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. As the LM74501-Q1 has an integrated VDS clamp control with threshold of 20 V (typical), MOSFETs with voltage rating of 40 V can be used with the LM74501-Q1. The maximum GATE pin voltage of the LM74501-Q1 can drive is 13.9 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 start-up, inrush current flows through the body diode to charge the bulk holdup 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, the lowest possible R_DS(ON) is preferred. Selecting a MOSFET with forward voltage drop of < 50 mV is a good starting point and gives good trade off between power dissipation and cost.

The BUK7Y3R0-40H MOSFET is selected to meet this 12-V reverse battery protection design requirements and it is rated at:

• 40-V V_DS(MAX) and ±20-V V_GS(MAX)
• R_DS(ON) 2.55 mΩ (typical) and 3-mΩ maximum rated at 10-V V_GS

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.