Hi all,

I recently came across a small 3-phase BLDC driver (Allegro A4949) circuit that has its IC ground separated from the power ground and connects the two grounds through a MOSFET. I'm new to electronics and wanted some insight to why this technique is used.

I have a schematic of the entire circuit for reference and attached it as an image.

I have read the datasheet for the A4949 and understand the component choices on each pin but cannot understand why the grounds are separated through a MOSFET. Furthermore, I am unsure about the type of the MOSFET, but when I search the markings, it shows as an N-Channel.

Any help would be appreciated. Thank you.

 

Good question.
I see that the chip is wired for motor speed control by varying the supply voltage but the mosfet doesn't do any of that.
Nothing in the data sheet that requires this mosfet switch.

 

Good question.
I see that the chip is wired for motor speed control by varying the supply voltage but the mosfet doesn't do any of that.
Nothing in the data sheet that requires this mosfet switch.

I cannot see anything like that either!

 

After a further look it's for reverse polarity protection.

 

Hmm that makes sense. I learned that ground planes (digital, analog, power, etc.) are separated sometimes to mitigate noise but I have never encountered this configuration with a MOSFET before. So i appreciate you looking into it.

Also I noticed that the load (IC ground) is connected to the source of the MOSFET rather than the drain, which has confused me a bit. Is this possible on N-Channel MOSFETs? Can current flow in either direction between source and drain?

 

 

I see. That explanation makes perfect sense.
Thank you for your help and taking the time to explain things in detail.