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Original question:

THVD2410: THVD2410 front end protection circuit

THVD2410: THVD2410 front end short circuit protection circuit

dineeshbabu chandran
Prodigy 70 points
Part Number: THVD2410

Hi ,

I am using THVD2410 in my design. I have connected PPTC infront for short circuit protection and i can seen my device itself support short circuit current 250mA.

I would like to know , If i use the THVD2410 with out PTC, during 32V short test, whether my device will be damaged?

In that case , if i need to protect my device , what will be the recommended hold current and trip current for PPTC?.

Thanks

Dinesh

  • 0
    TI__Guru 62910 points

    Hi Dinesh,

    A PTC would not be required when using this device, since it is rated to withstand short circuits up to 70 V.  A 32 V short should be no problem.

    Please let us know if you have any further questions.

    Regards,

    Max

  • 0 in reply to Max Robertson
    Prodigy 70 points

    Hi Max,

    Thanks for you clarification. For my case, if i do 32V short to the bus pin, How device will behave.

    1. current limit to 250mA and hold or

    2. Device will go for shutdown

    regards

    Dinesh

  • 0 in reply to dineeshbabu chandran
    TI__Guru 62910 points

    Dinesh,

    If the shorted line is driving high or disabled (DE pin low), there will be very little current flow.  If the shorted line tried to pull low (opposing the shorting voltage), the current will be limited to the "IOS" value.  While sourcing this larger current the power dissipated in the device is very high, so after some time in this state the device would likely go into over-temperature shutdown and make the outputs high-Z.  If that were to occur, the device would automatically re-enable whenever the internal temperature cooled beneath the shutdown threshold (minus some hysteresis).

    Please let me know if this isn't clear or if you have further questions.

    Regards,
    Max

  • 0 in reply to Max Robertson
    Prodigy 70 points

    Hi Max,

    Thanks for the Clarification and I really appreciate your quick response. It make sense, THVD2410 will be protected in short circuit condition. Let me ask a question specific to my design,

    In above circuit,

    Case 1: Consider 24V short connected to Pin 1 (RS485_B) and Pin A (THVD2410) is low. Maximum short circuit current (24V/120) 200mA will flow through 120 ohms resistor right? 

    If yes, maximum current flow through termination 120 ohm resistor is 200mA which is nearly 4.8W. Do I need really such big wattage resistor over there?. Please provide your recommendation to finalize my design.

  • 0 in reply to dineeshbabu chandran
    TI__Guru 62910 points

    Hi,

    Sorry I missed your follow-up post until now.  Yes, you are correct about the current into the A terminal (and through the resistance) in this case and the corresponding power dissipation.  (Well, it is a bit conservative, since in actuality the A terminal will not reach 0 V under such heavy current loading - reference Figure 1 in the datasheet for more info.)

    Something to consider when choosing the resistor wattage, though, is the duration for which it will be subject to this current.  The controller will not be sending a "0" to the bus constantly - it will only be for a portion of the time.  If you can estimate what percentage of time this line will be low, then you could compute the average power dissipation.  The average power is likely more critical in this case than the peak power.

    Another thing that will help the poor resistor in this case is the thermal shutdown feature of the IC.  In this case the power dissipation in the driver will be quite high and the internal temperature will rise quickly.  Once it reaches the shutdown threshold it will make the output high-Z and reduce the current being sunk to minimal levels.  It's hard to say that the transceiver would reach this point before the resistor would overheat, but I wanted to bring it up for consideration.

    Regards,
    Max