[HH] n channel mosfet questions

[Drew Van Zandt]

Do you specifically need an N-channel MOSFET?  I am fond of these:
http://www.findchips.com/avail?part=FAN3217TMX

A _lot_ of people use high-current buffers rather than N-channel MOSFETs
for low loads.  Or half-H drivers, as they're often easy to search for.[1]

There are also MOSFETs designed for logic-level drive, and driver ICs that
boost the drive voltage up nice and high with a charge pump so you can use
cheap N-channel MOSFETs for the high side as well as the low side, and
drive your volume up on one part rather than having e.g. 2 N's and 2 P's
for an H-bridge.

The resistor to the gate is most likely a throwback to BJTs, _but_ it also
does drop the edge rate a good bit and thus is good for EMI reduction at
the expense of slightly more power burned by the MOSFET during switching.
 It can let you reduce the switching speed of the FET as well, via RC time
constant.
GS resistor is not bad to have around while micro is booting.

[1] No, these things are not all the same animal, but they're useful for
very similar things.

*
Drew Van Zandt
Artisan's Asylum Craft Lead, Electronics & Robotics
Cam # US2010035593 (M:Liam Hopkins R: Bastian Rotgeld)
Domain Coordinator, MA-003-D.  Masquerade aVST
*



On Thu, May 17, 2012 at 2:41 PM, Greg London <email at greglondon.com> wrote:

> I'm looking for an N channel mosfet that can be turned on/off
> with 5v/0v on the gate and can sink a hundred milliamps or two
> of load.
>
> I found one example here:
>
>
> http://search.digikey.com/us/en/products/SI2312BDS-T1-E3/SI2312BDS-T1-E3CT-ND/1656844
>
> Rds On (Max) @ Id, Vgs   is listed as
> 31 mOhm @ 5A, 4.5V
>
> [question 1]
> I'm I making this too hard?
> I can't seem to find very many of these at all.
> Most datasheets list the "on" gate voltage at around 10v.
> Are there a bunch of mosfets I can use to switch 5v that
> I'm just not seeing?
>
>
> On a related n channel mosfet topic,
>
> [2] Can I simply hook a 5v TTL logic output to the gate of a mosfet?
> A lot of schematics show a resistor from gate to source,
> but usually say this is to drain the capacitance on the gate pin.
> If I use the output pin of a 7408 AND gate and tie it directly
> to the gate of the mosfet, won't the 7408 drain the capacitance
> when it outputs zero volts? It seems like the resistor is for
> when the thing driving the mosfet gate is either 5v or tristated, and
> doesn't actually drive to 0 volts.
>
> [3] Some schematics also show a resistor between whatever is driving
> the mosfet and the gate pin of the mosfet. If the gate to source
> impedance on most mosfets is 6.28 giga-watts, I mean giga-ohms,
> then I'm not sure why bother adding another resistor in there.
> A resistor into the base of a NPN BJT transistor will limit the
> current into the base, but that's because the base is just a diode
> drop away from the emitter.
>
> Thanks for any help,
> Greg
>
>
>
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