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DRV8848: Vref Current Control Accuracy

Part Number: DRV8848
Other Parts Discussed in Thread: DRV8881, DRV8830, DRV8832, DRV8832-Q1

I would like to use the Vref pin to control the speed of the motor. I have a DAC driving the Vref pin and a set sense resistor of 200mOhms.

In the DRV8848 datasheet, it is recommended to keep Vref between 1V and 3.3V. And Between 0V and 1V works but "accuracy is degraded". I did not see anything in the datasheet that specifies what the accuracy is in either range.

My application does not require high accuracy or anything, but it would be nice to know what is meant by degraded accuracy. Does TI have any number for this? Is it really significant? 

I plan on having a Vref low limit of 100mV. Which would be about 75mA current regulation. Full range would be 100mV to 2.7V.

  • Hi Matt,

    We will investigate and reply by Tuesday.
  • Hi Rick,

    Did you get a chance to look into the accuracy when Vref<1?

    Thank you!

    Regards,
    Ryan B.

  • Hello Matt and Ryan,

    I apologize for the delay. Rick and I looked for characterization data, but we could not find any for this condition. The device is only characterized according to the limits in the Recommended Operating Conditions Table. The current chopping is implemented with an amplifier and a comparator, so using low reference voltages causes the outputs to derate as the voltage approaches the ground rails.

    If you would like, I can try to look for a different device to support controlling the current over this range. Let me know if this is for a brushed-DC motor or stepper.
  • This is for two Brushed DC motors.

    See my other thread for my actual application. 
    https://e2e.ti.com/support/applications/motor_drivers/f/38/t/717511

    I will most likely just characterize it myself when I get the boards to see how it varies with lower voltage. If the amplifier is rail to rail, maybe the linear region is around +- 100mV from the rail? I will have to test at room temp, 0C and 60C and over a few units.

    Like I said, I don't need high accuracy, so as long as I set it to 100mA and it doesn't end up 200mA that is going to be ok. I'm hoping  for +-10mA accuracy or even +-20mA I will still try it out and maybe can compensate for extra inaccuracy in firmware. I wanted to make sure it wasn't a huge difference.

  • Hi Matt,

    The DRV8881 says that its xVREF pins are accurate down to 0.3 V.

    I just noticed in your original post that you said you want to control motor speed using VREF. I'm not sure that will work. Motor speed is typically a function of the voltage across the motor coil, while torque is a function of the current in the coil. The VREF pin is only meant regulate current. It will not change the PWM duty cycle to control speed.

    The DRV8832-Q1, DRV8830, and DRV8832 are the brushed-DC motor drivers in our portfolio that perform speed regulation.