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ISO1640-Q1: Request for Review of I2C Isolator ISO1640BD-Q1 Configuration

Natesan Sambasivam
Prodigy 190 points
Part Number: ISO1640-Q1
Other Parts Discussed in Thread: ISO1640, INA226,

Tool/software:

Hi,

We have used the I2C isolator ISO1640BD-Q1 in one of our designs.

Side 1 of the ISO1640 is connected to the microcontroller and an RTC circuit in parallel.
Side 2 of the ISO1640 is connected to the INA226.

Kindly review and share your comments. 

Is it acceptable to connect two I2C devices (MCU and RTC) in parallel on Side 1 of the ISO1640?

Regards,

Sivam Natesan

  • 0
    TI__Genius 12535 points

    Hi Sivam,

    Thanks for reaching out.

    The schematic looks mostly good. The only thing that is missing is the o.1uF decoupling capacitors from VCC1 to GND1 and VCC2 to GND2. Please refer to "detailed design procedure" section in the datasheet:

    Regards,
    Aaditya Vittal

  • 0 in reply to Aaditya V.
    Prodigy 190 points

    Hi,

    Thank you for the review comments.

    1. Could you please suggest an appropriate pullup resistor value for the I2C bus?

    2. Could you please explain this section from the datasheet?

    9.2.2 Detailed Design Procedure

    Although the ISO1640-Q1 features bidirectional data channels, the device performs optimally when side 1 (SDA1 and SCL1) is connected to a single controller or node of an I2C network while side 2 (SDA2 and SCL2) is connected to the I2C bus. The maximum load permissible on the input lines, SDA1 and SCL1, is ≤ 80 pF and on the output lines, SDA2 and SCL2, is ≤ 400 pF.

    It is stated in the datasheet that the device performs optimally only if side 1 is connected to the controller and side 2 to the I2C bus. Would it be acceptable to connect side 1 to a controller and an RTC in parallel? Kindly confirm.

    Regards,
    Sivam Natesan

  • 0 in reply to Natesan Sambasivam
    TI__Genius 12535 points

    Hi Natesan,

    Thanks for your reply.

    Natesan Sambasivam said:
    It is stated in the datasheet that the device performs optimally only if side 1 is connected to the controller and side 2 to the I2C bus. Would it be acceptable to connect side 1 to a controller and an RTC in parallel? Kindly confirm.

    As you have seen, the datasheet states that side 1 should be connected to a single controller while side 2 can be connected to the I2C bus. The main reason for this is due to the the capacitance load capability of each side. The datasheet states that the capacitive load for side 1 is 80pF while for side 2 it is 400pF. as you can see, side 2 capacitive load is much greater which allows for connection to multiple devices or the I2C bus. If you would like to connect multiple devices to side 1, please double check the capacitive load in relation to side 1 of ISO1640.

    In terms of the pull-up resistor for I2C bus, this mainly depends on your system. Please see the typical application diagram which shows a 3.3kohm resistor for pull-up resistor. This can be changed for a different rise/fall time requirement depending on your system:

    Regards,
    Aaditya Vittal