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DRV8323: DRV8323 Auto offset calibration

x z
Prodigy 130 points
Part Number: DRV8323

Hi Adam:

I found a problem with Auto offset calibration of DRV8323，

I did a test：DRV8323 is connect to a three-phase motor , the system  work in the state that the high side FETs is closed and the low side FETs is on,

pull the cal pin high  to do a calibration at 2s intervals,MCU uses ADC module to sample 8323 SOx

The results show that the ADC is different for each calibration, ADC jitter on 2045 and 2065，as shown in the figure below

Green data represents the state of the cal pin, 2050 is high, 0 is low

Black data represents ADC

Measured SOx with an oscilloscope, the voltage waveform is the same trend as the ADC，and the ADC reference voltage of my MCU is unchanged

I don't understand why the value of each calibration is different

I also made the following attempts, all the same phenomenon：

-Increase the time interval to 10s

-Calibration using SPI configuration

-Use the BOOSTXL-DRV8323Rx to do the same experiment

Is this phenomenon abnormal, can you explain why?

Looking forward to your reply

Thanks

ZX

  • 0
    TI__Guru 63280 points

    ZX,

    Can you explain the reason for the repeated calibration? Usually our customers calibrate once at power up and then occasionally recheck the calibration based on system temperature changes or similar.

    Regards,

    -Adam

  • 0 in reply to Adam Sidelsky
    Prodigy 130 points

    Hi Adam:

    Thanks 

    Because the temperature of our system increases during operation，There will be a temperature rise of at least 60 ℃ at room temperature

    Then we found that the SOx voltage varies with temperature causing our current to be inaccurate

    So we want to do multiple calibrations throughout the process，

    Do you have any other suggestions

    Can you explain the reason for the chang of SOx during each calibration

    Thanks

    ZX

  • 0 in reply to x z
    TI__Guru 63280 points

    ZX,

    It is normal for the change to occur with large temperature changes. I would recommend that you recalibrate each time the temperature changes significantly.

    Each time the calibration occurs a new offset is measured and stored so this is expected.

    Are you running the calibration when the FETs are switching?

    Regards,

    -Adam

  • 0 in reply to Adam Sidelsky
    Prodigy 130 points

    Hi Adam:

    Thanks for your reply

    I'm running the calibration when the low side FETS are on

    'Each time the calibration occurs a new offset is measured and stored so this is expected.' Does this sentence mean that my sampling result is based on calibration,

    So even if the each time the calibration result is different , it will not affect the current I calculated with the new offset

    So I don’t need to worry about why the calibration results are different every time

    It should be noted that the situation where the calibration results are different is at the same ambient temperature

    Thanks

    ZX

  • 0 in reply to x z
    TI__Guru 63280 points

    ZX,

    It is recommended to do the calibration with all the FETs OFF, please try this and let us know.

    If you want you can also use your MCU to read the SOx output when in cal mode and store this value yourself to then correct the offset in your own MCU and algorithm. 

    Regards,

    -Adam

  • 0 in reply to Adam Sidelsky
    Prodigy 130 points

    Adam:

    Thanks for your reply

    I have tried to do the calibration with all the FETs OFF and the result is the same

    According to the datasheet, is the state of Low side FETs on  also belonging to MOS are not switching

    According to your suggestion, if I need to turn off all FETs for calibration in the process of controlling the motor, it seems unreasonable

    Another problem is that when the Low side FETs are on, there is a difference between the calibrated ADC and the non-calibrated ADC.

    This is also the case for all FETs OFF，in this case , there is no current, right? Why is there a difference

    I also did the above experiment without connecting the load

    Thanks

    ZX

  • 0 in reply to x z
    TI__Guru 63280 points

    ZX,

    Did this get resolved? If not what are your outstanding questions?

    Regards,

    -Adam