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ISO1540: Interface to TCA9548A & TCA9517A

Steve Widener
Genius 9575 points
Part Number: ISO1540
Other Parts Discussed in Thread: TCA9517A, , TCA9548A

I have the following connections -> #1 ISO1540 side 2 going to the B-side of a TCA9517A (through a linking cable "A"). The A side of the TCA9517A goes trough PCB trace capacitance to a TCA9548A switch. Each of the 8 outputs of the switch go to Side 1 of another ISO1540, #2. 

We think that we've violated the input capacitance (40pF max) of the #2 ISO1540 since the output capacitance of the TCA9548A is 28pF + 7.5pF = 35.5pF when the switch is closed along with the #1 TCA9517A output capacitance (~ 13pF max) + the input capacitance of the # 2 ISO1540 side 1 (7pF typ) = 55.5pF and that's without trace or cable capacitance.

Please confirm this is correct. If so, do we have an alternate solution to fan out 8 I2C isolated I2C connections?

Is it ok to interface to the TCA9548A as described?

  • 0
    TI__Mastermind 47895 points

    Hi Steve,

    Side 2 of ISO1540 is capable of operating on a fully loaded I2C bus with a maximum capacitive load of 400pF. #1 ISO1540 should not have a problem with these connections. 

    Side 1 of ISO1540 is the low-load node side which also uses a static voltage offset (SVO). Side 1 of #2 ISO1540 not only would be overloaded by the capacitance you describe, but also would be incompatible with the B-side of TCA9517A which also uses an SVO (which it would be connected to when the switch enables that channel).

    Is it possible to switch the orientation of #2 ISO1540 to resolve the SVO and loading conflicts? If possible, please share a diagram or schematic so that I can better understand the connections and the purpose behind them. If you do not wish to share these on the public forum, you may send them to my email found by clicking my E2E name.

    Regards,
    Eric