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LM74700-Q1: recommended TVS in datasheet

Given Ding
Expert 7405 points
Part Number: LM74700-Q1

Hi Team, 

Here is a question on LM74700-Q1. 

In datasheet 10.1.1.4, it recommend using two uni-directional TVS diodes to protect from positive and negative transient voltages. And in Figure 10-3, it recommend using SMBJ58A. But this part has 64.4V-71.2V Vbr , and LM74700-Q1 has 65V abs max input voltage spec, so it seems the margin using SMBJ58A is not enough. Please kindly add your comments why we recommend that TVS part, is it because the 65V spec for LM74700-Q1 has margin ? Thanks. 

BRs

Given 

  • 0
    TI__Guru 57737 points

    Hi Given Ding,

    Thanks for reaching out. I will get back to you within a couple of days. 

  • 0 in reply to Praveen GD
    TI__Guru 57737 points

    Hi Given,

    Your observation is correct. The TVS may not really be useful.

    As you said, the part can withstand upto 65V at input. For most applications, this voltage rating should be good enough. The 24V suppressed load dump max voltage is also 65V which the LM74700 can withstand.

     Is there any scenario in your application which can cause the voltage at input to be > 65V.

  • 0 in reply to Praveen GD
    Prodigy 10 points

    Hi Praveen，

    In the 24V system 7637-2 pulse 2a test, the input voltage reaches 112 V. In this case, the SMBJ58A TVS is broken down and the voltage is higher than 65 V (us level). Can the LM74700 withstand voltage in this case? Whether transient withstand voltage margin is involved.

    In addition, surge voltage also occurs when the input is inserted or removed，the transient withstand voltage may cause the TVS tube to break down.

    Please answer the question about SMBJ58A TVS Selection.

    thanks

  • 0 in reply to user6564016
    TI__Guru 57737 points

    Hi,

    The Abs Max Rating of LM74700 (ANODE to GND) is 65V. Any voltage beyond 65V applied can damage the IC.

    Now regarding the TVS Selection procedure,

    1. The breakdown voltage of the TVS should be > Max operating DC voltage 
    2. Clamping voltage should be < Abs Max Rating of the LM74700
      1. For ISO 7637-2 pulse 2a, the abs max voltage is 112V and the impedance in the path is 2ohms. So the current that can flow into the TVS is 56A. The selected TVS should have a clamping voltage of < 65V when a current of ~56A is injected in it.

    TVS selection depends on the amount of energy it needs to clamp/absorb during transient events. The selection depends on the application (external transients), system functionality (system induced transients) and specs (voltage and current).

    The following article provides detailed explanation of TVS selection for hot-swap application but gives an overview on different parameters which influence TV selection.https://www.powerelectronics.com/technologies/circuit-protection-ics/article/21853356/tvs-clamping-in-hotswap-circuits 

  • 0 in reply to Praveen GD
    Prodigy 10 points

    Hi Praveen，

    Currently,the 56 V TVS tube is selected, the maximum VBR of the TVS is 58.8 V. If the TVS with a lower voltage is used, the load dump of the 24 V system may be risky.

    Based on the 8/20 μs parameter, the actual clamping voltage calculated by the maximum VBR is about 65 V at room temperature. According to calculation, the maximum voltage is 68.5 V at high temperature. In this case, the time when the voltage exceeds 65 V is less than 5 μs.

    Can you give an estimate of the short-time（<5us） transient voltage withstand?

    Thanks！！！

    Regards,

    Mr Wang.

  • 0 in reply to user6565198
    TI__Guru 57737 points

    HI Wang,

    Unfortunately, We do not have transient voltage spec for LM74700-Q1. You can consider using bigger TVS like SMCJ which will result in lower clamping voltage.