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LM7480-Q1: TVS diode selection

Juan Serrano
Prodigy 100 points
Part Number: LM7480-Q1

Tool/software:

Hi everyone, 

I have a doubt regarding some calculations within the LM7480-Q1 datasheet (in section 10.5.2: TVS Selection for 12V Battery Systems).

In order to select a proper TVS diode to clamp the negative voltage pulses, "the maximum clamping voltage of the TVS diode should not exceed -44V" for that specific use case (a 12V-battery system). However, the SMBJ33CA TVS diode is proposed, whose breakdown voltage is correctly written in the LM7400-Q1 datasheet (36.7V), but the clamping voltage is much higher (53.3V at Ipp = 11.3A). In automotive, the ISO pulses could be -600V, so I assume the currents are quite high (600V / 50Ohm impedance = 12A) during a very short pulse (~1ms). When that much current flows through the TVS diode, the voltage level will be clamped to 53.3V, not 36.7V, is that right? In that case, the instantaneous drain-source voltage applied to said MOSFET would be (16V + 53.3V ~ 70V), which would break the 60V-rated MOSFET recommended in the datasheet. Are the TVS diodes slower than the 1ms ISO pulses? Am I assuming anything wrongly? Could you elaborate on the calculations? 

 

Thank you for your time and I'm looking forward to your reply!

 

Best regards,

Juan

  • 0
    TI__Mastermind 23375 points

    Hi Juan,

    This TVS is recommended for a 12V system only. -600V pulse is seen in a 24V system and there we recommend a 75V MOSFET just to keep the VDS safe as you mentioned. (70V < 75V)

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 100 points

    Hi Shiven, 

    Sure, I'm only talking about the TVS selection criteria for the 12V-battery system. However, the following is written in the datasheet, where a 60V-rated MOSFET is recommended: 

    Is the datasheet wrong, then? Should the TVS diode selection be done considering the clamping voltage (as you confirmed in your previous answer) instead of the breakdown voltage for these ISO pulses? 

    Thank you again for your time! 

    Best regards,

    Juan

  • 0 in reply to Juan Serrano
    TI__Mastermind 23375 points

    Hi Juan,

    No, datasheet is not wrong. I quoted 75V rated FET for a 24V system. Sorry if I was unclear.

    The TVS needs to be selected based on clamping voltage for transients and breakdown for DC.

    Considering SMBJ33CA from datasheet, it is selected such that its VBR is greater than the load dump pulse (for TVS+) and clamping voltage is optimal for ISO pulse 1 (for TVS-).

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 100 points

    Hi Shiven, 

    Thank you again for your quick answer. However, there's something which is still unclear to me regarding to the following paragraph from the datasheet in Section 10.5.2: 

    During a negative pulse (ISO pulse 1 = -150V), Q1 is disabled quickly. Considering that VOUT = 16V (maximum working voltage from the alternator in 12V-battery systems), the maximum clamping voltage for a 60V-rated MOSFET (as suggested in the datasheet) would be -44V. Until here, I fully agree. But then, the SMBJ33CA is recommended in the datasheet, whose clamping voltage is 53.3V. However, in the datasheet it says "the SMBJ33CA clamps at -44V with 12A...). I don't know if I'm not understanding anything properly, but this seems contradictory to me. Is this really not a mistake? 

    Best regards,

    Juan