where 2.5 mA is minimum charge pump current at 7-V VDGATE - VA, FAC_RIPPLE is frequency of the AC ripple

superimposed on the battery and QGS_MAX is the QGS value specified in manufacturer datasheet at 6-V VGS. For

active rectification at FAC_RIPPLE = 30 KHz, QGS_MAX = 83 nC. Further for active rectification at FAC_RIPPLE = 200

KHz, QGS_MAX = 12.5 nC.

Based on the design requirements, BUK9J0R9-40H MOSFET is selected and its ratings are:

• 40-V VDS(MAX) and 16-V VGS(MAX)

• RDS(ON) 0.97-mΩ typical at 4.5-V VGS and 0.82 mΩ rated at 10-V VGS

• MOSFET QGS_MAX 30.2 nC

Thermal resistance of the MOSFET should be considered against the expected maximum power dissipation in

the MOSFET to ensure that the junction temperature (TJ) is well controlled.

9.2.5 MOSFET Selection: Hot-Swap MOSFET Q2

The VDS rating of the MOSFET Q2 should 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 and 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 could be reduced

to 40-V peak by placing sufficient amount of capacitance at input and output. For this 12-V design, a 40-V VDS

rated MOSFET is selected.

The VGS rating of the MOSFET Q2 should be higher than that maximum HGATE-OUT voltage 15 V.

Inrush current through the MOSFET during input hot-plug into the 12-V battery is determined by output

capacitance. External capacitor on HGATE, CDVDT is used to limit the inrush current during input hot-plug or

startup. The value of inrush current determined by Equation 2 need to be selected to ensure that the MOSFET

Q2 is operating well within its safe operating area (SOA). Considering COUT = 470 µF and inrush current of 2.5

A, the calculated value of CDVDT is 9.96 nF. Closest standard value of 10.0 nF is chosen for this design.

Duration of inrush current is calculated by:

(7)

Calculated inrush current duration is 2.36 ms with 2.5-A inrush current.

MOSFET BUK9J0R9-40H having 40-V VDS and 16 V VGS rating is selected for Q2. Power dissipation during

inrush is well within the MOSFET's safe operating area (SOA).

9.2.6 TVS selection

For 40-V rated MOSFET, two bi-directional 600-W SMBJ TVS, SMBJ33A and SMBJ16A are recommended for

input transient clamping and protection. For detailed explanation on TVS selection for 12-V battery systems,

refer to TVS Selection for 12-V Battery Systems and refer to TVS Selection for 24-V Battery Systems for 24-V

battery systems.

LM7481

SNOSDD9 – DECEMBER 2022

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