9.2.3.5 Overvoltage Protection and Battery Monitor

Resistors R1, R2 and R3 connected in series are used to program the overvoltage threshold and battery monitor

ratio. The resistor values required for setting the overvoltage threshold VOV to 37.0 V and battery monitor ratio

VBATT_MON : VBATT to 1:8 are calculated by solving Equation 4 and Equation 5.

(4)

(5)

For minimizing the input current drawn from the battery through resistors R1, R2 and R3, it recommended to

use higher value of resistance. Using high value resistors will add error in the calculations because the current

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 4 gives

R3 = 3.45 kΩ. Solving Equation 5 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.

9.2.4 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 LM74810 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

LM74810'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

2.5mA

Q

F

(6)

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SNOSDD9 – DECEMBER 2022

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