TI recommends using one of the middle layers
as a solid ground plane. The ground plane provides shielding for
sensitive circuits and traces. the ground plane also provides a
quiet reference potential for the control circuitry. Connect the
GND, AGND and PGND pins to the ground plane using via right next
to the bypass capacitors. PGND pins are connected to the source
of the internal LS switch. They must be connected directly to
the grounds of the input and output capacitors. The PGND net
contains noise at the switching frequency and can bounce due to
load variations.