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ISO7420E: Fail-safe behaviour

BO
Genius 12760 points
Part Number: ISO7420E

We have noticed a behavior in several of our devices that we cannot explain 100%.

We have a serial data link with four signals routed via two ISO7420ED.
Both isolators are supplied by the same voltage. They are directly next to each other in the layout, and are connected to the supply planes in the same way. So they both have the same supply voltage levels.

With power-down on the input side, dynamic signals (clock and a control signal) are present at the signal inputs of one of the two isolators. On the second isolator, the two signal inputs are static (low).
This ISO7420ED with the static input signals obviously goes into fail-safe state earlier than the one with the input signals that change their level continuously.

We have seen in the data sheet that also inputs with undefined levels at the ISO7420ED can lead to the fail-safe state at the outputs:



However, we were able to exclude this after our measurements.


Does the signal shape - static or clocked - at the signal inputs (INA, INB) of the ISO7420ED have an influence on the fail-safe behavior of the signal outputs (OUTA, OUTB) in case of input-side (INPUT SIDE Vcc) power-down?

Regards Bernd

  • 0
    TI__Mastermind 49495 points

    Hi Bernd,

    Thanks for reaching out and for sharing the details related to the customer issue.

    Please note that the absolute maximum input that can be applied to input pins (INA, INB) at any point of time shouldn't exceed "VCC + 0.5V". If VCC = 0V then INx < 0.5V, violating this can lead to permanent device damage or could affect long-term reliability. Outside of these absolute maximum ratings, the device behavior could be unpredictably and could vary from device to device. That's probably the reason why customer is seeing the device not follow the function table, as device is being operated outside of recommended operating conditions.

    I hope that answers your question, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Genius 12760 points

    Hi Koteshwar,

    in our circuit, the components that drive the signal inputs of the isocouplers are also powered by the same supply voltage. Therefore, the signal levels at the inputs of the ISO7420ED are never higher than its supply voltage. So we are within the specification of the component.

    Therefore, our question would still be the same: Is there a difference in the fail-safe behavior of the ISO7420ED's signal outputs (OUTA, OUTB) with input-side (INPUT SIDE Vcc) power-down when static or clocked signals are applied to the inputs (INA, INB)?

    Regards,
    Bernd

  • 0 in reply to BO
    TI__Mastermind 49495 points

    Hi Bernd,

    Thanks for the clarification.

    BO said:
    Therefore, our question would still be the same: Is there a difference in the fail-safe behavior of the ISO7420ED's signal outputs (OUTA, OUTB) with input-side (INPUT SIDE Vcc) power-down when static or clocked signals are applied to the inputs (INA, INB)?

    No, there is no difference in fail-safe behavior of the ISO7420ED signal outputs expected with its inputs being static or clocked.

    BO said:
    However, we were able to exclude this after our measurements.

    Could you please elaborate on the above point? What did you observe at the output when the input was clocked? Sharing a waveform showing the issue would really help containing input/output signals and input/output supply waveform. This help us if the device is not functioning the way it is expected or if any recommendations are violated. Thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Mastermind 49495 points

    Hi Bernd,

    Do you have this information from customer? This will help us understand the issue better and root cause it. Thanks.

    Koteshwar Rao said:
    BO said:
    However, we were able to exclude this after our measurements.

    Could you please elaborate on the above point? What did you observe at the output when the input was clocked? Sharing a waveform showing the issue would really help containing input/output signals and input/output supply waveform. This help us if the device is not functioning the way it is expected or if any recommendations are violated. Thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Genius 12760 points

    Hi Koteshwar,

    see offline mail.

    Regards
    Bernd

  • 0 in reply to BO
    TI__Mastermind 49495 points

    Thanks Bernd,

    Since we are discussing this issue offline, I will go ahead and mark this thread as closed. Thanks.


    Regards,
    Koteshwar Rao