The TPS54531 device sis designed to operate using an external catch diode between the PH and GND pins. The selected diode must meet the absolute maximum ratings for the application. The reverse voltage must be higher than the maximum voltage at the PH pin, which is V_IN(MAX) + 0.5 V. Peak current must be greater than I_O(MAX) plus on half the peak-to-peak inductor current. The forward-voltage drop must be small for higher efficiencies. The catch diode conduction time is (typically) longer than the high-side FET on time, so attention paid to diode parameters can make a marked improvement in overall efficiency. Additionally, check that the selected device is capable of dissipating the power losses. For this design, a CDBC540-G was selected, with a reverse voltage of 40 V, forward current of 5 A, and a forward-voltage drop of 0.55 V.