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, including any anticipated fault conditions. Using MOSFETs with voltage rating up to 60-V maximum with the LM74704-Q1 is recommended because the ANODE and CATHODE pin maximum voltage rating is 65 V. The maximum V_GS that the LM74704-Q1 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, the lowest possible R_DS(ON) is preferred, but selecting a MOSFET based on low R_DS(ON) is not always beneficial. Higher R_DS(ON) provides increased voltage information to the reverse comparator of the LM74704-Q1 at a lower reverse current. Reverse current detection is better with increased R_DS(ON). Operating the MOSFET in regulated conduction mode during nominal load conditions is recommended as well as selecting R_DS(ON) such that at nominal operating current the forward voltage drop V_DS is close to 20-mV regulation point and not more than 50 mV.

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

MOSFET manufacturers typically 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, operating the MOSFET close to 4.5-V V_GS is recommended, which is much higher than the MOSFET gate threshold voltage. Choosing a MOSFET with a typical gate threshold voltage V_th of 2 V to 2.5 V is recommended. Choosing a lower V_th MOSFET also reduces the turn ON time.

Based on the design requirements, the 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 / 3 A ) ≤ R_DS(ON) ≤ (50 mV / 3 A ) = 6.67 mΩ ≤ R_DS(ON) ≤ 16.67 mΩ
• MOSFET gate threshold voltage V_th: 2 V typical

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