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THVD2450: Back Powering Risks

Louca Tolosa
Prodigy 50 points
Part Number: THVD2450


Tool/software:

Do you know if there are any Back Powering Risks regarding to the design of the component ?

  • 0
    TI__Guru* 75391 points

    Hi Louca,

    I believe there is a weak internal resistor path from the A pin of the receiver circuit. This leakage current should follow the spec called out in I_I (Bus input current) where the max leakage will be 250uA at 25V. This won't damage our device but if your Vcc rail doesn't have a discharge path, you may need to place a weak pull down resistor from Vcc to GND if you are concerned about the Vcc rail having a slight bias.

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 50 points

    Hi Bobby,

    Thank you for your answer, my question is, if there is no voltage supply on the Vcc pin, can the component power itself from other signals on other pins ?

  • 0 in reply to Louca Tolosa
    TI__Guru* 75391 points

    This kind of depends on the if there is a leakage path for the current to move to once it gets to the Vcc supply rail. Like if your Vcc rail is an LDO and there is a parasitic diode from the LDO output to VIN, and from VIN to another current sink. (This is why I stated a weak pull down resistor on Vcc would fix this kind of situation). If there is no leakage path (impedance into Vcc rail is very high) then voltage can start to build up. 

    The device itself has an UVLO (somewhere above 2V) designed into it so it's not like it will power up if the voltage were at 1V. 

    I personally haven't seen or heard of a case where it occurs from the A/B pins of the device. 

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 50 points

    Thank you for your answer Bobby ! 

    One last question, do you have any data about the short-circuit / open-circuit failure rate of THVD2450 or other indications about other default mode we should care about during a design creation ?

    -Louca

  • 0 in reply to Louca Tolosa
    TI__Guru* 75391 points

    Hi Louca,

    This device is designed based off the RS485 standard which is quite robust and defines protections against short circuits.

    From the RS485 standard, the differential output pins must be able to handle/clamp +/-250mA. So our devices will clamp this current before it occurs (IoS spec in datasheet). 

    So long as the differential pins do not exceed +/-70V, the device will not be damaged. The THVD24xx family was designed for high fault conditions. Not really specific data, but we have done tests with long periods of a short on A/B at +/-40V (the max/min recommended operating conditions) and then test them against the ATE (automated test equipment screener) and all devices passed (checked against datasheet parameters). I believe this was done with 30x units. 

    For open circuit, the device itself will not be damaged from seeing an open circuit but if one of the cables were damaged and resulted in an open circuit, the device wouldn't really be functional. This would be true for all RS485 devices. 

    Feel free to post a schematic for review if you're uncertain and I or one of my colleagues can double check it. 

    -Bobby