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?
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