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DRV8848: nFAULT pin is pulled low at startup and when running motor. It should be Open drain and high with my 10KOhm pull-up to 3.3V but it isn't

Martha Calderon
Prodigy 80 points
Part Number: DRV8848

Hi,

I need resolution on this as soon as possible. We are using the DRV8848 IC on one of our projects and it is showing a nFAULT when I powered the board without moving motor or anything else. I can move the motor (H-bridge) active but the Fault still persists. I have a pull-up externally on my board of 10KOhms to 3.3V and I should be seeing nFAULT to be high but it isn't. Is there a reason why this is being pulled low?

Thanks,

Marth

  • 0
    TI__Guru** 114905 points
    Hi Martha,

    Is a load connected?

    If so, please refer to Table 2 (Fault Handling) and rule out the UVLO, OCP and Thermal shutdown faults.

    Also, are multiple boards showing the same behavior? If you have a working board, please compare the signals around the device to determine the difference.
  • 0 in reply to Rick Duncan
    Prodigy 80 points

    Hi Rick,

    There is no load (motor) connected. I have ruled out any of the Faults per the Datasheet. I am wondering if there is a initialization issue. All the boards tested so far experience the same issue.

    I also tried connecting a motor and running it. I was able to run the motor (which tells me the bridge is active) but the nFault continues to be driven low by the drv8848.  What else am I missing?

    Thanks,

    Martha

  • 0 in reply to Martha Calderon
    TI__Guru** 114905 points

    Hi Martha,

    If the motor is operating, there is no fault. Do you have a pullup resistor on nFAULT? Without a pullup, the nFAULT pin is probably low. 

  • 0 in reply to Rick Duncan
    Prodigy 80 points

    Hi Rick,

    We do have a pull-up resistor. We are only using one motor on AOUT1 and AOUT2 pins. We are not using the B bridge. I left the pins for B inputs and outputs including BISEN unconnected. Could this be an issue when the IC initializes? I am attaching our schematic for you to review.

  • 0 in reply to Martha Calderon
    TI__Guru** 114905 points
    Hi Martha,

    Floating BISEN is not recommended. It could be a problem, but we have not tried this in the past. Please connect BISEN to GND and report if this makes a difference.

    Ideally you may benefit by connecting the two outputs in parallel. This will reduce the power. In addition you can consider reducing VREF and the sense resistor by 50% to reduce the power further.
  • 0 in reply to Rick Duncan
    Prodigy 80 points
    Hi Rick,

    Connecting BISEN to GND appears to have solved the issue. I have two follow up questions on this:
    1. Would connection BISEN to GND interfere with AISEN functionality for current control to 0.5A as I have it set up?
    2. What would be the best way to test the FAULT pin, I was hoping to add a load tester and pull 2A to ensure the FAULT pin is asserted as expected. Does this make sense?
    3. Can you elaborate a bit further on why connecting the two outputs in parallel would reduce the power? I am connecting the same load (motor) so I would expect the same current consumption.
  • 0 in reply to Martha Calderon
    TI__Guru** 114905 points
    Hi Martha,

    We are glad to hear connecting BISEN to GND corrected your issue.

    1. Would connection BISEN to GND interfere with AISEN functionality for current control to 0.5A as I have it set up?

    Connecting BISEN to GND does not interfer with AISEN functionality.

    2. What would be the best way to test the FAULT pin, I was hoping to add a load tester and pull 2A to ensure the FAULT pin is asserted as expected. Does this make sense?

    Assuming your are trying to test the overcurrent fault, you may need to pull more than 2A. Please note this is a minimum value; the actual trip point can be higher.

    If you are trying to set any fault, lowering VM slowly from 3V down to 2V while nSLEEP is active also works. This creates a VM undervoltage fault.


    3. Can you elaborate a bit further on why connecting the two outputs in parallel would reduce the power? I am connecting the same load (motor) so I would expect the same current consumption.

    Sure. The current consumption is the same but the internal resistance is halfed.
    If we use the formula I2R where I = 0.5A and R in one bridge is typically .9 Ohms (.55 high side + .35 low side Rdson), the power is 225mW.
    When operating the bridge in parallel, R becomes .45 Ohms and the power is 112mW.

    The same calculation applies to reducing VREF and the sense resistor by 50%.

    These may not be important in your application, but the BOUTs are available to reduce the power if you desire. Operating the bridge in parallel mode becomes more important above 1A.