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DRV8711: Current regulation

Shunsuke Nagata
Genius 13100 points
Part Number: DRV8711

Hi, team,

Could you please give me your advice how to set current regulation value?

I use Full-scale current(IFS) formula x 71% (home state). But the measurement current is larger than calculation. It's depend on VM.
And when I change the motor, measurement current is increased.

Is my calculation correct? If it is incorrect, could you please give me advice how to calculate current regulation through VM=12 to 35V and dependency of Coil register? IFS formula doesn't have VM value and Coil register....

Here is test condition and result

DRV8711_test condition&measurement.xlsx

Regards,

Nagata.

  • 0
    TI__Guru 56650 points
    Nagata,

    PKP2913D56A motor has lower inductance. The winding current goes up quickly with a higher input voltage and small inductance winding. If the current goes up too high in tBLANK, DRV8711 cannot control it.

    After the current is enabled in an H-bridge, the voltage on the ISEN pin is ignored for a period of time before enabling the current sense circuitry. This blanking time is adjustable from 1 μS to 5.12 μs, in 20 ns increments, by setting the TBLANK bits in the BLANK register. Note that the blanking time also sets the minimum on time of the PWM.

    Would you reduce the tBLANK to let DRV8711 sense and control the current early?
  • 0 in reply to Wang5577
    TI__Genius 13100 points

    Hi,

    Thank you for your advice.

    Could you pleaes give below questions?

    1.  I would like to know how to set tBLANK depend on Winding Coil, external FET and VM. Is there a simulation tool or GUI to set tBLANK?
    2.  You said PKP2913D56A has lower inductance. But I have no idea about threshold to detect lower inductance or not. Is there guidline to detect lower inductance, resistance and VM?
    3. Do you have recommend device to work with PKP2913D56A?

    Regards,
    Nagata.

  • 0 in reply to Shunsuke Nagata
    TI__Guru 56650 points
    Nagata,

    1. dv=L*di/dt; In this case, L is winding inductance; dt is tBlANK; di is the winding current increasing value in tBLANK; dv is the voltage across the motor winding. If you want to regulate the current at 0.52A, you have to make sure the di<0.52A in tBLANK because the part cannot sense the current in tBLANK.
    2. Measure or calculate (dv=L*di/dt) the winding current in tBLANK. If the peak current at tBLANK is higher than the current regulation point, that means the winding inductance is small to limit the current in tBLANK.
    3. Please try to reduce tBLANK first.
  • 0 in reply to Wang5577
    TI__Guru 56650 points
    Nagata,

    Our senior engineer points out this issue is related to the decay mode setting. The setting is slow decay which turns on both low side FETs. For an inductor steady state operating, Ton * Von = Toff * Voff. The Voff is almost zero when both low side FETs are turned. So, Ton*Von>Toff *Voff, that drive the inductor current up. So, please set the decay mode to mix decay.
  • 0 in reply to Wang5577
    TI__Genius 13100 points

    Hi,

    Thank you for your support.

    In this time, I would like to know about tON of PWM.
    Do you have idea for setting tON? Should user set the time with Winding R and L? or is there a tool for the calculation?

    Regards,
    Nagata.

  • 0 in reply to Shunsuke Nagata
    TI__Guru 56650 points
    Nagata,

    After you select Toff and mix decay, the inductor current drop in Toff is same as the inductor current rise in Ton. So, you don't need to calculate Ton.