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ISO7220A: Unexpected output on power up

Part Number: ISO7220A

We are using an ISO7220 as a substitute part in an older design.  

We have seen on some of our boards that when the device first powers up, the output does not reflect the input. Once the input is changed, the output then follows the input correctly. 

Below is a scope shot showing the problem 


CH4 = VCC2(+3V3)



Here it shows that the input to INB is LOW however OUTB goes high and stays this way.   Not shown in the scope shot is that once INB is transitioned, OUTB then follows correctly. Is this expected behavior?  We have never seen this before with other parts.

  • Hi Dan,

    Thanks for reaching out and for sharing the schematic and waveform.
    ISO7720A is one of our older family of devices released about 16 years ago. Please allow me to check if this is an expected behavior from this device and come back to you. Thanks.

    Koteshwar Rao

  • Hi Dan,

    Thanks for your patience.

    I checked with the design and confirmed that this is not an expected behavior. The output is expected to follow the input even after the power-up with the exception of initial few µs where the output goes to default state (HIGH).

    We also quickly tested to verify this on ISO7220A, please find below the waveform captured. VCC1 = 5V, INx = LOW, VCC2 = 10kHz square wave for power cycling. As you can see, the output initially goes to the default state (HIGH) for about 8µs and then starts following the input (LOW).

    I see you also mentioned that only some of the devices show this behavior while others are working fine. Could you please confirm how many are tested, how many are working fine and how many are failing?

    Is it possible that the CH2 probe was not working fine in your previous testing and accidentally showing a LOW input when it is actually HIGH?

    Please do share as much information as possible to help resolve the issue faster, thanks.

    Koteshwar Rao

  • Hi,

    Thank you for attempting to replicate the problem.  So far, I have only one board that exhibits this behavior.  It is a difficult problem to catch. When the boards are tested, this input is toggled, and the problem goes unnoticed.

    I doubt that my CH2 probe stopped working, as this signal goes to other circuitry on the board and it agrees with the scope reading. I can double check. 

    There is another one of these parts on our board, used in the exact circuit, but with opposite logic. I see the correct behavior.

    I'll see if I can find another unit with the problem.

    Thank You,


  • Hi Dan,

    Thanks for your inputs and the update.

    Since this issue is happening only one device, I am not sure if that device is partially damaged. You can probably store this non-working device somewhere safe and continue your evaluation with other samples. If you encounter any other sample with a similar issue, then reach out to us and send us the sample. We can take a look at it on the bench and find out what's wrong.

    For now, it doesn't look like there is any action that needed in here. I will go ahead and mark this thread as closed. If you find another failing sample, please respond to this post or create a new post by clicking on "Ask a related question" so that we have this E2E post discussion as reference. Thanks.

    Koteshwar Rao