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'[EE]: Peripheral Power Options'
2002\06\05@104203 by Tim McDonough

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I'm working on a project where I would like to conserve battery power
by turning sensors and a radio module off when not needed. The items
don't require much current, less than 10mA each. I'm interested in
hearing thoughts on the following options:

1) Have the PIC control a small FET to switch device power on/off
2) Power the device directly from a port pin
3) Use a port pin to drive an inverter/buffer which powers the device

The PIC will periodically be in sleep mode.

Tim

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2002\06\05@110256 by John Walshe

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Hi Tim,
   I have found that driving such devices from a pic(876 in my case) port
pin, while power is ~3.6V (typical battery power), will not work. For some
reason the PIC cannot supply the current at this lower voltage. In one test
I had to parallel 5 pins to get this to work (10mA load @ 3.5V). I now use a
P channel FET (NDS0601T iirc) which seems to work a treat. Put the FET
between Vdd and the power pin of the load device. Pull the gate to 0v and
the device gets power. Set the gate high(~Vdd) and the power is removed from
the load.

John

{Original Message removed}

2002\06\05@110713 by M. Adam Davis

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All of these options are reasonable.  I suspect that the inverter/buffer
will draw the most current when the devices are off.  Driving the PIC
pin low would ideally be the lowest current, but if you have problems
with other devices powering through their pins to the pic pin then it
may end up using more current than the FET (which hopefully has a very
low leakage current).

In the end they are similar enough that you ought to set the circuit up
and measure the current draw of each method in both the on and off states.

You may find that they all provide equally low current draw when off but
one or the other might draw more current when in the on state than another.

This is assuming you are looking for low current draw.  If you want high
switching speed or some other variable this won't help you that much.

-Adam

Tim McDonough wrote:

{Quote hidden}

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2002\06\05@124919 by Harold M Hallikainen

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       Remembering that the ideal design has zero parts, I'd suggest powering
the part off the port pin. That's assuming the load is less than 25mA
(which you say it is), and is a 5V load.
       For larger loads that are not 5V, I've used the Micrel MIC2514. It's a
nice high side switch with overcurrent protection.

Harold


On Wed, 5 Jun 2002 09:38:08 -0500 Tim McDonough <.....timKILLspamspam.....MCDONOUGH.NET>
writes:
{Quote hidden}

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2002\06\05@140249 by Tim McDonough

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On Wed, 05 Jun 2002 09:43:03 -0700, Harold M Hallikainen wrote:
>Remembering that the ideal design has zero parts, I'd suggest
>powering the part off the port pin. That's assuming the load is less
>than 25mA (which you say it is), and is a 5V load.
>For larger loads that are not 5V, I've used the Micrel MIC2514. It's
>a nice high side switch with overcurrent protection.

The zero part count solution is appealing for both simplicity, cost,
and board real estate. Having no experience to draw on I just wasn't
sure if there might be any issues with noise getting back into the PIC
via the port pin circuits, etc.

All loads are 5 volt and all of them together are less than 25mA.

Tim

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2002\06\08@140622 by Tim McDonough

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Thanks to all those who replied both on and off list regarding powering
peripherals with port pins, FETs, and inverters. After considering the
emails I received and checking with Microchip I'm going for port pin
power since the external devices all run off 5 volts and the current
requirements were well under the 25mA per port pin specification.

Tim

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