What do PGND and AGND represent and how they are used?
The Power-Ground (PGND) is used as the return current path for high power components while the Analog Ground (AGND) is used as the return current path for the control/logic. The main reason why they are separated is because the PGND may see high di/dt noise spikes which can easily be in the range of 100mW. This is while the control logic signal is in the same range. Therefore, the PGND can NOT be used as the Ground for control/logic. Note, however, that both PGND and AGND must be connected together at a single point.