I'm using a PT4115 LED driver to power a 3 W LED (700 mA) over a 1 meter cable. I've already had help here to use the same VIN for the LED driver and LED+ pin to power an MCU on the end of the cable, and it worked perfectly, as in the diagram below (I have only 4 wires inside the cable):
Now I want to modify the circuit to PWM (dim) the LED not on the LED driver DIM PIN, which is 1 meter from the LED, (which will be set to 100% brightness), but with the MCU together with the LED at the end of the cable. So I chose an N-channel MOSFET with a low Rds (0.33 Ω at 4.5 V) for testing. It's the AO3400 MOSFET.
That extra wire from the cable will be used for other purposes, so I can't use it for dimming the LED with the driver DIM pin.
In my test circuit it works, but the MOSFET heats up a lot. Doing PWM, with whatever frequency I choose, or even using the MOSFET as an ON/OFF switch (to get full 700 mA current) it heats up a lot.
The negative pole of the LED is LED-, and not GND, so it goes to the LED driver and through another FET inside the driver. (set to allow 100% current, so no dimming in the FET inside the LED driver)
Is there a way to correct this excessive heat in the AO3400 mosfet? My space in the MCU/LED casing is limited.
Edit: I forgot to draw, but there´s a 10 kΩ resistor between the gate of the mosfet to GND, pulling it LOW. Also, this test circuit has the MCU pins at 3.3 V, not 5 V.
The MOSFET drawn in my schematics is an N-channel one, but I chose wrongly in TinyCAD and put a P-channel in the drawing. The correct MOSFET is an N-channel one, the AO3400.