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ISO7842: Can inputs and ouputs be over-driven and with how much current.

Part Number: ISO7842

I have an application where

  1. The input signal sources are 5V.  The supply is also 5V but may fall to say 3.3V in a fault condition.  Therefeore the inputs may be over-driven  but there are resistors to limit the current.  How much current can be supplied in these circumstances without damage to the isolation properties of the part (I don't care whether the signal is being transmitted correctly)?
  2. Similarly in a fault condition the ouputs may be pulled-up beyound the supply but there are resistors to limit the current.  How much current can be supplied to back-drive the outputs without damage to the isolation properties of the part (I don't care whether the signal is being transmitted correctly)?

  •  Robert,

    If the input or output pins experience a voltage higher than that of Vcc then current will flow through a protection diode at the pins as shown in the diagram below. Note that this is not normal operation and this diode is meant for ESD protection. The amount of current it can handle will depend on duration. How long do you suspect the proposed fault condition to last? Also, are you only concerned about the isolation barrier and not device functionality otherwise?

    Respectfully,

    Lucas

  • In reply to Lucas Schulte9:

    Dear Lucas,

    Thank-you for your prompt reply.

    " How long do you suspect the proposed fault condition to last?".  Forever.  Example fault conditions would be pasives or regulators failing to short-ciruit .

    "Also, are you only concerned about the isolation barrier and not device functionality otherwise?".  In the fault condition I am only concerned about the isolation barrier.

    I can fit resistors in series with the inputs and outputs and these can limit the over-drive/back-drive current to the low milliamp range. 

    My initial thought was that since the ouputs are rated for up to 15mA I would use that as the target value.

    Section 6.8 of the datasheet says that 288mA is the "Safety input, output, or supply current" however it's not clear whether this is one channel or all channels.  If it's all inputs and outputs then it suggests that 288mA/8=36mA is the limit for simultaneous over-drive/back-drive at any I/O - assuming that the supply is within range.

    So you see that the data sheet gives some hints but doesn't quite give me the parameter that I'm looking for.

    regards,

    Rob

  • In reply to Robert Pearce:

    Rob,

    A test concerning a similar situation has been done on our ISO77xx family which has the same input protection architecture. Data from those tests show that a theoretical current limit for your situation would be 500mA or less. However, the safest current limit as per the datasheet would be 288mA or less. Also, even if the protection diode from input to Vcc burns out it would not affect the isolation properties and the barrier would still be intact. Note that if and when this diode fails it would most likely fail short therefore shorting the 5V from the signal to the 3.3V faulted source.

    Respectfully,
    Lucas

  • In reply to Lucas Schulte9:

    Lucas,

    I'll limit the over-drive/back-drive current to 10mA and will be well.

    thanks

    Rob
  • In reply to Robert Pearce:

    Rob,

    Sounds good. Let me know if you have any more questions!

    -Lucas

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