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ISO7041DBQEVM: I am using two ISO7841, but I want to substitute one of those for a ISO7041.

Gustavo Guerrero1
Prodigy 60 points
Part Number: ISO7041DBQEVM
Other Parts Discussed in Thread: ISOW7841EVM, ISO7041, , ISO7841, ISOW7841, ISO6741, ISO7741, ISO7721, SN6501, SN6505B

I am isolating the SPI interface of a MAX30003 and a micro controller with two ISOW7841EVM (CSB, SCLK,SDO, SDI, INTB). INTB Is connected to the second ISOW7841EVM. This is working fine.

Now I want to reduce power consumption, so I change the second ISOW7841EVM for a ISO7041DBQEVM and I connected INTB and VCCISO from the other ISOW7841EVM to the ISO7041. This is not working.

I try changing the SDI and the INTB of isolator and this works.

My question is, if the transmission rate is less than 4 Mbps, is there a way where a configuration that works with the ISOW7841EVM should not work with the ISO7041DBQEVM? 

  • 0
    TI__Expert 6955 points

    Gustavo,

    Would you be able to share a basic block diagram of how these EVMs are being connected to each other as well how they are being powered? This will help eliminate confusion when trying to debug.

    One thing to keep in mind is the higher propagation delay that is associated with the ISO7041. This could affect the SPI communication if the prop delay of the ISO7041 is too large for the desired data rate.

    Another thing that can help the debug effort is capturing waveforms for these signals. Ideally these waveforms capture input vs output and help make sure that signals are passing through the isolators as expected. This would also show if there is significant prop delay  or any other physical layer issues.

    Respectfully,

    Lucas

  • 0 in reply to Lucas Schulte9
    Prodigy 60 points

    Hi Lucas,

     I solved the problem of substituting one of the ISO7841 for a ISO7041, but now I have another question

    The first circuit that I was testing is this one:

    Two ISO7841 and it worked fine.

    Then, I tried this circuit, replacing one ISO7841 for an ISO7041 and using the other ISO7841 as an isolated power supply for the ISO7041

    And this worked too.

    Now what I want to replace the other ISO7841 for a ISO7041, but I will use the ISO7841 as an isolated power supply for the moment:

    This configuration is not working. The ISO7841 isolated power supply has enough current for this application. I saw the delay of the ISO7041, but I was wondering, if I only use ISO7041, I will have the same delay in all the pins. But I will have the double of delay in the SDI, won't I? Do you have any ideas?

  • 0 in reply to Gustavo Guerrero1
    TI__Expert 6955 points

    Gustavo,

    Assuming all isolators and their Vcc pins are being powered, the primary suspect here is the propagation delay from using two ISO7041 devices.

    I recommend observing the input vs output signals with an oscilloscope to verify this, measure the propagation delay, and see if it's too much for the SPI data rate being used.

    Also, are 2 isolation devices in series needed for your application?

    Respectfully,

    Lucas

  • 0 in reply to Lucas Schulte9
    TI__Guru 54715 points

    Hi Gustavo,

    I just wanted to check back with you to see if you still are facing any issues. I understand that you are testing the following configurations,

    1. Two ISOW7841 EVMs for isolating 5 digital channels
      1. This is working fine.
    2. One ISOW7841 and one ISO7041 EVM each for isolating 5 digital channels
      1. This had an issue earlier but is working fine now. If possible, please do share what made this configuration work.
    3. One ISOW7841 for powering two ISO7041 EVMs that isolate 5 digital channels
      1. We expect this configuration also to work fine.
      2. As Lucas pointed out earlier, propagation delay plays a key role in supporting a specific datarate for SPI interface.
      3. SPI speed can be calculated as, fmax = 1 / (4*tpdmax) = 1 / (4*175ns) = 1.42 MHz

    From the above calculation, we expect ISO7041 to support a max SPI clock of 1.42 MHz while, using same calculations, ISOW7841 can support a max SPI speed of 14.2 MHz (for 5V). Hence, it is possible to see performance difference between the two device configurations.

    This should help clarify and answer your observations. I will go ahead and mark this post as closed, if you do have further questions please do respond back. Thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 60 points

    I think the problem is the propagation delay. Because if I connect all the cables in the SPI configuration to the ISO7041, except for the MISO (connected to the ISO7841), the connection works. 

  • 0 in reply to Gustavo Guerrero1
    TI__Guru 54715 points

    Hi Gustavo,

    Thanks for doing an experiment with MISO line connected to ISOW7841 and confirming. Yes, MISO is the line that sees the most return propagation delay and it does point out that the main cause for issue is the higher propagation delay of ISO7041.

    If you still like to meet 4 MHz of SPI speed, I would recommend using either ISO7741 or ISO6741 which support much higher datarates with much better propagation delay specifications. I understand these digital isolators are not as low power compared to ISO7041 but I believe the power consumption by these devices is significantly low compared to the isolated power supply device ISOW7841.

    I hope the results have helped you choose the right devices for your application. Let me know if you have any other questions, thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 60 points

    Maybe you can help me with another question related to the same issue. I think the Isolated DC/DC of the ISO7841 consumes too much. So I am planning to use one ISO7721 for the MISO and the rest of the digital channels connected to an ISO7041. Now, I am using an sn6501 with a transformer for the isolated power supply, but I was wondering if you have a more efficient solution or one with a smaller footprint.

  • 0 in reply to Gustavo Guerrero1
    TI__Guru 54715 points

    Hi Gustavo,

    Unfortunately, achieving lower power for isolation power supply solution would be a challenge. Since SN6501 based power supply solution is a discrete solution with external transformer, it achieve higher efficiency than ISOW7841 and hence, is going to be lower power consuming, relatively. This lower power consumption solution of SN6501 comes at a cost of higher space compared to ISOW7841.

    Apart from ISOW7841 and SN6501 / SN6505B, I do not have any other lower power power supply solutions to propose. Thanks.

    Regards,
    Koteshwar Rao