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through the resistors at higher value will become comparable to the leakage current into the OV pin. Maximum

leakage current into the OV pin is 1 µA and choosing (R1 + R2 + R3) < 120 kΩ ensures current through resistors

is 100 times greater than leakage through OV pin.

Based on the device electrical characteristics, VOVR is 1.23 V and battery monitor ratio (VBATT_MON / VBATT) is

designed for a ratio of 1/8. To limit (R1 + R2 + R3) < 120 kΩ, select (R1 + R2) = 100 kΩ. Solving Equation 3 gives

R3 = 3.45 kΩ. Solving Equation 4 for R2 using (R1 + R2) = 100 kΩ and R3 = 3.45 kΩ, gives R2 = 9.48 kΩ and R1

= 90.52 kΩ.

Standard 1% resistor values closest to the calculated resistor values are R1 = 90.9 kΩ, R2 = 9.09 kΩ and R3 =

3.48 kΩ.

10.2.4 MOSFET Selection: Blocking MOSFET Q1

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), the

maximum source current through body diode and the drain-to-source ON resistance RDSON.

The maximum continuous drain current, ID, rating must exceed the maximum continuous load current.

The maximum drain-to-source voltage, VDS(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 VDS voltage rating of 60 V along with a single bidirectional

TVS or a VDS rating 40-V maximum rating along with two unidirectional TVS connected back-back at the input.

The maximum VGS LM7480 can drive is 14 V, so a MOSFET with 15-V minimum VGS rating should be selected.

If a MOSFET with < 15-V VGS rating is selected, a zener diode can be used to clamp VGS to safe level, but this

would result in increased IQ current.

To reduce the MOSFET conduction losses, lowest possible RDS(ON) is preferred, but selecting a MOSFET based

on low RDS(ON) may not be beneficial always. Higher RDS(ON) will provide increased voltage information to

LM7480's reverse comparator at a lower reverse current. Reverse current detection is better with increased

RDS(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 QGS of Q1

must be selected to meet the required AC ripple frequency. Maximum gate-source charge QGS (at 4.5-V VGS) for

active rectification every cycle is

GS_MAX

AC_RIPPLE

1.3mA

(6)

Where 1.3 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 = 43 nC.

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

• 60-V VDS(MAX) and ±20-V VGS(MAX)

• RDS(ON) 5.0-mΩ typical at 5-V VGS and 2.9-mΩ rated at 10-V VGS

• MOSFET QGS 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 (TJ) is well controlled.

10.2.5 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

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