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THVD1520: Replacing MaxLinear SP485ECN-L/TR with TI THVD1520DR

Ray Vincent Aranzaso
Mastermind 18180 points
Part Number: THVD1520


Hi,

Good Day.

Customer have a board design that uses a MaxLinear SP485ECN-L/TR RS485 transceiver. Over the past year, that part has moved from very scarce to not available. The TI cross-reference tool lists THVD1520DR as a pin-for-pin replacement. Yet output current for that part is significantly lower (60mA vs 250mA). Has THVD1520DR been validated as a drop-in replacement for SP485ECN-L/TR? Are there other known significant differences? 

Please advise. Thank you very much.

Best Regards,

Ray Vincent

  • 0
    TI__Guru 52532 points

    Hi Ray,

    They are essentially pin to pin.  The specification you point to in the comp datasheet isn't the same spec you are comparing it to in the TI datasheet. The 250mA current is the maximum current that will be sourced/sunk into the bus pins during a short circuit for comp. We usually spec that as well but on this part we didn't - however it is fail-safe so it can handle a short circuit event. The specification you are looking at in our datasheet is a recommendation not a spec per se - as in we suggest this is the range that the device operates as it works best in this range - abs max ratings are the actual ratings of the device. Comp. doesn't include recommended operating so they don't have this spec either. Essentially in RS-485 standard the impedance between A and B is 54 Ohms (which in practice is the two end points on the bus each have a 120 ohm resistor in parallel) with at most 375 Ohms to ground from A or B - the current under normal operating conditions will be much less than the short spec. Impedances below this level are not standard - the short spec is to show how the device responds if there is a bus short (such as due to insulation failure) so the current spec really shouldn't affect operation of device. 

    They are both spec'd to work in RS-485 applications (and RS-485 compliant) and are spec'd at the same data-rate - functionally there should be very little difference in any standard use case. 

    The biggest difference in operation that I am seeing is that our device has much higher input impedance (ours in min 96K (256 nodes * 375) where comp is min 12K (32 nodes * 375)) which means less loading of the bus from the transceiver itself - this could potentially affect fail-safe resistor sizing if an external fail-safe was applied to the system but this effect is negligible if there are <= 3-4 nodes on the bus (and that is still probably a pretty conservative).

    Do you have any more information about the application - because while the differences are slight and under normal RS-485 it really shouldn't be an issue - but there are always edge cases and if you more information I could give you a more definitive answer - but generally they are p2p (but more of a Q rating than an S).

    Please let me know!

    Best,

    Parker Dodson