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Original question:

LM74500-Q1: MOSFET selection recommendations?

LM74500-Q1: Can LM74500-Q1 pass ISO7637 pulse 1 transient (Harsh conditions)

Kami Huang60
Genius 3830 points
Part Number: LM74500-Q1

Dear Sir/Ms.

It is stated in the specification that LM74500 can pass the ISO76730 pulse 1 test. For the 12V system, the input load is 12V/2.5A~3A. The waveform of the test is as shown in the figure below.

The customer's terminal product is the headlight of the car, and he hopes that the lights will not flash or flash during the test. Stop. Can LM74500 pass the following waveform test?

Best Regards,

Kami Huang

  • +1
    TI__Guru 57737 points

    Hi Kami,

    There are two aspects here,

    1. The IC has to withstand the input -600V pulse

    • The ISO pulse 1 generator applies a -600V trough a series impedance of 20 ohms. 
    • Therefore we need to use a TVS to clamp the -ve voltage at the input to within the Abs Max rating of the device (-65V).
    • The TVS has to be selected such that,
      • The breakdown voltage of TVS- should be higher than maximum reverse battery voltage –16 V, so that the TVS- is not damaged due to long time exposure to reverse connected battery.
      • The clamping voltage of the TVS at max reverse voltage should be within the Abs Max rating of the device (-65V). In this case the input voltage goes up to –600 V with a generator impedance of 20 Ω.This translates to 30 A flowing through the TVS - . 
    • A TVS like SMDJ33CA should be able to meet this requirement. 

    2. The output load should not be interrupted during this test.

    • During ISO Pulse 1 , the input voltage goes negative. 
    • Once the input goes negative, the LM74500-Q1 disconnects the power path.
    • Now, the output load has to be supplied completely by the output cap. Therefore the output cap has to be sized appropriately to supply the load during the the duration when the input is not within the output load range.