analog switch

R

ravi

Guest
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
 
On Aug 6, 6:59 pm, ravi <ravi2n...@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
Click to expand...
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.

G˛
 
On Fri, 6 Aug 2010 21:17:56 -0700 (PDT), stratus46@yahoo.com wrote:

On Aug 6, 6:59 pm, ravi <ravi2n...@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.
Click to expand...
---
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
 
"ravi" <ravi2neha@gmail.com> wrote in message
news:dd623d43-6c24-4300-afd4-944764ccc40c@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?
Click to expand...
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.
 
On Sat, 07 Aug 2010 08:47:31 -0500, John Fields <jfields@austininstruments.com> wrote:

On Fri, 6 Aug 2010 21:17:56 -0700 (PDT), stratus46@yahoo.com wrote:

On Aug 6, 6:59 pm, ravi <ravi2n...@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
Click to expand...
Unless you're switching between regulating down from a higher
voltage, then the LN317 approach is useful. But OP asked to
switch between two voltages, not control a regulator. So
obviously an SDPT switch is either two p-channel or pnp devices
forming high side switch, or perhaps an 'hc405{2,3} if the load
is in the mA range.

Grant.
 
On Aug 6, 6:59 pm, ravi <ravi2n...@gmail.com> wrote:
How can design analog SPDT switch which switches VCC between +3.3V and
5V depending on application?
Click to expand...
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).
 
ravi 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
Feed the 5V through a couple or 3 diodes for the low side and bridge
Click to expand...
this link with a PMOS high side to deliver the full 5? Depending on the
selected diodes, they can drop your voltage on the average of .3 to .6
volts each.

I don't know how stable your circuit must be but, its a simple
solution and the diodes will give you some kind of reg over a R
net work. How ever, if you know your load is constant, then use a
R inplace of the Diodes..

etc..

Just a thought..
 
On Fri, 6 Aug 2010 18:59:05 -0700 (PDT), ravi <ravi2neha@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
Click to expand...
Is the application the availability of two different supplies and
something makes it desirable to have the ability to switch between
them, or a necessity to HAVE two different supplies?
--
 
On Sat, 7 Aug 2010 13:38:54 -0500, "Jeff Johnson"
<Jeff_Johnson@Hotmail.com> wrote:

"ravi" <ravi2neha@gmail.com> wrote in message
news:dd623d43-6c24-4300-afd4-944764ccc40c@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.
Click to expand...
---
Except that it won't work because of the MOSFET's body diode.
---

You can do similar things with BJT's.
Click to expand...
---
This seems to work:

Version 4
SHEET 1 880 724
WIRE 224 16 -144 16
WIRE 224 96 224 16
WIRE 48 144 0 144
WIRE 160 144 128 144
WIRE 0 192 0 144
WIRE 224 224 224 192
WIRE 400 224 224 224
WIRE 224 256 224 224
WIRE 0 304 0 256
WIRE 48 304 0 304
WIRE 160 304 128 304
WIRE 400 336 400 224
WIRE -144 432 -144 16
WIRE 0 432 0 304
WIRE 224 432 224 352
WIRE -144 624 -144 512
WIRE 0 624 0 512
WIRE 0 624 -144 624
WIRE 224 624 224 512
WIRE 224 624 0 624
WIRE 400 624 400 416
WIRE 400 624 224 624
WIRE -144 704 -144 624
FLAG -144 704 0
SYMBOL pnp 160 192 M180
SYMATTR InstName Q1
SYMATTR Value 2N3906
SYMBOL pnp 160 256 R0
SYMATTR InstName Q2
SYMATTR Value 2N3906
SYMBOL voltage 224 416 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 3.3
SYMBOL voltage 0 416 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value PULSE(0 5 0 1e-6 1e-6 .1 .2)
SYMBOL voltage -144 416 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 5
SYMBOL res 384 320 R0
SYMATTR InstName R1
SYMATTR Value 100
SYMBOL Digital\\inv -64 256 R270
WINDOW 3 8 104 Invisible 0
SYMATTR InstName A1
SYMATTR Value trise 1e-7 tfall 1e-7 vhigh 5V
SYMBOL res 144 128 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R3
SYMATTR Value 1000
SYMBOL res 144 288 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R5
SYMATTR Value 1000
TEXT -88 656 Left 0 !.tran 1


Whether the OP is going to get something that works for him is going
to depend on the load current and what's available in terms of
Vce(sat) at that current.

JF
 
On Sun, 08 Aug 2010 08:03:10 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Sat, 7 Aug 2010 13:38:54 -0500, "Jeff Johnson"
Jeff_Johnson@Hotmail.com> wrote:



"ravi" <ravi2neha@gmail.com> wrote in message
news:dd623d43-6c24-4300-afd4-944764ccc40c@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.

---
Except that it won't work because of the MOSFET's body diode.
---

You can do similar things with BJT's.

---
This seems to work:
Click to expand...
---
Snip...

Even better:

Version 4
SHEET 1 880 724
WIRE -176 -32 -304 -32
WIRE 288 -32 -176 -32
WIRE -176 0 -176 -32
WIRE 288 96 288 -32
WIRE -176 144 -176 80
WIRE -112 144 -176 144
WIRE 224 144 -32 144
WIRE 288 224 288 192
WIRE 416 224 288 224
WIRE -176 256 -176 144
WIRE 288 256 288 224
WIRE -80 304 -112 304
WIRE 48 304 0 304
WIRE 112 304 48 304
WIRE 224 304 192 304
WIRE -304 432 -304 -32
WIRE 48 432 48 304
WIRE 288 432 288 352
WIRE 416 432 416 224
WIRE -304 624 -304 512
WIRE -176 624 -176 352
WIRE -176 624 -304 624
WIRE 48 624 48 512
WIRE 48 624 -176 624
WIRE 288 624 288 512
WIRE 288 624 48 624
WIRE 416 624 416 512
WIRE 416 624 288 624
WIRE -304 704 -304 624
FLAG -304 704 0
SYMBOL pnp 224 192 M180
SYMATTR InstName Q1
SYMATTR Value 2N3906
SYMBOL pnp 224 256 R0
SYMATTR InstName Q2
SYMATTR Value 2N3906
SYMBOL voltage 288 416 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 3.3
SYMBOL voltage 48 416 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value PULSE(0 5 0 1e-6 1e-6 .1 .2)
SYMATTR InstName V2
SYMBOL voltage -304 416 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 5
SYMBOL res 400 416 R0
SYMATTR InstName R1
SYMATTR Value 100
SYMBOL npn -112 256 M0
SYMATTR InstName Q3
SYMATTR Value 2N3904
SYMBOL res -192 -16 R0
SYMATTR InstName R2
SYMATTR Value 1000
SYMBOL res -16 128 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R3
SYMATTR Value 1000
SYMBOL res 208 288 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R4
SYMATTR Value 1000
SYMBOL res 16 288 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R5
SYMATTR Value 1000
TEXT -264 656 Left 0 !.tran 1

JF
 
"John Fields" <jfields@austininstruments.com> wrote in message
news:ugat56lah2i1c9d2c5mqkvcd8slj2pbmnq@4ax.com...
On Sat, 7 Aug 2010 13:38:54 -0500, "Jeff Johnson"
Jeff_Johnson@Hotmail.com> wrote:



"ravi" <ravi2neha@gmail.com> wrote in message
news:dd623d43-6c24-4300-afd4-944764ccc40c@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.

---
Except that it won't work because of the MOSFET's body diode.
Click to expand...

Huh? The body diode is in cutoff as it is reversed biased in both cases.
Maybe you are getting the orientation backwards.

Small signal mosfets are almost entirely symmeterical so one can reverse the
drain and source at will excluding the body diode polarity. Since the
maximum of 5V is well within the any maximum voltage requirements there
should be no problems.

In any case one simply has to choose the right combinations of p-ch and n-ch
to get the right combination of body diode orientations to prevent forward
conduction.

One can use two n-ch mosfets or two p-ch, etc...

If you have m branches such as

V1---Mosfet1---+
|
V2---Mosfet2---+
....... |
Vm---Mosfetm---+---Load

Then as long as the mosfet's body diode are forward conducting from the to
the load they will block all over branches. e.g., looks something like

Vk--|>|---+

Current can't flow into any source. It won't work with diodes but does with
mosfets. The biggest problem with a 5V or less system is driving the gates
enough to reduce R_dson.
 
Jeff Johnson wrote:
"John Fields" <jfields@austininstruments.com> wrote in message
news:ugat56lah2i1c9d2c5mqkvcd8slj2pbmnq@4ax.com...

On Sat, 7 Aug 2010 13:38:54 -0500, "Jeff Johnson"
Jeff_Johnson@Hotmail.com> wrote:



"ravi" <ravi2neha@gmail.com> wrote in message
news:dd623d43-6c24-4300-afd4-944764ccc40c@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.


---
Except that it won't work because of the MOSFET's body diode.



Huh? The body diode is in cutoff as it is reversed biased in both cases.
Maybe you are getting the orientation backwards.

Small signal mosfets are almost entirely symmeterical so one can reverse
the drain and source at will excluding the body diode polarity. Since
the maximum of 5V is well within the any maximum voltage requirements
there should be no problems.

In any case one simply has to choose the right combinations of p-ch and
n-ch to get the right combination of body diode orientations to prevent
forward conduction.

One can use two n-ch mosfets or two p-ch, etc...

If you have m branches such as

V1---Mosfet1---+
|
V2---Mosfet2---+
....... |
Vm---Mosfetm---+---Load

Then as long as the mosfet's body diode are forward conducting from the
to the load they will block all over branches. e.g., looks something like

Vk--|>|---+

Current can't flow into any source. It won't work with diodes but does
with mosfets. The biggest problem with a 5V or less system is driving
the gates enough to reduce R_dson.



Proper selection of components out of the junk box makes a nice 3.3 to 5
Click to expand...
switch.. V2 would be the uC line to select voltage. The current source
is just a test load. PMOS selected to have its turn on Gate voltage in
range of this voltage window.. Op-amp selected where there is no rail
output and thus gives you the resulting drive voltage that is needed.
Run the sim, you'll see using these components, it comes close for what
I found in the libs.

Version 4
SHEET 1 892 680
WIRE 80 32 0 32
WIRE 192 32 80 32
WIRE 448 32 288 32
WIRE 0 48 0 32
WIRE 448 96 448 32
WIRE 208 112 208 80
WIRE 208 112 160 112
WIRE 80 144 80 32
WIRE 192 144 80 144
WIRE 288 144 288 32
WIRE 128 192 16 192
WIRE 80 208 80 144
WIRE 160 208 160 192
WIRE 240 208 160 208
WIRE 448 208 448 160
WIRE 16 224 16 192
WIRE 48 224 16 224
WIRE 128 240 128 192
WIRE 128 240 112 240
WIRE 240 240 240 208
WIRE 240 240 128 240
WIRE 48 256 -96 256
WIRE 80 288 80 272
FLAG 0 128 0
FLAG 448 208 0
FLAG 80 288 0
FLAG -96 336 0
SYMBOL voltage 0 32 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 5
SYMBOL pmos 288 80 M270
SYMATTR InstName M1
SYMATTR Value FQB11P06
SYMBOL npn 192 208 R270
WINDOW 0 48 54 Left 0
WINDOW 3 -8 122 Left 0
SYMATTR InstName Q1
SYMATTR Value 2N2222
SYMBOL load 432 96 R0
WINDOW 3 48 56 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName I1
SYMATTR Value PULSE(.010 .050 0 500us 500us 20ms 20ms 60)
SYMBOL Opamps\\LT1006 80 176 R0
SYMATTR InstName U1
SYMBOL voltage -96 240 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value PULSE(0 5 0 1ms 1ms .100 .200)
SYMBOL res 144 96 R0
WINDOW 0 -7 -23 Left 0
WINDOW 3 -9 4 Left 0
SYMATTR InstName R1
SYMATTR Value 47
TEXT 326 302 Left 0 !.tran 1 startup
 
On Sun, 8 Aug 2010 12:37:37 -0500, "Jeff Johnson"
<Jeff_Johnson@Hotmail.com> wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
news:ugat56lah2i1c9d2c5mqkvcd8slj2pbmnq@4ax.com...
On Sat, 7 Aug 2010 13:38:54 -0500, "Jeff Johnson"
Jeff_Johnson@Hotmail.com> wrote:



"ravi" <ravi2neha@gmail.com> wrote in message
news:dd623d43-6c24-4300-afd4-944764ccc40c@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.

---
Except that it won't work because of the MOSFET's body diode.


Huh? The body diode is in cutoff as it is reversed biased in both cases.
Maybe you are getting the orientation backwards.

Small signal mosfets are almost entirely symmeterical so one can reverse the
drain and source at will excluding the body diode polarity. Since the
maximum of 5V is well within the any maximum voltage requirements there
should be no problems.
Click to expand...

---
Good point.

This seems to work quite well:


Version 4
SHEET 1 880 724
WIRE 224 80 -144 80
WIRE 224 96 224 80
WIRE 176 112 0 112
WIRE 0 144 0 112
WIRE 224 224 224 192
WIRE 496 224 224 224
WIRE 224 256 224 224
WIRE 0 272 0 208
WIRE 176 272 0 272
WIRE 496 320 496 224
WIRE -144 400 -144 80
WIRE 0 400 0 272
WIRE 224 400 224 352
WIRE -144 512 -144 480
WIRE 0 512 0 480
WIRE 0 512 -144 512
WIRE 224 512 224 480
WIRE 224 512 0 512
WIRE 496 512 496 400
WIRE 496 512 224 512
WIRE -144 592 -144 512
FLAG -144 592 0
SYMBOL voltage 224 384 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 3.3
SYMBOL voltage 0 384 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value PULSE(0 5 0 1e-6 1e-6 .1 .2)
SYMATTR InstName V2
SYMBOL voltage -144 384 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 5
SYMBOL res 480 304 R0
SYMATTR InstName R1
SYMATTR Value 10
SYMBOL Digital\\inv -64 208 R270
WINDOW 3 8 104 Invisible 0
SYMATTR Value trise 1e-7 tfall 1e-7 vhigh 5V
SYMATTR InstName A1
SYMBOL pmos 176 352 M180
WINDOW 0 -42 25 Left 0
WINDOW 3 -97 58 Left 0
SYMATTR InstName M1
SYMATTR Value FDS4465
SYMBOL pmos 176 192 M180
WINDOW 0 -49 31 Left 0
WINDOW 3 -101 58 Left 0
SYMATTR InstName M2
SYMATTR Value FDS4465
TEXT -88 544 Left 0 !.tran 1


--
JF
 
"John Fields" <jfields@austininstruments.com> wrote in message
news:d6du565r15g56mgo8n6b2b50tbmi5s3mde@4ax.com...
On Sun, 8 Aug 2010 12:37:37 -0500, "Jeff Johnson"
Jeff_Johnson@Hotmail.com> wrote:



"John Fields" <jfields@austininstruments.com> wrote in message
news:ugat56lah2i1c9d2c5mqkvcd8slj2pbmnq@4ax.com...
On Sat, 7 Aug 2010 13:38:54 -0500, "Jeff Johnson"
Jeff_Johnson@Hotmail.com> wrote:



"ravi" <ravi2neha@gmail.com> wrote in message
news:dd623d43-6c24-4300-afd4-944764ccc40c@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.

---
Except that it won't work because of the MOSFET's body diode.


Huh? The body diode is in cutoff as it is reversed biased in both cases.
Maybe you are getting the orientation backwards.

Small signal mosfets are almost entirely symmeterical so one can reverse
the
drain and source at will excluding the body diode polarity. Since the
maximum of 5V is well within the any maximum voltage requirements there
should be no problems.


---
Good point.

This seems to work quite well:


Version 4
SHEET 1 880 724
WIRE 224 80 -144 80
WIRE 224 96 224 80
WIRE 176 112 0 112
WIRE 0 144 0 112
WIRE 224 224 224 192
WIRE 496 224 224 224
WIRE 224 256 224 224
WIRE 0 272 0 208
WIRE 176 272 0 272
WIRE 496 320 496 224
WIRE -144 400 -144 80
WIRE 0 400 0 272
WIRE 224 400 224 352
WIRE -144 512 -144 480
WIRE 0 512 0 480
WIRE 0 512 -144 512
WIRE 224 512 224 480
WIRE 224 512 0 512
WIRE 496 512 496 400
WIRE 496 512 224 512
WIRE -144 592 -144 512
FLAG -144 592 0
SYMBOL voltage 224 384 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 3.3
SYMBOL voltage 0 384 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value PULSE(0 5 0 1e-6 1e-6 .1 .2)
SYMATTR InstName V2
SYMBOL voltage -144 384 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 5
SYMBOL res 480 304 R0
SYMATTR InstName R1
SYMATTR Value 10
SYMBOL Digital\\inv -64 208 R270
WINDOW 3 8 104 Invisible 0
SYMATTR Value trise 1e-7 tfall 1e-7 vhigh 5V
SYMATTR InstName A1
SYMBOL pmos 176 352 M180
WINDOW 0 -42 25 Left 0
WINDOW 3 -97 58 Left 0
SYMATTR InstName M1
SYMATTR Value FDS4465
SYMBOL pmos 176 192 M180
WINDOW 0 -49 31 Left 0
WINDOW 3 -101 58 Left 0
SYMATTR InstName M2
SYMATTR Value FDS4465
TEXT -88 544 Left 0 !.tran 1


--
JF
Click to expand...
What program to you use to view the circuit?
 
On Mon, 9 Aug 2010 17:08:13 -0500, "Jeff Johnson"
<Jeff_Johnson@Hotmail.com> wrote:


What program to you use to view the circuit?
Click to expand...
---
Not only just view, but also draw and simulate.

A beautiful piece of work available free from Linear Technologies at:

http://www.linear.com/designtools/software/


and, if you can't figure it out all by yourself, a wonderful support
group at:

http://groups.yahoo.com/group/LTspice/


If you enjoy this kind of thing, they'll get you pretty close to
nirvana.

--
JF
 
"John Fields" <jfields@austininstruments.com> wrote in message
news:mqv066po4uo2khrbtngn5t0p5nq6pqhn4r@4ax.com...
On Mon, 9 Aug 2010 17:08:13 -0500, "Jeff Johnson"
Jeff_Johnson@Hotmail.com> wrote:


What program to you use to view the circuit?

---
Not only just view, but also draw and simulate.

A beautiful piece of work available free from Linear Technologies at:

http://www.linear.com/designtools/software/


and, if you can't figure it out all by yourself, a wonderful support
group at:

http://groups.yahoo.com/group/LTspice/
Click to expand...
I tried to use it once but it's interface is so 60's...
If you enjoy this kind of thing, they'll get you pretty close to
nirvana.

--
JF
Click to expand...
 
Jeff Johnson wrote:
"John Fields" <jfields@austininstruments.com> wrote in message
news:mqv066po4uo2khrbtngn5t0p5nq6pqhn4r@4ax.com...

On Mon, 9 Aug 2010 17:08:13 -0500, "Jeff Johnson"
Jeff_Johnson@Hotmail.com> wrote:


What program to you use to view the circuit?


---
Not only just view, but also draw and simulate.

A beautiful piece of work available free from Linear Technologies at:

http://www.linear.com/designtools/software/


and, if you can't figure it out all by yourself, a wonderful support
group at:

http://groups.yahoo.com/group/LTspice/


I tried to use it once but it's interface is so 60's...

Well, stop using the 60's version then!
Click to expand...
 
Jeff Johnson wrote:
"John Fields" <jfields@austininstruments.com> wrote in message
news:mqv066po4uo2khrbtngn5t0p5nq6pqhn4r@4ax.com...
On Mon, 9 Aug 2010 17:08:13 -0500, "Jeff Johnson"
Jeff_Johnson@Hotmail.com> wrote:


What program to you use to view the circuit?

---
Not only just view, but also draw and simulate.

A beautiful piece of work available free from Linear Technologies at:

http://www.linear.com/designtools/software/


and, if you can't figure it out all by yourself, a wonderful support
group at:

http://groups.yahoo.com/group/LTspice/


I tried to use it once but it's interface is so 60's...
Click to expand...

What SPICE program were you using in the '60s, 'Pencil & Paper 1.0'?
 
Jeff Johnson wrote:
"John Fields" <jfields@austininstruments.com> wrote in message
news:mqv066po4uo2khrbtngn5t0p5nq6pqhn4r@4ax.com...

On Mon, 9 Aug 2010 17:08:13 -0500, "Jeff Johnson"
Jeff_Johnson@Hotmail.com> wrote:


What program to you use to view the circuit?


---
Not only just view, but also draw and simulate.

A beautiful piece of work available free from Linear Technologies at:

http://www.linear.com/designtools/software/


and, if you can't figure it out all by yourself, a wonderful support
group at:

http://groups.yahoo.com/group/LTspice/


I tried to use it once but it's interface is so 60's...
Click to expand...
Well, if you want to read the stuff JF posts that way (always
highly recommended reading, by the way) it's the solution.
The content/capability is worth overcoming whatever negative(s)
you think about the interface.

Ed

If you enjoy this kind of thing, they'll get you pretty close to
nirvana.

--
JF
Click to expand...
 
On Tue, 10 Aug 2010 22:05:09 -0400, ehsjr <ehsjr@nospamverizon.net>
wrote:

Jeff Johnson wrote:


"John Fields" <jfields@austininstruments.com> wrote in message
news:mqv066po4uo2khrbtngn5t0p5nq6pqhn4r@4ax.com...

On Mon, 9 Aug 2010 17:08:13 -0500, "Jeff Johnson"
Jeff_Johnson@Hotmail.com> wrote:


What program to you use to view the circuit?


---
Not only just view, but also draw and simulate.

A beautiful piece of work available free from Linear Technologies at:

http://www.linear.com/designtools/software/


and, if you can't figure it out all by yourself, a wonderful support
group at:

http://groups.yahoo.com/group/LTspice/


I tried to use it once but it's interface is so 60's...

Well, if you want to read the stuff JF posts that way (always
highly recommended reading, by the way) it's the solution.
The content/capability is worth overcoming whatever negative(s)
you think about the interface.
Click to expand...
---
Thanks Ed, that's very kind of you! :)

---
JF
 
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