correct value for pull down?

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I'm using an at90s1200 output line to switch the gate
of a N-channel enhancement mode mosfet (VN2222L) to
drag a 12V line at the source down to the drain at ground.
The current going through the mosfet source to drain is
about 20mA in this automotive app, so there is no current
limiting resistor in that path since the mosfet can
handle about 200mA.

This little device is really easy to turn on when
driven directly from an atmel output pin.. too easy
in fact. The gate acts sort of like a capacitor and
keeps it's charge. Without using a pull-down resistor at the
I/O pin, the mosfet doesn't always switch off
when the I/O pin is given a logic low.

So I just guessed with a 10K pulldown resistor, and things
seem to work fine. The mosfet performs it's duties
when I set the output high, and the load shuts off
when I set it back to zero.

But can anyone comment on the merits of using higher or
lower values for a pull down in this case? I would like to
give the gate as much juice as the atmel has to offer
to turn it on, but at the same time be sure it turns off
when "let go".

The specs for VN2222L are found at
http://www.vishay.com/docs/70213...
I just don't know how to interpret all the graphs
at the bottom.

Appreciated,
Scott

Regards,
Scott

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Hi Scott,

I'm not familiar with the specs for VN2222L, but I always use logic level FETs with logic level type source.
I've played around with the higher powered devices like Philips BUK553-60B etc. with micros and they work fine. The gate threshold voltage for this particular device is typically 1.5 volts. No resistor required by the way.

Cheers Jim

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As you said the gate behaves like a cap, there is no gate current other than a very very small current when the devices is turned on/off.You only need a pull down if the gate can foat e.g. if the I/O goes tri-state. The pulldown value you use is not really that important, if you use a very high value the gate may be a little slow to turn off. 10K is a good value.

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Hi Scott,
it seems your description is wrong. If you connect the source to your 12V-load and the drain to ground, your load is always on due to the internal bypass-diode of the VN2222L.
But with the right connection it is also a critical design. In worst case (at high temperature) the gate-threshold of the VN is very low (less than 0.6V at 1mA drain-current).
The value of the pull-down is not so important (the output of the AVR is in low-state less than 100 Ohm and its parallel to your 10k), but you have to connect it nearby the transistor. The capacity of a layout-track is some pF/cm, the inductance is about 10nH/cm. Cross-talk in the tracks is the reason that your transistor doesn't turns off.
The lowest value of the pull-down is given by the current the AVR can drive, the highest value is given by the distortions you have to unload. As Colin Meikle mentioned, 10k is a good value, (upto 47k?), but put it very close to your gate-source.
Best regards
Wolfgang

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