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Original question:

DRV8873-Q1: DRV8873 PWM mode function

DRV8873-Q1: Losing torque in brushed DC motor when PWM

user5983645
Prodigy 75 points
Part Number: DRV8873-Q1
Other Parts Discussed in Thread: DRV8873, LMD18200

I redesigned a pcb for my client who used the THT IC LMD18200T for driving his motor.
The redesigned board has now the DRV8873-Q1 chip, but when I PWM the inputs like 12% ON to slowly drive the brushed geared motor, I'm basically lost all torque.
I tried PWM frequenties between 60Hz to 300kHz, but no results.

I was actually surprised to see that the PWM signals are passed on to the OUT pins, while the LMD18200T measures a continuous voltage output.
Can the DRV8873-Q1 operates the same way PWM in and variable but continuous voltage out?

I'm sure the torque loss is caused by PWM pulsing the DC brushed motor.
Or what other SMD chip would you recommend?

  • 0
    TI__Guru** 114905 points

    Hi user5983645,

    The DRV8873-Q1 should be able to match the behavior of the LMD18200T.

    Which version of the DRV8873-Q1 are you using (H or S)?

    If S, what are the register settings?

    If H, what are the configuration inputs?

    Is the device operating properly above 12% duty cycle?

    How were the LMD18200T inputs (DIR, PWM, BRAKE)?

  • 0 in reply to Rick Duncan
    Prodigy 75 points

    Thanks for your fast reply, much appreciated since this board is close to mass production

    In the old situation with the same geared 12V DC motor we drove the PWM line of the LMD18200T as low as 17% duty cycle = 2.1V at 7812.5 Hz frequentie and got enough torque.
    What does "continuous output" mean in the datasheet? There is no PWM pulse on the DC motor terminals?

    This is my new situation with the DRV8873 Hardware version:

    I PWM between drive and brake.
    I lowered R4 and R5 to 390ohm on the board but when I measure with my multimeter the motor never gets more then approx 1.6A while in the old situation I reach 3 Ampere during load conditions.

    I found it surprising to see the PWM pulse on OUT1 or OUT2, I probably lose massive torque because of that? Or is the LMD18200T also outputting PWM pulse ?

    Here is my scope image of the new situation.
    IN2 permanent set high.
    IN1 PWM 17% 7.8kHz (motor is not turning and high pitched sound from the motor)

    Yellow line is IN1 17% PWM
    Blue and Purple are each a motor terminal

  • 0 in reply to user5983645
    TI__Guru** 114905 points

    Hi user5983645,

    Thank you for the additional information.

    The image do not appear to match the description provided.

    Can you confirm that you are PWM'ing IN1 as stated?

    The image makes more sense if MODE selected PH/EN mode, IN1/EN is set high, and IN2/PH is being PWM'ed.
    Is it possible the MODE pin is not connected to DVDD?

    The image shows each output PWM cycle appears to be drive forward (Ch2 high/Ch3 low), drive reverse (Ch2 low/Ch3 high), Hi-z.

    Also, yes the LMD18200 also PWM's the outputs. The PWM and DIR inputs control which output is PWM'ed.

  • 0 in reply to Rick Duncan
    Prodigy 75 points

    Yes the yellow line is 17% duty cycle PWM of IN1
    One engineer told me that the DRV chips are using inverse PWM duty cycle.
    So my 17% duty cycle in the scope image should mean a fast driving motor.

    That when I double checkt IN2 and measured 0 volts on there.
    So in the scope image I was PWMing IN1 and had IN2 low.

    I noticed that STM controllers ignore the following line:
    HAL_GPIO_WritePin(GPIOA, MOTOR_IN2_Pin, GPIO_PIN_SET);

    Maybe because it is in PWM output mode that it isn't accepting WritePin commands.

    So now I have IN1 PWM duty cycle 83% and IN2 PWM duty cycle 100% and get very close to the torque I want.

    What other tricks can I do to increase torque even more?

    Lower slew rate?

  • 0 in reply to user5983645
    TI__Guru** 114905 points

    Hi user5983645,

    Glad to hear you are very close to the torque you want.

    How close are you?

    Have you measured the average voltage and average current between the DRV8873 and LMD18200?

    There could be some small differences due to input to output synchronization in the DRV8873. You may need to increase the duty cycle slightly in the DRV8873 to match the LMD18200.

    Please let us know what you find.