protection during various automotive transient tests as per ISO 7637-2 and ISO 16750-2 standard as well as

other automotive OEM standards. For more information, see the Automotive EMC-compliant reverse-battery

protection with ideal-diode controllers article.

LM74810 gate drive output DGATE controls MOSFET Q1 to provide reverse battery protection and true

reverse current blocking functionality. HGATE controls MOSFET Q2 to turn off the power path during input

overvoltage condition. Resistor network R1, R2 and R3 connected to OV and SW can be configured for

overvoltage protection and also for battery monitoring under normal operating conditions as well as reverse

battery conditions. Bi-directional TVS D1 clamps the automotive transient input voltages on the 12-V battery,

both positive and negative transients, to voltage levels safe for MOSFET Q1 and LM74810.

Fast reverse current blocking response and quick reverse recovery enables LM74810 to turn ON/OFF MOSFET

Q1 during AC super imposed input and provide active rectification of the AC input superimposed on DC battery

voltage. Fast reverse current blocking response of LM74810 helps to turn off MOSFET Q1 during negative

transients inputs and input micro short interruption conditions where input can fall to 0-V for short duration.

9.2.3 Detailed Design Procedure

9.2.3.1 Design Considerations

Table 9-1 summarizes the design parameters that must be known for designing an automotive reverse battery

protection circuit with overvoltage cut-off. During power up, inrush current through MOSFET Q2 needs to be

limited so that the MOSFET operates well within its SOA. Maximum load current, maximum ambient temperature

and thermal properties of the PCB determine the RDSON of the MOSFET Q2 and maximum operating voltage

determines the voltage rating of the MOSFET Q2. Selection of MOSFET Q2 is determined mainly by the

maximum operating load current, maximum ambient temperature, maximum frequency of AC super imposed

voltage ripple and ISO 7637-2 pulse 1 requirements. Overvoltage threshold is decided based on the rating

of downstream DC/DC converter or other components after the reverse battery protection circuit. A single

bi-directional TVS or two back-back uni-directional TVS are required to clamp input transients to a safe operating

level for the MOSFETs Q1, Q2 and LM74810.

9.2.3.2 Charge Pump Capacitance VCAP

Minimum required capacitance for charge pump VCAP is based on input capacitance of the MOSFET Q1,

CISS(MOSFET_Q1)and input capactiance of Q2 CISS(MOSFET).

Charge Pump VCAP: Minimum 0.1 µF is required; recommended value of VCAP (µF) ≥ 10 x ( CISS(MOSFET_Q1) +

CISS(MOSFET_Q2) ) (µF).

9.2.3.3 Input and Output Capacitance

A minimum input capacitance CIN of 0.1 µF and output capacitance COUT of 0.1 µF is recommended.

9.2.3.4 Hold-up Capacitance

Usually bulk capacitors are placed on the output due to various reasons such as uninterrupted operation during

power interruption or micro-short at the input, hold-up requirements for doing a memory dump before turning

of the module and filtering requirements as well. This design considers minimum bulk capacitors requirements

for meeting functional status "A" during LV124 E10 test case 2 100-µs input interruption. To achieve functional

pass status A, acceptable voltage droop in the output of LM74810 is based on the UVLO settings of downstream

DC-DC converters. For this design, 4.0-V drop in output voltage for 100 µs is considered and the minimum

hold-up capacitance required is calculated by

(3)

Minimum hold-up capacitance required to hold output with 4.0-V drop at 18-A current for 100 µs is 450 µF. A

470-uF electrolytic capacitor is a closest standard value that can be placed at the output. Note that the typical

application circuit shows the hold-up capacitor as optional because not all designs require hold-up capacitance.

LM7481

SNOSDD9 – DECEMBER 2022

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