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TIOL111: NFAULT behavior during wake-up pulse

Oba
Genius 17345 points
Part Number: TIOL111

Hello

 
My customer has an question for NFAULT behavior during wake-up pulse in TIOL111.
TIOL111 on their board detects wake-up pulse correctly, but NFAULT signal also goes to low at the same timing as the wake-up pulse like the below figure.

They can see this behavior only if ILIM_ADJ is open. It never happen if ILIM_ADJ is connected to L-.
What do you think is the root cause of this NFALUT strange behavior?

 

Regards,

Oba

  • 0
    TI__Mastermind 33430 points

    Hello Oba-san,

    It is hard to tell what exactly is happening just by the hand-drawn diagrams.  Can you please provide actual scope waveforms that show the timing and voltage levels of the CQ, WAKE and NFAULT signals? 

    The Wake Up pulse will be by definition greater than the current limit.  When the device detects an over current condition and determines if the condition meets the criteria for a possible Wake Up pulse, it will start the counter to determine if it is a Wake Up pulse or a Fault condition.  From your description the device appears to be passing the Wake Up pulse correctly on the WAKE pin.

    When the ILIM_ADJ pin is left floating the Output Disable and Auto Recovery feature is disabled and the device will not turn off the output if the over current condition exceeds the Current Fault Blanking Time.

    When the ILIM_ADJ pin is not left floating, the over current condition is masked to make sure a Wake Up pulse does not get treated as a current fault that is reported on the NFAULT pin.  This appears to be working as expected from your description as well.

    What I think is happening, but will need to verify before giving you a definitive answer, is whether the Over Current condition of the wake up pulse is still masked when the ILIM_ADJ pin is left floating and the Output Disable and Auto Recovery feature is disabled, or if all Over Current conditions including the WAKE pulse is reflected as an over current fault. 

    I would like to verify the length of your wake pulse and see the voltage levels to help verify the observed behavior reported by the customer.  I will also verify the behavior of the device and try to replicate the test results for myself.

    Regards,

    Jonathan

  • 0 in reply to Jonathan Nerger
    TI__Genius 17345 points

    Hello Jonathan,

    I'll send the actual waveform in separated E-mail.

    Regards,
    Oba 

  • 0 in reply to Oba
    TI__Mastermind 33430 points

    Hello Oba-san,

    Thank you for sending me the actual waveforms for review.

    I can confirm that the device is working as designed and that when the ILIM_ADJ is left floating, the Current Fault Blanking Time (Tsc) is not applicable and any Over Current condition will be immediately reported on the NFAULT pin.  The Wake Up pulse is by definition an over-current condition created as a signal to the device that it needs to disable the driver and place the CQ pin into a high impedance state.

    Most applications will want to treat a wake up pulse differently than an over current condition due to some short or some other fault in the system and it will use the Current Fault Blanking Time (Tsc) as a way to filter out the Wake Up pulses and delay reporting the over current condition.  When the ILIM_ADJ pin is connected to GND or through any resistor value to GND, the wake up pulses will be filtered out and not reported as an over current fault on the NFAULT pin.

    However, some applications such as Safety applications want immediate reporting of an over current condition and eliminate any delay in reporting it to the MCU on the NFAULT pin.  Therefore the Current Fault Blanking Time was disabled when the ILIM_ADJ is left floating for systems that require this type of operation.  However, as observed, the wake up pulses will be reported as a fault on the NFAULT pin and the MCU firmware will need to handle this appropriately for their application.

    I apologize that this feature was not well documented in the datasheet.  I will be working to address it in a future datasheet update.

    I hope this answers the question.  If you have any additional questions, please let me know.

    Regards,

    Jonathan

  • 0 in reply to Jonathan Nerger
    TI__Genius 17345 points

    Hello Jonathan,

    Thanks for your answer.
    I understand this behavior itself well. I have an additional question.

    So if MCU software don’t care this NFAULT signal during wake-up pulse, there is no other problems for IO link operation. Correct?

    Regards,
    Oba

     

  • 0 in reply to Oba
    TI__Mastermind 33430 points

    Hello Oba-san,

    There is no problem with IO Link operation when the ILIM_ADJ pin left floating.  The communication protocol will function the same as when the ILIM_ADJ pin is connected to GND or through a resistor to GND.

    The only factor that needs to be understood is that when the ILIM_ADJ pin is left floating, the device will not disable the output in case of an short circuit or over current condition.  If there is an over current condition occurs that is not associated with a wake up pulse that comes from a short circuit on the wires or other fault, then the device will source a large amount of current and heat up quickly.  The only protection for the device at this point is the thermal shutdown feature of the device unless the MCU will also be monitoring the NFAULT pin and disable the driver through the EN pin when an over current condition not associated with a wake up pulse occurs.

    I would not recommend using the device with the ILIM_ADJ pin floating unless there is a very specific reason to do so because the Output Disable and Auto Recovery feature is an important safety feature of the device.  But as long as this is understood, there is no reason why the device cannot be used with the ILIM_ADJ pin floating.

    Regards,

    Jonathan