My questions are about the additional zener diode on the right attached to the USB lines: 1. Is this for specific ESD protection on the USB lines? 2. If so, i've seen ESD circuits without this zener and just diodes, so what does this do and why is it needed?
The Zener turns on when the inputs rise (Vzener + Vdiode) above GND.
If 6volt zener (a typical number, for highly doped junctions) + 1volt Diode (at human-body currents of 1amp), as Vesd rises to 7 volts, the Zener comes into action.
The ESD energy has to be dissipated DEEP into the substrate; the ESD design task included steering the current flows into deep resistive regions and dumping that energy.