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DRV8885: 2-phase excitation of full step

Yasushi Fujiwara
Genius 11685 points
Part Number: DRV8885
Other Parts Discussed in Thread: DRV8880, DRV8886AT, DRV8886, DRV8711, DRV8840, DRV8842

Hi Experts,

My customer is interested in DRV8885 because of this device does not require an external resistor for current motor.
Customer will use Full-Step and customer's motor is needed 1.5Arms output current. According to datasheet, Table 2, p14, output current is limited by 71% full scale. (stated by DRV8885 recommended operating conditions.) Does it mean 1.5Arms*0.71%=1.065Arms, correct? If so, why is limitation? Is it thermal limitation?

DRV8880 achieved over 1.5Arms output current but it requires a large external resistors for current monitor. It does not prefer for customer.

Do you have any alternative device?

Your support would be much appreciated.
Best Regards,
Fujiwara

  • 0
    TI__Guru** 114905 points
    Hi Fujiwara-san,

    The DRV8885 is recommended for stepper motors that require 1A rms or less. The limit is based on the thermal limitations.

    The DRV8886 or DRV8886AT is the alternative to the DRV8880.
  • 0 in reply to Rick Duncan
    TI__Genius 11685 points

    Hi Rick-san,

    Thanks for answer.
    I want to understand IFS definition of recommended operating condition. IFS is set by external resister connecting GND. Can customer set IFS above recommended operating condition? In particular, DRV8885 and DRV8886 can set IFS=2.1A? My understanding is that IFS of recommended operating condition is defined full-scale current which is an actual current flow through motor and device. Is my understanding correct? If so, is there any limitation for an external resistor value?

    Your support would be much appreciated.
    Best Regards,
    Fujiwara

  • 0 in reply to Yasushi Fujiwara
    TI__Guru** 114905 points

    Hi Fujiwara-san,

    Can customer set IFS above recommended operating condition? In particular, DRV8885 and DRV8886 can set IFS=2.1A?


    The DRV8886 may operate at 2.1A. From section 7.1 of the datasheet "The actual full-scale and rms current depends on the ambient temperature, supply voltage, and PCB thermal capability."

     

    My understanding is that IFS of recommended operating condition is defined full-scale current which is an actual current flow through motor and device. Is my understanding correct?


    Yes. Please note the Microstepping Current Waveform on the first page of the datasheet. Full scale current is the 100% setting. RMS current is .707 * the full scale current.

    If so, is there any limitation for an external resistor value?


    If you trying to lower the resistor to achieve more current, the limit will depend on the factors listed above: ambient temperature, voltage, PCB thermals, and Rdson variation. For the DRV8886, the lower limit is 10kOhm because overcurrent can activate at 3A. In reality, the thermals will probably cause problems before then.

    If you trying to increase the resistor to lower current, the limit will depend on the voltage, motor inductance, and decay mode.These factors will determine how well the steps are defined when microstepping.

     

     

  • 0 in reply to Rick Duncan
    TI__Genius 11685 points

    Hi Rick-san,

    DRV8886 can operate/work correctly a following conditions?

    - Full-step, (Microstepping settings: M0=M1=0)
    - IFS=2.1A, (RREF=14.2Kohm@1% (TRQ is 100%))
    - VM=24V
    - Ta: -40C ~ +125C

    Best Regards,
    Fujiwara

  • 0 in reply to Yasushi Fujiwara
    TI__Guru** 114905 points

    Hi Fujiwara-san,

    It is unlikely the DRV8886 will operate correctly at high ambient temperatures and high RMS current. 

    There are few options available.

    The DRV8711 with external FETs is one.
    Two DRV8840 or DRV8842 is another.