• State
  • Locked Locked
  • Replies 11 replies
  • Subscribers 30 subscribers
  • Views 1763 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DRV8801 OCP fault because of noise?

Igor Cerdic
Intellectual 360 points
Other Parts Discussed in Thread: DRV8801, DRV8871, DRV8870, DRV8872

Hello Forum, I need your help!


I made a circuit that moves a 24V 3A DC Motor with a DRV8801 motor Driver. I have the sense output connected directly to ground. A microcontroller ramps up a 31KHz PWM on the Enable Pin (To limit the startup current of the motor). If it detects an Fault signal, the PWM is started at 0 and ramped up again. The IC is making overcurrent faults when the current is <2A. I attached an 1 Ohm resistor before the motor and measured the current with an oscilloscope. The cursors are located at 3V, showing the 3A theoretical minimum peak current limit of the DRV8801, where I am expecting the OCP to trigger.

After the fault is detected, the motor stalls and the current is increased again because of the pwm ramp up. The voltage on the resistor looks much more noisy when I am driving the motor with the H-Bridge than when I drive it with a DC source. This is the graph of the motor being drived by the H Bridge:

I also captured a graph showing the motor current at the same resistor, powering it with an external supply and no driver. The spike is when the motor starts lifing a weight. The voltage over the resistor has also much less noise than with the driver.


Does anybody has an Idea if the fault is produced by noise in the current? The datasheet says something about putting inductors in series to the motor, can this be a solution for me? How do I have to size them?

Thank you very much for your help, and have a very nice weekend!

  • 0
    Intellectual 360 points

    Sorry, the images were not uploaded. Here is the graph showing a fault using the driver:

    And here is the graph of the current feeding the motor with a DC source, please notice the much less noisier curve:

  • 0 in reply to Igor Cerdic
    TI__Guru** 114905 points

    Hi Igor,

    OCP is a fast event (>IOCP for TDEG, then repeats after TOCP). Can you zoom in to the fault and determine if an OCP sequence is occurring?

    It you do not see this sequence, you may be experiencing thermal shutdown. 

  • 0 in reply to Rick Duncan
    Intellectual 360 points

    Hello Rick, thank you for your reply! 

    Here is a graph of the Fault. As far as I can see, the current does not reach 3A. I don't think it is a thermal shutdown, because the driver instantly continues its operation (much lower start current due to PWM ramp up).. Do you have any other ideas what the problem can be? I switched to fast decay mode, but it makes no difference. Thank you and have a nice weekend!

    I 

  • 0 in reply to Igor Cerdic
    Intellectual 360 points

    Hello Forum,

    I noticed one more thing: Before the Fault, the current goes up and down, partially exceeding the 3A limit:

    The Frequency of the rising Current (~51KHz):

    Frequency of falling current (~83KHz):

  • 0 in reply to Igor Cerdic
    TI__Guru** 114905 points
    Hi Igor,

    Is it possible to continue to drive the motor after the fault? If you can do this, the duration of the fault will help identify the type of fault.

    If the fault is approximately 1.2ms, it is most likely an OCP. If it is longer, it could be undervoltage, overtemp, charge pump fault, or short. The most common of the remaining faults are undervoltage and overtemp. With VBB = 24V, it is unlikely. The device can cycle in and out of overtemp shutdown.
  • 0 in reply to Rick Duncan
    Intellectual 360 points
    Hello Rick,

    yes, I can drive the motor after the fault. The Fault duration is 1.2ms, so it is a overcurrent fault without doubt.

    When I drive the motor with a DC source, I dont see current going over 3A, but when I drive it with the H bridge, the current is noisier and some peaks exceed 3A (as can be seen in the graphs I sent). Happens also with the PWM duty cycle is set to 100% and the motor is moving.

    Do you have any ideas how the current spikes are produced by the bridge?

    Thank you very much, have a nice day,

    Igor
  • 0 in reply to Igor Cerdic
    TI__Guru** 114905 points

    Hi Igor,

    Thanks for the confirmation. Since this is an OCP event, it appears the DRV8801 is marginal in this application.

    The OCP may show up more frequently across devices and motors.

    There are a few options:

    1) You can place two DRV8801s in parallel to increase the current. Please see the application note ( LINK )

    2) You can evaluate other devices such as the DRV8870, DRV8871, or DRV8872. The DRV8870/1/2 have higher OCP values, lower RDSon, a smaller package. They also have the ability to limit the current to a specific value. This may eliminate the requirement to increase the PWM on startup. The devices performs the current limiting function on startup.

  • 0 in reply to Rick Duncan
    Intellectual 360 points

    Hello Rick,

    Thank you very much for your reply. I connected the DC source again, and I noticed the only moment when the motor consumes >3A is the startup. I zoomed into the graph and i don't see spikes in the current, not even in us range. In my opinion, the IC with Imax=3A should move the motor if I help it with startup, or not? Do you know why the bridge is faulting and I have spikes of current when I use the bridge, and not with the DC source?

    This is the whole travel of the motor, measured over 1ohm in series to the motor:

    Thank you,

    Igor

  • 0 in reply to Igor Cerdic
    TI__Guru** 114905 points

    Hi Igor,

    The DC source does not have protection logic like the DRV8801, so it should work unless the current exceeds the dc source capability for an extended period of time.

    For the DRV8801, an OCP event is occurring based on your description of the1.2ms retry time. You should be able to see the OCP event if you zoom in on the current(10us/div or less).  Use nFAULT falling as the trigger, making sure you have at least 10us prior to nFAULT falling visible.

  • 0 in reply to Rick Duncan
    Intellectual 360 points

    Hi Rick, thank you again very much for your reply!!

    What wonders me, is that when I drive the motor with the DC source, the current never goes over 3A. If I use the Bridge (Feeded by the same source), I have spikes of >3A. This spikes make the fault happen. What I don't understand, is why they are spikes when I drive the motor with the bridge, and when I drive it with the same DC source without bridge, no current >3A can be seen... Do you have any idea how this spikes can happen?

    Thank you,

    Igor

  • 0 in reply to Igor Cerdic
    TI__Guru** 114905 points
    Hi Igor,

    I can only speculate, but one possible difference is how the motor is driven with the DC source vs the DRV8801.

    If the DC source ramps from 0 to 24V slowly, this may prevent large current spikes that could be seen in the DRV8801. The DRV8801 output ramp from off to 24V can be much faster than the DC source. This can lead to higher current spikes.

    What is the resistance and inductance of the motor?