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DRV8870: usage issues

Cherry Zhou
Mastermind 22235 points
Part Number: DRV8870

Hi team,

1) What are the differences between coast and brake (fast current decay versus slow decay) and the pros and cons? 

coast （00） fast current decay

brake（11）Brake; low-side slow decay

2) Normal operating current is 120mA, hardware stall current Itrip 800mA, measured effective value is 330mA, software overcurrent 270mA. Is it reasonable?

3) The electrolytic capacitor on VM is 47uF, what is its purpose? Is it ok to remove it?

4) When current exceeds Itrip, hardware protection is triggered, how does the corresponding OUT1,OUT2 change? The 00 mode is being used.

5) Below is the measured waveforms, could you help review is there any issues?

At normal operation

At stall

Could you help check this case? Thanks.

Best Regards,

Cherry

  • 0
    TI__Mastermind 37683 points

    Hi Cherry,

    Cherry Zhou said:
    1) What are the differences between coast and brake (fast current decay versus slow decay) and the pros and cons? 

    This app-note explains the main differences between the decay modes. The terminology is a bit different but asynchronous decay stands for coast and slow decay for brake.

    Cherry Zhou said:
    2) Normal operating current is 120mA, hardware stall current Itrip 800mA, measured effective value is 330mA, software overcurrent 270mA. Is it reasonable?

    Are these peak current or average values? There is always some difference due to errors.

    Cherry Zhou said:
    3) The electrolytic capacitor on VM is 47uF, what is its purpose? Is it ok to remove it?

    This is the bulk capacitance that provides instantaneous current to the motor during switching and it also filters out low frequency ripple that may be on the VM line. We recommend a 0.1µF cermaic cap and at least 10µF of bulk capacitance. 47µF is a reasonable capacitance value.

    Cherry Zhou said:
    4) When current exceeds Itrip, hardware protection is triggered, how does the corresponding OUT1,OUT2 change? The 00 mode is being used.

    When ITRIP is reached, the device enforces slow current decay by enabling both low-side FETs, and it does this for a time of tOFF (typically 25 µs). OUT1 and OUT2 are shorted to GND during the TOFF time and re-enabled afterwards. 

    Cherry Zhou said:
    5) Below is the measured waveforms, could you help review is there any issues?

    What are the waveforms showing? Is it output voltage?

    Regards,

    Pablo Armet