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ISO7821: Output voltage level discussion

Part Number: ISO7821

Hi team,

Long time no see. I have one question about the output voltage level. When VCC1 short to GND, apply INx=GND, from the function table, the output shoule be high. Could you please share why the output voltage is high? Thanks.

BR,

CL

 

  • When the input side is powered down, no signal is transmitted over the isolation barrier. In that case, the output pin has no choice but to show the default value.

  • Hi Charles,

    Thanks for posting on E2E! As mentioned in the comments, under the condition where Vcc_input = INx = GND and Vcc_output is powered, output OUTx will match the default output state of the isolator (HIGH for "non-F" devices, LOW for "F"-suffix devices).

    The "H" in this table is misleading,  so please consider the comments as definitive. The function table is clearer on the ISO77xx datasheets, and it also applies for ISO78xx devices:




    Respectfully,
    Manuel Chavez

  • Hi Manuel,

    Thank you for your response here. Could you share me the internal circuit about how to keep default state? Is it something like pull up resistor in the output stage? Would appreciate it if you can explain more. Thanks.

    BR,

    CL

  • Hi Charles,

    No problem!

    The ISO7821's On-Off-Keying signal transmission structure inherently gives the devices its default state by design. Once the output side is powered up, each output will continuously output a signal corresponding to the device's default state (HIGH for "non-F" and LOW for "F" devices) unless told otherwise by a valid input signal. This is explained further in Section 8 of the ISO7821 datasheet.

    When no signal is transmitted across the isolation barrier because the input is disconnected, the entire input side is unpowered, or the input signal matches the default output state, the output does not rely on the input signal to output the default level.


    Please let me know if this is a sufficient explanation.


    Thank you,
    Manuel Chavez