What is the purpose of the corner pad of the CP2120 QFN-20 package? Can it be left disconnected? What material is it made of?
This pad (shown below, highlighted as "DETAIL 1") is merely used to signify the orientation of the package. It is not connected to the CP2120, so it can be left unconnected. It is composed of the same material of the other pads, so it can be soldered down, if desired.
Please note, however, that the corner pad is connected through the lead frame to the center pad of the device, which we suggest connecting to PCB ground in the datasheet. By doing so, you are unavoidably also connecting this corner pad to ground. Be aware of nearby traces/vias that you aren't inadvertently shorting something to PCB ground through this pad.
Note: The center pad of the device is not connected to any of the CP2120 device connections, not even device ground (pin 2). For this reason, you cannot rely on the ground pad or corner pad to supply GND to the device, you must connect pin 2 to GND for this purpose. It is still recommended to connect the center pad to GND, however, for heat dissipation purposes.