Re: Spartan3 driving mosfets

John Larkin wrote:
On Fri, 15 Sep 2006 12:40:54 +1200, Jim Granville
<no.spam@xxxxxxxxxxxxxxxxxxxxxx> 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

.

Relevant Pages

  • Re: LED Questions (just two: )
    ... the operating range of the LED. ... If the LED Vf spec is 2 volts, ... The 470 ohm resistor will limit the ... you would change the resistor to 4700 ohms. ...
    (sci.electronics.basics)
  • Re: Which way is better, zener or resistor
    ... The relay operates with 20mA current. ... their ratings - since the rating is usually "must pull in by X volts." ... You choose a resistor that allows the hold voltage to develop across ... Calculate the resistor using Ohms law - calculate for hold current ...
    (sci.electronics.basics)
  • Re: Basic resistor question
    ... at the body of the resistor and to bend out past the needle nose. ... If the lamp is a low voltage lamp, let's say 6 volts as an example. ... out that the resistance of the lamp when it is operating is 300 ohms. ...
    (rec.models.railroad)
  • Re: Gonna change the oil
    ... voltage drop of 3.4 volts for the white ultrabrights leaving about a 9 ... through each resistor and LED. ... even as alrge as 1/4 watt less than 1000 ohms. ...
    (rec.boats)
  • Re: Gate drive resistor question.
    ... is driving a fetvia a 47R resistor. ... both turn on and turn off (although life is easy at turn-on for DCM). ... look carefully at the turn-on switching loss. ... The fet is not getting to ...
    (sci.electronics.design)