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DRV8825: About current control PWM and step frequency

Tao_2199
Genius 5660 points
Part Number: DRV8825

Hello,

 

Regarding to internal current control PWM frequency on DRV8825, my customer is asking some question.

They are understanding as following.

The step frequency is used to control the indexer logic. The indexer can be advanced at a 250kHz rate.
The  current control PWM frequency is used to regulate the current on the output. It is up to 30kHz.

(Question)

If the step frequency(indexer) is used on 250kHz rate, how behaver will current control get?

Can current control (30 kHz) support to indexer with 250 kHz rate?

 

Regards,

Tao2199

  • 0
    TI__Guru** 114905 points
    Hi Tao2199,

    (Question)
    If the step frequency(indexer) is used on 250kHz rate, how behaver will current control get?
    Can current control (30 kHz) support to indexer with 250 kHz rate?

    The indexer (step rate) can be accepted at 250kHz, but the current may not follow the indexer at this rate.

    In most cases, such a high step rate will not be used. There are potential uses of this high step rate, such as setting the indexer to a previous condition upon awakening the device. For example:

    The last position of the stepper motor is known when the outputs are disabled and the DRV8825 is placed in sleep mode. After some time, the DRV8825 is awaken to move the motor. Prior to enabling the DRV8825, the desired number of steps can be issued at 250kHz to set the current. When the device is enabled, the motor should remain in the same position.
  • 0 in reply to Rick Duncan
    Prodigy 180 points

    Hi Rick DunCan,

    I have same question about the step rate and pwm chopper frequency, when we set DRV8825 to 32 microstepping mode, then issue a 40khz (greater than chopping frequency 30khz) pulse to it, in this case the speed of stepper motor will be only 375RPM, but  as my understanding, the coil current is regulated by pwm cycle, and microstepping is realized by different current level changed by step pulse, so step rate should be less than pwm chopper frequency, then what will happen ? Thanks!

  • 0 in reply to liur
    TI__Guru** 114905 points
    Hi Liur,

    The response will depend on the system voltage, the motor resistance, the motor inductance, and the motor speed.

    The DRV8825 will attempt to drive the correct amount of current based on the indexer table and VREF, but may not be able to. To achieve the desired current, the system voltage may need to be increased or the microstepping level may need to be reduced.
  • 0 in reply to Rick Duncan
    Prodigy 180 points

    Hi Rick Duncan,

    Thanks for your feedback. Could you help to confirm whether the PWM period is fix or variable(automatically) in DRV8825? if the PWM period is fix, I think the current chopper will cannot follow the step pulse when it greater than PWM frequency(30khz), despite the supply voltage or motor parameter, because one PWM period cannot produce two or more different current level, is it right?

  • 0 in reply to liur
    TI__Guru** 114905 points
    Hi Liur,

    The internal PWM clock is fixed.

    As the STEP input is asserted, the current will change to the next level. If the current level is reached, the decay mode will begin for the remainder of the internal PWM clock.

    If the STEP input is not asserted, the current will be regulated for the full PWM cycle. If the STEP input is asserted, a new current level will begin.