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SN65HVD10: What kind of performance degradation can be expected if the driver output current remains at or slightly exceeds 60 mA?

Nabeel Mahmood
Prodigy 10 points
Part Number: SN65HVD10

Tool/software:

One of our 485 network designs has baselined termination resistors at every node as opposed to a single 100ohm termination at the end of the trunk. This causes increased current at the driver which we are currently simulating at slightly above 60 mA (on the order of 65 mA). The datasheet specifies -60mA as the recommended high level output current at the driver, but does not specify an absolute max rating for the driver. 

Does TI have an absolute max rating available, or any recommendations on if this nominal operating condition is too high (and as a result we should look to rework our termination scheme)?

Thanks!

  • +1
    Guru 316690 points

    An RS-485 driver tolerates shorts. The datasheet specifies the short-circuit current as less than 250 ms (to allow you to design your power supply for that case). However, this can eventually lead to thermal problems, and correct operation is not guaranteed.

    The higher load reduces the voltage drop over the termination, and thus the noise margin. (And you're wasting power.) You should correct this.

  • +1
    TI__Prodigy 430 points

    Hello Nabeel, 
    Yes, Clemens is right. We have a max short circuit output current at 250 mA.
    Best, 
    Leah J

  • 0 in reply to Leah Jones
    Prodigy 10 points

    Thank you! Does TI have any numeric or empirical information on degraded lifetime/performance if the driver is run consistently at 66 mA?

  • 0 in reply to Nabeel Mahmood
    TI__Prodigy 430 points

    I can tell you that the 100 ohms is RS-422 and not RS-485. RS-485 has a minimum of 54 ohms between pins A and b and RS-485 can support 32 unit loads. Drawing 66 mA will probably not be that big of a deal, but we don’t have an equations for this, and if you follow RS-485 standard it should work at documented speeds. I do have questions about the design that may be able to help you get a more detailed answer:

    How many nodes are in your system and what is the termination value?

    What network are you using that requires more than 2 resistors that 485 usually has in the network?

    • Most applications do not do this, those are there for reflection mitigation, so we ask this for some context to check.
  • +1 in reply to Leah Jones
    Prodigy 10 points

    There are 4 total nodes and the termination value at each is 100 ohms. This was more so a design error than anything else. 

    We actually approved a design change to eliminate the extra resistors so I think we are in the clear now. Appreciate the support either way!

    -Nabeel