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Original question:

[FAQ] Why is the logic LOW level output voltage, VOL1, up to 0.8V on Side1 of the ISO1540/ISO1541 and ISO1640/ISO1641 bidirectional...

ISO1641: Do we need a buffer between INA237 and ISO1641

Diang Xing
Mastermind 25651 points
Part Number: ISO1641
Other Parts Discussed in Thread: INA237, , ISO1640, INA226, TCA9517A, TCA9800

Hi Team,

We want to use I2C to output power telemetry data. But checked their H/L threshold voltage, INA237 consider 0 - 0.4 V as L. Will ISO1641 be able to output the L less than 0.4 V? Adding buffer will be a negative effect for the customers since cost is one of their major considerations.

In the datasheet, since SDA/SCL are open-drain pins, if there is no PN junction in the loop at the low side, I think we can make the ISO1640/1’s low level much lower than 0.57 – 0.71 V by adjusting the pull-up resistance (the SDA/SCL current will be reduced too). Since we do not need a high data rate (1 - 10 kHz), large pull-up resistance should be acceptable.

Best regards,

Diang

  • +1
    TI__Guru 54245 points

    Hi Diang,

    Thanks for reaching out.

    Diang Xing said:
    Will ISO1641 be able to output the L less than 0.4 V?

    If Side1 of ISO1641 is being interfaced to INA237 then the two devices won't be compatible to each other as you pointed in your drawing. This can be addressed in 3 ways as listed below.

    1. If the ISO1641 is connected to INA237 on one side (Side1) and connected to only one node (like an MCU) on the other side (Side2), then you can swap the sides of ISO1641 and have the MCU on Side1 while making sure they are compatible with each other. If direction of SCL becomes a concern, you can consider using ISO1640 which has bidirectional clock.
      1. If the MCU is the one that is going to send clock then Side1 of ISO1641 should ideally be connected to MCU to allow SCL transmission.
    2. Check if customer can use INA226 instead of INA237. INA226 is compatible with ISO1641 and they will not have any issues in communicating.
    3. If INA237 needs to be connected only to Side1 of ISO1641 then you will have to use an I2C buffer like TCA9517A or TCA9800.

    Let me know if you have any other questions, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Mastermind 25651 points

    Hi Koteshwar,

    Really appreciate your answer!

    The bus voltage for our measurement is 57 V. So IN226 could not be directly used. If we need at least two items among Voltage / Current / Power, could we use IN226 with a voltage divider to measure +dc voltage?

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    TI__Guru 54245 points

    Hi Diang,

    I understand your concern on INA226 not supporting 57V. Could you please post this question related to INA226 on the Amplifiers forum to get the most accurate answer?

    https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Mastermind 25651 points

    Hi Koteshwar,

    Sure. Thanks for your help. No more questions for now and please close this thread.

    Best regards,

    Diang