• State Answer Suggested
  • Locked Locked
  • Replies 8 replies
  • Answers 1 answer
  • Subscribers 29 subscribers
  • Views 1966 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

DRV8842 goes too fast in nFAULT conditions after start the Motors movements

Michele Bonollo
Prodigy 160 points
Other Parts Discussed in Thread: DRV8842, DRV8842EVM

Hy, i have a big problem with a Motors (is a Motorgear) control that use DRV8842 (attach the drawing) pilot normally 24 VDC Motor:

 24 VDC Motor 4 Ampere Coil  3,2mH at 1Khz  5,8 Ohm Motors DC resistance, the VM Supply is about 36 Volt.

It seems to be all right everything. But somewtimes after start Motor little seconds (0,4 / 0,5 sec ....) the Driver STOP motor with nFAULT protections active.

I think is for the OCP protection! overcurrent. Because i must active nRESET OFF/ON for restart the Motor. 

I have 2.200 uF capacitor in the VM Power Supply, and 2,2 uF plus 220nF near the 8842 IC.

I don't think to have exessive peak motor current, and with Vref setting at 3,3 A max is possible to reach the protection?

Pwm setting in 5Khz, Slow or Fast Decay seems indifferent ??, How can i do for not have the STOP Motor ??

Thank you

  • 0
    TI__Guru** 114905 points

    Hi Michele,

    This does appear to be an overcurrent event. Overcurrent can occur during the blanking time of 3.75us when the comparator is not limiting the current.

    To confirm this can you provide either:
    a) a scope capture of the current at the time of the fault or
    b) a scope capture of the voltage across the sense resistor with respect to the DRV8842 GND at the time of the fault
    If possible, please add the output voltages and the VM voltage.

    The critical time window is 20us before the fault until the fault activates.

  • 0 in reply to Rick Duncan
    Prodigy 160 points

    Ok i provide this information,  attach the draw, and some scope for now in the Shunt ISEN  ( 0,2 Ohm )  is very strange that in one direction the motor Stop frequently,  i don't know the real peak value, seem few microsecond .... 

    What is the best methods, for control the motor robustness to avoid false alarms in nFAULT with Hardware/Firmware:

    Capacitor, resistance value, shunt, filter,  Decay Fast or Slow, VREF, Sleep pin, PWM Frequency    ???     Thank you

  • 0 in reply to Michele Bonollo
    TI__Guru** 114905 points
    Hi Michele,

    Thank you for the scope captures. Can you provide a few more details?
    What signal is each image? Is there one image that shows the current and the nFAULT?

    To avoid false OCPs, please check for capacitance on the outputs. Placing a cap across the motor can cause false OCP.
    Is the motor stalling after 0,4 seconds? If not the current is normally is decreasing.
  • 0 in reply to Rick Duncan
    Prodigy 160 points
    Good morning, yes sure:
    All the image refer to the Pin 6/9 Rshunt of the DRV8842, in the normal cicle used. The Fault condition is not predictable but sporadic.
    I have do some test and have good news, It seems that some my motors have a light loss versus ground system. I have see that, with a ground connections in the PIN 5/10 or 7/8 (depend of the motor versus) >> sends in nFAULT the DRV8842 after the star cycle!!!, just as happens to me!!
    Now the my system work very better. I have only very rarely nFAULT conditions that appear and disappear very few times.

    Ok, i have 1nF very low capacitors for EMI, across the motors,l i try to put and remove this capacitor without variations, this isn't a problem i think!!.
    The my PWM frequency is 5 Khz, enought low... Can you, please, suggest me, the best robustness configurations for this device in terms of frequency, hardware, firmware, setting, ecc....
    Thank you
  • 0 in reply to Michele Bonollo
    TI__Guru** 114905 points
    Hi Michele,

    Is it possible to lower the VM voltage? As an experiment lowering the voltage to 24 V should reduce or eliminate overcurrent events.
  • 0 in reply to Rick Duncan
    Prodigy 160 points

    Hy, unfortunately NO, because it is derivated from the classic  24 Vac line rectifier and stabilized with big capacitors,  24 * 1,41  About  34 VDC with tolerance.

    But, now is not bad.  The my actual test is with worst case tolerance ecc...  about  38 VDC on VM and the maximun weight on the my mechanical load.

    For Now the movements dont't stop never. Only sometimes there is a little trigger on nFAULT that don't stop the my cycle use of mechanism.

    This is a little strange, because seem that nFAULT goes down for few milliseconds and return up, but not affect the movements,. The my microprocessor see this

    situation and report on a test Led, is normal ??

    Unfortunately isn't possible adjust with resistor the overcurrent time trigger like other driver (over 6A for tot microseconds).  

    I don't know what is the best methods, to avoid spurius false trigger. For example continuos go in nSLEEP mode and return before movements is Bad??

    change the decay mode during movement slow <> fast ecc... is Bad???  change Vref during movements??  Others ??

    Thank you

  • 0 in reply to Michele Bonollo
    TI__Guru** 114905 points
    Hi Michele,

    The best method to avoid the nFAULT is to understand the cause.

    From your description, it could be overtemperature conditions. How long does nFAULT assert?

    Do you have a good thermal connection from the DRV8842 thermal pad to the board? How large is the ground plane?

    Please refer to the DRV8842EVM as an example of the connection to the thermal pad.
  • 0 in reply to Rick Duncan
    Prodigy 160 points
    Hi, Ok. Yes i have a large and good Thermal pad on the PCB. The cycle is very quite (like time) and i continuos monitoring the temperature of the driver, it is very low < 50°C. The short nFAULT OFF/ON trigger very sometimes duryng the motor cycle but not affect the movements. While if i ground in short circuit a motor output, the cycle STOP as long as i do nRESET OFF/ON ....