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DRV8842: A possibility nFAULT becomes Low at VM1V ?

Shinichi Inoue
Guru 20681 points
Part Number: DRV8842
Other Parts Discussed in Thread: DRV8840,

Dear Specialists,

My customer is evaluating DRV8840 and DRV8842, they have a question about nFAULT.

I would be grateful if you could advise.

---

Some VM devices have nFAULT going low when a voltage 1V is supplied.

Could you please advise the condition of VM voltage that nFAULT becomes L.

According to the datasheet, the operating voltage of VM is MIN: 8.2V ~ MAX: 45V.
On the other hand, the VM undervoltage lockout voltage is TYP: 7.8V ~ MAX: 8.2V.

*I thought that the motor driver would not be able to pull nFAULT low (do not operate) if it was about 1V.

Is there a possibility  nFAULT becomes L at VM1V ?

System behavior

After turning on the power to the device, the power (3.3V system) that controls the signal of the CONTROL system of the motor driver is turned on immediately.

・nFAULT is connected to CPLD and pulled up to 3.3V system by CPLD (several tens to hundreds of kΩ).

・24V is supplied to VM at a certain timing after powering on the equipment, but until then a low voltage of about 1 to 1.1V is supplied to VM.

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Mastermind 37683 points

    Hi Shinichi,

    The driver is going into undervoltage lockout which pulls nFAULT low when the VM voltage falls below ~7V as specified in the datasheet.

    What you are observing is normal and expected operation.

    Regards,

    Pablo Armet

  • 0
    Genius 5040 points
    Shinichi Inoue said:

    Some VM devices have nFAULT going low when a voltage 1V is supplied.

    Could you please advise the condition of VM voltage that nFAULT becomes L.

    According to the datasheet, the operating voltage of VM is MIN: 8.2V ~ MAX: 45V.
    On the other hand, the VM undervoltage lockout voltage is TYP: 7.8V ~ MAX: 8.2V.

    *I thought that the motor driver would not be able to pull nFAULT low (do not operate) if it was about 1V.

    Is there a possibility  nFAULT becomes L at VM1V ?

    System behavior

    "Is there a possibility  nFAULT becomes L at VM1V ?" 

    How important is it to know the answer? Obviously since VM is below 7.8V and so it's not a big surprise that nFAULT is low. Regardless if the DRV8842 is capably to pull the nFAULT pin low with VM=1v, or not, the logic is correct, so what is the problem?

    In other words, since VM=1 which is way below 8.2V minimum operating voltage, no one should be surprised if nFAULT is either open drain or pulled low, as the operating voltage VM  is invalid outside the spec.

    Brian

  • 0 in reply to Brian Dang
    Guru 20681 points

    Hi Pablo and Brian,

    Thank you for your reply.

    I understand that nFAULT can be either high or low when VM is below the minimum voltage (8.2V).

    If the customer has an additional question, I consult you again.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi