LM7480 datasheet(21/43 Pages) TI

21 / 43 page

10.2.3 Detailed Design Procedure

10.2.3.1 Design Considerations

Table 10-1 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 LM7480.

10.2.3.2 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)

10.2.3.3 Input and Output Capacitance

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

10.2.3.4 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 LM7480 is based on the UVLO settings of downstream

DC-DC converters. For this design, 2.5-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 for 2.5-V drop in 100 µs is 200 µF. Note that the typical application circuit

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

10.2.3.5 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 3 and Equation 4.

(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

www.ti.com

LM7480

SNOSDD8 – DECEMBER 2022

Submit Document Feedback

Product Folder Links: LM7480

LM7480 Datasheet(PDF) 21 Page - Texas Instruments

LM7480 Datasheet(HTML) 21 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

Datasheet Download

Link URL