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 would include any anticipated fault conditions. It is recommended to use MOSFETs with voltage rating up to 60-V maximum with the LM74500-Q1 because SOURCE pin maximum voltage rating is 65-V. The maximum V_GS LM74500-Q1 can drive is 13 V, so a MOSFET with 15-V minimum V_GS rating should be selected. If a MOSFET with V_GS rating < 15 V 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.

Based on the design requirements, preferred MOSFET ratings are:

• 60-V V_DS(MAX) and ±20-V V_GS(MAX)

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 to ensure lower power dissipationa cross the FET

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