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VBST shawtooth shaped

Rui Chen58
Prodigy 240 points
Other Parts Discussed in Thread: DRV8662

Hi,

I am using the DRV8662 to drive a piezo bender (CMPB4). This bender needs to be connected to GND, SIGNAL and VBST. When I measure the voltage on VBST (with no load connected) VBST is not a constant signal but has this sawtooth shape. Is this normal? If not, what could cause it? See images below for the VBST waveform and my schematics.

Thanks!

VBST:

Schematics:

  • 0
    TI__Expert 3280 points
    Hello Rui,
    The sawtooth ripple is because that is the ripple of the hysteretic boost converter. If you take a look at section 8.3.1 of the datasheet, it will provide a little more detail of why it has that ripple.

    Also, I will provide some more details of the output configuration in our other post shortly.
  • 0 in reply to Rodney Miranda
    Prodigy 240 points

    Ok, thanks. Then I understand the behaviour better. This ripple is visible also in the outputs so I guess I need to put a low pass filter on the outputs to eliminate the ripples in the output. Below is an image of the OUT+ signal.

  • 0 in reply to Rui Chen58
    TI__Expert 3280 points
    Rui,
    If the load is between OUT+ and OUT- then the ripple is not observed because both channels have it and therefore is removed.
    [(OUT(+) + ripple) - (OUT(-) + ripple)] = [(OUT(+) - OUT(-)) + (ripple-ripple)] = OUT(+) - OUT(-)
    However due to your output configuration, you will see it. All you can do is increase the BST capacitor to lower the ripple however this will increase the amount of time needed to have VBST up to it's DC level.