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DRV11873: Question of the abnormal FG

user4661991
Prodigy 90 points
Part Number: DRV11873

Hi ,

I am using DRV11873 to drive the color wheel motor on our projector project. 

I needs a constant and stable rotating speed to protect  the color wheel motor. so,I detect the pin of FG  to get the rotating speed of the color wheel motor.

The attach is the schematic .

Through observing oscilloscope ,I found out the sudden change of FG signal in a few hours .

The FG frequency change dramatically In two seconds,then turn into normal frequency .

The attach is the FG and RD waveform of two circuit boards.

Oscilloscope channel 1 and channel 4 represents the RD signal.

Oscilloscope channel 2 and channel 3 represents the FG signal.

  • 0
    TI__Intellectual 1555 points

    Hi,

    What is the FG frequency prior to the event occurs?

    Thanks,

    seil

  • 0 in reply to Seil Oh
    Prodigy 90 points

    Hi,

    The FG frequency prior is 560 Hz.

    Now,I am looking for the problem to change the FG.

    The following is the layout.

  • 0 in reply to user4661991
    TI__Intellectual 1555 points

    Can you verify that device is not detecting false lock condition? If it does, device will stop driving motor for lock release time which is based on FS setting.

    Thanks,

    seil

  • 0 in reply to Seil Oh
    Prodigy 90 points
    Hi:
    I have a question.why is one part of DRV11873 device working well for several days.
    From DRV11873 datasheet, for high speed fan motors with low motor winding resistance and low inductance, the FS pin should be pulled high. FG frequency is 550 Hz, so FS pin is pulled high.
    Excuse me,what is the lock condition ?The DRV11873 datasheet is not very clear about this.When the motor is blocked or stopped by an external force, I obeseve the FG signal to change and the current is nealy zero.Now ,there is only one Low-power fan used for coolling the color wheel motor.
    Can you explain the lock condition in detail? I'd like to know more about lock condition.
    Thanks.
  • 0 in reply to user4661991
    TI__Intellectual 1555 points

    If device detects, motor is not spinning. The device will stop driving the motor for 3 or 6 seconds depending on the FS setting and restart the motor operation.

    That is lock detection function. I wanted to make sure device is not falsely detecting lock detection.

    Thanks,

    Seil

  • 0 in reply to Seil Oh
    Prodigy 90 points

    Thank you for your reply.

    The time of FG singal change is almost 3 second.FS setting is setted high.Maybe the decice is lock detection.

    I don't quite understand that the motor is always spinning when the FG signal has changed.

    Can you tell me how to judge the device being falsely detecting lock detection.

    there is another question about FS setting.

    RPM = (FG *60)/ pole pairs. pole pairs is 4. RPM is 7200.FG is 480 Hz in theory。

    The question is whether the FS pin should be pulled high。

  • 0 in reply to user4661991
    TI__Intellectual 1555 points
    here is the paper on the lock detection. I agree with you device seems to catch false lock detection.

    www.ti.com/.../slva778.pdf

    Seil
  • 0 in reply to Seil Oh
    Prodigy 90 points
    I have read slva778.pdf in detail.
    I'm assuming that my device seems to catch false lock detection.
    According to the datasheet ,during the lock protection condition, the RD output remains high until the lock protection is dismissed and restart is completed.
    Throughout the motor spinning ,I didn't observe any change of RD singal.
    Does that also mean that my device does not catch false lock detection.
    This is not the real reason.I have to find other reasons.
    Thank you for your thoughtful reply.