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DRV2605L: Need help with open-loop PWM driven mode

Mitch Cairns
Prodigy 20 points
Part Number: DRV2605L

Tool/software:

I have my DRV2605L set to open loop mode so I can control an LRA driving frequency manually. I have the mode set to 0x3 for PWM input.

The datasheet says that you must set your PWM frequency to 128x the desired driving frequency but this does not seem correct and I'm hoping some engineers can share some insights.

Similarly, the BIDIR_INPUT bit and related documentation does not seem correct. When I set the PWM to a given frequency, and provide a 50% duty cycle, the output frequency when measured with an audio tuning device is equal to the PWM frequency. The expected result based on the datasheet would be NO output at 50% duty cycle, and as I increase the duty cycle approaching 75 or 90%, it would drive the LRA to output a signal. Similarly, it is the exact frequency, not the signal divided by 128 times. 

Putting all of this aside, lets say I want to just drive the LRA at 320Hz consistently, I can do this by setting my PWM to 320Hz and sending a 50% duty cycle. This works, but there is a constant 'pulsing' happening on the haptic actuator that is quite irritating. This pulsing is not reflected in the PWM output under oscilloscope going into the PWM IN pin.

Is there any explanation for this?

  • 0
    TI__Mastermind 18385 points

    Hi Mitch,

    LRAs have a very narrow bandwidth to produce any meaningful vibration. Why do you wish to run in open loop and control the frequency manually? you will get better performance(i.e. vibration strength) by using the closed loop auto resonance tracking.

    I am suspecting the pulsing you are feeling could be due to driving the LRA slightly off resonance. does the pulsing change or lessen if you adjust the frequency by a few Hz?

    Regards,
    Arthur

  • 0
    Prodigy 20 points

    Arthur, I'm using the haptic actuator to play tones and other effects based on audio formats. When I drive the actuator directly with a mosfet I know it works. 
    Even when driving the actuator at the resonant frequency it still does the pulsing.

    The pulsing is only present when driving the LRA with the DRV2605L in PWM mode, leading me to think it's some oscillation within the driver chip itself.

    All that being said, the documentation seems left-over or just incorrect. There are quite a few places with typos and what seems to just be copy/pasted information from older models. Measuring the LRA with an oscilloscope shows that the frequency matches exactly what I'm putting in to the PWM in open-loop mode... It's just confusing because the documentation clearly says the driving frequency needs to be 128x the desired frequency which isn't right Disappointed

  • 0 in reply to Mitch Cairns
    TI__Mastermind 18385 points

    Hi Mitch, 

    OK, understand your point. Sometimes people have similar requests and once we probe a bit deeper, we find that they may just be concerned about additional complexity of the resonance track (however its quite simple to implement). I understand your use case is different

    I will check this on the EVM tomorrow and get back to you. 

    Regards,

    Arthur

  • 0 in reply to Arthur Brown
    Prodigy 20 points

    Arthur, I'm not sure if it was some configuration bit or whatever, but I re-wrote my driver code and the strange pulsing is gone.

    That being said, the PWM mode documentation isn't a match and I'm hoping the documentation could be updated for anyone else who may run into this issue.