What is the purpose of this zener diode from USB Vbus to Vcc?
It appears as though the diode D3 is to protect the VBUS from &quot;feedback&quot; from the case scenario where you would be connecting an external 5V supply to the EXT_5on connector J2.
External supply still will need to share/be referenced to the same Ground GND
Supply current from USB devices is &quot;normally&quot; limited to 500mA max but depending on how many devices are hanging on the hub you may have a lower limit.
If you have higher current requirements than are available from your host USB connection, you can provide power externally via this pin.
Note that the devices on the board are powered via 3V3 behind the regulator U2 which can be powered via VBUS from your USB connection or from externally provided power via EXT_5
If using externally powered devices make sure that the outputs that you connect to the Inputs of the microcontroller are within the voltage range of these inputs.
Although not recommended, since you could damage you usb hosts port, iif you are not going to use any external power source, you could bypass the diode with a short.