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LM74700-Q1: 24-V battery protection

John Pan
Intellectual 990 points
Part Number: LM74700-Q1

Hello,

I have two questions related to Fig. 21 in LM74700-Q1 datasheet.

1. The recommended TVS+ is SMBJ58A, with the breakdown voltage of 64.4-V (minimum), 67.8 (typical). It it were connected to the ground alone, it meets the requirements that "the breakdown voltage of the TVS+ should be higher than 48-V jump start voltage, less than the absolute maximum ratings of anode and enable pin of LM74700-Q1 (65 V) and should withstand 65-V suppressed load dump." However, there is a TVS- in series with the TVS+ and the TVS- has a forward voltage drop of 3.5-V or 5-V. Thus, the TVS+ will start to break down when the battery voltage at 64.4-V +3.5V, or 67.9-V.  it seems to me that the recommendation shown in Fig. 21 won't protect LM74700-Q1 and other downstream devices. I am wondering whether it is an incorrect recommendation, and TVS+ should be changed to SMBJ52A?

2. In this design, The breakdown voltage of the TVS+ is recommended to be less than the maximum voltage of downstream devices such as LM74700-Q1. However, in page 2 of TI application report SLVA930 attached, it said, "For the most reliable operation, the maximum V_CLAMP of a TVS diode should be below the absolute maximum rating of the downstream device. Any condition violating the absolute maximum ratings results in a risk of damaging the component." I am wondering whether the breakdown voltage or the clamp voltage of a TVS should be below the absolute maximum rating of the downstream device.

Could you please answer the above two questions/comments?

Thanks,

John

slva930.pdf

  • 0
    TI__Guru 57737 points

    Hi John,

    Thanks for reaching out to us.  We will get back to you within a couple of days after the weekend.

  • 0 in reply to Praveen GD
    TI__Guru 57737 points

    Hi John,

    Your understanding of the TVS circuit is absolutely right. As you said, the below statements need to be considered while selecting a TVS.

    • For the most reliable operation, the maximum V_CLAMP of a TVS diode should be below the absolute maximum rating of the downstream device. Any condition violating the absolute maximum ratings results in a risk of damaging the component.
    •  The breakdown voltage or the clamp voltage of a TVS should be below the absolute maximum rating of the downstream device.

    You can select SMBJ52A if it suits your application.

  • 0 in reply to Praveen GD
    Intellectual 990 points

    Hi Praveen,

    Sorry, your reply confused me even more. It seems that you don't distinguish the difference between the breakdown voltage of a TVS and the clamp voltage of a TVS. If you don't know the answer, please have one of your colleagues reply.

    Thanks,

    John

  • 0 in reply to John Pan
    TI__Guru 57737 points

    Hi John,

    In my second point, I intended to mean that the clamp voltage (not the break down voltage) of a TVS should be below the absolute maximum rating of the downstream device. This was a typo and apologies  for creating more confusion. This point also needs to be corrected in the datasheet.

    SMBJ58A cannot be used because the Max breakdown voltage (71.2V ) is greater than the Abs Max (65V) rating of the LM74700. Using SMBJ58A will not help protect the device.

    If you can help provide more information below, I can help you suggest a suitable TVS.

    • What is the max operating voltage in your system ?
    • What is the magnitude and time duration of the over voltage that can occur in your application ?
      • If the Max over voltage that can occur in your system in less than 65V, you can just use a normal diode instead of the TVS+ as a TVS in the +ve direction is not required.