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DRV8870: The board doesnt work with the new PCN

Riccardo Cortinovis
Prodigy 100 points
Part Number: DRV8870
Other Parts Discussed in Thread: DRV8251, DRV8251A,

Tool/software:

I have a lot of 1000 boards with new version and all are not working. All board works if I change the driver with the old one.

This is the schematic.

The image shows scope screen with driver outputs top and driver inputs bottom. The expected behavior is an output always low and the other output with PWM.

The image shows the signals. As the previous image, the signals are:

Channel 1 OUT2

Channel 2 OUT1

Channel 3 IN2

Channel 4 IN1

The switching sequence is between FWD/BRAKE

The threshold is higher than the current.

We have already produced more than 20000 boards without problems.

After some other tests, I think the problem is in OCP. I can say that because the driver retries to start after 3ms, equal to tRETRY.

I find peaks of current when the motor is rotating of about 20 A but they are 20-30ns long.

They should not trigger the OCP because tOCP is 1.5us and shorter impulse should be filtered.

The other problem is that it doesn’t restart after 3ms and there isn’t any overcurrent fault.

  • 0
    TI__Mastermind 19430 points

    Hi Riccardo,

    Please allow me some time to review. I've reached out to the design team for any and all differences.

    Best,

    David

  • 0 in reply to David Medis
    Prodigy 100 points

    I have some updates.

    I think I can use the sleep function to reset the driver when it doesn't work properly. It work well when the driver is in forward/breake but it doesn't so well in reverse.

     

    In the image C1 is In1, C2 is In2, C3 is Out2 and C4 is Out1.

    The driver try to restart every 3ms but it cannot.

    After putting both in to 0 for 10ms, the driver powers the motor.

    After that I added a 1nF ceramic capacitor on every IN to GND near the DRV and the DRV begin to work properly.

    I'm testing more this configuration to confirm that is a solution. 

  • 0 in reply to Riccardo Cortinovis
    Prodigy 100 points

    After more test, the configuration with capacitors in inputs is always fine.

    I have tested DRV8251 and DRV8251A and both work only with the capacitors.

    So the question is: why the old DRV8870 works and the new one and all the similar drivers work only with capacitor at the inputs?

  • 0 in reply to Riccardo Cortinovis
    TI__Mastermind 19430 points

    Hi Riccardo,

    In some bench testing I did yesterday, I verified that the pin impedance on the inputs is higher on the new devices. I will have to do some more testing to say with confidence why this solution works. 

    Does your design allow for caps on the inputs for this application? Does this solution work for your system?

    Best,

    David

  • 0 in reply to David Medis
    Prodigy 100 points

    Hi David,

    We are doing more tests to be confident that the capacitors are the solution in any condition. After that, I need to obtain the acknowledgement from the customer.

    More important for me now is to fully understand the issue to be sure that this problem cannot happen again in the future with another batch of drivers.

  • 0 in reply to Riccardo Cortinovis
    TI__Mastermind 19430 points

    Hi Riccardo, 

    Thanks for sharing your testing results. We will also try to do some testing on our end to replicate this issue. Please let us know the results of your testing as well.

    Best,

    David