• State TI Thinks Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 35 subscribers
  • Views 475 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN65HVD1781: SN65HVD1781 70V fault protection

Michael Shne
Prodigy 100 points
Part Number: SN65HVD1781
Other Parts Discussed in Thread: SN65HVD1780, SN65HVD1782

Hi. I have question about functionality of SN65HVD1781 70V fault protection. Unfortunately I have no internal schematic of this chip to understand how it protects himself in following situation : DE=H, D=H, A=H,B=L   and some 60V (or surge) is aplied to the B in this state. So ,even for recommended 60mA output current it will be  60V*60mA =3.6W . So , the transceiver should limit the current to survive, correct?

Thanks

Michael

  • 0
    TI__Guru 53575 points

    Hi Michael,

    This device has an internal current limiting feature which can limit the power drawn during a short-circuit condition. This current limit is specified in the datasheet as Ios or Driver short-circuit output current (different than the recommended driver current). This is sufficient to protect the device dring mild faults in all device states or more serious faults in most device states. In the case of a fault while the transceivers is in a state that pulls the faulted bus line to ground (e.i. 60V fault on B pin which is pulling LOW), the device will limit the current through the driver pin to its specified limit value. However, the power dissipation in this case can still be quite high and would put the device in a thermal shutdown case quickly and damage may occur. This and other cases are highlighted on the datasheet on Table 1 for more reference.

    Let me know if this makes sense.

    Regards,
    Eric

  • 0 in reply to Eric Schott1
    Prodigy 100 points

    Hi, Eric

    Does Table 1 related to "The devices designed for fast signaling rate (10 Mbps) will not survive a bus pin fault with a direct short to voltages above

    30V "  like SN65HVD1783 only?

    In our application we have SN65HVD1781 protected with ( Smbj12CA + SM712+surge arrestor in parallel )with some 2 ohm serial resistor.

    So, the Voltage to GND should not exceed 26V.  Anyway ,we have RMA with SM712+SN65HVD1781  wirebonds evaporation when 3W serial resistor+ ,Smbj12CA +surge arrestor  are Ok.  

    Thanks

    Michael.

  • 0 in reply to Michael Shne
    TI__Guru 53575 points

    Hi Michael,

    Thanks for sharing your surge protection scheme. I agree it is unclear which device(s) the table pertains to. I presume that any of these devices exposed to a 70V in a low-driving state would experience at least significant functional upset and possibly damage due to the large power dissipation requirements even with the current limit of 200mA. In either case, all devices are rated to withstand faults below +30V in any state. If the voltage is not expected to exceed 26V in your system, any of the devices in this family seems suitable. 

    Regards,
    Eric

  • 0 in reply to Eric Schott1
    TI__Guru 62920 points

    I can clarify on Table 1.  It only applies to the SN65HVD1782 device.  The other part numbers in this datasheet (SN65HVD1780 and SN65HVD1781) can support +/-70 V faults even with the driver active.  This condition can lead to high power dissipation in the device (i.e., if it is trying to pull a high-level short low or a low-level short high), but when this occurs the thermal shutdown function should disable the driver before damage occurs.

    Regards,
    Max