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THVD1439V: Is protective earth connection requiered for the integrated TVS didoes?

Lars Schmitz
Prodigy 10 points
Part Number: THVD1439V

Hello,

1. Is the protective earth connection requiered for surge protection with THVD1439V or is a connection to local/signal ground working?

2. Is it generally requiered to connect the protection circuit (e.g. tvs diodes) to protective earth if i want to protect a rs485 IC? Does the shield of RS485 needs to be connected to earth oder can i just connect it to local/signal ground of the pcb?

Kind Regards

  • 0
    TI__Guru 52172 points

    Hi Lars,

    Thanks for reaching out!

    1. No it's not required -you can connect it to the ground as long as the ground potential shift doesn't cause devices on the line to be outside of the common mode range (for this part that is -12V/12V). However if there are a lot of sensitive components on the same ground plane the additional surge current could potentially cause ground shifting issues that could damage parts. So it depends on the specific application - the part itself should be fine - however sensitive components sharing the same ground plane may have some issues.

    2. Please see above. It depends on system - if there are sensitive components on the ground plane and the surge current causes them to violate spec (due to ground shifting - as a lot of current could be shunted to ground). The THVD1439V should be okay  as long as the surge current doesn't cause the the voltages to exceed common mode rating. For the most robust system - this shunted current could be separated.

    Please let me know if you have any other questions!

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 10 points

    Hi Parker,

    thanks for the fast reply.

    When the emc lab perform a surge test on RS485 (symmetrical signal, with a cableshield) according to IEC 61000-4-5 and IEC 61000-6-2. It is discribed that the Surge is coupled between all lines an earth (If the shield is only earthed at one point or no connected to safety earth).

    Please take in to consideration that earth is protective earth (safety) and signal ground is the pcb ground, which has no connection to the safety ground (isolated!).


    We want to connect the tvs diode only to signal ground of the pcb. So when the emc test lab wants to make a surge on RS485.....one side of the surge generator is connected to A, B (symmetrical signal) and the other side is connected to the reference plane(connected to safety earth) or signal ground? I think it should be safety earth....Since our product has no connection to safety earth/reference plane....where does the surge couple into since there is no return path....?

    Kind Regards,

    Lars

  • +1 in reply to Lars Schmitz
    TI__Guru 52172 points

    Hi Lars,

    The TVS diodes are connected from the A/B pins to ground. The surge current gets shunted to whatever the ground pin is connected to. There isn't a separate return for the TVS diodes in the device. 

    Generally the other solution is to have a resistor from the GND to EARTH to mitigate high loop current by ground. However since it sounds like there is no earth in the system the next best solution is to use a digital isolator to help isolate the PCB ground from the THVD1439V - but that may also not be practical in the system.

    If connected to signal ground the surge current will be dumped into the PCB ground plane which - depending on the ground plane properties itself - there could be voltage differences between ICs on the same ground plane (this is depending on ground plane impedance).

    I believe I am answering your question - but I may have misread it. So please if any other clarifications / or questions are need please let me know.

    Best,

    Parker Dodson