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DRV8848: DC Motor Controllers

Part Number: DRV8848
Other Parts Discussed in Thread: DRV8801, DRV8843, DRV8873, DRV8840, DRV8412, DRV8881

 

Hi

In using Brushed Motor Controllers DRV 88xx (such as DRV8801, 8840...DRV8848....etc);

·        None of these DRV88** can report the actual value of the current to the MCU. Correct?

·        We cannot control the current of the motor all the time. Correct?

·        At best we can set the current level to 1 one of 32 values (as happens in DRV8843. A feature that is not available in every DRV88**). Correct?

·        Even if we PWM the enable pin we still cannot control the current accurately because the MCU does not get current feedback and hence has no way to accurately control that PWMed signal to generate accurate current control.

·        The only exception to all the above is DRV 887*. Correct?

·        Is there any other DRV88** in the family that provides current feedback to the MCU?

·        Is there any DRV88** in the family that provides can adjust the current level continuously (rather than to 32 levels)?

·        If we use one of the evaluation board (say EVM DRV8848)….it has MCU on-board….what is the easiest way to get access to one of the analog pins of that MCU…we would like to provide current sensor feedback to that pin ?

 

Regards

Sam

 

  • Hi Sam,

    In using Brushed Motor Controllers DRV 88xx (such as DRV8801, 8840...DRV8848....etc);

    (1) None of these DRV88** can report the actual value of the current to the MCU. Correct?

    >> The DR8412 with amplifier and sense resistors on each ground, or a INA1xx current sense amplifier in series with each winding, can provide the actual currents.
    >> The DRV8873 can report the current during Drive and High side slow decay, but not during fast decay or low side slow decay.
    >> The DRV8801 can report the current during Drive, but not during decay.
    >> With external amplifiers, other devices like the DRV8840, DRV8848, etc can do the same.


    (2) We cannot control the current of the motor all the time. Correct?

    >> If you add the ability to monitor the current as described in the DRV8412 in (1), you can.

    (3) At best we can set the current level to 1 one of 32 values (as happens in DRV8843. A feature that is not available in every DRV88**). Correct?

    >> Many of the devices have a VREF pin that allows the current level to be adjusted.

    (4) Even if we PWM the enable pin we still cannot control the current accurately because the MCU does not get current feedback and hence has no way to accurately control that PWMed signal to generate accurate current control.

    >> Correct. You will need external circuitry as described above

    (5) The only exception to all the above is DRV 887*. Correct?

    >> Even the DRV8873 has limitations, as mentioned in (1)


    (6) Is there any other DRV88** in the family that provides current feedback to the MCU?

    >> Please refer to (1)

    (7) Is there any DRV88** in the family that provides can adjust the current level continuously (rather than to 32 levels)?

    >> Any DRV88xx device with a VREF input has this ability. Adjusting the VREF can be achieved by using a DAC, PWM, or other method.

    (8) If we use one of the evaluation board (say EVM DRV8848)….it has MCU on-board….what is the easiest way to get access to one of the analog pins of that MCU…we would like to provide current sensor feedback to that pin ?

    >> The EVM were not designed for this. You can solder a small wire to the pin of the mcu.
  • Hi 

    Thank you for your clear answers.

    I would like to follow up on one of the questions.

    Is there any DRV88** in the family that provides can adjust the current level continuously (rather than to 32 levels)?

    >> Any DRV88xx device with a VREF input has this ability. Adjusting the VREF can be achieved by using a DAC, PWM, or other method.

    • Can we supply VREF with PWMed signal? ...does this mean DRV88** (8848 for example) will filter this signal or do we have to filter that PWMed signal ourselves?
    • In case of DRV8848 (Dual Motor) VREF will control both motors (or the Chopping limit of both motors). Correct? The motors are not independent anymore. Correct?
    • Without connecting this sense resistor DRV8848 will run in an open loop manner. Correct? and in that case if Vref changes all the time the value of the current will be struggling to follow. Correct?
    • If we connect the current sense resistor is there anything that would stop us supplying square wave to VREF at a frequency close to 1KHz? 
    • If we are not running in closed loop mode (current is not regulated).Ain1 & Ain2 can be supplied with PWMed signal and hence they vary the motor current. Correct?
    • The only way to have independent current of both motors independently is by closing the current loop inside an MCU and supplying the DRV8848 with PWMed based on that external close loop. Correct?

    Regards

    Sam

  • Hi Sam,

    Can we supply VREF with PWMed signal? ...does this mean DRV88** (8848 for example) will filter this signal or do we have to filter that PWMed signal ourselves?

    >> You will need to filter the PWM externally to create a stable VREF voltage

    In case of DRV8848 (Dual Motor) VREF will control both motors (or the Chopping limit of both motors). Correct? The motors are not independent anymore. Correct?

    >> Correct, The DRV8848 VREF controls both motors. The DRV8881 has two VREFs inputs to allow separate control.

    Without connecting this sense resistor DRV8848 will run in an open loop manner. Correct? and in that case if Vref changes all the time the value of the current will be struggling to follow. Correct?

    >> If the xISEN pins are connected to GND directly, VREF no longer controls the current. The current is controlled by the inputs.

    If we connect the current sense resistor is there anything that would stop us supplying square wave to VREF at a frequency close to 1KHz?

    >> It might work in some cases, but is not recommended. At low VREF voltages, there could still be some current flowing through the load due when the outputs are enabled during the blanking time.

    If we are not running in closed loop mode (current is not regulated).Ain1 & Ain2 can be supplied with PWMed signal and hence they vary the motor current. Correct?

    >> Correct.

    The only way to have independent current of both motors independently is by closing the current loop inside an MCU and supplying the DRV8848 with PWMed based on that external close loop. Correct?

    >> Correct for the DRV8848. As mentioned above there are other devices like the DRV8881 with two VREF inputs.