Switch off unneeded periperals - how?

[Heinz Liebhart]

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

 

[Robert Monsen]

Heinz Liebhart wrote:

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
+
|
1k |
___ |<
PIC -|___|--| PNP like 2N4403 or 2N3906
|\
|
| 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.