Spartan3 driving mosfets

I've got some DACs that I'd like to switch gain ranges on, and it
turns out I can do it nicely using a single 2N7002 (sot23 n-channel
mosfet) to switch a resistor to ground in each reference circuit. So
I'd like to turn each 7002 on and off from a pin on an XC3S400. But
3.3 volts is a marginal high for this fet... 4 volts looks safe.

So, how about running Vccio a bit high, 3.5 maybe, and adding an
external pullup resistor to +5. If I tristate the pin, I should
forward-bias the upper esd diode and get 4.2 roughly, right? I'm
thinking maybe a half milliampere or so pullup current. Doing this 8
times only dumps 4 mA into the Vccio rail, no hazard there.

I could use a lower-threshold fet, I guess, but the 7002's are in
stock and cost 3 cents each.

John

John Larkin wrote:
> I've got some DACs that I'd like to switch gain ranges on, and it
> turns out I can do it nicely using a single 2N7002 (sot23 n-channel
> mosfet) to switch a resistor to ground in each reference circuit. So
> I'd like to turn each 7002 on and off from a pin on an XC3S400. But
> 3.3 volts is a marginal high for this fet... 4 volts looks safe.
> 
> So, how about running Vccio a bit high, 3.5 maybe, and adding an
> external pullup resistor to +5. If I tristate the pin, I should
> forward-bias the upper esd diode and get 4.2 roughly, right? I'm
> thinking maybe a half milliampere or so pullup current. Doing this 8
> times only dumps 4 mA into the Vccio rail, no hazard there.
> 
> I could use a lower-threshold fet, I guess, but the 7002's are in
> stock and cost 3 cents each.

  You can also switch with NPN transistor + resistor, and a reverse 
connected one has lower saturation voltage ( and lower beta too ).

  What resistance do you expect of the on devices ?

  -jg

Hi John,
your idea might work for awhile, but not for the 20 years of lifetime
that is our criterion.
I would pay a little more for a proper level converter...
Hope to see you some time again!
Peter Alfke, Xilinx
===========================
John Larkin wrote:
> I've got some DACs that I'd like to switch gain ranges on, and it
> turns out I can do it nicely using a single 2N7002 (sot23 n-channel
> mosfet) to switch a resistor to ground in each reference circuit. So
> I'd like to turn each 7002 on and off from a pin on an XC3S400. But
> 3.3 volts is a marginal high for this fet... 4 volts looks safe.
>
> So, how about running Vccio a bit high, 3.5 maybe, and adding an
> external pullup resistor to +5. If I tristate the pin, I should
> forward-bias the upper esd diode and get 4.2 roughly, right? I'm
> thinking maybe a half milliampere or so pullup current. Doing this 8
> times only dumps 4 mA into the Vccio rail, no hazard there.
>
> I could use a lower-threshold fet, I guess, but the 7002's are in
> stock and cost 3 cents each.
> 
> John

A more constructive idea:
connect the cathode of a simple diode to each FPGA output, and pull the
anode through 470 Ohm to 5 V. Then use the diode-resistor node to drive
your FET.
This means you have shifted the voltage up by 0.7 V.
Pool Low voltage of ~0.9 V,but fairly good High voltage, 3.3 V + two
diodes = 4.7V
Peter Alfke
============
John Larkin wrote:
> I've got some DACs that I'd like to switch gain ranges on, and it
> turns out I can do it nicely using a single 2N7002 (sot23 n-channel
> mosfet) to switch a resistor to ground in each reference circuit. So
> I'd like to turn each 7002 on and off from a pin on an XC3S400. But
> 3.3 volts is a marginal high for this fet... 4 volts looks safe.
>
> So, how about running Vccio a bit high, 3.5 maybe, and adding an
> external pullup resistor to +5. If I tristate the pin, I should
> forward-bias the upper esd diode and get 4.2 roughly, right? I'm
> thinking maybe a half milliampere or so pullup current. Doing this 8
> times only dumps 4 mA into the Vccio rail, no hazard there.
>
> I could use a lower-threshold fet, I guess, but the 7002's are in
> stock and cost 3 cents each.
> 
> John

On 14 Sep 2006 19:09:58 -0700, "Peter Alfke" <alfke@sbcglobal.net>
wrote:

>Hi John,

>your idea might work for awhile, but not for the 20 years of lifetime
>that is our criterion.

Is there a failure mechanism?

John

On Fri, 15 Sep 2006 12:40:54 +1200, Jim Granville
<no.spam@designtools.maps.co.nz> wrote:

>John Larkin wrote:
>> I've got some DACs that I'd like to switch gain ranges on, and it
>> turns out I can do it nicely using a single 2N7002 (sot23 n-channel
>> mosfet) to switch a resistor to ground in each reference circuit. So
>> I'd like to turn each 7002 on and off from a pin on an XC3S400. But
>> 3.3 volts is a marginal high for this fet... 4 volts looks safe.
>> 
>> So, how about running Vccio a bit high, 3.5 maybe, and adding an
>> external pullup resistor to +5. If I tristate the pin, I should
>> forward-bias the upper esd diode and get 4.2 roughly, right? I'm
>> thinking maybe a half milliampere or so pullup current. Doing this 8
>> times only dumps 4 mA into the Vccio rail, no hazard there.
>> 
>> I could use a lower-threshold fet, I guess, but the 7002's are in
>> stock and cost 3 cents each.
>
>  You can also switch with NPN transistor + resistor, and a reverse 
>connected one has lower saturation voltage ( and lower beta too ).
>

Once I start adding parts, I may as well go with an octal level
shifter and get all the way to 5 volts. The pcb would route nicer if I
could use just the pullups.

>  What resistance do you expect of the on devices ?

The 2N7002's go to about 2 ohms with 4 volts or so on the gates, and
that 2 ohms will have a roughly +6000 PPM/K tc, net 12 mohm/K, which
is good enough here... that will cause just a few PPM/K dac gain
error.

John




>
>  -jg

John Larkin wrote:
> On Fri, 15 Sep 2006 12:40:54 +1200, Jim Granville
> <no.spam@designtools.maps.co.nz> wrote:
>
> >John Larkin wrote:
> >> I've got some DACs that I'd like to switch gain ranges on, and it
> >> turns out I can do it nicely using a single 2N7002 (sot23 n-channel
> >> mosfet) to switch a resistor to ground in each reference circuit. So
> >> I'd like to turn each 7002 on and off from a pin on an XC3S400. But
> >> 3.3 volts is a marginal high for this fet... 4 volts looks safe.
> >>
> >> So, how about running Vccio a bit high, 3.5 maybe, and adding an
> >> external pullup resistor to +5. If I tristate the pin, I should
> >> forward-bias the upper esd diode and get 4.2 roughly, right? I'm
> >> thinking maybe a half milliampere or so pullup current. Doing this 8
> >> times only dumps 4 mA into the Vccio rail, no hazard there.
> >>
> >> I could use a lower-threshold fet, I guess, but the 7002's are in
> >> stock and cost 3 cents each.
> >
> >  You can also switch with NPN transistor + resistor, and a reverse
> >connected one has lower saturation voltage ( and lower beta too ).
> >
>
> Once I start adding parts, I may as well go with an octal level
> shifter and get all the way to 5 volts. The pcb would route nicer if I
> could use just the pullups.
>
> >  What resistance do you expect of the on devices ?
>
> The 2N7002's go to about 2 ohms with 4 volts or so on the gates, and
> that 2 ohms will have a roughly +6000 PPM/K tc, net 12 mohm/K, which
> is good enough here... that will cause just a few PPM/K dac gain
> error.
>
> John
>
>
>
>
> >
> >  -jg

You could always use something like a 74AC05 open drain buffer - the
device has about 4 ohms of resistance to ground (at 12mA sink). If a
couple more ohms isn't an issue, you can dispense with the FETs
entirely, but it would add more error than the typical 2 ohms of a 7002
in full drive.

Of course, the output FET in a logic gate isn't usually characterised
'thoroughly' :)

Cheers

PeteS

Use an other FET as FDV301 ... in sot23 too !
The Vgs of this one is much better than your one.

Laurent
http://www.amontec.com

John Larkin wrote:

> I've got some DACs that I'd like to switch gain ranges on, and it
> turns out I can do it nicely using a single 2N7002 (sot23 n-channel
> mosfet) to switch a resistor to ground in each reference circuit. So
> I'd like to turn each 7002 on and off from a pin on an XC3S400. But
> 3.3 volts is a marginal high for this fet... 4 volts looks safe.
> 
> So, how about running Vccio a bit high, 3.5 maybe, and adding an
> external pullup resistor to +5. If I tristate the pin, I should
> forward-bias the upper esd diode and get 4.2 roughly, right? I'm
> thinking maybe a half milliampere or so pullup current. Doing this 8
> times only dumps 4 mA into the Vccio rail, no hazard there.
> 
> I could use a lower-threshold fet, I guess, but the 7002's are in
> stock and cost 3 cents each.
> 
> John
> 
> 
> 

Have a look at out TechiTip 
http://www.enterpoint.co.uk/techitips/Previous_TechiTips/5p_charge_pump.html 
for a charge pump style solution that does not cost much. We did this bus 
switch (FET) gate driving.

John Adair
Enterpoint Ltd. - Home of Tarfessock1. The Spartan-3E Cardbus Development 
Board.
http://www.enterpoint.co.uk


"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message 
news:fhojg2l323ts6bvpqompi3flcj5fcphlbv@4ax.com...
> I've got some DACs that I'd like to switch gain ranges on, and it
> turns out I can do it nicely using a single 2N7002 (sot23 n-channel
> mosfet) to switch a resistor to ground in each reference circuit. So
> I'd like to turn each 7002 on and off from a pin on an XC3S400. But
> 3.3 volts is a marginal high for this fet... 4 volts looks safe.
>
> So, how about running Vccio a bit high, 3.5 maybe, and adding an
> external pullup resistor to +5. If I tristate the pin, I should
> forward-bias the upper esd diode and get 4.2 roughly, right? I'm
> thinking maybe a half milliampere or so pullup current. Doing this 8
> times only dumps 4 mA into the Vccio rail, no hazard there.
>
> I could use a lower-threshold fet, I guess, but the 7002's are in
> stock and cost 3 cents each.
>
> John
>
>
> 

John Larkin wrote:
> I've got some DACs that I'd like to switch gain ranges on, and it
> turns out I can do it nicely using a single 2N7002 (sot23 n-channel
> mosfet) to switch a resistor to ground in each reference circuit. So
> I'd like to turn each 7002 on and off from a pin on an XC3S400. But
> 3.3 volts is a marginal high for this fet... 4 volts looks safe.
>
> So, how about running Vccio a bit high, 3.5 maybe, and adding an
> external pullup resistor to +5. If I tristate the pin, I should
> forward-bias the upper esd diode and get 4.2 roughly, right? I'm
> thinking maybe a half milliampere or so pullup current. Doing this 8
> times only dumps 4 mA into the Vccio rail, no hazard there.
>
> I could use a lower-threshold fet, I guess, but the 7002's are in
> stock and cost 3 cents each.

Does a FET work better in this app than a bipolar would?  I guess it is
better to have a resistance (FET) in the path than a low constant
voltage (saturated NPN).

Personally, I would find a FET with a lower Vgsth.  They are not hard
to find.  I seem to recall the only thing you give up is a higher Vdss
rating.