Heinz Liebhart
Guest
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 Posted:
Wed Feb 16, 2005 12:02 am Post subject:
Switch off unneeded periperals - how? |
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Hi Folks,
I'm looking for a solution to switch off not used peripheral components in a
portable uP system.
My first thought was ULNxxxx, but, cause they have a rather hi C-E-drop
voltage, they are not suitable.
Does anybody have a suggestion how to do this?
Current is about 20mA...
I'm thinking on something with a (mos)fet, but I'm not really fit with that
semi-analog stuff.
TIA for any hints,
Heinz
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Robert Monsen
Guest
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| Posted:
Wed Feb 16, 2005 2:22 am Post subject:
Re: Switch off unneeded periperals - how? |
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Heinz Liebhart wrote:
| Quote: | Hi Folks,
I'm looking for a solution to switch off not used peripheral components in a
portable uP system.
My first thought was ULNxxxx, but, cause they have a rather hi C-E-drop
voltage, they are not suitable.
Does anybody have a suggestion how to do this?
Current is about 20mA...
I'm thinking on something with a (mos)fet, but I'm not really fit with that
semi-analog stuff.
TIA for any hints,
Heinz
|
If the current was less, you could simply power it from a PIC pin...
I've done that before. They will source or sink up to 20mA.
A PNP transistor will give you a pretty good 'high side' switch,
particularly for such a low current.
VCC
+
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1k |
___ |<
PIC -|___|--| PNP like 2N4403 or 2N3906
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| Output
'-------------
Drop is about 0.2V, which is Vce(sat) for the transistor.
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
Turn it *off* by setting the pin high. Turn it on by setting the pin to
ground.
You can use a MOSFET as well. In that case, you don't need a gate
resistor, and it's possible the V drop won't be as severe. However, make
sure you get a 'logic level' MOSFET. Otherwise, you won't be able to
turn it all the way on, and the drop across it will be bigger. Logic
level mosfets turn on at a Vgs of less than 5V, as specified in the
datasheet.
An N-channel mosfet will go between ground and your circuit, a P-channel
mosfet will go between VCC and your circuit. N or NPN are both active
high, whereas P and PNP are active low.
Reed switches may also work, but I don't have any experience with them,
so I can't help there.
--
Regards,
Robert Monsen
"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God. |
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