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questions are regarding the SN65HVD1780 RS-485 interface chip

Keith Taylor
Prodigy 50 points
Other Parts Discussed in Thread: SN65HVD1780

1) What additional external protection components does TI recommend? We were thinking of using SMA30CA (30V bi-directional 400W TVS). If you see a problem with this (or a better implementation), I would like to hear it.

2) On the datasheet (Absolute Maximum Ratings), what is the difference between the specification for "Voltage range at A and B inputs" and "Voltage input range, transient pulse, A and B, through 100

Ω". The values are identical, aren't these specs redundant?

  • 0
    TI__Mastermind 28265 points

    Keith,

    1) don't use the old SMAs, use PSM712 from ProtekDevices.

    2) The transient spec is actually nonsense. Applying a transient pulse through a current limiting resistor has absolutely nothing to do with the real world application.

    I don't know who came up with this test, but please look into the attached apps-report and see whether this one helps.

    Best regards,

    Thomas

     

    RS-485 Transient Protection.pdf
  • 0 in reply to Thomas Kugelstadt
    TI__Prodigy 50 points

    Feedback from customer:

    Thanks for the advice on #1....however the voltage levels (7/12V) conflict with the intrinsic 70V fault protection of the device that you claim. That is, if an external short to a 24VDC power supply is applied across A/B, it will cook the protection diodes, where in the case of the SN65HVD1780, it would survive by its own fault tolerance characteristics. Thereby, unless I am mistaken, using the PSM712 negates the fault tolerance of the chip.

    Question #3 still remains unanswered, and may help in deciding which TVS to use. That is, if the SN65HVD1780 can survive +/-70V between (A to B) and (A or B to GND), then from my interpretation, we should focus on components that will protect the device outside this range.

  • 0 in reply to Keith Taylor
    TI__Mastermind 28265 points

    Unfortunately it was never mentioned that there was a concern of  the bus pins being shortened to the 24V supply. Many of our HVD17xx customers however do use the PSM712.

    Nevertheless I did check out the SMA30CA TVS which has an impressively lower capacitance of 200pF compared to the standard SMA TVS devices whose capacitance can be up to 5 times higher. Still 200pF is quite a chunk of capacitance that, when applied to multiple bus nodes can convert a transmission line into a lumped circuit, causing your data rate going down the drain.

    Another TVS of significantly lower capacitance is the SEMTECH SDC36C with 45pF only (pls see attached data sheet), which enables the protection of 4 times the number of bus nodes as the SMA device does. The question of course is: How many nodes does the network support ?

    There was also the mentioning of a Question #3 which only showed up now in this forum. whether the HVD1780 can survive differential voltages of 70V is better answered by the product champion Clark Kinnaird.

    Best regards,

    Thomas

     

    SEMTECH SDC36.pdf