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MCT8316A: Fault occurring during temporary high current load request

Michael Marini
Prodigy 33 points
Part Number: MCT8316A

Hello, I'm hoping you can help determine what fault might be occurring in our application of the MCT8316AV. We have tested this in both inductor and resistor mode and received the same results. The motor starts up and runs just fine, but when it encounters an unexpected increase in load request, (similar to when a car encounters a speed bump, for example) it struggles to overcome this, the recorded speed increases greatly, a fault is triggered, the buck voltage begins to decline down to match the DVDD voltage. The buck voltage seems to automatically recover after about 2ms when the fault automatically clears. We were suspicious that the MTR_LOCK fault was the cause of the this, but our register values don't align with that theory. Here are our current register values for FAULT_CONFIG1, FAULT_CONFIG2, and GD_CONFIG1:

FAULT_CONFIG1:       0x78F43000
FAULT_CONFIG2:       0x17C5A009
GD_CONFIG1:             0x1C441600
I was also monitoring the I2C bus to see if we could read any faults. The only faults read were:
GATE_DRIVER_FAULT_STATUS:   0x90550000    Which is an "overcurrent condition is detected", but based on our register values, this is report only and would not trigger a fault.
CONTROLLER_FAULT_STATUS:   0x80040000    Which is a "lock current limit fault detected", but based on our register values, this is report only and would not trigger a fault.
Based on all of the information provided above, I am hoping to determine 2 things:
1. What could be the actual cause of the fault pin driving low and automatically recovering after 2ms based on our config register values?
2. What is causing the buck voltage to drop when the fault occurs and then recovering when the fault clears? Hopefully the answer to the first question will also answer the second.
 
Let me know if you need any other information to help solve the problem, Thanks!
  • 0
    TI__Mastermind 24360 points
    Michael Marini said:
    1. What could be the actual cause of the fault pin driving low and automatically recovering after 2ms based on our config register values?

    From the GATE_DRIVER_FAULT_STATUS register information you provided, there are 4 faults being reported, OCP of all the low side MOSFETs and an Overtemperature Shutdown Fault (bit 22). The overtemperature shutdown (TSD) fault is what is driving the fault pin low, as part of the TSD fault action the device will clear the fault once the junction temperature cools below T_TSD.

    Michael Marini said:
    2. What is causing the buck voltage to drop when the fault occurs and then recovering when the fault clears? Hopefully the answer to the first question will also answer the second.

    What is connected to the bucks output? 

    Could you provide the value of register GD_CONFIG2?

    Regards,

    Joshua

  • 0
    Prodigy 33 points

    Thank you for your reply, Joshua.

    - I neglected to mention that we are monitoring the I2C bus when the fault occurs in the attempt to gather the register values of the GATE_DRIVER_FAULT_STATUS and the CONTROLLER_FAULT_STATUS, BUT we did not receive these register values consistently EVERY single time the fault occurs. Sometimes the registers would return those fault status register values and sometimes that would return empty fault status registers. Do you think it could be possible that the TSD is causing the fault every single time? We have monitored the temperature with a thermal camera, and it did not seem to ever pass the 100c temperature mark, let alone going over the 150c die temperature limit for T_TSD.

    - If you believe that the TSD is the cause of the fault, do you have a hypothesis on why we are exceeding that temperature limit so frequently?

    - We are not using the buck's output currently in our application design. It only feeds back into FB_BK after the inductor. No external load on V_BK.

    - GD_CONFIG2:         0x03200000
  • 0 in reply to Michael Marini
    TI__Mastermind 24360 points

    Hi Michael,

    Please allow me some time to look into this. I will get back to you by next Thursday at the latest.

    Regards,

    Joshua

  • 0 in reply to Joshua Horlander Cruz
    TI__Mastermind 24360 points

    Hi Michael,

    Could you provide a capture of the current and voltage of VM and the bucks output?

    Could you try to incrementally increase the load on the motor and monitor the current to see if it gets to or exceeds 8A? If the current is exceeding 8A peak this could be what is causing the faults.

    Michael Marini said:
    If you believe that the TSD is the cause of the fault, do you have a hypothesis on why we are exceeding that temperature limit so frequently?

    Currently we believe that the increase in load is causing an increase in current demand which is also leading to a temperature increase.

    Regards,

    Joshua

  • 0
    Prodigy 33 points

    Hello Joshua, 

    I believe Andrew Lui CC'd you on an email thread that includes snapshots and explanations of the snapshots for the VM and VBK voltages during our tests, as well as the single-phase motor current. 

    In terms of the theory that the current exceeding 8A is causing the fault; what protection fault would this be triggering?

    Based on our GD_CONFIG1 register values, an overcurrent condition (OCP_MODE) is set to "report only but no action is taken".

    Same thing goes for our FAULT_CONFIG1 register values, (LOCK_ILIMIT_MODE) is set to "Ilimit lock detection is in report only but no action is taken".

    For those reasons, I don't understand how the overcurrent condition could be triggering the fault pin.