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Original question:

DRV2605L: Vybronics VLV101040A LRA Motors Getting Extremely Hot

DRV2605L: Wide Bandwith LRA getting hot in Open Loop mode

Tomi Kuye
Prodigy 20 points
Part Number: DRV2605L
Other Parts Discussed in Thread: DRV2605

Tool/software:

Hi, I asked this question a couple of months ago but have encountered a new development.

I mentioned that I was driving 4 LRAs continuously in open loop mode at 250 HZ with the DRV2605L and that they were getting extremely hot, almost dangerously so.

I have been running some tests and noticed that when in closed loop mode, the motors stay cool despite the long drive time. I am struggling to find an explanation as to why considering the Vybronics motors are wide bandwidth .

Can someone please help me out here, and explain why at 250 hz these motors get so hot. The duty cycle is as low as possible without going to negative drive. The DRV2605 is in PWM mode , and the frequency of the signal is 32,000 (128*250)

The resonant frequency on the website says it’s 170 Hz. Is it because I’m driving so far off resonance? I haven’t seen this problem anywhere else and there is such limited information on LRAs that I could read up on to find a suitable explanation. I have contacted the manufacturers themselves and have heard nothing back.


Thanks you for your help.

  • 0
    TI__Genius 10512 points

    Hi Tomi,

    Yes, I think you can expect to see this behavior driving off-resonance. Sorry, I didn't really think about the impact here when you mentioned driving open-loop in the other thread. Here are a few reasons for the heating - first few are related to each other.

    1. Reduced efficiency. Driving off resonance will require more current/voltage depending on how strong you want the vibration
    2. Increased losses. Driving the higher current will lead to higher I*I*R losses. 
    3. Mechanical dampening. More energy will be lost in the suspension and internal dampening systems as heat, rather than being converted into mechanical motion.
    4. Lastly, when driven in closed-loop, the driver only drives for a portion of the full period and is always driving the most efficient, resonance, frequency. The driver spends a portion of the waveform sensing the Back EMF of the motor. 

    This is an older document and more related to narrow band LRAs: Benefits of Auto-Resonance Tracking (Rev. A). Precision Microdrives has some good papers/guides on LRA's: Technical resources - Precision Microdrives (search LRA). 

    You could try getting closer to the resonance and see if the heating issue is better. It is difficult to find any online documentation for off-resonance driving.

  • 0 in reply to Kelly Griffin
    Prodigy 20 points

    Thank you! One last, hopefully final question fingers crossed. One thing that really influenced my decision with these particular motors was the really small rise time I was able to get (<2ms) While other motors with a resonant frequency of close to 250 exist, their rise times on the datasheet are much higher. 

    Is rise time something I could improve with the DRV2605 L? Like with an overdrive at startup or something. If so, how. Or is it something dependent on the technology of the LRA itself. I plan on doing some testing by just buying a bunch of LRAs and characterizing their performance but I wanted to go into that having some idea what is possible and expected.

    Thanks again for the help and resources you’ve provided.

  • 0 in reply to Tomi Kuye
    TI__Genius 10512 points

    No problem. I would recommend moving to a LRA in the 250Hz range. 

    Yes, you can decrease the rise time with our drivers. The rise time in datasheet is typically with just a sine wave signal - no closed loop driver. For our drivers, the overdrive feature in closed loop can significantly reduce the rise time from the LRA datasheet value. The typical rise time value should be in the 10-30ms range (10% to 90% acceleration). The thing to pay attention to here is the available supply voltage range. Make sure it is sufficient for your Overdrive voltage level, typically 1.5x or 2x rated voltage. 

    These drivers calibrate to the LRA's back EMF magnitude at the rated voltage. When you drive a waveform, you will see the overdrive voltage initially, and as the back EMF magnitude of the LRA increases, the overdrive voltage will drop down to the rated voltage level.