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TIOS101: NFAULT behavior

Atsushi Hirai
Expert 5130 points
Part Number: TIOS101


Hi team,

The NFAULT behavior seems different from datasheet description. Could you give me your advice?

There is 400mA-500mA output current when they drive a relay unit. ILIM_ADJ is open. NFAULT is pulled up to 3.3V.

They expect NFAULT is driven low after 200us but, it was driven low immidiately(around 20us) and return to High after 250us.

What shoud they check?

Regards,

  • 0
    TI__Guru 74322 points

    Hirai-san,

    An engineer has been notified of this thread and will respond by end of business 11/3/2022 CST.

    Regards,

    Eric Hackett

  • 0
    TI__Mastermind 34560 points

    Hi Atsushi,

    This was by design and referred to as fast-detect mode but it is not documented in the datasheet.  Because the Output Disable and Auto Recovery feature is disabled when the ILIM_ADJ pin is floating (open), there exists a possibility of damage to the device during a short circuit condition allowing large amounts of current to flow through the device.  

    When the ILIM_ADJ pin is floating (open) the current-fault is detected immediately and there is NO blanking time. The fast-detect mode is an immediate indication to the MCU that an over-current condition may exist and pull the EN pin low if this condition was not expected.  This is why the NFAULT pin is pulled low approximately 20uS after the driver is enabled and the current exceeds the limit threshold.

    In this mode the NFAULT pin will directly reflect the logical-OR status of all possible fault conditions (current, power, and temperature).

    Regards,

    Jonathan

  • 0 in reply to Jonathan Nerger
    TI__Expert 5130 points

    Jonathan-san,

    Thank you for the answer!

    Could you explain the reason why the NFAULT signal returned to High even though the output current is still seems higher than Io(LIM)?

    Does the threshould have a temperature drift? I think the output current is very close to the threshold. If the threshould has positive relation with the die temperature, I think the thredhoud may go higher and exceed the output current level by the high output current.

    Regards,

  • +1 in reply to Atsushi Hirai
    TI__Mastermind 34560 points

    Hi Atsushi-san,

    When the ILIM_ADJ pin is floating (Rset resistor is open), the output current limit will be somewhere between 300mA and 400mA for a given device.  It may differ from device to device due to process, voltage, and temperature conditions.

    From the scope plot, it appears the average output current initially rises to approximately 500mA, and then reduces down to slightly below 400mA.  For this particular device and test, the average output current must have dropped below the current limit threshold causing the NFAULT pin to return to a high state.  This threshold may be lower on other devices and could be closer to 300-350mA and in that case the NFAULT pin may not return high under the same current load.

    Regards,

    Jonathan

  • 0 in reply to Jonathan Nerger
    TI__Expert 5130 points

    Jonathan-san,

    Could you review below statemachine chart? Left side is from datasheet.

    Best Regards,

  • +1 in reply to Atsushi Hirai
    TI__Mastermind 34560 points

    Hello Atsushi-san,

    That looks correct to me.

    Best Regards,

    Jonathan

  • 0 in reply to Jonathan Nerger
    TI__Expert 5130 points

    Jonathan-san,

    Thanks.

    I received one more question.

    Customer tested the NFAULT behavior with OUT-GND short. The ILIM_ADJ is Open.

    The NFAULT signal goes low by current fault but, returned to High by thermal shutdown even though the die temperature is higher than thermal warning threshould.

    Is it expected behavior?

    Regards,

  • +1 in reply to Atsushi Hirai
    TI__Mastermind 34560 points

    Atsushi-san,

    The NFAULT is an open drain pin the datasheet says it should be driven low during a thermal warning event.  If the temperature does not exceed the shutdown threshold, the NFAULT will remain low.  

    However, if the device temp exceeds the shutdown temp, the device will completely shutdown except for the LDO causing the NFAULT to get pulled high through the external pullup resistor.  Once the device temperature drops low enough to re-enable the device, the driver will turn on and the process will repeat itself until the short is removed.

    If you adjust the scale of your scope during this test, you will likely see the cycle repeat itself every 30-50mS, but this time depends on how fast the die temp cools in the application.

    This is normal operation.

    Regards,

    Jonathan