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DRV8840 Internal Current control PWM frequency

Yuriy Suhinin
Prodigy 20 points
Other Parts Discussed in Thread: DRV8840

Hello.

I have DRV8840 on TI CPG005_DRV88xx Evaluation Module.

Load: DC motor R = 6.9 Ohm, L = 0.309 mH and inductor R = 2.6 Ohm, L= 1.36 mH. Total R = 9.5 Ohm, L= 1.669 mH.

VM = 24V.

Current sense resistor RS = 0.25 Ohm.

I (0-4) = HI.

DECAY = HI (fast decay).

VREF = (0 - 3.3) V.

Peak current control quality is an evaluation goal.

At low Ipeak = (10 - 200) mA PWM frequency is 50 KHz, as stated in datasheet. When I increase VREF (and Ipeak) PWM frequency

lowers twice, to 25 KHz. Increasing current further lowers PWM frequency twice again, to 12.5 KHz.

Is that behavior normal?

Thank you,

Yuriy

  • 0
    TI__Guru** 114905 points
    Hi Yuriy,

    When running the motor, what is state of the ENABLE pin. Is it high or toggling?

    Can you provide scope captures of the motor current and output voltages when the frequency is 25kHz and 12kHz?
  • 0 in reply to Rick Duncan
    Prodigy 20 points

    Hi Rick.

    Thank you for reply.

    Both ENABLE and PHASE are HI - full forward current limiting mode, I would say.

    Ok, the first picture is for Ipeak = 150 mA (VREF = 198 mV). 49 KHz. Everything seems OK.

    The second one for Ipeak = 330 mA (VREF = 422 mV). Frequency is changed to 22 KHz.

    And the last one for Ipeak = 966 mA (VREF = 1.215 V).

    It is not exactly 12 KHz as I said, but 16 KHz shown is also questionable.

    Thank you,

    Yuriy

  • 0 in reply to Yuriy Suhinin
    TI__Guru** 114905 points
    Hi Yuriy,

    The scope captures are very helpful.

    You are seeing the effect that the back emf of the motor has when injecting current into the motor. As the motor spins, the back emf makes it easier to remove current in fast decay and harder to inject current. In the last scope capture, you can see it takes almost twice as long to restore the current as it did to remove it.

    Also, depending on the time at which fast decay is triggered, current could be removed for almost a full 50kHz cycle.

    Many customers use slow decay instead of fast to avoid this. But it can create another problem at low currents where the current in the motor can be higher than the desired current.

    An alternate device is the DRV8881P, which has an additional decay mode 30% fast/70% slow which tends to prevent both problems. The DRV8881P has two full bridges that can be paralleled to achieve the full 5A.