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ISO7721: Level-translation

Bhavesh Tailor
Intellectual 290 points
Part Number: ISO7721
Other Parts Discussed in Thread: ISO7720

To whom it may concern,

I was looking to use the ISO7721DBQ, but would like to confirm if the part performs level-translation if I was to connect VCC1 to 3.3V and VCC2 to 5V?

It is not clear from the datasheet as reference to different voltages on VCC1 and VCC2 is only made to ISO7720 and not the ISO7721.  All references to ISO7721 have VCC1 and VCC2 at the same voltage.  Please confirm.

Look forward to your response.

Many Thanks,

Bhav

  • 0
    TI__Intellectual 2325 points
    Hi Bhavesh,

    Thank you for using E2E.

    All parts in the ISO77xx family are indeed able to support separate voltages on Vcc1, Vcc2 (provided each of these is within the Recommended operating range). This level translation between the two domains (Vcc1, Gnd1 <-> Vcc2, Gnd2) is handled internally.

    Vcc1 = 3.3V and Vcc2 = 5V is within this range and so will be okay for use.
    Hope this helps!

    Thanks and regards,
    Abhi
  • 0 in reply to Abhi Aarey
    TI__Intellectual 2325 points
    Hi Bhavesh,

    Please let us know if this resolved your question. I will mark it as such for now, but of course, you are always welcome to post any additional questions.

    Thank you,
    Abhi
  • 0 in reply to Abhi Aarey
    Intellectual 290 points

    Hi Abhi,

    Thanks for your prompt response, but I don't feel this is quite resolved yet.

    This indeed was my assumption too, however the following items in the datasheet made me think otherwise:

    a) Section 6.19: Supply currents given for VCC1@3v3 and VCC2@5V specifically for ISO7720, but only VCC1=VCC2= 3v3 or 5V given for ISO7721

    b) Section 9.2: Example using the ISO7721 is given using VCC1=VCC2= 3v3

    Please confirm if specifically the ISO7721 is designed to perform level-translation and it is acceptable to use VCC1=3v3 and VCC2=5V?

    Also, wrt point a) above, could you provide a similar graph (i.e. Supply current Vs Data Rate, with and without loading) for  ICC1@VCC=3v3 and ICC2@VCC=5V?

    Look forward your response.

    Many Thanks,

    Bhav

  • 0 in reply to Bhavesh Tailor
    TI__Intellectual 2325 points
    Hi Bhavesh,

    No problem at all and happy to clarify further.
    Yes, we can confirm specifically for the ISO7721 that it can handle 3.3V on Vcc1 and 5V on Vcc2.

    For all our digital isolators, the isolation barrier inside the chip completely separates the power domains on one side of the chip from the other. Therefore Vcc1, Gnd1 is a unique, independent domain from Vcc2, Gnd2 (and the respective I/Os on each side).

    Using section 6.19 , Figures 5, 6 as an example (7720). The graph is intended to show _six_ different unique curves and _not_ Icc1, Icc2 pairs as such (Icc1 and Icc2 are independent variables here). So we have 3 supply values and 2 independent variables to give us 6 curves. The way the graphs are formatted, it is very easy visually to treat those as Icc pairs (like Icc1 at 2.5 and Icc2 and 3.3V, for example), but that is not the intent - we are sorry for the confusion here.

    Figures 7 and 8 need additional explanation. The 7720 has two channels, both facing in the same direction. The 7721 also has 2 channels, but with a more symmetric configuration with one 'forward' and one 'backward' channel. Because of this symmetry, Icc1 and Icc2 are similar and therefore 6 curves from the 7720 example above can be combined into 3 curves. Again, the device can be configured to any of the allowed supply values with one side independent of the other.

    Therefore, figures 7 and 8 can still be used to assess Icc1, Icc2 independently and see how they vary with Data Rate, which I believe was what you wished to see? Please let me know if you need additional clarifications.

    Happy holidays!
    Regards,
    Abhi
  • 0 in reply to Abhi Aarey
    Intellectual 290 points

    Thank you for clearing this up Abhi.

    Many Thanks,

    Bhav