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DRV8702-Q1: Current Chop and Motor Voltage Shutdown

Amirul Kamaruzaman
Prodigy 90 points
Part Number: DRV8702-Q1
Other Parts Discussed in Thread: DRV8703-Q1

Tool/software:

Hi E2E mods, 

Provided below is the motor driver schematic for your reference

There are 2 questions I would like to gain clarification:

1) After few testing, I have found some inconsistency in the current chop limit between few tested boards.

Based on calculation my current chop setting should be at 23A, but across few units I saw that the current chop varies between 18.5A to 21A.

I compared 2 units which had the lowest and highest current chop, and I found that the their Vso reading is entirely different during same motor load current of 13A.

I have verified current chop setting resistors (R73,R77) and current sense resistors (R10) to be comparable values.

My best assumption now for this issue is because the Vio (input referred offset) range is too wide as per in datasheet (5 to 10mV). *Please correct me if this is incorrect.

This results in variant of Vso, and also the Ichop across all units. If this is the case, can you recommend how may I ensure the Vio to be stable across all units?

2) When I probe the voltage across motor terminals (P32 +ve & P42 -ve), I observe a voltage spike during motor shutdown.

I have tried to remove all capacitors (C77,C78,C84,C85) across all of the power mosfet used and the voltage spike still persists during motor shutdown.

Refer graph below to see (yellow probe: voltage across motor, red probe: motor current)

Test procedure: Turn on motor signal and run for 3s before turn off.

Is this normal behavior of motor driver shutdown? Hope you may help to give some clarifications.

Thank you in advance!

Best Regards,

Amirul

  • 0
    TI__Genius 14541 points

    Hi Amirul,

    Thanks for your question.

    I am assigning this to the device expert from my team for support.

    Thanks,

    Ibinu

  • 0 in reply to Ibinu Alaudeen Nadeera
    TI__Guru 51085 points

    Hi Amirul,

    I'll review this and get back to you. 

    Regards, Murugavel 

  • +1 in reply to Murugavel4637
    TI__Guru 51085 points

    Hi Amirul,

    Amirul Kamaruzaman said:

    My best assumption now for this issue is because the Vio (input referred offset) range is too wide as per in datasheet (5 to 10mV). *Please correct me if this is incorrect.

    This results in variant of Vso, and also the Ichop across all units. If this is the case, can you recommend how may I ensure the Vio to be stable across all units?

    Your assumption is correct. 

    The current regulation formula has a VIO x AV term. In your schematic VREF = 555.5 mV. For devices where VIO is around 10 mV, this term would be around 20 x 10 = 200 mV. This would be a significant ratio of the VREF voltage. To achieve smaller variation you may want to keep VREF voltage at a higher multiple of this offset term. You are already using 1 mΩ, so you may be at the limit already. Would it be possible to reduce this sense resistor further? Like two of them in parallel? Then you could increase VREF voltage.  

    With the DRV8703-Q1 you could select a gain of 10 V/V using SPI register setting.

    Amirul Kamaruzaman said:

    2) When I probe the voltage across motor terminals (P32 +ve & P42 -ve), I observe a voltage spike during motor shutdown.

    I have tried to remove all capacitors (C77,C78,C84,C85) across all of the power mosfet used and the voltage spike still persists during motor shutdown.

    Refer graph below to see (yellow probe: voltage across motor, red probe: motor current)

    Test procedure: Turn on motor signal and run for 3s before turn off.

    Is this normal behavior of motor driver shutdown? Hope you may help to give some clarifications.

    While the motor is spinning and the power FETs are turned off the BEMF of the motor takes the path to the power supply via the FET body diodes. This would result in a spike VM + body diode voltage drop for a brief period of time until the BEMF of the motor reduces due reduction in its speed due to system friction. This is expected behavior. To avoid this you can turn off the motor using the driver to perform slow-decay or low side brake - see below. In this case the motor would stop instantaneously. Alternatively you can reduce the PWM duty cycle to ramp down the speed of the motor and then turn it off. 

    Regards, Murugavel