Should bulk of Power NMOS be connected to PGND or AGND?

[rom0011]

hi all :

For a switching dc-dc buck converter,the bulk of its power NMOS should be
connected to power GND or analog GND ?

If the bulk of the power NMOS is connected to power GND,the substrate
will become noisy by switching power NMOS.
Thus analog GND will become noisy if the bulk of analog nmos is connected to
substrate.That is mean that the analog GND and power GND is connected by
substrate.

So if without buried layer,what should we connect the power NMOS bulk to
analog GND or power GND?

Someone have any opinion?

 

[dick_freebird]

The bulk (body) should be connected to the source. Consider
the case that you tie source to PGND and body to AGND,
and have 1V of PGND bounce. The negative PGND bounce
would stand a very bad chance of lighting up the parasitic
BJT (or maybe it, and some of its friends, latching up).

I would recommend stiff body guardrings tied hard to the close-in
source metallization. In a high current design this may not
cost you much, as fat metal is liable to set the cell pitch.