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PIC pin sinking capacity

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Mosaic

Well-Known Member
Hi all:

I know the 16Fxxx has around a 25mA capacity on it's output drives, however, can it handle grounding a 5V charged 10uF cap via a 1n4148 diode? LTspice shows a spike of about 1.8Amps. With a 100ohm added in series with the diode the spike drops to around 40mA.

The purpose is driving an Nfet into soft on or (faster off) conditions.
 
LTspice isn't taking On resistance of the pin FETs into account.

At 5v, the pin driving FETs have about 70Ω for the Nfet (low) and 170Ω for the Pfet (high).
Charging a cap to 5v, then shorting it to ground through a Low pin would yield about 70mA spike.

While it may handle it, you'll be shortening it's lifespan.
It may be better to use an external transistor to supply that high spike.
 
I made a dummy bellbox with a twin flashing blue led circuit using a 10f200, theres a charge pump to double the battery voltage to increase battery usage, the circuit dumps the 3v charge from a 47u electrolytic cap through an led straight to gnd via the pic i/o without a resistor.
Theres a couple of these now been running a couple of weeks or so and they are still running.
All this said the technique probably violates several ratings from the datasheet.
 
Actually the PIC is driving an NPN so the actual voltage is around 0.7V. That makes it within spec for a sink thru a 70 ohm NFET.
 
Take care not to be caught up with the good old rmw effect, writing a 1 would read back a 0
 
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