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ISO7731-Q1: Using Mixed Vcc with Digital Isolators

David Schabel
Prodigy 40 points
Part Number: ISO7731-Q1

Hi, 

I'd like to know how to calculate the maximum propagation delay for a digital isolator when using different supply voltages.  

I'm presuming that the device is intended to be able to run with different voltages on the two sides.  I've used it this way, and I've seen it used this way.  The datasheets, however, don't actually give any ratings or examples of this, other than saying "2.25-V to 5.5-V level translation" in the features column.  

The datasheets all supply the prop delay times when VCC1 = VCC2 = 5 V, and when VCC1 = VCC2 = 3.3V, and when VCC1 = VCC2 = 2.5V.  

How about when VCC1 = 5 V and VCC2 = 2.5V, for instance?

Thanks,

Dave

  • 0
    TI__Genius 17100 points

    Hi Dave,

    Thank you for posting, and welcome to E2E! Digital isolators are designed to operate with independent voltage supplies, so connecting VCC1 and VCC2 to different voltage levels (like 5V and 2.5V respectively), is within recommended use as mentioned in the FAQ below:



    Unfortunately, our ISO77xx datasheets do not have a switching characteristics table for when different supply voltages are given, like VCC1 = 5V and VCC2 = 2.5V, but we do expect these specs to be in between the switching characteristics data available from Sections 7.15 and 7.17 in the ISO7731-Q1 datasheet. Please use these values as approximation, and let us know if you have any additional questions.


    Respectfully,
    Manuel Chavez