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ISO7721: Digital Isolator Instead of Optocoupler

Part Number: ISO7721
Other Parts Discussed in Thread: TPS1H000-Q1, TPS4H000-Q1

Hi everyone,

I have an application for my microcontroller application. I need to enable some pins related with microcontroller but I don't want to use optocoupler. Instead of it, I would like to use digital isolator. I attached the my application schematic. Please check. 

Instead of using optocoupler like in the schematic, I want to use digital isolator if possible. 

Thanks!

  • Hi Alikaan,

    Digital isolators offer several advantages over optocouplers, and their implementations are quite different. A digital isolator could definitely be used instead of the optocoupler above with modifications to the circuit. Please let me know the following information, and we can make this happen!

    • Is this design flexible? The components R25A, R26A, R31A, and C1 can be removed for digital isolator input and output lines
    • On the output side, 24V is too high to supply our ISO77xx and ISO78xx devices. If this voltage is not critical, could we reduce it to 3.3V or 5V? Alternatively we can use an LDO.
    • How is the isolated power supply generated?
    • Do the 12 MCU data lines need isolation? They can all be isolated using two of our six-channel isolator devices, ISO776x.
    • In which direction does the data flow? Our isolators have forward (left to right) and reverse (right to left) channels for data transmission and receiving.

    If there is anything I missed, feel free to ask me! I am looking forward to your response.


    Thank you for posting to E2E,
    Manuel Chavez

  • Hi Manuel,

    Here the answers:

    -Is this design flexible? The components R25A, R26A, R31A, and C1 can be removed for digital isolator input and output lines

    +Yes, design is flexible so I can remove the unnecessary components.

    -On the output side, 24V is too high to supply our ISO77xx and ISO78xx devices. If this voltage is not critical, could we reduce it to 3.3V or 5V? Alternatively we can use an LDO.

    +We have to  use 24V for the second side, so if possible we can change the part number for 24V

    -How is the isolated power supply generated?

    +I used isolated power supply from TI.

    -Do the 12 MCU data lines need isolation? They can all be isolated using two of our six-channel isolator devices, ISO776x

    +It is not necessary but we can use.

    -In which direction does the data flow? Our isolators have forward (left to right) and reverse (right to left) channels for data transmission and receiving.

    +Data flowing from left to right.

    Please let me know reference EVM for this kind of application.

    Thanks!

  • Hi Manuel,

    I replied your questions, do you have any update for these answers?

    Thanks
  • Digital isolators are designed for logic signals; there are none for 24 V.

    It would be possible to use the output of the isolator to control a separate switch (e.g., TPS1H000-Q1, or TPS4H000-Q1 for more channels, or built from discrete transistors). But what kind of load are you switching; could a low-side switch be used?

    Why do you want to replace the optocoupler? As far as I can see, it would be the simplest solution.
  • Hi Alikaan!

    I apologize for the delay, below are my responses:


    Yes, design is flexible so I can remove the unnecessary components.

    That's perfect! The ISO77xx requires 0.1uF capacitors on Vcc1 and Vcc2. Layout will change with respect to the optocoupler as well since only one pin is needed for data transmission. This is shown in the datasheet clip below:



    We have to  use 24V for the second side, so if possible we can change the part number for 24V

    Unfortunately our digital isolators cannot be powered directly by 24V, but an LDO like the TLV7605 can be used for Vcc2. I will include a diagram further below

    I used isolated power supply from TI.

    If you don't mind sharing, which device?

    It is not necessary but we can use.

    That's okay! If preferred, combinations of one, two, four, or six channels can be isolated with ISO devices. Ensure that they all end in ISO77x0 so that every channels enable data to flow from left to right. The following diagram demonstrates a possible solution to replace your optocoupler:



    Data flowing from left to right. Please let me know reference EVM for this kind of application.

    Although not identical due to power constraints, some helpful EVMs for this design are the ISO7721 EVM and ISO7762 EVM.

    On the original schematic, there is an MCU_D_OUT bus/connection that could be crossing the isolation barrier. I admit that the path's termination is unclear, but if any lines for power or data cross the isolation barrier (dotted red line above), the entire system is compromised. Please feel free to share a larger schematic via PM.


    Let me know your initial thoughts and concerns you have regarding the design diagram above. Does this model your desired system correctly? If it's acceptable we can move on to a schematic.


    Thank you for your time,
    Manuel Chavez

  • Hi Alikaan,

    Has the post above been helpful? Please let me know! If there is no response soon, this thread will be closed due to inactivity.


    Respectfully,
    Manuel Chavez