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SN65HVD1781: reliability concerns if above operating but below max ( when not operating)

ajayt
Genius 9555 points
Part Number: SN65HVD1781
Other Parts Discussed in Thread: SN65HVD1786,

I have a question regarding RS485 transceivers and voltage rating. On one of our interfaces we are running standard RS485 communication but also applying 12-16V when not communicating (driver disabled). The duty cycle is approximately 50%, so this is not something that happens occasionally. Currently we are using SN65HVD1786 for this specific interface:

Both input voltage and differential input voltage is within range when apply 16V.

SN65HVD1786 is a 5V part, but having a 3.3V part would make interfacing to 3.3V logic easier. We are looking at SN65HVD1781 and run with 3.3V supply, but then we will no longer operat withing recommended operation conditions:

However 16V is still within absolute maximum ratings:

Is it safe to operate in the area between recommended operating conditions and absolute maximum ratings?

Will it limit the component expected liftime in any way and/or stress the component?

 

  • 0
    TI__Guru 62920 points
    Hello,

    Forgive me if I am misunderstanding your question, but it sounds like you are concerned about running SN65HVD1781 at VCC = 3.3 V. The recommended operating range actually extends down to VCC = 3.15 V, though, and there would be no additional stress to the device at the lower voltage. Similarly, the device is rated to handle bus voltages up to 70 V, so 16 V should not be a concern.

    One thing to note is that the output differential voltage amplitude of SN65HVD178x scales with the VCC level. The device is designed to satisfy the minimum requirements of the RS-485 standard when VCC is >= 4.75 V. At lower VCC levels the output differential voltage amplitude will reduce (see VOD spec in the datasheet). In many cases this is fine (RS-485 specifies a worst-case receiver sensitivity of 200 mV, so there is some margin inherent in the standard), but could be a concern in applications with high loss (e.g., due to long cables or the use of series resistances for current limiting).

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    TI__Genius 9555 points

     

    My concern is that we will be running outside recommended operation condition (-7V to +12V) when applying 16V accross the output on a SN65HVD1781  (but inside absolute maximum rating). So what do you base the figures for recommended operation condition? Are we safe when operating outside recommended operation condition? 

    There is a comment to the absolute maximum rating indicating that it is not all good: "These are stress ratings only and functional operation of the device at these or any other conditions beyond those indicated under Recommended Operating Conditions is not implied".

    When using SN65HVD1786 16V is inside the recommended operation condition (-20V to 25V).

     

  • 0 in reply to ajayt
    TI__Guru 62920 points
    Thanks for clarifying - I understand the concern now. In this case the recommended operating range for the common mode voltage means the range over which communication functions (transmit and receive) continue to work normally. With the 'HVD1781 device you may not be able to properly communicate when 16 V is applied, but it will return to normal operation once the bus voltages return to normal levels. Applying this voltage should not have any significant impact on the device lifetime or reliability.

    I hope that answers your question - please let me know if not.

    Max