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For example, when driving a logic-level MOSFET directly from a microcontroller, what is the smallest gate resistor that can be used?

Specifically for AVR microcontrollers; i.e. this datasheet, the maximum current that can be sourced or sunk from a pin is 40mA, for a logic-level MOSFET where the gate is driven between ground and +5V, this would mean a gate resistor of at least 125 Ohms is necessary.

However, I also see other anecdotal evidence (not in the datasheet) that the pin itself has an internal resistance of 25 Ohms; this would mean that only 100 Ohms is necessary for the gate resistor.

I can't find any references to this internal resistance in the datasheet; but it does seem to have some value. Are there any canonical references?

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It's right there, in section 21. See it?

Sink Current

\$\frac{0.25\text{V}}{10\text{mA}}= 25\Omega\$

Source Current

\$\frac{5\text V - 4.75\text{V}}{10\text{mA}}= 25\Omega\$

Therefore yes, the output has a resistance of 25\$\Omega\$ at 25\$^{\circ}\$C.

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  • \$\begingroup\$ Definitely not obvious to me, but great find! I wonder if it's linear for bigger differences, as the 3.3V curves definitely aren't. Also, that looks like it's coming from a different datasheet, but I see similar figures in the ATtiny44A I'm looking at. \$\endgroup\$ – Andrew Mao Nov 20 '14 at 7:21
  • \$\begingroup\$ Those are from the '84A, but yeah, they should be very similar. \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 20 '14 at 7:38

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