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From: ravi on 6 Aug 2010 21:59
How can design analog SPDT switch which switches VCC between +3.3V and
5V depending on application? I've an available pin on micro which can
select the VCC. There are some ICs available to do this job but I
would prefer to do it FET or Transistor. Anybody has idea or circuit?

Thanks
From: stratus46 on 7 Aug 2010 00:17
On Aug 6, 6:59 pm, ravi <ravi2n...(a)gmail.com> wrote:
> How can design analog SPDT switch which switches VCC between +3.3V and
> 5V depending on application? I've an available pin on micro which can
> select the VCC. There are some ICs available to do this job but I
> would prefer to do it FET or Transistor. Anybody has idea or circuit?
>
> Thanks

I wold use an adjustable regulator like an LM317 and use the FET
switch to add a second resistor in parallel for the lower voltage so
that the voltage will never exceed the higher of the two selections.
The resistor calculation is slightly more complicated but you can
handle it.

G²
From: John Fields on 7 Aug 2010 09:47
On Fri, 6 Aug 2010 21:17:56 -0700 (PDT), stratus46(a)yahoo.com wrote:

>On Aug 6, 6:59�pm, ravi <ravi2n...(a)gmail.com> wrote:
>> How can design analog SPDT switch which switches VCC between +3.3V and
>> 5V depending on application? I've an available pin on micro which can
>> select the VCC. There are some ICs available to do this job but I
>> would prefer to do it FET or Transistor. Anybody has idea or circuit?
>>
>> Thanks
>
>I wold use an adjustable regulator like an LM317 and use the FET
>switch to add a second resistor in parallel for the lower voltage so
>that the voltage will never exceed the higher of the two selections.
>The resistor calculation is slightly more complicated but you can
>handle it.

---
The problem with that approach, if the OP is trying to switch between
a 5V and a 3.3V supply, is the LM 317's dropout voltage, which will
vary from about 1.3V at 75C with 20mA out, to about 2.2V at 75C with
1.5A out:

http://www.national.com/ds/LM/LM117.pdf

From: Jeff Johnson on 7 Aug 2010 14:38


"ravi" <ravi2neha(a)gmail.com> wrote in message
news:dd623d43-6c24-4300-afd4-944764ccc40c(a)y11g2000yqm.googlegroups.com...
> How can design analog SPDT switch which switches VCC between +3.3V and
> 5V depending on application? I've an available pin on micro which can
> select the VCC. There are some ICs available to do this job but I
> would prefer to do it FET or Transistor. Anybody has idea or circuit?
>

Must easier than you might think! A simple mosfet will do the trick.



VL---MOSFET---+---Vout
|
R
|
VH

If the gate of the mosfet is off then Vout = VH. If the mosfet is on then
Vout = VL.

If VL = 3.3V and VH = 5V then applying 0V to the gate will give Vout = 5V
and supplying 5V to the gate will hopefully give you Vout = 3.3V.

There are some issues with this as the mosfet R_DSON and R form a voltage
divider. If you cannot get a low enough turn on voltage then Vout may be
significantly degraded. Also if you need to source appreciable current then
R may end up being to low. R can be replaced with a p-ch mosfet with similar
effect.

VL---MOSFET1---+---Vout
|
VH---MOSFET2---+

looks very similar to a SPDT switch.

You can do similar things with BJT's.




From: whit3rd on 7 Aug 2010 15:20
On Aug 6, 6:59 pm, ravi <ravi2n...(a)gmail.com> wrote:
> How can design analog SPDT switch which switches VCC between +3.3V and
> 5V depending on application?

You want a 'high-side switch'; either a PNP or PMOS switch device
will do it. This assumes that your intent is to create a voltage
source to a load (the load always draws current), not an analog
switch.

Two sources, so two PMOS devices; they'll work better if you can
bias 'em with -5V on the gate to turn them fully ON, otherwise you'll
have to spend extra money (or use PNP and expect 0.1V drop).
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