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SN6501 Overcurrent protection

user576915
Prodigy 20 points
Other Parts Discussed in Thread: SN6501

Hello,

I have just designed a RS422 interface with SN6501+ wurth trasformer + generic rs422/7485 transceiver.

We've done some tests to the interface with several loads=transceivers, and we have seen how the SN6501 was DESTROYED twice.

What we beleive is that SN6501 could be damaged if there is an overcurrent in one RS422 line because it doesn't have any protection.

1).  I was wondering what is the best way to protect SN6501 againts OVERCURRENTs

     + polyswitch?

     + an LDO with overcurrent protection?

     + My product is very cost sensitive, any discrete solution?

2) Could SN6501 have a load with 5V-0.350mA permanently?

Could you help me, please?

Thanks a lot in advance

José López

  • 0
    TI__Intellectual 2200 points

    Hi Jose,

    Using an LDO with overcurrent protection is definitely an option. The LDOs in the TPS763xx series have built in over current protection. The SN6501 datasheet has typ voltage curves with SN6501 paired with LDOs from this series. I'll check and get back to you if there is a simpler alternative.

    Yes, it is ok to use SN6501 with a 5V, 350uA continuous load.

    Rgds,

    Anant

  • 0 in reply to Anant Kamath
    Prodigy 20 points

    Hello Anant

    + OK, If you know some simpler overcurrent method, let me know.

    + I understant, but If we take into account the losses at the transformer, maybe it could manage loads 5V and 300mA aprox.  It's quite enought

    thank you very much

    jose

  • 0 in reply to Anant Kamath
    Expert 1961 points

    Hi Anant,

    The data sheet for the TPS763XX series says "During normal operation, the TPS763xx limits output current to approximately 800 mA".  The data sheet also says "Current limiting and thermal protection prevent damage by excessive output current and/or power dissipation. The device switches into a constant-current mode at approximately 1 A; further load reduces the output voltage instead of increasing the output current".  These values far exceed what the SN6501 is capable of driving.  The SN6501 will only be saved if the regulator cuts out before the SN6501 does.  Would you please look at this again and correct me if I'm wrong?  José says he has destroyed two SN6501 devices.  I know that the RS-485 spec has a maximum short-circuit spec of 250 mA.  I have looked at a lot of RS-485 chips on the TI website and they all follow this short circuit spec, except for a few devices (particularly 3.3V devices) that limit at a lower value.  So I am wondering why the SN6501 devices failed?  Maybe more particular information is needed from José in order to understand why those devices failed.

    You said that it is OK to use the SN6501 with a continuous 5V 350uA load, which is what José asked (he said 0.350 mA).  But I suspect he meant 350 mA.  I think the answer is that it depends upon the ambient temperature and airflow, and the PCB that the SN6501 is mounted to.  The SN6501 data sheet does not list a recommended maximum junction temperature to operate at.  An absolute maximum temperature of 170 degrees C is listed, but that would not be a good value to design to.  I would use 125 deg C as the max, and to be conservative I would use an even lower value.  Using the theta-JA of 208.3 for the DBV package, a max junction temperature of 125 C, the max ambient temperature to keep the dissipation below 250 mW would be about 75 degrees C.  That is a ballpark figure, but it does highlight the issue of operating at high currents in a warm environment.  It is possible to exceed the maximum continuous power dissipation for the device.

    Regards,

    Greg