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DRV8332 Passive Sizing

Eric LaFay
Prodigy 30 points
Other Parts Discussed in Thread: DRV8332, DRV8814, DRV8312, DRV8313, TMS320F28069, DRV8412

We are using planning to use a DRV8332 to drive Maxon EC45 70W Brushless, Maxon EC13 12W and/or hobby motors Exceed RC Rocket 3520-800KV Brushless Motor.

So I have two questions:

1. How does one go about sizing the bulk cap(s) for high voltage PVDD line. The drv8312evm has a pair of two 330uF 63V caps. These seem rather large. However my only experience until now has been with brush motors & drv8814 where I use a single bulk 47uF cap. 

2. We are switching away from the Maxon epos2 45/2 drivers where Maxon adds 10-100uH of extra inductance to make the motors more controllable.  The datasheets for drv8312/drv8313 outline adding ferrite beads for EMC, but I wonder anyone or TI had any input on this.

2.A. we plan on using a ti piccolo TMS320F28069 running at 80kHz PWM and 3 kHz control loop to overcome not needing these inductors. 

All help would be greatly appreciated!

-Eric

  • TI__Expert 8885 points

    Eric,

    There are various factors which decide the value of bulk cap at PVDD.

    1. To filter out ripple in PVDD due to ripple present incoming power source
    2. To filter out ripple in PVDD due to high frequency switching current drawn by 3-pahse H-bridge circuit. Two things affect this, PWM switching frequency and power rating. Higher switching frequency leads to lesser cap, but higher power rating means higher peaky current, which would require bigger capacitor value.   
    3. To meet filtering for any  standard EMI requirements
    4. Hold-up requirement in case of power interruption.
    5. Breaking requirement, if you need to break the motor with high deceleration rates, you will face over-voltage across PVDD, if cap is small.

    From first two factors, I can recommend based on my experience that if you’re input is DC source (without much ripple i.e. from Battery) and you are going to operate above 10Khz, 2uF per  watt would good starting point. Note that this is ball-park figure solely based on my experience; you would be better judge to select right value based on your end application needs.     

    Ferrite beads are recommended in data-sheet for proper functioning of over-current protection circuit in case of dead-short at outputs. A ferrite bead limit the rate of rise of output short-circuits current thus reduces the magnitude of peak current in such event.  I would request you to refer to DRV8412 data-sheet for more detail on selection of this ferrite bead. I would recommend you to use these additional inductors from protection point of view even if you do not need from current control perspective.

    Best Regards

    Milan-Motor Application Team

  • in reply to Milan Rajne
    Prodigy 30 points

    Hi Milan,

    I had one more question. I'm trying to keep the height of circuit to a very minimum. Is there any draw back to have 4-5 smaller electrolytic capacitors in parallel? I'm curious about switching and esr. 

    Thanks!

    -Eric

  • in reply to Eric LaFay
    TI__Expert 8885 points

    Eric,

    Using multiple capacitors in parrallel is infact better than using single higher value becuase it help in reducing the over-all ESR.

    Best Regards

    Milan-Motor Control Application Team