I'm a newbie at EE, and just I wanted to verify that the design I came up with for a MOSFET circuit that drives a 12V 500mA water pump will work. I'd also like to confirm I can run the N-Channel MOSFET in question (RUR020N02) without a heatsink.
P8 is a two-pole terminal block which the pump will be attached to. The MOSFET has a 1.5V Logic Level gate, which will be driven by a 3.3V I/O pin from an Atmega328p.
By what research I've done, I've learned that it's good to have a pull-down resistor (R9) in the event the digital I/O pin on the microcontroller (PUMP) is left floating, as well as a limiter resistor (R10) to keep the uC I/O pin within spec (20-40mA). I think this can also act as a voltage divider, but the voltage drop should be negligible in this case and still well over the gate threshold.
I've added a flyback diode just in case, though I'm not sure if one is truly needed. The pump is an impeller so I assume it has the same problems as running a DC motor--I'm not sure if the pump has its own internal protection circuitry.
So, my main question is whether or not this design is adequate for my needs.
However, I'd also like to know if I would need to use a heatsink for this particular SOT-23 MOSFET.
Based on the datasheet (linked above), it has a power dissipation of 0.54W at 25°C. Since the MOSFET has a RDS(on) value of around 105mΩ and my pump draws 500mA, I should be OK without a heatsink since it will only need to dissipate 52.5mW, correct?
0.105Ω x 0.500A = 0.0525W
Assuming the MOSFET turns on fully in an acceptable timeframe.