EP
is the power ground that all return (power) paths of bucks are joined at, and AGND
and BUCK_AGND
are signal grounds.
The application schematic on p.55 clearly shows this distinction. The ground that the bucks' SR MOSFETs on the right half are connected to is named PGND
and has the same GND symbol as EP
whilst all other signal-related grounds have a different symbol. To the right of the EP
connection, it's shown that these grounds are joined together. NOTE: I didn't read the entire datasheet so I don't know if it's indicated somewhere, but these grounds may have been kept separately at the chip level, meaning that it is your responsibility to join them "properly".
The main idea behind this is to keep the signal grounds clean. Because the power grounds carry switching currents and all are joined at EP
, it may get "polluted". Tying the AGND or other signal grounds directly to the EP
is to ask for trouble because there's a risk of having the signal grounds polluted as well.
See the simplified schematic below:
simulate this circuit – Schematic created using CircuitLab
The switching currents flowing through the EP
pin will induce voltage due to the presence of the via parasitic. This voltage may be too small to measure, but it's there, and it has a lot of high-frequency components. Tying the signal grounds here will make the signal grounds fluctuate (i.e. dirty). This may cause regulation issues. The easiest solution is to connect all grounds to a big enough plane (presumably with the lowest impedance possible) having no discontinuities.