For selecting the blocking MOSFET Q1, important electrical parameters are the maximum continuous drain current ID, the maximum drain-to-source voltage VDS_(MAX), the maximum drain-to-source voltage VGS_(MAX), Safe Operating Area (SOA), the maximum source current through body diode and the drain-to-source ON resistance R_DS(ON).

The maximum continuous drain current (I_D) rating must exceed the maximum continuous load current.

To reduce the MOSFET conduction losses, MOSFET with the lowest possible R_DS(ON) is preferred, but selecting a MOSFET based on low R_DS(ON) cannot be beneficial always. Higher R_DS(ON) provides increased voltage information to LM74721-Q1 reverse current comparator at a lower reverse current. Reverse current detection is better with increased R_DS(ON). Choosing a MOSFET with forward voltage drop of less than 50 mV at maximum current is a good starting point.

The maximum drain-to-source voltage, VDS_(MAX), must be high enough to withstand the highest differential voltage seen in the application. With LM74721-Q1, the maximum differential voltage across the MOSFET is V_CLAMP (maximum) of 43 V. TI recommends a minimum of 60-V VDS rated. This includes all the automotive transient events and any anticipated fault conditions.

During the ISO7637 pulse 1, the maximum VDS seen by the external MOSFET Q1 is VDS_{CLAMP (max)} that is 43 V. Use Equation 4 to calculate the peak current during ISO7637-2 pulse 1.

Where

• V_ISO is the negative peak of the ISO7637-2 pulse 1
• V_OUT is the initial level of the VBATT before ISO pulse is applied
• VDS_CLAMP is maximum VCLAMP threshold of LM74721-Q1
• R_S is the ISO7637 pulse generator input impedance (10 Ω)

For ISO7637-2 pulse 1 with amplitude of –100 V, V_OUT nominal voltage of 13.5 V the peak current seen by MOSFET Q1 comes around 7 A.

The current profile tapers down from 7 A to 0 A from the peak of 7 A as shown in Figure 9-5. The resulting average current (I_{ISO_AVG}) can be approximated as one third of the peak current that is around 2.4 A. The VDS clamp operation lasts for about 1 ms (maximum). Selecting a MOSFET with SOA characteristics covering the load line of 43 V which can support drain current greater than (I_{ISO_PEAK} / 2) for 1 ms is a good starting point. For this particular design example, MOSFET which can support greater than 3.5 A of drain current at 43-V V_DS on SoA curve is suitable.

Figure 9-2 shows typical SoA characteristics plot highlighting maximum drain current supported by the MOSFET for the duration of 1 ms. MOSFET data sheet SoA curves are typically plotted at ambient temperature, so consider sufficient margin over MOSFET parameters calculated values to ensure safe operation over desired operating temperature range.

Figure 9-2 Typical MOSFET SoA Characteristics

As external MOSFET dissipates ISO7637-2 pulse 1 energy, a special attention must be given while calculating maximum power dissipation and effective temperature rise. Use Equation 5 to calculate an average power dissipation across the MOSFET.

For given design example, average power dissipation comes around.

Typical ISO7637-2 pulse 1 transient lasts for 2 ms with total time period of 200 ms between two consecutive pulses (duty cycle of 1%). The effective temperature rise due to power dissipation across MOSFET during ISO7637-2 pulse 1 event can be calculated by looking at transient thermal impedance curve in a MOSFET data sheet. Figure 9-3 shows an example of how to estimate transient thermal impedance of a MOSFET for ISO7637-2 pulse 1 event.

Figure 9-3 Typical MOSFET Transient Thermal Impedance

The maximum V_GS LM74721-Q1 can drive is 13.9 V, so a MOSFET with 15-V minimum V_GS rating must be selected.

Based on the design requirements and MOSFET selection criteria BUK7Y4R8-60E, SQJ460AEP, STL130N6F7 are some of the 60-V MOSFET options that can be selected.