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ISOW7821: Potential Failure modes?

Dan Ringger
Intellectual 1800 points

Part Number: ISOW7821

I am looking for a device to isolate a microcontroller signal for the external use by customer.  ISOW7821FDWE seems to be fitting the application.  Two pins would be provided to customer: GND2 and OUTA (no access to VISO).

Could you help me to answer couple of questions related to this device?

    1. What are the potential failure modes of this device if customer mis-wires the signal:

Shorts OUTA to GND2

Wires OUTA and GND2 in reverse

    1. Is there any advantages of using the Enable pin if I am using only one channel (INA-OUTA)?

  • 0
    TI__Guru 57475 points

    Hi Dan,

    Thanks for listing out customer questions clearly.
    Please find below my inputs to the questions listed,

    a. Potential failure modes.

    i. Since the typical source impedance of OUTA is ~50 Ohm, shorting OUTA to GND2 will lead to an increased current flow from OUTA to GND2 violating datasheet spec. This can heat-up the device and may lead to device damage.

    ii. I am assuming the wrong connection of wires to be as OUTA of ISOW connected to GND of MCU and GND2 of ISOW to IN of MCU. Such a connection will be equivalent to applying a negative voltage at IN/GND pins of MCU. This might lead to MCU damage which in-turn can cause excess current to be flown out of OUTA of ISOW causing it also to be damaged.

    b. Please note that ISOW7821 has enable pin only on Side1 as EN1. EN pin only controls the output pins that are available on its side. In case of ISOW7821, EN1 can only enable/disable OUTB. Thus the device doesn't have any control for OUTA which you intend to use. If customer needs to use EN pin for OUTA then they can choose a digital isolator like ISO773x or ISO774x that supports EN pins on both sides of the isolation barrier.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Intellectual 1800 points

    It looks like some external resistance on the OUTA output may help with the first failure mode (shorting OUTA and GND2). Still an issue with the reverse connection of the pins (negative voltage on the MCU input).  Do you have any suggestions? 

    What is a source impedance of OUTA when INA is in low state?  Thank you

  • 0 in reply to Dan Ringger
    TI__Guru 57475 points
    Hi Dan,

    You are right, using a series external resistance on OUTA to limit the current to datasheet limits will avoid device damage.
    For the reverse connection, the external resistance used might prevent the damage to ISO device but I am not sure if the MCU will be protected. Maybe using an external diode followed by a pull-down resistor at IN pin of MCU might prevent the MCU from seeing negative voltage.

    The typical source impedance of OUTA is also going to be about ~50 Ohm when INA is in LOW state as well. Thanks.


    Regards,
    Koteshwar Rao