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?
1 Answer 1
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– Andrew Mao2014年11月20日 07:21:53 +00:00Commented Nov 20, 2014 at 7:21
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\$\begingroup\$ Those are from the '84A, but yeah, they should be very similar. \$\endgroup\$Ignacio Vazquez-Abrams– Ignacio Vazquez-Abrams2014年11月20日 07:38:19 +00:00Commented Nov 20, 2014 at 7:38
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