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ISO7761: Device Functional Modes

Akira Nonoyama
Prodigy 10 points
Part Number: ISO7761

I am using ISO7761.
The data sheet states the following, but I think the actual operation is different from the description.
Attached is the circuit diagram and waveforms.

According to the data sheet, when VCCI is PD (<1.7V) and VCCO is PU (>2.25V), the output should be Default (High without F suffix).
However, according to the attached waveform, it looks like the Output is being driven low.

Q1:
What causes the behavior to differ from the datasheet?

Q2:
Is there a way to prevent Output from going L when VCCI is PD?
Pulling up Output with VCCO had no effect.

thank you.

  • 0
    TI__Genius 13365 points

    Hi Nonoyama-san,

    Welcome to E2E and Thank you for your questions.

    In terms of the behavior of the device, One thing to note is the difference between the F suffix device versus the non-F suffix devices, which is the default state. For the ISO7761 (non-F suffix) the default state is LOW, while the ISO7761F (with F suffix) is HIGH.

    It looks like you require the state to be HIGH but the default state of the non-F suffix device is LOW. If you use the ISO7761F device, the default state will be HIGH, which could be more fitting for your application.

    (For more information, please feel free to look at this FAQ: [FAQ] What is the difference between ISOxxxxF and ISOxxxx devices (Ex. ISO7741 vs. ISO7741F)? - Isolation forum - Isolation - TI E2E support forums)

    Regards,
    Aaditya Vittal

  • 0 in reply to Aaditya V.
    Prodigy 10 points

    Mr. Aaditya Vittal

    Thank you for your prompt reply.
    There is the following description in the data sheet, and I think the default for non-F suffix is High.

    Is my understanding wrong?

    If the power is turned off while Input is High, the following behavior will occur.


    When the power is turned off, the output is momentarily reversed.


    This becomes a very important issue on my device.
    Is there a way to fix the output when the power is turned off?

    thank you.

  • 0 in reply to Akira Nonoyama
    TI__Genius 13365 points

    Hi Nonoyama-san,

    Thank you for your question.

    Looking back at my response, looks like I had a mix-up and I am sorry about that. Here is the corrected response: 

    In terms of the behavior of the device, One thing to note is the difference between the F suffix device versus the non-F suffix devices, which is the default state. For the ISO7761 (non-F suffix) the default state is HIGH, while the ISO7761F (with F suffix) is LOW

    For the ISO7761, The behavior of the output momentarily going to HIGH when powering down the VCCI is expected. As shown in the below image (section 8.4 from datasheet), when VCCI is powering down and VCCO is powered up, the output is at default state (HIGH for the ISO7761). When the VCCO is powering down, there is no power to VCCI and the state is undetermined:

    In order to solve your experience in your system, we recommend to use the F-suffix device, as the default state is already LOW. This means that when VCCI is powering down, the output state will remain LOW and will not momentarily flip.

    Regards,
    Aaditya Vittal

  • 0 in reply to Aaditya V.
    Prodigy 10 points

    Dear Mr.Aaditya Vittal

    thank you for your reply.
    Returning to the first question.
    I'm expecting a waveform (assumude output) like the one below, but it's actually driven low when the power is turned off.

    What could be the cause?
    Also, is there any way to avoid this phenomenon?

    ISO7761F cannot be used because Default High is required for other channels.

  • +1 in reply to Akira Nonoyama
    TI__Genius 13365 points

    Hello,

    Thank you for your reply.

    After taking a closer look at your waveforms, this issue may be cause by a margin of timing when VCCI is powered down versus when VCCO is powered down.

    When VCCI is powered down, the input voltage is also decreasing, but the VCCO is still powered. So when input decreases enough to cross the low level input threshold, the output can still give a LOW signal until the VCCO is fully powered down. 

    To possibly avoid this, you can delay the VCCO power down by 10ms or 20ms. Another way is to power down the VCCO first before powering down the VCCI.

    Regards,
    Aaditya Vittal

  • 0 in reply to Aaditya V.
    Prodigy 10 points

    Dear Mr. Vittal

    Thank you so much for your suggestions.
    I understand the cause.
    I would like to turn off VCCO before VCCI.

    Have a nice weekend!