For thermal and ESD benefits, the following PCB design is recommended to provide thermal and electrostatic paths:
1. Design the PCB to conduct heat away from the device using thermal vias under the QFN IC to the digital ground plane.
2. Mount the metal shells of the USB and Ethernet connectors to a separate Chassis / Earth ground.
3. Place the chassis / earth ground metal on one PCB layer, digital ground on another PCB layer and connect through a zero ohm resistor located away from sensitive electronic devices as much as possible.
4. Place a large metal trace for the Earth ground all the way surrounding the PCB, except under the Ethernet connector.
As an example, see the schematic and an example PCB layout for the XR22800 Evaluation Board below:
Layer 1 Chassis / Earth ground layout with mounting pins for Ethernet and USB metal shells:
Layer 2 Digital ground layout:
Stackup layout with zero ohm resistor connecting Earth and Digital ground: