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DRV2665: Issues when load is attached

Drew Hamilton
Prodigy 60 points
Part Number: DRV2665
Other Parts Discussed in Thread: DRV2667

Hi,

I have been using the DRV2667 to generate a square wave type signal 0->80V for an inchworm Piezo actuator type application. I have been getting a variety of different results. A little over a month ago, I had a new spin of PCB's running successfully for about a 30 hours non-stop stress-test without any issues, but now I'm finding that the output waveform is very strange with a load attached.

I am using a PK3CMP2 Piezo stack from Thorlabs with 240nF Capacitance and 0 to 100V drive voltage range. At the upper end of it's voltage range, the DRV2667 shouldn't have any problem driving this load, and it seems to have done it just fine for quite a while. Now, all of the sudden, the output waveform gets incredible noisy and the voltage is much lower, but only with the load attached. 

Instead of the usual "click" I expect out of the piezo at the rising and falling edge of my square-wave, I get a buzzing sound that alternates between very quiet and "loud", the RMS voltage for which is about 50% of the unloaded output waveform. The waveform on the scope is about what you would expect from that description (I can upload a JPEG if necessary).

Schematic (there are pullups for I2C elsewhere on the board):

The PCB is a 2 layer board with 2oz copper.

PCB Layout (Frontside):

PCB Layout (Backside):

Piezo information:

Any help would be very much appreciated. I have really struggled with this part of the design.

Thanks in advance,

Drew Hamilton

  • 0
    TI__Mastermind 18545 points

    Hello Drew, 

    I am looking into this thread with my colleagues and will provide a response as soon as possible. In the meantime, the waveform capture would be helpful to have if you don't mind uploading it. 

    Best Regards, 
    Justin Beigel

  • 0 in reply to Justin Beigel
    TI__Mastermind 18545 points

    Hello Drew, 

    We are still looking into this. In the meantime, what is the frequency that you are driving the piezo at? 

    Thank you, 

    Justin Beigel

  • 0 in reply to Justin Beigel
    Prodigy 60 points

    Hi Justin,

    I actually got everything working fine, so it would be quite a bit of trouble to reproduce the issue now that I have fixed it. I appreciate your responses.

    The issue seems to have been current draw. I was not able to provide enough current and that was causing issues. When I put this section of the board on it's own supply, the buzzing disappeared and everything worked as expected.

    This actually brings me to another question: how much current exactly does this system draw if I am driving a 100V Piezo load? I read somewhere on the forums that the I(batmax) current is what I need to look at. Before, I had accounted only for I(out_max), which is significantly lower. From my Design Equations spreadsheet it looks like:

           Ibatmax = 323.135mA

           Iout_max = 11.309mA

    this is for a 240nF, 100V Piezo that I am putting 90V across.

    Am I getting all this right? Would I see effects like this if I wasn't able to source enough current to the chip? Are my new estimates of the current requirements correct?

    Thanks!

    Drew

    As a postscript: I can't figure out how to change the title of this post, but I'm actually using a DRV2667, not a 2665.

  • 0 in reply to Drew Hamilton
    TI__Mastermind 18545 points

    Hello Drew, 

    Glad it is working for you now. It makes sense that you would see a decreased output if their was not enough current available to the device. The lack of current is likely creating an unstable boost voltage that translated to your decreased output. I don't have any data that matches your load, but your Ibatmax value is around what I would expect. 

    Also, don't worry about changing the title. I will see if we can alter it internally, but it is not a big deal since the DRV2667 is explicitly mentioned. 

    Best Regards, 

    Justin Beigel