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DRV8881: VREF and TRQ

Shuji Ishiwata
Mastermind 7265 points
Part Number: DRV8881

Tool/software:

Hi All,


I have a question about the DRV8881.

I have a DAC connected to VREF to dynamically change the current limit value.

If I change the VREF voltage, how long does it take for the output to stabilize?

Also, if I change TRQ, how long does it take for the output to stabilize?


Best Regards,
Ishiwata

  • 0
    TI__Mastermind 36675 points

    Hi Ishiwata-san,

    Thanks for posting in this forum.

    Shuji Ishiwata said:

    I have a question about the DRV8881.

    I have a DAC connected to VREF to dynamically change the current limit value.

    If I change the VREF voltage, how long does it take for the output to stabilize?

    Internally to the device AVREF and BVREF are immediately updated at the Itrip comparator, so the changes would reflect immediately. It is fast enough to support microstepping indexing using an external MCU. Please see this application note, High Resolution Microstepping Driver.

    Shuji Ishiwata said:
    Also, if I change TRQ, how long does it take for the output to stabilize?

    Similar to the above the response will be immediate as well. May I know the purpose of this request? What is the desired response rate and why it is needed? Thank you.

    Regards, Murugavel 

  • 0 in reply to Murugavel4637
    Mastermind 7265 points

    Hi Murugavel-san,


    The customer is changing the current limit by dynamically changing VREF using a DAC.

    When the customer actually checked, there seemed to be no problem with TRQ, but the time it took for VREF to rise did not seem to be fast.

    The customer requested to know the specific time it takes for VREF to rise.

    Please tell us the specific time.


    Best Regards,
    Ishiwata

  • 0 in reply to Shuji Ishiwata
    TI__Mastermind 36675 points

    Hi Ishiwata-san,

    Thanks for checking with the customer and getting back to us. 

    We do not have a specification for this parameter for this device. We can do an example calculation based on the 256 microsteps example waveform in the application note which was implemented and tested to work with this device. See below snapshot. One half-wave is 32 ms. Within that half-wave the current level changes 512 steps using a sine table driven DAC driving the VREF. So VREF was updated every 32 / 512 =  62.5 μs with no issues. CH1 blue trace is real-time VREF change in this example.

    Shuji Ishiwata said:
    When the customer actually checked, there seemed to be no problem with TRQ, but the time it took for VREF to rise did not seem to be fast.

    Did the customer have any capacitor to GND on AREF and BREF? This can slow down the VREF voltage change rate. Also the settling time of the DAC used is important. The application note uses an MSP430 integrated DAC supports 1 μs settling time. 

    Please let us know what is the desired response time and purpose for it. Thank you.

    Regards, Murugavel