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DRV8701: How to control a DC motor by PWM

Franz Giesen
Prodigy 130 points
Part Number: DRV8701

We would like to use the DRV8701 as an alternative to a currently used discrete circuit. However, we found the following significant problem in the test setup. In the de-energized phase of PWM we leave the high side MOSFET switched on in our discrete configuration. The DRV-chip switches off the bridge completely. In our discrete circuitry, a certain motor type starts reliably under identical conditions with a duty cycle of approx. 20% (33kHz clock frequency). The DRV chip must output a duty cycle of approx. 70% under otherwise identical conditions. In the meantime, we have found that this is due to the fact that the high MOSFET, which is switched off by the DRV-chip, converts the counter-EMK completely into a braking effect.
What proposal does TI have to avoid this problem? We have to control the engine. However, a controller then only has the difference of 70% to 100% duty cycle available as a control interval. I look forward to receiving a more detailed answer.
F. Giesen

Translated with www.DeepL.com/Translator

  • 0
    TI__Guru** 114905 points
    Hi Franz,

    Are you using the DRV8701E or DRV8701P?
    What signals are toggled at 70% on the DRV8701x to reliably start?
    Are you using current control in the DRV8701x?
    If yes, what is the sense resistor and VREF voltage? Have you confirmed the control control is not affecting the outputs?
  • 0 in reply to Rick Duncan
    Prodigy 130 points

    Hello Mr. Duncan,

    we are using DRV8701P. We toggle IN 2. IN1 is during PWM phase constant low. After about 6-8ms we switch off by setting both inputs to low. In a time off about 1ms we measure the EMK through a Resistor divider with about 38k-10k. We have tried al modi of current controll. We have also switched off by setting SP=SN=0V. But we see that the current is about 1/10 of the expected current.

    If you  need further information I will cooperate.

    Best regards

    Franz Giesen2045.Motor sheet V1M4.pdf

  • 0 in reply to Franz Giesen
    TI__Guru** 114905 points
    Hi Mr. Giesen,

    Thank you for the additional information.
    If I understand your discrete circuit, the current is re-circulating through the high side FETs.

    The DRV8701P has a similar mode using the low side FETs by setting IN1 and IN2 high.
    Have you tried setting IN2 as a constant high, and PWM'ing IN1 at 80%? This setting would drive current for 20% and re-circulate current for 80% of the time, which should be similar to the discrete circuit.

    Do you agree that this setting should be similar?
  • 0 in reply to Rick Duncan
    Prodigy 130 points

    Thank you very much. The problem is solved.

    Best regards

    Franz Giesen