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DRV8884: Motor Current not stable

Part Number: DRV8884

DRV8884 is used for driving a stepper motor. The design parameters are,

1. RREF = 562K

2. TRQ = 1

3. DECAY = tied to DVDD (slow decay)

4. We use both 1/4 and 1/2 stepping modes in our design and two PWM frequency - 400Hz and 800Hz.

Theoretically, by using a 562k RREF resistor should result in 53mA max current when the motor is running (37.7mA RMS).

When measured the max. current appears to be close to this - 60mA  (there may be some error in the 562k resistor, the DRV8884 and the probe.)

But the measure data of motor current for the same motor but different hardware board, shows larger than 10% variation. Could you please suggest how to let the output current more stable?

9 Replies

  • The current measured in 30 boards are attached.

    Motor drive current.xlsx

  • In reply to uhtis:

    Hi Uhtis,

    Can you provide a scope capture of the current through the winding for the min and max boards?

    Have you tried setting the decay mode to mixed 30%/mixed 30%?

    Rick Duncan
    Motor Applications Team

  • In reply to Rick Duncan:

    Hi Rick,

    The testing is done at customer site and I have asked to share the waveforms. 

    Regarding the decay mode, the setting is slow decay. The DECAY pin is tied to 3.3V in design and we have not tried any other options. 

    Meanwhile, what all factors affect the output current stability?

    Regards,

    Uthis

  • In reply to uhtis:

    Hi Rick,

    Please see the waveforms attached.

  • In reply to uhtis:

    Why the driving current of DRV8884 is not stable?

  • In reply to uhtis:

    Hi uhtis,

    Can you confirm the TRQ pin is set to 1? If TRQ=1, the current should be set to 26.5mA.

    When operating at 53mA, this is similar to operating at the 5% current at 1A. Please note the current trip accuracy (IRREF = 1.0A, 10% to 20% current setting). At such a low level of current is +/- 25%.

    There are a couple of options to try, assuming the TRQ pin is set to 0 instead of 1.

    1) Change the RREF resistor to 281k and the TORQUE setting to 1. This will reduce the blanking time to 1us and may help.
    2) Consider a different device like the DRV8880 with external sense resistors to boost the voltage to the comparator. The DRV8880 with a 2.37V VREF, 6.8 Ohm sense resistors, and a TRQ setting of 11 will increase the voltage to the comparator.

    Rick Duncan
    Motor Applications Team

  • In reply to Rick Duncan:

    Hi uhtis,

    I forgot to mention the actual RMS current will typically be less than the calculated RMS current.

    This is because the current regulation begins when the current reaches the trip point. The current begins to decay, so the average current at each step is less than the current desired.

    To compensate for this, the full scale current can be increased.

    Rick Duncan
    Motor Applications Team

  • In reply to Rick Duncan:

    Hi,

    But even if we increase the full scale current, it may vary from board to board. Do you have any suggestion to make the drive current stable in all boards?

    Regards,

    uhtis

  • In reply to Rick Duncan:

    Confirmed that TRQ = 0. Sorry for the misunderstanding.

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