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ISO7142CC: Current Limiting Spec

Part Number: ISO7142CC

Hi Team,

I'm using the ISO7142CC to send signals between two ICs and my application is in a safety critical, therefore I need to account for incorrect connections. I don't see any current limiting spec in the abs max datasheet, only max output current.

What would happen if two outputs of the ISO7142CC were connected together and 'driving' into each other.

Thanks,

Hayden

  • Hi Hayden,

    The safety limiting values of ISO7142 is mentioned in section 8.3.3 of the data sheet. Limiting currents as mentioned in the table will prevent potential damage to the isolation barrier upon failure of input or output circuitry.

    Two outputs of the Isolators should not be connected together. For instance, if one channel is driving high and the other channel is driving low there is a short between supply and ground that will cause excessive current and may damage the device.

    Regards

    Tejas

  • In reply to Tejas Hommaradi:

    Hi Tejas,

    Thanks for the info, I must of missed that in the datasheet.

    Do you have an idea of what the assumed source impedance of the outputs is? If two outputs were connected directly to each other with no current limiting resistor between, the source impedance would be the only current limiting factor, correct?

    Regards,
    Hayden
  • In reply to Hayden_Hast:

    Hi Hayden,

    The source impedance should be around 40 ohm. So if two outputs are connected together the total resistance in path should be around 80 ohm.

    Can you please explain more about the system, what is condition where two outputs get shorted in your system? 

    Regards

    Tejas

  • In reply to Tejas Hommaradi:

    Tejas,

    That’s better than I expected!

    So, if I have two shorted to each other and worst case one is high and one is low, that would result in:
    5.5V/80 = 69mA.

    Per the Safety curve below, the case can be over 100C and be able to handle that much current. So, that seems like not a huge issue, though we want to correct in next board rev to avoid unnecessary current draw.

    At present, we haven’t been using the outputs that are tied together. We are just developing the software functionality now. So, they have typically been driving into each other with about the same voltage, which would be even less current draw than the above calculation.

    System wise - this is for an aerospace electrical engine control module. It's a system that interfaces between the cockpit controls, main flight engine (previously only mechanically controlled) and other actuators around the airplane.

    Regards,
    Hayden
  • In reply to Hayden_Hast:

    Hi Hayden,

    The current mentioned in safety limiting parameter would ensure that the "Isolation barrier" doesn't get damaged due to failure of input or output side circuitry. And safety limiting parameter doesn't convey that the input or output side circuitry are protected. To  protect input and output circuitry ensure that all the parameters are withing the recommended operating condition mentioned in the datasheet.

    If system has two outputs shorted, there will be around 69mA of current flow from VCC to GND on the same side(lets say output side circuit). The output side side circuit cannot handle this high current and  will get damaged and may loose its functionality. Further if the current exceeds beyond 'Is" mentioned in safety limiting parameter, the barrier also may get damaged. 

    So, in the  application mentioned if two outputs drive each other, you may notice output side circuitry damage.

    Regards

    Tejas