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Digital isolator with fail-safe condition when VCC(Output) is powered down

Fabian Scheidl
Prodigy 80 points
Other Parts Discussed in Thread: SN74LVC1T45, ISO7221A, ISO7321C

I am looking for a digital isolator (2 channel) that outputs logic low when VCC(output) is powered down.

All digital isolators from TI I looked at have undetermined behavior in this case.

The only other possibility I see is putting a SN74LVC1T45 after the outputs and make use of its VCC-Isolation feature with both VCCs tied to VCC(output) of the digital isolator. According to the absolute maximum ratings of the SN74LVC1T45, it shouldn't be a problem to keep the Input for example at 5V while VCC is at GND. I don't necessarily like putting two "voltage translators"/isolators in series, but it would be a possibility.

Is there a dedicated IC for this? Is there a more elegant solution?

  • 0
    TI__Mastermind 49535 points
    Hi Fabian,

    VCCO (VCC on the output side) is powered down, the output circuitry is basically OFF hence there is no powered circuitry to drive output to any particular state and hence the undetermined state. Knowing that there is no power on VCCO, the potential at OUTx will be close to 0V and it is very unlikely for this pin to go HIGH without power being applied. Hence you should be able to use the device without any additional device / circuitry. Let me know if you still any question on this.

    Regards,
    Koteshwar Rao
  • 0 in reply to Koteshwar Rao
    Prodigy 80 points
    So I should be able to use ISO7221A in this way, one side powered off, without having to worry about getting a high signal on the output at that (powered off) side?
  • 0 in reply to Fabian Scheidl
    TI__Mastermind 49535 points
    Fabian,

    That is correct. ISO7221A is a device from one of older families, I would recommend you to use ISO7321C which has many improvements in performance over ISO7221A.

    Regards,
    Koteshwar Rao