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Original question:

ISO7760: Device Functional Modes

ISO7741-Q1: Default Output When VCCI is Between 1.7V and 2.25V

Utku Karakas
Prodigy 50 points
Part Number: ISO7741-Q1
Other Parts Discussed in Thread: ISO7741

Hello,

I'm using ISO7741-Q1 (non-F version) in an automotive project.

From Table 8-2 Functional Table it is clear that the output will be HIGH, when VCCI is "powered down (< 1.7V)" and VCCO is powered up. 

What happens when VCCI is falling and it is between 1.7V and 2.25V?

Can the output be set to HIGH in this undefined region? Or does VCCI really have to fall below 1.7V before the default output mode can trigger?

Thanks!

Utku

  • 0
    TI__Guru 52530 points

    Hi Utku,

    Thank you for reaching out.

    When VCCI is falling from 2.25V to 1.7V, the device is going to go from ON to OFF state which can happen at any voltage between this range. Typically, the voltage where it turns OFF is 1.8V (spec'd in datasheet as UVLO-) and that's where device goes from ON to OFF. Before this voltage, the device is ON and output will follow the input. After this voltage, the device is OFF and output will go to default state. Since it could be in either state based on the actual UVLO threshold value of that unit and the input pin state, we list it as undetermined in the Function Table.

    I would recommend keeping your input pulled-up through a resistor to VCCI so that the output stays HIGH when it is ON while VCCI is falling and is still HIGH when it is OFF while VCCI is falling.

    Let me know if this answers your question, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 50 points

    Hello Koteshwar,

    Thank you for the quick response. 

    Just to make sure: UVLO- is typically 1.8V, but it can be as high as 2.15V (2.25V minus 100mV Hysteresis), is this correct? And if that's the case, the ON to OFF transition would happen as early as at 2.15V when VCCI is falling and ISO7741 would pull its output to default HIGH at VCCI=2.15V?

    This is the situation I'm trying to avoid. My input is 0V, and will stay at 0V while my VCCI is falling and I have a strong pull-down resistor at the output. So I already have the desired configuration, but if ISO7741 is outputting default HIGH at 2.15V, instead of at < 1.7V as I assumed, then the pull-down resistor won't help.

    Best regards,

    Utku 

  • +1 in reply to Utku Karakas
    TI__Guru 52530 points

    Hi Utku,

    Thank you for your inputs, please find my comments below.

    Utku Karakas said:
    Just to make sure: UVLO- is typically 1.8V, but it can be as high as 2.15V (2.25V minus 100mV Hysteresis), is this correct? And if that's the case, the ON to OFF transition would happen as early as at 2.15V when VCCI is falling and ISO7741 would pull its output to default HIGH at VCCI=2.15V?

    Yes, your understanding is correct.

    Utku Karakas said:

    This is the situation I'm trying to avoid. My input is 0V, and will stay at 0V while my VCCI is falling and I have a strong pull-down resistor at the output. So I already have the desired configuration, but if ISO7741 is outputting default HIGH at 2.15V, instead of at < 1.7V as I assumed, then the pull-down resistor won't help.

    Your understanding is correct again. But I would like to mention that the output will go to default state when the input side power is lost and that can happen for any voltage <2.15V. This is expected behavior.

    If a HIGH output is considered not safe, then I recommend choosing a device with LOW default output state, ISO7741F. This makes sure the output stays LOW when VCCI is falling both when device is ON and OFF. Let me know if this is what you would like to consider doing, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 50 points

    Hello Koteshwar,

    Thank you for the confirmation. 

    Default output HIGH is indeed my preferred safe state; I'll find another way to mitigate the glitch in the undefined range of 1.7V...2.25V.

    Have a nice day,

    Utku