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Original question:

ISO1540: ISO1540 in series

ISO1540: Connect Side 2 to Side 2

Will DeCook
Prodigy 240 points
Part Number: ISO1540
Other Parts Discussed in Thread: ISO1640

Hi I have an I2C bus with multiple slaves. Some of the slaves are in different voltage/ground domains as the master, some are in the same domain. I'm wondering if I can put an ISO1540 in front of each node with Side 1 facing the node device and Side 2 facing the common bus (on which there will be multiple other ISO1540 devices with Side 2 facing out). An example is shown below.

This is not possible with other I2C buffers because the implementation of the bidirectionality relies on offsets from GND and VCC on both sides. From what I've read in the ISO1540 datasheet, the offset is only implemented on Side 1 and it appears that it would be ok to hook Side 2 up to the Side 2 of another ISO1540. Is this do-able?

Thanks,
Will

  • +1
    TI__Genius 17100 points

    Hi Will,

    Thank you for posting to E2E and for sharing the circuit schematic. As you mentioned, ISO1540 has a voltage offset on Side 1 of the isolator to prevent latch-up of the bidirectional channel, so the Side 2 thresholds are more lenient and thus, the Side 2 pins of multiple ISO1540 devices can be connected together. This is confirmed in Section 6.9 of the ISO1540 datasheet since V_OL2 is lower than the V_ILT2 spec, and when the pins are Hi-Z, the V_IHT2 threshold will be exceeded by the bus voltage:



    You may also consider using ISO1640 instead of ISO1540 in your design. This newer device is hot-swap capable, features integrated IEC-ESD protection up to 8kV on the I2C pins, has higher isolation ratings than ISO1540, and is available in multiple package and channel configuration options. Please let us know if you have any further questions.


    Thank you,
    Manuel Chavez

  • 0 in reply to Manuel Chavez
    Prodigy 240 points

    Thank for confirming this for me, Manuel.

    Best,
    Will