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Original question:

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

LM74500-Q1: TVS diode for 48V input

Joselito Go
Expert 3660 points
Part Number: LM74500-Q1
Other Parts Discussed in Thread: LM74700-EP, LM74700-Q1

Hi Experts,

Good day.

This is regarding the LM74500-Q1 component. You recommended that we use two unidirectional TVS devices at the input of the circuit for voltages that exceed 24V. I want to use it for 48V, but I am having difficulty determining the appropriate size of the TVS device. Is it crucial? Could you provide some advice regarding these TVS devices?

The same for LM74700-EP

Please advise.

Regards,

Josel

  • 0
    TI__Guru 57737 points

    Hi Josel,

    The reason for using TVS is to limit the transient voltages that can appear at the input of LM74700-Q1 and limit them to less than the Absoulte Maximum Rating of the IC.

      

    Below are the points that need to be taken into account while selecting a TVS,

    • The breakdown voltage of the TVS has to be greater than than the maximum DC voltage that can be applied at the input - both positive and negative side.
    • All the input positive transient voltages that can occur at the input have to be clamped to less than +65V and all the negative transients voltages have to be clamped within  -65V. 
    • The TVS needs to be properly rated for power handling capacity to withstand power dissipation in it during transients
  • 0 in reply to Praveen GD
    TI__Expert 3660 points

    Hi Praveen,

    Good day.

    We have read Figure 9-6. Typical 24-V Battery Protection with Two Uni-Directional TVS on the datasheet: LM74500-Q1 Reverse Polarity Protection Controller datasheet,

    For 24-V, During ISO 7637-2 pulse 1, the input voltage goes up to –600 V with a generator impedance of 50 Ω. Single bi-directional TVS cannot be used for 24-V battery protection because the breakdown voltage for TVS+ ≥ 48V, maximum negative clamping voltage is ≤ –65 V . Two uni-directional TVS connected back-back needs to be used at the input. For positive side TVS+, SMBJ58A with the breakdown voltage of 64.4 V (minimum), 67.8 (typical) is recommended. For the negative side TVS-, SMBJ26A with breakdown voltage close to 32 V (to withstand maximum reverse battery voltage –32 V) and a maximum clamping voltage of 42 V is recommended.

    How about for the 48-V?

    Please advise.

    Regards,

    Josel

  • 0 in reply to Joselito Go
    TI__Guru 57737 points

    Hi Josel,

    The TVS selection is specified in the datasheet for 24V Battery in Automotive Applications. 

    What is your end Application? Is it Automotive ? What transient voltages can be applied at the input ? Are there any IEC / other standard transient voltages that can be applied at the input ?

  • 0 in reply to Praveen GD
    TI__Expert 3660 points

    Hi Praveen,

    Thank you for explaining the functioning of your device. However, I have a question regarding the TVS diode's negative voltage clamping.

    As per your datasheet, the maximum input voltage example is 32V, and the MOSFET Vds voltage is 75V. Therefore, the voltage clamping should not exceed 75V - 32V = 43V.


    In my case, I will have an input voltage of 48V with a MOSFET Vds voltage of 100V. Hence, the voltage clamping of the TVS- diode should not exceed 100V - 48V = 52V. However, I could not find any diode in the market with a breakdown voltage near 48V and a clamping voltage less than 52V.


    Could you please advise me on whether I should follow the indications on your datasheet? Can I exceed the voltage clamping of the TVS diode, or could you provide me with an alternative that I can use?

    Thank you for your time and assistance.

    Regards,

    Josel

  • 0 in reply to Joselito Go
    TI__Guru 57737 points

    Hi Josel,

    Thanks for your response. Let me review and get back to you within couple of days.

  • 0 in reply to Praveen GD
    TI__Guru 57737 points

    Hi Josel,

    You need to select a TVS such that the clamping voltage satisfies both the conditions below,

    • Clamping voltage within Abs Max rating of the IC --> -65V
    • Clamping voltage such that FET Vds rating is not violated --> 'FET Vds rating' - 'Vin(max)' is greater than the TVS clamping voltage.
      • To satisfy this condition - You can consider  using a Mosfet with Vds rating >100V