For selecting the blocking MOSFET Q1, important electrical parameters are the maximum continuous drain current ID, 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 can include all the automotive transient events and any anticipated fault conditions. TI recommends to use MOSFETs with VDS voltage rating of 60 V along with a single bidirectional TVS or a VDS rating 40-V maximum rating along with two unidirectional TVS connected back-back at the input.

The maximum VGS LM74930-Q1 can drive is 14 V, so a MOSFET with 15-V minimum VGS rating must be selected. If a MOSFET with < 15-V V_GS rating is selected, a Zener diode can be used to clamp VGS to safe level, but this action can result in increased IQ current.

To reduce the MOSFET conduction losses, lowest possible RDS(ON) is preferred, but selecting a MOSFET based on low R_DS(ON) can not be beneficial always. Higher R_DS(ON) provides increased voltage information to LM74930-Q1 reverse comparator at a lower reverse current. Reverse current detection is better with increased RDS(ON). Choosing a MOSFET with < 50-mV forward voltage drop at maximum current is a good starting point.