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DRV8870: VREF pin issue

Cherry Zhou
Mastermind 22235 points
Part Number: DRV8870

Hi team,

Customers are using the DRV8870 to drive a 12V DC motor spin without speed adjustment and IN1 and in2 level switching enables forward and reverse switching.

1) They consider using Vref as a general EN, without Vref, pin 2 and pin 3 cannot be operated with any level change. It was found that VM input 12 V and Vref no-load voltage is 0 V, as long as the in2 IN1 level is not equal the motor will act. In2 IN1 is a 3.3-V monolithic drive. 

Current equation: I=VRef/10* current limiting resistor, Vref = 0V, there should be no current. Does the DRV8870 have a fixed voltage internal when Vref<0.3V? 

2) Measuring the short circuit overcurrent protection, short circuit OUT1 and OUT2, and the chip emits a whistle. Would the chip be damaged if the short circuit condition was not handled? 

Could you help check this case? Thanks.

Best Regards,

Cherry

  • 0
    TI__Mastermind 20886 points

    Hey Cherry,

    Thank you for your question. I will aim to provide a response  before the end of the week.

    Best,

    Akshay

  • 0 in reply to Akshay Rajeev Menon
    TI__Mastermind 20886 points

    Hey Cherry,

    1) You cannot supply below 0.3V at VREF. As the current regulation is dependent on VREF which is being supplied externally.

    So you will hit I trip instantaneously.

    2) Could you explain a bit more on what test was performed? If there is an OCP event then the FETS will be disabled and the device will retry after some time.

    Shorting the outputs is not recommended as you can potentially damage the internal FETS. Was your chip functional after the 'whistle' was heard?

    Best,
    Akshay

  • 0 in reply to Akshay Rajeev Menon
    TI__Mastermind 22235 points

    Hi Akshay,

    Thank you for the support.

    Akshay Rajeev Menon said:

    1) You cannot supply below 0.3V at VREF. As the current regulation is dependent on VREF which is being supplied externally.

    So you will hit I trip instantaneously.

    When Vref = 0V, the Itrip is reached instantaneously according to the datasheet. But the current does not decay and the motor can still startup. 

    Akshay Rajeev Menon said:

    2) Could you explain a bit more on what test was performed? If there is an OCP event then the FETS will be disabled and the device will retry after some time.

    Shorting the outputs is not recommended as you can potentially damage the internal FETS. Was your chip functional after the 'whistle' was heard?

    OUT1 and OUT2 use wires to test the short-circuit protection, after short circuits, OUT1 and OUT2 enter OCP overcurrent protection, and the chip emits a whistle of inductance. And it will recover after disconnecting the shorted chip.

    Thanks and regards,

    Cherry

  • +1 in reply to Cherry Zhou
    TI__Mastermind 20886 points

    Hey Cherry,

    1) That is interesting. 0V VREF might not exactly correspond to a 0A current trip (due to offset of the internal comparator) which might explain why we see motor spin but it should be a very small current. Would you be able to provide the motor datasheet?

    2) The chip itself does not contain anything that can cause a 'whistling'. I am suspecting that it might be from your power supply during the current short.

    Best,
    Akshay

  • 0 in reply to Akshay Rajeev Menon
    TI__Mastermind 20886 points

    Hey Cherry,

    Has your question been answered? If so, please mark the thread as resolved.

    Best,

    Akshay