The important MOSFET electrical parameters are the maximum continuous drain current I_D, the maximum drain-to-source voltage V_DS(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 includes any anticipated fault conditions. TI recommends to use MOSFETs with voltage rating up to 60-V maximum with the LM74700D-Q1 because anode-cathode maximum voltage is 65 V. The maximum V_GS LM74700D-Q1 can drive is 13 V, so select s MOSFET with 15-V minimum V_GS. If a MOSFET with < 15-V V_GS rating is selected, a Zener diode can be used to clamp V_GS to safe level. During start-up, inrush current flows through the body diode to charge the bulk hold-up capacitors at the output. The maximum source current through the body diode must be higher than the inrush current that can be seen in the application.

To reduce the MOSFET conduction losses, lowest possible R_DS(ON) is preferred, but selecting a MOSFET based on low R_DS(ON) cam not be beneficial always. Higher R_DS(ON) provides increased voltage information to the LM74700D-Q1 reverse comparator at a lower reverse current. Reverse current detection is better with increased R_DS(ON). TI recommends to operate the MOSFET in regulated conduction mode during nominal load conditions and select R_DS(ON) such that at nominal operating current, forward voltage drop V_DS is close to 20-mV regulation point and not more than 50 mV.

As a guideline, TI suggests to choose (20 mV / I_{Load(Nominal)}) ≤ R_DS(ON) ≤ ( 50 mV / I_{Load(Nominal)}).

MOSFET manufacturers usually specify R_DS(ON) at 4.5-V V_GS and 10-V V_GS. R_DS(ON) increases drastically below 4.5-V V_GS and R_DS(ON) is highest when V_GS is close to MOSFET V_th. For stable regulation at light load conditions, TI recommends to operate the MOSFET close to 4.5-V V_GS, that is, much higher than MOSFET gate threshold voltage. TI recommends to choose MOSFET gate threshold voltage V_th of 2-V to 2.5-V maximum. Choosing a lower V_th MOSFET also reduces the turn-ON time.

Based on the design requirements, preferred MOSFET ratings are:

• 60-V V_DS(MAX) and ±20-V V_GS(MAX)
• R_DS(ON) at 3-A nominal current: (20 mV / 3A ) ≤ R_DS(ON) ≤ ( 50 mV / 3A ) = 6.67 mΩ ≤ R_DS(ON) ≤ 16.67 mΩ
• MOSFET gate threshold voltage V_th: 2-V maximum

DMT6007LFG MOSFET from Diodes Inc. is selected to meet this 12-V reverse battery protection design requirements and is rated at:

• 60-V V_DS(MAX) and ±20-V V_GS(MAX)
• R_DS(ON) 6.5-mΩ typical and 8.5-mΩ maximum rated at 4.5-V V_GS
• MOSFET V_th: 2-V maximum

Consider thermal resistance of the MOSFET against the expected maximum power dissipation in the MOSFET to make sure that the junction temperature (T_J) is well controlled.