DRV8889-Q1: Unexpected reports open

Hi Zhenghao,

Thank you for the post. 

wang Zhenghao said:

1. What are the possible OL bit  be set except current less than 30mA?

       2. Whether TRQ_DAC will affect OL judgment?

TRQ_DAC setting will not influence OL detection as long as every step or microstep current is > 30 mA.

Other than this the following highlighted conditions could trigger a false OL.

wang Zhenghao said:

3.The exception only occurs on the vehicle, and the same fault is not repeated separately. Is it related to the vehicle EMC environment?

It is possible the cable harness could be causing current to go < 30 mA during microsteps current regulation.

What is the VREF voltage, microstep settings and step rate? 

Regards, Murugavel 

Hi Murugavel ,

           microstep: 1/8 

           SLEW_RATE: 00b = 10-V/μs
           VREF voltage 2.14V

            In my opinions, the OL will be set in any run condition when the electrical Angle is equal to 0,90,180 or 270. But this issue only occur in vehicle environment. We tried thousands of tests with the abnormal parts and nothing happened.

Regards, Zhenghao

Hi Zhenghao,

Thanks for further details. 

wang Zhenghao said:

In my opinions, the OL will be set in any run condition when the electrical Angle is equal to 0,90,180 or 270. But this issue only occur in vehicle environment. We tried thousands of tests with the abnormal parts and nothing happened.

With VREF = 2.14 V and TRQ_DAC = 15 (6.25 %), the current amplitude would be ~ 60 mA. Which means with 1/8 uSteps depending on where the motor is stopped the current can go below the OL threshold of 30 mA. Lowest current would be 60/8 = 7.5 mA, Only after four steps from the lowest current the value would be > 30 mA. Assuming the TRQ_DAC = 15 was used for hold current.

While running with TRQ_DAC = 10 (37.5 %) the amplitude would be 365 mA, lowest current would be 365/8 = 45.6 mA which would be > 30 mA. 

Was this test done with the same cable harness? 

wang Zhenghao said:

But this issue only occur in vehicle environment.

If this is suspected as false readout due to noise you can read the fault register three to five consecutive times and see if the results or the same. If it was noise only one of them or may be two out of five may have a false report. This could be a simple noise filter to implement. Thank you.

Regards, Murugavel 

Hi Murugavel:

      Thanks for your response.

      We used the same faulty part for repeated tests and there was no abnormality, which was consistent with the cable harness for the step motor and 8889.

      If the noise is misread, will the motor drive chip actively stop working?

Regards, Zhenghao

Hi Zhenghao,

wang Zhenghao said:

If the noise is misread, will the motor drive chip actively stop working?

The OL detection is for report only. The device will continue to work whether OL is 1 or 0. Only the ECU determines the outcome once OL is set. Thank you.

Regards, Murugavel  

Hi Murugavel ,

      The DRV8889-Q1  as an open circuit , How can software avoid " If the motor is held at position corresponding to 0°,90°，180°or  270° electrical angle ..."? 

Regards, Zhenghao 

Hi Zhenghao,

wang Zhenghao said:

The DRV8889-Q1  as an open circuit , How can software avoid " If the motor is held at position corresponding to 0°,90°，180°or  270° electrical angle ..."? 

The following approaches could be of help.

1. Disable OL before stopping the STEP input. Enable after STEP input pulses are applied. This approach cannot OL while the motor is stopped.

2. Keep track of the indexer steps in the MCU as per Table 7-3. Relative Current and Step Directions in the datasheet. Avoid stopping at these angles. Most customers use this approach. With this approach OL is possible even when the motor is in hold current mode (stopped). 

Regards, Murugavel