Hi Everyone, hope you’re all ok with the COVID pandemic.

i’ve been searching about the auto voltage select mechanism used on the UNO, MEGA and other boards.

There are some diferences in betwen schematics… Some of them uses de FDN340 and others FDN304. On the Micro board an PMV48XP is used.

Since there are some differences in RdsOn, Gate source voltage and the lack of FDN340P, Is there any special consideration to choose one of them? will they work on the same way?

please, excuse my english… i’m trying to do my best. Best Regards from Chile!

Thanks so much!

Keep Rds(on) low at logic levels and you are okay.

Some logic level (Vgs ~ 5volts or less) MOSFETs I’ve used.

Make sure you do not exceed Vgs(maximum).

Well... That's simple! haha... Thanks so much!

I'm attempting to adapt this mechanism of auto voltage selecting on a stand alone ATMEGA328P and other circuits using the blue pill) to select betwen Vin and USB.

i understand how the p-mosfet and the comparator mechanism works unisg LM385...perhaps not in especific but general. Googling i've found this:

"Due to the construction of the MOSFET there is still a resistance between the source and drain, even when fully activated (saturated) and this resistance causes some of the current being passed through the device to be wasted and thus produce heat. Reducing this resistance reduces the amount of wasted power and so also reduces the amount of heat generated by the MOSFET. Having a lower Rds basically means that less power is going to be lost across the MOSFET as per ohms law and by saying that their MOSFET are low-Rds they are basically saying that their boards are more power efficient and will thus produce slightly less heat as a by-product of the MOSFET.

Rds(on) is basically just saying that the Rds is low when the MOSFET is in the "on" state. In the Off state the MOSFET will not be conducting so you don't care about the resistance."

From that... I've understand your statement larryd. To keep RDson low.

Now...How much is "low" enough?

Are any other parameters that need to be considered?

Thanks so much!!

“ Now...How much is "low" enough?”

It depends on the current being drawn.

Let’s say Rds(on) was .080Ω and you were pulling 1 Amp through the MOSFET.

Voltage drop would be .08Ω * 1A = .08v across the transistor.

Let’s say you were pulling 10 amps, .08Ω * 10A = .8v which would be significant if Vcc was 5v ???.

If Rds(on) would have been .008Ω in the 10A example, we get .08 volts, much better than .8v ;).

“ Are any other parameters that need to be considered? ”

V(DS) supply voltage maximum
I(D) operating current maximum
V(GS) gate to source voltage maximum
P(D) power dissipation maximum

For most hobby situations, the above maximums must ‘never’ be exceeded.

Thank’s larryd i really apreciate your help.

Well i’ve found a nice and cheap MOSFET on stock. The DMP1045U

RDS(on) 26-31mOhm at -4.5V
V(DS) -12V
V(GS) -+8/V
I(D) 3-4A
p(D) 0.8W

From what i’ve understand it will work since i’m atempting to switch between USB 5v and 9v DC wall adapter on a design that draws no more than 500mA max.

i’m going to use this arrangement.

DMP1045U looks okay.

Always confirm the footprint.