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ISOW7742: vs ISOW7842? What would instruct selecting one over the other?

ken krieger
Prodigy 20 points
Part Number: ISOW7742
Other Parts Discussed in Thread: ISO1432, ISO7842, ISOW7741, SN6501,

Tool/software:

Hello Folks,

We need to select a single-ended isolator, and note that TI is offering two different solutions. 

the 78xx series seems to have the Viso connected internally, and requires only decoupling. 

the 77xx series seems similar to the ISOW14x2 series with the isolated supply fed back to the device externally. 

From an EMC, power consumption, or other factor, can one be recommended over the other? If they are the same, we prefer the smaller package as our board is dense. 

Edit:

Another semi-related question: For the 78xx series, it seems there is an excess of Viso capacity when operating from 5V. Very curious if we could power 1 or 2 ISO1432 from this supply to limit dissipation to one component for the Viso supply. This all is related to helping us a: protect that GPS module mentioned in our other question, and b: helping to create a more compact layout given the challenging real estate! 

Very curious if this is not advisable for some reason.

Much thanks for the wisdom!

  • 0
    TI__Mastermind 29275 points

    Hello Ken,

    ISOW77xx and ISOW14x2 is the newest generation ISOW technology, which offers significantly improved overall performance including radiated emissions performance. However, as you have noted, ISO7842 is available in a smaller package if smaller board space is a priority.  

    ISOW77xx can be used power another logic device. There shouldn't be an issue to use ISOW7741 as a method of power and isolation for a discrete RS-485 transceiver. However, the SN6501 and transformer driver family should be used if a larger amount of isolated power is needed for the isolated side.

    Best,
    Andrew

  • 0 in reply to Andrew J
    Prodigy 20 points

    Hi Andrew,

    Thanks for the reply! Yeah, I understand that the ISOW77xx series is considered "newer/better", however the specs show that the efficiency and "spare power" of the converter in the ISOW78xx are better. (by a little... perhaps enough in our case). 

    We only want to power two add'l ISO1432s, so not a lot of power is needed. The signals on the ISOW77xx/78xx will be static most of the time, so it won't be consuming a lot doing switching/driving, meaning most of the converter output would be useful for the external devices. 

    When you quote the ISOW77xx as "better", can you be a bit more specific? It seems like a "close race" between them. Also, the layout techniques for the ISOW77xx/14x2 seem quite a bit more rigorous, and I wonder how much adhering to such techniques for the ISOW78xx could bring it "closer" to the ISOW77xx in terms of EMC performance. 

    Sorry if it seems like hair splitting, just trying really to understand why I'd choose the ISOW77xx over the ISOW78xx. We're still flexible if there are very good reasons, it just makes the layout tighter to have to use the 20-pin package.

    Also, just for clarification, the isolated I/O drivers inside the ISOW14x2 vs the ISO14x2 are the same, right? It's just the DC DC converter that is the improvement. Did I understand your comment on that above correctly?

    Gratefully,

    ken

  • 0 in reply to ken krieger
    TI__Genius 17605 points

    Hi Ken,

    Thanks for your response. 

    Andrew will get back to you tomorrow with a response.

    Regards,
    Aaditya

  • 0 in reply to Aaditya V.
    TI__Mastermind 29275 points

    Hello Ken,

    Thank you for waiting. You are correct that the ISO-"W" devices add a DC/DC converter and the isolated I/O drivers inside the ISOW14x2 and the ISO14x2 are the same. ISOW77xx has had major improvements to the DC/DC converter section, and it allows the device to easily meet the CISPR 32 emissions standards. 

    You also can power another device off of the VISO rail as long as its current consumption is below the ISOW's current output. Usually this is enough for the isolated side of the ISOW and one other device. The spec comparison is shown below. 

    The ISO1432 (no-'W') will probably consume >100mA on the secondary side which is why I mention that there will probably not be enough power for a second device according to the above specs. 

    I hope this helps. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Prodigy 20 points

    Thanks Andrew,

    Please again correct me if I'm wrong...

    If we look over the datasheets, we also note the current consumption is assuming a 54Ohm impedance load. In our application we are using 100-120Ohm lines & termination. Hence, part of the envelope calculation is derating the max current by about 50% from the datasheet value. 

    Is it correct to make that inference? If it's not precisely scaled to the driven impedance/load, could you illuminate what the more correct scaling factor might be? (teasing apart any internal consumption vs. what is driven into the load). 

    Much thanks!

    ken

  • 0 in reply to ken krieger
    TI__Mastermind 29275 points

    Hello Ken,

    An inference can be made however it won't be precisely scaled to load impedance. You can use the various curves and specs to make assumptions. 

    1. To estimate ISOW7742 I/O current usage w.r.t. data rate see - Figure 7-15. ISOW7742 Channel Supply Currents vs Data Rate for CL = 15 pF
    2. To estimate ISO143x current with different loads see - Figure 7-7. ISO143x Supply Current Vs. Data Rate - 120Ω||50pF Load
    3. Keep in mind that these specs are typically specs and will vary with temperature. 

    Best,
    Andrew