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ISO1541: Using Multiple ISO1541 from a Single I2C Bus on the MCU to Isolate Different System Segments

Ivan Vargas
Prodigy 65 points
Part Number: ISO1541

Tool/software:

Hi everyone,

I'm working with an MCU (3.3V) as an I2C master, and I need to isolate two different segments of the bus to interact with:

  1. Two DACs (0x20 and 0x21, powered at 5V) for analog outputs (0–10V and 4–20mA).
  2. Two I/O Expanders (0x14 and 0x15, powered at 5V) for digital signals up to 24V.

My idea is to use two ISO1541s, both connected to the same ESP32 I2C bus on the non-isolated side (SDA/SCL at 3.3V), and then isolate each I2C segment separately on the 5V side as shown in the following schematic.

  • Is this configuration valid from an ISO1541 operation perspective?
  • Are there any limitations when connecting multiple ISO1541s to the same MCU I2C bus as shown here?

Thank you in advance for your help.

  • 0
    Guru 316920 points

    This will work.

    Two sides 1 must not be connected together, so side 2 must go towards the MCU (and the buses on side 1 must be short).

  • 0
    TI__Expert 7585 points

    Hi Ivan,

    Thank you for reaching out. Please find my inputs below:

    1. The Side1 of ISO1541 should be connected to only one node, which should be MCU in this case. Hence this configuration is not valid for ISO1541 due to 2 ISO1541 Side1 connected to each other along with MCU.
    2. The Limitation exists on SIde1 for one node because of the way bi-directionality is achieved. The FAQ talks about how bidirectional function is being achieved and this makes the device talk to only device on Side1 ( ESP32 I2C bus in this case) and not to any more devices and not even to a second ISO154x Side1 : https://e2e.ti.com/support/isolation-group/isolation/f/isolation-forum/757564/faq-why-is-the-logic-low-level-output-voltage-vol1-up-to-0-8v-on-side1-of-the-iso1540-iso1541-and-iso1640-iso1641-bidirectional-i2c-isolators
    3. Alternate Solutions:
      1. Since both the IO Expanders and DACs are powered through same 5V rail on Side2, you can connect the I2C_io to I2C_dac and use only one ISO1541. Make sure that the Side2 capacitance is <400pF combining all the 4 I2C peripherals.
      2. As Clemens suggested, swapping it should also work fine as long as the cable length is kept short to limit 40pF Cap Load value for Side1. Make Sure the logic compatibility is taken care for Side1 VOL/VOH and Peripherals VIL/VIH.

    Regards
    Varun

  • 0 in reply to Varun Kumar
    Prodigy 65 points

    Hi Clemens, Varun;

    Thank you very much for your previous response; it was very clear and helpful in understanding the limitations of ISO1541.

    We are currently in an early stage of development, and I need to isolate two I2C segments, each with its own power supply (5V_dac and 5V_io), but the MCU only has a single I2C bus. I understand that it is not possible to connect two Side1 of ISO1541 to the same bus due to its internal architecture.

    Therefore, I wanted to ask:

    Is there an ICC isolator (such as an ISOxxx) that allows multiple isolated segments to be connected to the same master bus without violating its operating conditions?

    I am also open to considering other alternative solutions that TI may recommend for this specific case, as long as they respect the isolation between both segments and operate with a single I2C bus on the MCU side.

    Due to logic compatibility restrictions, I cannot invert Side1 and Side2 or use independent I2C buses.

    Thank you very much for your help and suggestions.

    Greetings,
    Ivan

  • +1 in reply to Ivan Vargas
    TI__Expert 7585 points

    Hi Ivan,

    Thanks for further inputs.

    Ivan Vargas said:
    Is there an ICC isolator (such as an ISOxxx) that allows multiple isolated segments to be connected to the same master bus without violating its operating conditions?

    Currently, we don't have any I2C Isolator with above requirement.

    I have following ideas which can be implemented:

    1. One Suggestion that I have is that you can use an I2C Switch to split the I2C_master into 2 separate I2C lines. An example part is TCA9543ADR.

    For any further questions on the I2C switch I recommend to direct the questions through separate e2e post using the I2C device so that the appropriate experts can answer your questions.

    Regards
    Varun