The important MOSFET electrical parameters are the maximum continuous drain current, I_D, the maximum drain-to-source voltage, V_DS(MAX), and 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 would include any anticipated fault conditions. TI recommends to use MOSFETs with voltage rating up to 60-V maximum with the LM74700-EP because anode-cathode maximum voltage is 65 V. The maximum V_GS LM74700-EP can drive is 13 V, so a MOSFET with 15-V minimum V_GS 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. During startup, 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) can not always be beneficial. Higher R_DS(ON) will provide increased voltage information to LM74700-EP's 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 it 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

Thermal resistance of the MOSFET must be considered against the expected maximum power dissipation in the MOSFET to ensure that the junction temperature (T_J) is well controlled.