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DRV8662 switching frequency

Alexander Kaliadin
Prodigy 130 points
Other Parts Discussed in Thread: DRV8662, DRV8662EVM

Hi Brian-

What is DRV8662's switching frequency at the default values (Lb=4.7uH; Cb=100nF) and how it is going to change if I use 3.3uH and 56nF?

Thanks,

Alex

  • 0
    TI__Expert 3280 points

    Hello Alex,

    The boost converter for the DRV8662 is a hysteretic converter, therefore the switching frequency is not set and will vary based on not only the inductor, but also the loading of the output. To give a sense of direction, using the generic formula of a inductor,  L*(di/dt)=V, if the L decreases then the dt will decrease (frequency increase). The Rext resistor that sets the current limit, will change the switching frequency because it sets the threshold at which it switches (di)

    If the Cb you are referring to is the Vbat capacitor, this will not change the switching frequecy as it is a decoupling cap. We recommend not changing this cap.

    Additionally, simulations can be run in TINA using the DRV8662 simulation model found on the DRV8662 product page on TI.com.

    Please let us know if you have any more questions.

     

    Best regards,

    Rodney Miranda

    Haptics & Piezo Applications

  • 0 in reply to Rodney Miranda
    Prodigy 130 points

    Thanks Rodney.

    So it seems that replacing 4.7uH with 3.3uH will not change much then. For Cb I have been referring to Cboost. 

    I am looking for the way of reducing switching noise in the output. Placing resistor in series with piezo did help but it is still above my acceptance criteria.

    Kind regards,

    Alex

  • 0 in reply to Alexander Kaliadin
    TI__Expert 3280 points

    Hello Alex,

    If you want to change the switching frequency then the inductor and Rext are the ways to go.

    If you are looking to minimize switching noise, then you will need additional decoupling caps. Since there is already a 0.1uF capacitor on Vbst (assuming you are using the DRV8662EVM), you could add an additional 10nF capacitor for better decoupling. Ideally, you would like these to be small size caps (0402) and close to the DUT so that the inductance is minimized.

    Here is a good app note describing switching noise. http://www.ti.com/lit/an/slva219a/slva219a.pdf

    Let us know if you have any more questions.

     

    Best regards,

    Rodney Miranda

  • 0 in reply to Alexander Kaliadin
    TI__Genius 9670 points

    Hi Alex,


    Just to add to Rodney's comment.  The DRV8662 is a discontinuous boost, which means the current goes from 0A to the peak current every switch cycle.  If you are most concerned with switching noise, then increasing REXT, which decreases the maximum switch current, will result in a higher switch frequency for the same load.

    Typically higher switching frequencies will result in better voltage ripple on the output. This will come at the cost of higher switching losses.

    In addition, higher capacitance on the output (Cboost) and a larger inductance will help as Rodney mentioned.

    Thanks,
    Brian