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DRV8701: DRV8701P- No motor movement in Coast

tom prohaszka77
Prodigy 170 points
Part Number: DRV8701


We have a pcb design with the DRV8701.  I need some clarification.   We want to PWM our BDC in both directions.   Focusing on one direction, I set IN2 to 0, and PWM IN1.   Per the data sheet, it places the part into coast when IN1/IN2=0, and IN2=0 and IN1=1 (during PWM high), into forward motion.  

When in PWM high, I wee SH1 high, SH2 low.  During coast I see SH1 low and SH2 High.  The motor only turns when IN2=0 and IN1=1 (100% duty).  Other duty cycles, the motor is not turning.

The motor will turn at commanded duty cycle when we set IN2 =1, and on low cycle IN2=0.  During off time IN2=1 and IN1 (brake mode).

Any clarification on why the motor fails to turn in coast at duty <100%

  • 0
    TI__Guru** 114905 points
    Hi Tom,

    What is your PWM frequency?

    PWM'ing with coast causes the current to be removed from the motor quickly. This reduces the torque and prevent motor movement.
    PWM'ing with brake recirculates the current creating more torque in the motor. This allows the motor to move.

    If you have a current probe or a voltage probe across the sense resistor, you can see this difference.
  • 0 in reply to Rick Duncan
    Prodigy 170 points
    We are using 16KHz pwm. I tried 8KHz, same result. Probing the Gates, GH1 goes high with PWM high, and low on PWM low, GL1 tracks. I'm also seeing GH2 go low on PWM High, and GH2 High on PWM low (only pwming IN1, IN2 =0), GL2 stays low. GH2 should not be driven per datasheet. Coast should have all fets in off state.
  • 0 in reply to tom prohaszka77
    TI__Guru** 114905 points
    Hi Tom,

    The outputs are off, although some explanation is needed.

    Please refer to Figure 24 of the datasheet. The high side gates connect to SHx when the output is off (VGS = 0) and to VCP when the outputs are on (VGS = ~10V).

    Assuming you have the load connected, you are seeing is the current flowing through the body diodes when entering coast mode. Please refer to Figure 19.

    As you switch from Forward Drive (1) to High-Z (3), the voltage at SH1 changes from ~VM to -0.7V to allow the current to continue to flow. This causes GH1 to go low. The voltage on SH2 changes from ~0V to VM+0.7V to allow the current to flow. GH2 follows SH2 to ~VM, but the VGS of the FET is 0V.
  • 0 in reply to Rick Duncan
    Prodigy 170 points
    Just a follow up. It appears for driving the motor with PWM, we engage low side braking on PWM ON cycle. This results in inverted Duty cycle control. Not that this is an issue, but is there a way for positive duty cycle control with DRV8701?

    Also, i did lower the PWM frequency to 1KHz, and with Coast mode the motor did turn. This is not an option though since this puts out an audible tone.
  • 0 in reply to tom prohaszka77
    TI__Guru** 114905 points
    Hi Tom,

    If using the DRV8701P, the duty cycle is inverted as you noted.

    If you want the duty cycle to be positive, please consider the DRV8701E. The PH pin sets the direction and the EN pin is PWM'ed. When high, the outputs are driving. When low, the outputs are in brake mode.