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ISO7840:OUT signal is unstable

Teruyuki Wada
Prodigy 80 points
Part Number: ISO7840
Other Parts Discussed in Thread: UCC27714, ISO7740, ISO6740,

Tool/software:

The OUT signal of the ISO7840F occasionally becomes unstable.
When VCC1 = 3.3V, VCC2 = 5V, EN2 = 5V, and INA = 0V (Low), OUTA is output at a fixed 2.5V.
When this happens, the ISO7840F becomes hot.
Please advise what the cause and solution for this phenomenon are.

  • 0
    TI__Guru 55525 points

    Hi Wada-san,

    This can happen if you have connected two outputs like OUTA and OUTB together where one output is forced to be HIGH while the other to LOW. In such a situation, there is VCC2 to GND2 short through these two output pins and therefore, the output voltage settles to the mid-point of VCC2 which is 5V. Due to VCC2 and GND2 short created through two outputs, a large current will be flowing through these pins leading to excessive heating of device.

    Please confirm if you have connected two outputs together or share the schematic for us to verify, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 80 points

    Thank you for your answer.
    OUTA and OUTB are not connected on the circuit.
    OUTA and OUTB are connected to UCC27714DR.
    I have attached a connection diagram. The circuit is a Phase-Shifted Full Bridge DC/DC.
    QA, QB, QC, and QD form a full bridge.

    The symptom where ISO7840F's OUTA is fixed at 2.5V occurs after overcurrent is detected in Phase-Shifted Full Bridge DC/DC.

    ISO7840F.pdf

  • 0 in reply to Teruyuki Wada
    TI__Guru 55525 points

    Hi Wada-san,

    Thank you for the clarification and for sharing the schematic.

    Yes, I can see that the outputs are not connected together. Do you know if UCC27714 gets damaged when overcurrent occurs?
    When this happens, does OUTA stay 2.5V even when INA = HIGH (3.3V)?
    Does the device come back to normal when device is power cycled?

    One of the ways that could create about 2.5V on OUTA is when OUTA is HIGH while the internal pin of UCC27714 is pulling HI (pin1) LOW. When OUTA is both pulled HIGH and LOW at the same time, then the outputs end-up at a mid-point.

    Let me know your inputs, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 80 points

    Thank you for your answer.

    Do you know if UCC27714 gets damaged when overcurrent occurs?
    →UCC27714 is not damaged even if overcurrent occurs.
    　Is it possible that UCC27714 malfunctions and outputs 2.5V from HI (OUTA) or LI (OUTB)?
    　(I apologize for the content of another forum, but I would like to receive an answer if possible.)

    When this happens, does OUTA stay 2.5V even when INA = HIGH (3.3V)?
    →When INA = HIGH (3.3V), OUTA = HIGH (5.0V).
    　Also, if a HIGH (3.3V) ⇔ LOW (0V) PWM is applied to INA, a HIGH (5.0V) ⇔ unstable (2.5V) signal is output to OUTA.

    Does the device come back to normal when device is power cycled?
    →When the device is restarted, it returns to normal operation.

    One of the ways that could create about 2.5V on OUTA is when OUTA is HIGH while the internal pin of UCC27714 is pulling HI (pin1) LOW.
    When OUTA is both pulled HIGH and LOW at the same time, then the outputs end-up at a mid-point.
    →Is it possible that the UCC27714 malfunctions and pulls down HI (pin1) to LOW?
    　(I apologize for the content of another forum, but I would like to receive an answer if possible.)

  • 0 in reply to Teruyuki Wada
    TI__Guru 55525 points

    Hi Wada-san,

    Thank you for your inputs.

    I can't comment on UCC27714, although, it is unlikely for a device affect its input pin.
    Just for the debug purposes, can you please consider testing ISO6740 or ISO7740 in place of ISO7840 and confirm if you still see the issues?

    Let me know, thanks.


    Regards,
    Koteshwar Rao