Schottky diodes are used on VBUS to help absorb large GND currents when a Type-C cable is removed while drawing high current. The inductance in the cable will continue to draw current on VBUS until the energy stored is dissipated. Higher currents could cause the body diodes on IC devices connected to VBUS to conduct. When the current is high enough it could damage the body diodes of IC devices. Ideally a VBUS Schottky diode should have a lower forward voltage so it can turn on before any other body diodes on other IC devices. Schottky diodes on VBUS also help during hard shorts to GND which can occur with a faulty Type-C cable or damaged Type-C PD device. VBUS could ring below GND which could damage devices hanging off of VBUS. The Schottky diode will start to conduct once VBUS goes below the forward voltage. When the TPS65988 is the only device connected to VBUS place the Schottky Diode close to the VBUS pin of the TPS65988. The two figures below show a short condition with and without a Schottky diode on VBUS. In Figure 9-5 without the Schottky diode, VBUS rings 2 V below GND and oscillates after settling to 0 V. In Figure 9-6 with the Schottky diode, VBUS drops 750 mV below GND (Schottky diode Vf) and the oscillations are minimized.

TVS Diodes help suppress and clamp transient voltages. Most TVS diodes can fully clamp around 10 ns and can keep the VBUS at their clamping voltage for a period of time. Looking at the clamping voltage of TVS diodes after they settle during a transient will help decide which TVS diode to use. The peak power rating of a TVS diode must be able to handle the worst case conditions in the system. A TVS diode can also act as a “pseudo schottky diode” as they will also start to conduct when VBUS goes below GND.