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ISO7421: level shift design

Richie Zhao
Prodigy 20 points
Part Number: ISO7421
Other Parts Discussed in Thread: ISO7721

I want to design a level shift circuit with ISO7421, VCC1=5V, GND1=0V; VCC2=-15V, GND2=-20V.

could you help check if this device can do this application?

  • 0
    Guru 244290 points

    The typical application of an isolator is to get galvanic isolation, where you do not even know the voltage difference between the two sides.

    A circuit with a constant difference of 20 V is just a special case of that, and will work fine.

  • 0
    TI__Mastermind 49535 points

    Hi Richie,

    Yes, the voltage levels that you described are fine and the device work just fine. Even though VCC2 is -15V and GND2 is -20V with respect to GND1, the potential at VCC2 with respect to GND2 is still 5V which is within datasheet recommended limits. While the potential difference between GND1 and GND2 is 20V which is also within datasheet limits and the device can support upto a potential difference of 566V.

    Please do make sure even the input signals connected to VCC2 side input pins are also within datasheet limits with respect to their GND reference, which is GND2. This means that the input signal voltages on VCC2 side should be between -20V and -15V with respect to GND1.

    ISO7421 is one of our older device families, I would recommend customer to use ISO7721 instead as the ISO77xx family is our latest digital isolator family offering the most optimum performance and cost benefits. Thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Prodigy 20 points

    Thanks a lot for the reply.

    I will apply some samples for a try.