For selecting the blocking MOSFET Q1, important electrical parameters are the maximum continuous drain current I_D, the maximum drain-to-source voltage V_DS(MAX), the maximum drain-to-source voltage V_GS(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 all the automotive transient events and any anticipated fault conditions. It is recommended to use MOSFETs with V_DS voltage rating of 60 V along with a single bidirectional TVS or a V_DS rating 40-V maximum rating along with two unidirectional TVS connected back-back at the input.

The maximum V_GS LM74900-Q1 can drive is 14 V, so a MOSFET with 15-V minimum V_GS rating should 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, but this would result in increased IQ current.

To reduce the MOSFET conduction losses, lowest possible R_DS(ON) is preferred, but selecting a MOSFET based on low R_DS(ON) may not be beneficial always. Higher R_DS(ON) will provide increased voltage information to LM74900-Q1's reverse comparator at a lower reverse current. Reverse current detection is better with increased R_DS(ON). Choosing a MOSFET with < 50-mV forward voltage drop at maximum current is a good starting point.

For active rectification of AC super imposed ripple on the battery supply voltage, gate-source charge Q_GS of Q1 must be selected to meet the required AC ripple frequency. Maximum gate-source charge Q_GS (at 4.5-V V_GS) for active rectification every cycle is

Equation 16.

Where 1.3 mA is minimum charge pump current at 7-V V_DGATE-V_A, F_{AC_RIPPLE} is frequency of the AC ripple superimposed on the battery and Q_{GS_MAX} is the Q_GS value specified in manufacturer datasheet at 6-V V_GS. For active rectification at F_{AC_RIPPLE} = 30 KHz, Q_{GS_MAX} = 43 nC.

Based on the design requirements, BUK7Y4R8-60E MOSFET is selected and its ratings are:

• 60-V V_DS(MAX) and ±20-V V_GS(MAX)
• R_DS(ON) 5.0-mΩ typical at 5-V V_GS and 2.9-mΩ rated at 10-V V_GS
• MOSFET Q_GS 17.4 nC

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.