Another N channel enhancement mode MOSFET driven directly by a PIC12F629

[vintagecarguy]

I have had success using the 12F629 at 5v to replace the 555 and now am looking at driving the
load - 30 red LED's at 20mA for a total of 600mA - with a logic level power MOSFET - Fairchild
FDD8778 in a TO252 package. The gate capacitance is nominally 700pF and nominal gate charge is 9.4nC.
Running through the math the PIC should be able to switch the MOSFET in less than 10 microseconds. The
equivalent capacitance of the LED's is ~600pF so this should not be a problem.

I plan on turning the LED's on for 1 milli-second and off for 1 millisecond so effectively
at a 500Hz rate and am wondering if there will be any issues directly driving the FET.
Some have mentioned needing to have a pull down resistor and a series limiting resistor; looking
at the I/O driver for the 12F629 it looks like it should be able to source/sink 25mA without
an issue. The MOSFET is placed close to the PIC and I'n running wide traces to the drain.

The question is will the PIC be able to directly drive the FET and have I missed anything with my
calculations?

[Dave]

At least, you should have a pull down resistor of about 10K ohms so to give the FET gate some kind of state during startup or if by change in your code the pin is made an input.

[vintagecarguy]

At least, you should have a pull down resistor of about 10K ohms so to give the FET gate some kind of state during startup or if by change in your code the pin is made an input.

Thanks for the input; that makes sense to keep the gate low at startup. The system turns on from a cold start so it's good to
set the state low with a pulldown.

[andywpg]

I have been told (by an electronics engineer that I know) that, along with the 10K pull down resistor, you should have something in series with it. His explanation was that a MOSFET can occasionally 'run wild' and the gate essentially shorts to source and will draw amazing amounts of current.

His suggestion was 1K to 5K or so, try it, if it works, great. Since then, I've used a 2.2K and have never had any problems.

Just my 2 cents

[vintagecarguy]

I have been told (by an electronics engineer that I know) that, along with the 10K pull down resistor, you should have something in series with it. His explanation was that a MOSFET can occasionally 'run wild' and the gate essentially shorts to source and will draw amazing amounts of current.

His suggestion was 1K to 5K or so, try it, if it works, great. Since then, I've used a 2.2K and have never had any problems.

Just my 2 cents

The series resistor is between the I/O line from the PIC and the gate of the MOSFET? That makes
sense but I'm thinking it shouldn't be too large because that would limit the charge current to the gate. A 220 ohm
limits the current to 23mA which shouldn't be a problem for the PIC.

Thanks for your comments.

[Dave]

Well actually, The resistor should be rather small, in the order of 10 to 100 ohms Steven. When switching the gate at high frequencies you need to overcome the gate to source capacitance issue. Even at low frequencies if you take a scope and look at the gate, you will notice a delay when switching it on when using a resistor. The larger the resistor the slower the turn on time as well as discharge time. In fact when switching the gate at frequencies of approx.15+ KHz. you should be using some kind of push/pull driving circuit. In the past I have used the IR2183 mosfet driver ic's on all of my PWM full bridge drivers. They will deliver up to 1 amp to the gate. There are others available. Microchip also makes single mosfet drivers.