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DRV8871: PWM signal gets distorted when motor is connected

robin corn
Prodigy 200 points
Part Number: DRV8871

We have an issue with the DRV8871. We input a PWM signal as can be seen in image 1. This shows the inputs to both in1 and in2, one is our PWM input signal and the other is ground. But then when we measure out1 we see something strange as can be seen in image 2. The out1 pin gives a PWM signal which doesn't go all the way to 0, the minimum is ~5V. Then when we connect the brushed motor we get a distorted PWM signal as can be seen in the last image. Also notice that without the motor connected the mean voltage is 14V, but when the motor is connected it suddenly rises to 20.8V. 

The input PWM doesn't change when the motor is attached so the problem seems to be in the chip. The schematic below shows the values of the caps and resistor we use. It also shows the DPDT switch that we use to change the direction in which the motor spins. 

SCHEMATIC:

IMAGE1:

IMAGE 2:

IMAGE 3:

  • 0
    TI__Guru 56650 points

    Robin,

    The motor winding has a higher inductance. After PWM high signal drive the current up, the winding current may keeps the same direction. But, the PWM low signal turns off the FET. To find out why the output low is not on ground and why it is high when PWM signal is low, please check PGND and GND pin voltage.

    Do you test on an EVM or your board? If possible, please check it on an EVM to compare with your board's result.

  • 0 in reply to Wang5577
    Prodigy 200 points

    Dear Wang Li,

    Thank you for your reply. We have measured both GND and PGND and they are both indeed 0V compared to the input GND of the board. We also tried adding a higher bulk capacitor which also didn't change anything. We are using our own board now.

    Do you have any more suggestions on how we can get the low output to GND and how comes that the PWM signal gets distorted once we attach a motor?

  • 0 in reply to robin corn
    Prodigy 200 points

    Dear Wang Li,

    We found that the IN2 should always be high, instead of GND such that is switched between driving and braking, then it works. Coast-mode, resulted in this faulty result. Is this what you expect of using coast-mode? I Quote from the datasheet "When using PWM, it typically works best to switch between driving and braking." ... "Alternatively, the coast mode (IN1=0, IN2=0) for fast current decay is also available." It seems that both should work, can you explain?

    Kind regards

  • 0 in reply to robin corn
    TI__Guru 56650 points

    Robin,

    The INx H-Bridge control table is attached. IN1=IN2=1 can run into brake mode. When IN1=IN2=0, all FETs are off.  All your description matches the H-bridge control table.