The V_DS rating of the MOSFET Q2 must be sufficient to handle the maximum system voltage along with the input transient voltage. For this 12-V design, transient overvoltage events are during suppressed load dump 35 V 400 ms and ISO 7637-2 pulse 2 A 50 V for 50 µs. Further, ISO 7637-2 pulse 3B is a very fast repetitive pulse of 100 V 100 ns that is usually absorbed by the input and output ceramic capacitors. The maximum voltage on the 12-V battery can be limited to < 40 V the minimum recommended input capacitance of 0.1 µF. The 50-V SO 7637-2 pulse 2 A can also be absorbed by input and output capacitors and its amplitude can be reduced to 40-V peak by placing sufficient amount of capacitance at input and output. Choose a MOSFET with ≥ 40-V V_DS rating .

The VGS rating of the MOSFET Q2 must be higher than that maximum boost drive output of 15.5 V. FET with VGS absolute maximum rating of +/– 20 VGS is selected.

Inrush current through the MOSFET during input hot-plug into the 12-V battery is determined by output capacitance. External capacitor on HGATE, C_DVDT, is used to limit the inrush current during input hot-plug or start-up. The value of inrush current determined by Equation 1 must be selected to ensure that the MOSFET Q2 is operating well within its safe operating area (SOA). To limit inrush current to 250 mA, value of C_DVDT is 10.43 nF. The closest standard value of 10.0 nF is chosen.

Duration of inrush current is calculated by Equation 6

Calculated inrush current duration is 2.36 ms with 250-mA inrush current.

MOSFET BUK7Y4R8-60E having 60-V V_DS and ±20-V V_GS rating is selected for Q2. Power dissipation during inrush is well within the MOSFET safe operating area (SOA).