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ISO6721-Q1: What's the device output OUTB state when VCC = 0V

Cheng Zhang
Prodigy 120 points
Part Number: ISO6721-Q1
Other Parts Discussed in Thread: ISO6721

Tool/software:

HI Team,

I have an application using ISO6721 as isolated UART, the OUTB is connect to an UART slave's RX pin. And when VCC= 0V on both side of the ISO6721, my device RX pin are internally pull up to 5V through a 230Kohm internal resistor. And I found that the RX pin is draining about 20+uA of the internal 5V. OUTB is about 1.1V. When I disconnect the OUTB from the device RX, the current is gone and OUTB return to 0V.

What's the state of the OUTB pin when no supply applied on the ISO6721, and why the OUTB is pulled up to 1.1V?

Thanks

  • 0
    TI__Mastermind 28745 points

    Hello Cheng, 

    The OUTx of a digital isolator will be undetermined (theoretically high impedance) when the local VCCx is unpowered. The OUTx pin ideally should not be pulled up to a voltage while unpowered (see abs. max. conditions for OUTx). The 1.1V is likely due to a leakage path of the ISO6721 ESD protection that is biased dues to the Rx pin's 20uA leakage. 

    Please include a schematic and connection diagram if you have further questions. 

    Best,
    Andrew