A TVS, D1, across the rectified PoE voltage per Figure 28 must be used. TI recommends a SMAJ58A, or a part with equal to or better performance, for general indoor applications. If an adapter is connected from VDD1 to RTN, as in ORing option 2 above, voltage transients caused by the input cable inductance ringing with the internal PD capacitance can occur. Adequate capacitive filtering or a TVS must limit this voltage to be within the absolute maximum ratings. Outdoor transient levels or special applications require additional protection.
Use of diode DVDD for PoE priority may dictate the use of additional protection around the TPS23754. ESD events between the PD power inputs, or the inputs and converter output, cause large stresses in the hotswap MOSFET if DVDD becomes reverse biased and transient current around the TPS23754 is blocked. The use of CVDD and DRTN in Figure 28 provides additional protection should over-stress of the TPS23754 device be an issue. A SMAJ58A would be a good initial selection for DRTN. Individual designs may have to tune the value of CVDD.