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Original question:

LM74720-Q1: Automotive transient protection ICs -ISO-16750 /ISO-7367

LM74703-Q1: 24V protection - why 58V TVS diode?

Tomasz Jastrzebski
Prodigy 202 points
Part Number: LM74703-Q1
Other Parts Discussed in Thread: TVS1800,

Tool/software:

Hi TI,

Why for 24V battery protection application 58V TVS diode is recommended?
Does not such diode have typical clamping voltage of over 90V which far exceeds LM74704 65V ANODE to GND limit?
What am I missing?

What nominal voltage is recommended for 100nF CCAP?

Kind regards,

Thomas

  • 0
    TI__Mastermind 22925 points

    Hi Thomas,

    It is true that the clamping voltage will be ~90V. But in reality, we don't want to clamp in positive side transient. 58V diode is present such that it does not blow during load dump pulse of a 24V system.

    100nF cap needs to be rated for > VIN (max) including all the transients.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 202 points

    Hi Shiven,

    I see, consequently, as the result of sufficiently strong impulse the circuit may easily burn out while TVS diode may survive.
    I think it is very important to keep it in mind.
    The more I study ideal diode drivers and catches like this one the more I think that I may be better off using a simple, non-ideal diode.

    Kind regards,

    Thomas

  • +1 in reply to Tomasz Jastrzebski
    TI__Mastermind 22925 points

    Hi Thomas,

    Surely a non-ideal diode is easier to implement. Ideal diodes being more expensive have its own pros like smaller size and much better thermals.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 202 points

    Hi Shiven,

    I would not mind higher price and extra complexity (PCB space gain is questionable) as long as solution is robust - and here I have some doubts.
    I start to think that dropout reduced to near near zero is not a sufficient reason to go this path.

    Kind regards,
    Thomas

  • 0 in reply to Shiven Dhir
    Prodigy 202 points

    Hi Shiven,

    One more question if I may; would you agree that when 48 V protection level is required (48V TVS diode), LM5050 may be a better choice since it can survive 100 V transient voltage?

    Typical 48V TVS diode clamping voltage is 77.4 V so with LM5050 there is no danger of circuit damage, which may not be the case if LM7470x is used since its max voltage is 70 V.

    Is it possible that figure 9-3 in datasheet has a mistake and 58V TVS diode should NOT be used with LM7470x since such diode typically has clamping voltage 93.6 V which far exceeds LM7470x max voltage? I guess, I just do not completely understand one part of your response:
    "58V diode is present such that it does not blow during load dump pulse of a 24V system."
    If the above is true, why use TVS diode at all when its purpose is just not to blow? Perhaps it is a stupid question and I am just missing something.

    Kind regards,
    Thomas

  • 0 in reply to Tomasz Jastrzebski
    TI__Mastermind 22925 points

    Hi Thomas,

    Yes, for a 48V system, LM5050 maybe a better choice.

    If the positive side TVS is not present, we will just short the input supply via TVS-. Hence TVS+ is present to block the power path during normal operation.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 202 points

    Hi Shiven,

    There is no doubt about TVS diode function. What I struggle to understand is why 58 V diode is suggested.
    58 V TVS diode allows for the voltage to go up to 93.6 V (typ) while LM7470x can survive 70 V only.
    I hope now I made my point clear.

    Kind regards,
    Thomas

  • 0 in reply to Tomasz Jastrzebski
    TI__Mastermind 22925 points

    Hi Thomas,

    I understand now. We dont really need any positive side clamping. The TVS are recommended for 12V/24V automotive systems where the expected transients are known and within the range of controller ratings. Surely 58V TVS will clamp at 93.6V but that is a case which will never be seen. Only function of the TVS is to block the power path as drawn.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 202 points

    Hi Shiven,

    Why do you say that that 96.3 V will never be seen?

    Standard automotive ISO tests (ISO 7637-2) use pulses as higher as 139 V (Pulse2a) and 327 V (Pulse3b). LM74703 datasheet section "9.2.2.5 Selection of TVS Diodes and MOSFET for 24-V Battery Protection Applications" mentions ISO 7637-2 standard explicitly.

    Ofc, ISO 7637-2 pulse generator has strictly defined impedance (2 Ω for Pulse2a). If my math is correct, to pull the voltage down from 139 V to allowable 70 V 34.5 A current is required while SMBJ58A diode has Ipp (peek pulse current) 6.5 A only.
    At 6.5A SMBJ58A diode can pull voltage down to 126 V only. Probably 48 V TVS diode like SMBJ48A could do a better job since its Ipp is 7.8 A so it could pull voltage down to 123.4 V. It is still well above LM74703 max voltage but it is a better result.

    Maybe instead of SMBJ58A three TI 18 V TVS1800 flat-clamp diodes (8/20 µs pulse 40 A) connected in series would do the job and allow to meet ISO 7637-2 standard but it is just my guess/hope.

    LM74703-Q1 datasheet describes -600 V pulse testing only (Pulse1) which, according to my math, should pass with the recommended SMBJ26A diode but Pulse1 test it just a small part of the IOS 7637-2 standard.

    Kind regards,
    Thomas

  • 0 in reply to Tomasz Jastrzebski
    TI__Mastermind 22925 points

    Hi Thomas,

    Pulse2A and pulse3B are low energy pulses which are suppressed by the input and output capacitors.

    Majorly we need to take care of load dump pulse and pulse 1.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 202 points

    Hi Shiven,

    I think it all boils down to question: does the design from LM74703 datasheet section "9.2.2.5 Selection of TVS Diodes and MOSFET for 24-V Battery Protection Applications" meet complete ISO 7637-2 standard requirements? If not, which tests should pass and which may not?

    Kind regards,
    Thomas

  • +1 in reply to Tomasz Jastrzebski
    TI__Mastermind 22925 points

    Hi Thomas,

    The design takes care of all the transients expected in ISO 7637-2 standard.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 202 points

    Hi Shiven,

    Thank you, this is a very important statement, which allows me to implement the design presented in LM74703 datasheet section 9.2.2.5 Selection of TVS Diodes and MOSFET for 24-V Battery Protection Applications" in hope that it will pass the actual tests and meet ISO 7637-2 standard.
    This means a lot.

    Kind regards,

    Thomas