For selecting the blocking MOSFET Q1, important electrical parameters are the maximum continuous drain current I_D, the maximum drain-to-source voltage V_DS(MAX), the maximum drain-to-source voltage V_GS(MAX), the maximum source current through body diode and the drain-to-source ON resistance R_DSON.

The maximum continuous drain current, I_D, rating must exceed the maximum continuous load current.

The maximum drain-to-source voltage, V_DS(MAX), must be high enough to withstand the highest differential voltage seen in the application. This action includes all the automotive transient events and any anticipated fault conditions. TI recommends to use MOSFETs with V_DS voltage rating of 60 V along with a single bidirectional TVS or a V_DS rating 40-V maximum rating along with two unidirectional TVS connected back-to-back at the input.

The maximum V_GS LM74720-Q1 can drive is 14 V, so a MOSFET with 15-V minimum V_GS rating must be selected. If a MOSFET with < 15-V V_GS rating is selected, a zener diode can be used to clamp V_GS to safe level, but this results in increased I_Q current.

To reduce the MOSFET conduction losses, lowest possible R_DS(ON) is preferred, but selecting a MOSFET based on low R_DS(ON) cannot be beneficial always. Higher R_DS(ON) provides increased voltage information to LM74720-Q1's reverse comparator at a lower reverse current. Reverse current detection is better with increased R_DS(ON). Choosing a MOSFET with < 50-mV forward voltage drop at maximum current is a good starting point. Based on the design requirements, BUK7Y4R8-60E MOSFET is selected