Nch mosfet direct driven with pic

[longpole001]

hI GUYS , looking for a bit of input on using these Nch Mosfet ( DMN3404L) and this Pch Mosfet( NX2301P) - see spec sheet attached

and if the use of resistors in series, pic to the gate and or resistor pullup ( nch ) / pull down (Pch ) on gate is required

The N ch has 500pf capacitance with a gate voltage threshold of 1.4 - 2v max , very low on Ohms

the current PCB design is such that the Mosfet is close the LED or other device it switches ,
with the CPU a max of 1200mm away and the tracks feeding the MOSFET are thick as possible ( board Layout to be reviewed )

the Nch mosfets used to drive IR leds at about 100ma pulsed 38Khz one at a time

regards

Sheldon

[longpole001]

it seems 100R in series with the NCh that drives just a normal LED is the go else when the pic is told to turn off the mosfet i need a pause after the command to allow the LED to turn off

[Art]

Why can't you switch the IR LED with a signal transistor, or directly from the pic pin?

[Charlie]

I have not looked at the datasheets for your specific devices, but in general you don't need any configuration of resistors - you can drive the gate directly from the PIC pin, as long as the pin can pull high and low. I do this all the time with N channel devices. If you intend to tristate the pin, you would want to add a resistor to the appropriate rail to make sure it is off (or on) in that situation.
If you need 100 mA, directly driving simply won't work.

[longpole001]

yes the devices work ok with nch mosfets , connected to the IR LEDs , without a series resistors from the PIC , but as this will be in service a while i was wanting it to be robust

as this board is 1.2 meters long with IR LEDS ( 15 off) along the the length it was practical for board layout and other components to put the mosfets close to the IR LED , this mean a long signal length from the pic , something i would have not normally done , where i would normally put the mosfet close to source and switched output can be a distance away.

I have found that by putting series resistors on the NCh RBG led solves a problem of where a color , which was turned on/off using toggle ,and then another color turn on , would not , without a pause between the commands

[richard]

I like to put a 39 ohm r between the pic and the mosfet gate. I often find that failed fets love short drain to gate on failure and in low volt apps a small r will save the pic chip. its also nice to have a 1k r between source and gate to ensure fet is kept "off " when pic output is in high impedance state .thats probable whats keeping your leds turned on. its important to remember that mosfets are gate charge dependent devices and that correct switching means rapid and thorough charge/discharge of the gate capacitance

[longpole001]

thanks guys , i tending toward a series resister of about 100R and the 1K pull up( Nch)/ down ( PCh) on the gate just be safe , as fixing problems down the road , is not a good design make , its pain on so many fets but better to be safe , and 4 and 8 resistor arrays make it bit less work for smd

[Charlie]

You can certainly add the extra components - it's your budget - but series resistors are usually left overs from transistor designs. If you are having an issue with turn on / off times, technically, series resistors will make it worse. They will, however, help improve noise immunity if that's a concern, and it might be with your long runs.
Pull up / down resistors are only useful if you ever tri-state the output. If you always drive either high or low, all they do is waste current in one of those states. They are useful during start-up if there's a risk of damage with an unknown gate state, (say where you could short the supply by having 2 devices on in some designs).
I used to make those same circuit choices (series resistors and pull up/down) automatically, until I sat down and actually did the circuit analysis. In some cases they hurt more than help. (The same is true of liberally sprinkling decoupling capacitors, but I digress)

[longpole001]

yes the only issue i am having is having to add the pauses to turn off /on when driving the the Nch on the rgb at the moment added the series resistors solved that , those are only 200mm runs , the IR leds seem fine without resistors and those signal runs are long at up to 1200mm ,

i have been looking at moving the mosfets closer to the cpu on the next board layout.

not seen much that much noise in the switching even with the long runs , but ill look at it closer