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DRV8846: DRV8846 use problems

Rock Su
Intellectual 2220 points
Part Number: DRV8846

Hi!

We have a customer plan to use DRV8846, but they have some questions as below. 

Could you please reply these questions? Customer will use fstep=39.024KHz, stepping level = 1/32, rising and falling edges.

1. In datasheet, fstep max = 250KHz. 

    If fstep = 250KHz, Tsw = 4us, but toff min = 10us. If Tsw < toff, is this OK? Why? Can it result that current can't decrease timely? Because in toff, there will be 2 step pluse(assumption: rising-edge only).

2. How to choose Decay mode?

    Customer asked that since we have AutoTune and why we supply other decay modes to them.

3. How to select tBLANK?  It means how to select Torque DAC.

4. Attachment is stepper datasheet that customer will use, how to select RxISEN according stepper datasheet?

Appreciate it for your help!

Best Regards,

Rock Su

  • 0
    TI__Guru 56650 points
    Rock,

    Sorry for the delay. I thought it was replied on last Saturday.
    1. In general, 250kHz fstep is not for motor motion control because the motor inductor current may not get the setting value when the next step signal is received. Datasheet 8.2.2.1 Stepper Motor Speed gives a equation.
    250kHz setting could be used in another scenario: the motor stops at a position A which is different than the initial default 45 degree. After sleep, if customers want to quickly get the previous position A, they can use this 250kHz fstep to quickly get there before enable driver output.
    2. AutoTune is what we want for decay mode control. But, DRV8846 offers manually decay mode setting for some customers who want to use their own decay control.
    3. Table 6 gives the tBLANK selection according to Torque DAC percentage. In our case: If the input supply is 5V, motor DCR is 14 ohm and 1ohm sense resistor is used. The winding current is 5V/15ohm=333mA. The maximum allowing voltage on the sense resistor is 500mV; maximum VREF is 3.3V. According to the equation (1), it allows us selecting 75% TORQUE to support 333mA IFS and narrow tBLANK range.
    4. The item3 just talks about the RxISEN selection. If we use 1ohm sense resistor:
    a. it almost use full 500mV range which is good.
    b. The worst case the power dissipation on that sense resistor is 111mW when the winding current is at peak current. When motor is rotating, the wind current is sine waveform. So, the average sense resistor power dissipation could be lower than 111mW.