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DRV8886: Why are torque different between DRV8886 and DRV8813

Koyo Asaishi
Intellectual 1710 points
Part Number: DRV8886
Other Parts Discussed in Thread: DRV8813, , DRV8813EVM, DRV8818

Hi team,

I have a question about the difference between DRV8886 and DRV8813.
The boards to measure the 2 devices are similar conditions but the torque of DRV8886 shows 10-20% higher than DRV8813 at 10kHz as you see the following table.

I checked the current waveform but both waveform at 11993Hz are similar.
Therefore I guess this difference comes from other factor.

Could you estimate why the difference is appeared?

I use DRV8886EVM and DRV8813EVM to evaluate.

Best regards,
Koyo

  • 0
    TI__Guru 56650 points

    Koyo-san,

    Higher torque means higher winding current.

    At 10kHz step rate, the output current cannot reach the regulation point even the FETs are fully on. In this case, only the loop resistance, motor inductance and back EMF limit the current increasing.

    DRV8813 internal high side FET and low side FET total Rds_on is about 0.4ohm. DRV8886 total Rds_on is about 0.54ohm.

    The lower Rds_on can make the winding current a little bit higher.

    Also, please compare the input voltage and input cable resistance carefully. That can also affect the winding current a lot when the output current cannot reach the regulation point.

  • 0 in reply to Wang5577
    TI__Intellectual 1710 points

    Hi Wang,

    Following table shows wire resistance.

    DRV8813 is lower Rds_on than DRV8886 and both wire resistances are not so big difference.
    Therefore DRV8813 should be higher torque because lower Rds_on make the winding current a little bit higher and higher torque means higher winding current as you said.

    Also, input power supplies are same value.

    Could you tell me more detail about DRV8886 is higher torque?

  • 0 in reply to Koyo Asaishi
    TI__Guru 56650 points

    Koyo-san,

    1. The winding average winding current is related to the torque. Higher winding current has a higher torque. Can you save DRV8813 winding current waveform in the scope first and put DRV8886 winding current waveform comparing with it?

    2. At 10k step rate, both DRV8813 and DRV8886 winding current doesn't reach the setting current and high side FETs are fully on. I would think the current is related to the input voltage and loop resistance. If you have checked the input voltage and loop resistance, I am wondering the winding current. DRV8818 is higher than DRV8886 or not?

    From previous picture, DRV8813 current looks higher than DRV8886's with 11993Hz step rate. If DRV8813 current is higher and both DRV8813 and DRV8886 internal FETs are on continually to drive the current up, the winding current is not related to motor driver devices.

  • 0 in reply to Wang5577
    TI__Intellectual 1710 points

    Hi Wang,

    The waveform I attached at first post is not winding current?

    1. Both input voltage are 24Vdc and both motor are same.
    If loop resistance means the following resistance of current loop (high and low side FET and motor), difference will be only Rds_on because same motor is used.

    2. What is the setting current?
    Sorry for my poor understanding.

    "the winding current is not related to motor driver devices" means you guess this issue is not related to ICs but related to peripheral factor, correct?
    DRV8886EVM and DRV8813EVM is used to measure the waveform and the table.

    Best regards,
    koyo

  • 0 in reply to Koyo Asaishi
    TI__Guru 56650 points

    Koyo,

    "the winding current is not related to motor driver devices" means you guess this issue is not related to ICs but related to peripheral factor, correct?

    Correct. As picture you draw, if the FETs are full on and Rds_on is similar, the winding current is not related to the motor driver.

    "The setting current"

    DRV8813 uses xVREF pin to set the output current. DRV8886 uses RREF to set the output current. 

    With a low step rate (1790Hz in your attached waveform), the output current reaches the current regulation setting point. The motor driver FETs do the chopping to maintain the output current.

    With a high step rate (11993Hz), the output current is not chopping. That indicates the FETs fully on.

  • 0 in reply to Wang5577
    TI__Intellectual 1710 points

    Hi Wang,

    I could understand thanks to your answers.

    Thanks and Best regards,
    Koyo