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ISO7421: High Level at Input when Vcc is turned off

Tobias Hensle
Prodigy 95 points
Part Number: ISO7421
Other Parts Discussed in Thread: ISO7721,

Hi,

Due to the current market situation I wanted to replace the ISO7421D with an ISO7721.

For power reduction during sleep mode, the Vcc is turned of via a µC-Pin (ATMEGA168-PA) but the input pin of the ISO remains in high state. Replacing the ISO7421 with the ISO7721 resulted in a much higher current consumption during sleep mode and the main reason is clamping diode from the input (Anode) to VDD (Kathode). While Vcc is in this case shorted to GND via µC, current is flowing throw this clamping diode.
But this additional clamping diode is not available in the ISO7421D, see pictures below:

Iso7421D Input

And this is the input of the ISO7721:

Iso7721 input

As you can see, replacing the ISO7421D with the ISO7721 is not possible, it would result in software changes which is not an option.

But as I noticed in the "Absolute Maximum Ratings", the ISO7421D is still used outside the specification. The logic level is 5V and results in a voltage which is at least 4.5V higher when Vcc is 0V, but there hasn't been an issue for several years now. From an electrical perspective the design is save at this point, a small current of 5µA is flowing throw the 1MOhm resistor to Vcc, if the input is high and Vcc low.
Some measurements also gave some certainty that this shouldn't be an issu at all.

Yellow: Vcc-Voltage

Green: Input-Pin

Orange: Vcc-Current

My questions is now: Is the design of the ISO7421D still robust enough to last for a lifetime (10 years) and in case of a "yes" why is it so (silicon design etc.) ?

kind regards

Tobias

  • +1
    TI__Guru 53925 points

    Hi Tobias,

    Thanks for reaching out and for sharing details information related to your question, appreciate it.

    The bypass diode that you see in the input circuit between INx pin and VCC is part of the ESD circuit. Most commonly we use the ESD circuit that is similar to what is in ISO7721 while ISO74xx family is one which uses a different ESD design that doesn't result in a diode connection between INx pin and VCC. For this reason, the absolute max ratings of ISO7421 are retained to be similar to other devices.

    Tobias Hensle said:
    My questions is now: Is the design of the ISO7421D still robust enough to last for a lifetime (10 years) and in case of a "yes" why is it so (silicon design etc.) ?

    Since there is no ESD bypass diode between INx and VCC and since the device is not getting power or passing excessive current through any circuit, we do not expect the device to have any damage. Hence, it should be fine to continue using ISO7421 in your application.

    If you are redesigning your system we recommend changing the software such that this situation doesn't occur as it is outside recommended operating conditions and most isolators do not support such operation. Thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 95 points

    Hi Koteshwar,

    Thank you for your reply.
    Your information was helpfull and we can live with this issue Slight smile

    kind regards

    Tobias