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I2C Isolation and Mux usage (ISO1540 + PCA9518)

robb fisher1
Prodigy 150 points
Other Parts Discussed in Thread: ISO1540, PCA9518

Hi, we are using 2 PCA9518 I2C expanders to create a 1 to 4 I2C expander.  That circuit is working good, but now we have to isolate the I2C interface and want to use the ISO1540 in front of the PCA9518 which runs at 3.3V.  Will those 2 parts play nice together?  I'm concerned as the PCA9518 creates a 0.5V ledge on the SDA bus and not sure if that will interfere with the ISO1540 direction detection. The side 2 spec of the ISO1540 says 0.3 x VCC2 (0.99v) for Vil2, so it appears that it would meet that spec, but not sure......

Thanks - Robb

  • 0
    TI__Guru 55795 points
    Hi Robb,

    I do see that PCA9518's VOL can be at max of 0.7V with 0.52V being typical for VCC between 3V to 3.6V. 0.52V typical should still work fine for ISO1540 under normal operating conditions. If VCC is maintained at 3.3V for PCA9518, I guess VOL should remain around 0.52V which should be fine for ISO1540 as input.
    Alternatively if you connect PCA9518 on Side 2 of ISO1540 and MCU / host on Side 1 of ISO1540, you shouldn't see any issues with these devices. Let us know if you this helps answer your question and if you have any more concerns on this?

    Regards,
    Koteshwar Rao
  • 0 in reply to Koteshwar Rao
    Prodigy 150 points
    Koteshwar,
    thanks for the info - we will go with this solution.