• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 33 subscribers
  • Views 790 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.

SN65HVD72 Extreme environment failsafe resistors to 3.3V rail

David Mckinney
Expert 6840 points

We are operating in an extreme environment with the part placed nearby a high power LF transmitter and require fail-safe resistors.  On TI’s half Duplex +3.3V RS485 transceivers, if I add pull-up and pull-down failsafe resistors to the HI/LO output pins with the High side connected to +3.3V, will there be an issue with other devices on the bus that may be  powered with +5V instead of 3.3V? Or if other devices on the bus have failsafe pull-up resistors connected to +5V?

  • 0
    TI__Guru 62920 points
    David,

    Typically the resistors used for failsafe biasing are large enough in value that the currents involved are too low to cause much of an issue. It depends on how the power regulators at either end of the link work, though. Some are not able to sink current at all, and so there is a risk that the 3.3-V side may be biased up by the 5-V failsafe. You could avoid this issue by placing a diode in series with the failsafe resistances or by ensuring that the current load on the 3.3-V rail is large enough that the IR drop across the failsafe resistances knocks the voltage down to reasonable levels.

    Max