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ISOW7840: Isolation betwen output lines

Emanuele Giacomozzi
Prodigy 10 points
Part Number: ISOW7840
Other Parts Discussed in Thread: ISO7840, ISO7740, ISO6740, ISO7741

Hi,

I would like to use the isow7840 digital isolator to drive with an FPGA, the synchronization lines of four hydrophones submerged in water. The hydrophones are powered by an external DC/DC converter so I don't use the isolated dc/dc power from the chip. If water enters in one hydrophone, its line could short out to ground but our system can switch off each single hidrophone. The iso7840 guarantees that the short circuit does not propagate towards the FPGA but what happens to the other working output lines? Could they short out to ground too? Thank you. Regards, Emanuele Giacomozzi

  • +1
    TI__Mastermind 49485 points

    Hi Emanuele,

    Thanks for reaching out to us and for your interest in isolators.

    ISOW7840 is a digital isolator with an integrated DC-DC converter but if you are looking only for a digital isolator as you have the isolated DC-DC already available then you can also look at ISO7740 and ISO6740.

    When the output lines are shorted to GND, then there is going to be a high current flowing from the output pin to GND (>100mA). This can cause damage to the output circuit and the damage may also extend to the rest of the circuit on the same output die. If the total current into device is limited to safety limiting values (<200mA at an ambient temperature of 25C), then the device damage is expected to be limited only to the output die. But if these limits are not maintained, then the excess heat of device could cause the junction temperature of the input die to exceed its maximum value of 150C. If this happens, then the input die can get damaged due to thermal spec violation.

    To summarize, please restrict the maximum current into device to less than the current stated in the section titled "Safety Limiting Values" (7.8 in ISO7741 datasheet). When the current is limited to such levels, the damage will be restricted only to the output circuit and input circuit remains intact. Please refer to below article for some additional information in this regard, thanks.

    https://e2e.ti.com/blogs_/b/analogwire/posts/understand-and-apply-safety-limiting-values-for-digital-isolators


    Regards,
    Koteshwar Rao