected load.

When PIC sleeps tri-state pin. The transistor will go into cutoff and load
will draw no power.

Note that your load will see a voltage slightly less than the Vdd voltage by
the amount Vce(sat) (the saturation voltage of the transistor).

Bob Ammerman
RAm Systems



{Original Message removed}

2002\05\19@002634 by Matt Pobursky

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On Sun, 19 May 2002 10:19:19 +0000, Peter Grey wrote:
>I have an application where I am running a PIC in SLEEP mode and
>want to turn the power on to other chips when the PIC wakes up
>then when finished I turn the power off. I am trying to get zero
>current draw from these other chips. I also want to have minimum
>size of the switching element and minimum weight. Cost is
>important but it is secondary to the weight and size
>configurations. I am running at 3 volts and the remainder of the
>chips run at 3 volts. I can achieve this with a PNP & NPN
>transistor but is there a way with 1 transistor??

P-channel Mosfets with a low Vgs(on) threshold work nicely as a
zero current (from PIC pin) power switch. As long as all the
other devices run 3V as well, you shouldn't have any trouble.
Zetex, Fairchild and others all make suitable devices.

You can also use the PIC output port pin to power other devices
as long as the current required is relatively small.

Matt Pobursky
Maximum Performance Systems

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2002\05\19@003712 by tundra

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face
> On Sun, 19 May 2002 10:19:19 +0000, Peter Grey wrote:
> >I have an application where I am running a PIC in SLEEP mode and
> >want to turn the power on to other chips when the PIC wakes up
> >then when finished I turn the power off. I am trying to get zero
> >current draw from these other chips. I also want to have minimum
> >size of the switching element and minimum weight. Cost is
> >important but it is secondary to the weight and size
> >configurations. I am running at 3 volts and the remainder of the
> >chips run at 3 volts. I can achieve this with a PNP & NPN
> >transistor but is there a way with 1 transistor??

Something like a 2N7000 might do nicely.  I use these for muxing
common-cathode 7-Segment displays running at 5V.


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2002\05\19@011743 by Peter Grey

picon face
At 11:59 PM 18/05/2002 -0400, you wrote:

Thanks Bob. I should have mentioned that I am chasing every little uA of
current. Do you think there will be any leakage in this case? I did not
consider running the other chips off a PIC output because I thought there
would be some leakage. Looking at the data sheet of 1 device it suggests
running the Vdd off a micro output but says the leakage current is some
5uA. Would I be better off using a FET as someone else has suggested?

Thanks,


Peter

{Quote hidden}

>{Original Message removed}

2002\05\19@083134 by Bob Ammerman

picon face
> Thanks Bob. I should have mentioned that I am chasing every little uA of
> current. Do you think there will be any leakage in this case? I did not
> consider running the other chips off a PIC output because I thought there
> would be some leakage. Looking at the data sheet of 1 device it suggests
> running the Vdd off a micro output but says the leakage current is some
> 5uA. Would I be better off using a FET as someone else has suggested?
>
> Thanks,
>
>
> Peter

A decent transistor in cutoff should have little or no leakage.
I'm not sure what the 5uA spec is about. If the device is powered from a PIC
pin, then the only thing that matters is the leakage on that PIC pin. If the
PIC pin is driven active low then you shouldn't have any leakage at all.

Watch out for devices being powered by their inputs thru their protection
diodes.

Bob Ammerman
RAm Systems

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2002\05\19@093445 by Olin Lathrop

face picon face
> I have an application where I am running a PIC in SLEEP mode and want to
> turn the power on to other chips when the PIC wakes up then when finished
I
> turn the power off. I am trying to get zero current draw from these other
> chips. I also want to have minimum size of the switching element and
> minimum weight. Cost is important but it is secondary to the weight and
> size configurations. I am running at 3 volts and the remainder of the
chips
> run at 3 volts. I can achieve this with a PNP & NPN transistor but is
there
> a way with 1 transistor??

It sounds like all you need is a single PNP in series with the supply to the
other chips if you can tolerate the 200mV - 300mV drop.  In that case the
emitter goes to + supply, the collector the the supply input of the other
chips, and the base to a PIC pin via a suitably large resistor.

A P channel FET might give you a smaller voltage drop, depending on the
current draw of the other chips.  The problem is that 3V will not be enough
to really turn the FET on, so you have to find one that has a sufficiently
low guaranteed on resistance at -3V on the gate.  It might be possible, but
many FET data sheet won't guarantee anything at -3V gate.


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(978) 742-9014, http://www.embedinc.com

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2002\05\19@093458 by Olin Lathrop

face picon face
> Thanks Bob. I should have mentioned that I am chasing every little uA of
> current. Do you think there will be any leakage in this case? I did not
> consider running the other chips off a PIC output because I thought there
> would be some leakage. Looking at the data sheet of 1 device it suggests
> running the Vdd off a micro output but says the leakage current is some
> 5uA. Would I be better off using a FET as someone else has suggested?

First, all PIC outputs have low side drivers, so when the other circuit is
off the PIC output pin will be actively driven to 0V.  Leakage into the
other circuit is not an issue.  Second, whatever "leakage" there is thru the
disabled high side drivers when a totem pole output is low is there anyway,
it just goes thru the low side driver instead of to the output.

Again, how much current does the other circuit draw when on?


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(978) 742-9014, http://www.embedinc.com

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2002\05\19@222609 by Peter Grey

picon face
At 09:01 AM 19/05/2002 -0700, you wrote:
>> Thanks Bob. I should have mentioned that I am chasing every little uA of
>> current. Do you think there will be any leakage in this case? I did not
>> consider running the other chips off a PIC output because I thought there
>> would be some leakage. Looking at the data sheet of 1 device it suggests
>> running the Vdd off a micro output but says the leakage current is some
>> 5uA. Would I be better off using a FET as someone else has suggested?
>
>First, all PIC outputs have low side drivers, so when the other circuit is
>off the PIC output pin will be actively driven to 0V.  Leakage into the
>other circuit is not an issue.  Second, whatever "leakage" there is thru the
>disabled high side drivers when a totem pole output is low is there anyway,
>it just goes thru the low side driver instead of to the output.
>
>Again, how much current does the other circuit draw when on?

about 3-5 mA is all.

Peter
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2002\05\20@084016 by Olin Lathrop

face picon face
> >Again, how much current does the other circuit draw when on?
>
> about 3-5 mA is all.

In that case I think the best bet is to drive the circuit directly from a
PIC pin.  I've done this for some low power battery operated projects.
There is really nothing wrong with this since everything remains well within
the manufacturers specs.


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(978) 742-9014, http://www.embedinc.com

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2002\05\20@200317 by Harold M Hallikainen

picon face
Also have a look at the Micrel MIC2514 high side switch. It's pretty
neat, and has a current protected output so you don't accidentally blow
the switch.

Harold


On Sat, 18 May 2002 22:32:23 -0400 "M. Adam Davis" <adampicspamKILLspamUBASICS.COM>
writes:
{Quote hidden}

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