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.