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DRV8312: Recommendation of BST cap

Kazuki Kuramochi
Genius 10545 points
Part Number: DRV8312

Hello expert,

I have question for DRV8312's BST cap.
In the datasheet p26 schematics example used 100V 100nF MLCC.

However, we'd like to use lower voltage rating MLCC on this portion.
We are plan to use PVDD = 40V and GVDD = 12V.
So we guess BST cap is applied approx. 12V. There for we think we can change this cap to other cap which have lower voltage rating.
Then, I'd like to know about following points.
・Why schematics example used 100V MLCC?
・Is there recommended capacitance of BST cap as actual effective capacity.

Could you provide answer for above question?


Best regards,
Kazuki Kuramochi

  • 0
    TI__Mastermind 21805 points

    Hello Kuramochi-san,

    The bootstrap capacitors should be rated with tolerance for GVDD with some additional margin due to output switching. I think a 25V rating is acceptable.

    The schematic referenced in the datasheet appears to be the EVM . You can see that the EVM designer used the same capacitor component on PVDD and BST_X for simplicity (i.e. C42 & C43).

    0.1uF appears to be the most popular value for the BST capacitor.

    Thanks,

    Matt

  • 0 in reply to Matt Hein
    TI__Genius 10545 points

    Hello Matt-san,

    Thank you for your swift reply.
    I understood that we can change BST cap voltage rating based on GVDD voltage.
    However, would you provide recommended "effective" Capacitance of BST cap?

    You mentioned 0.1uF is popular in this application.
    However, I worried DC bias performance of BST cap.
    EVM used C1608X7R1H104K080AA and this cap has good DC bias performance that it is 80% effective capacitance @ 1/2 rated voltage.
    However, customer tend to use the MLCC which have worse DC bias performance than EVM's cap. For example, they have used the cap which have less than 40% effective capacitance @ 1/2 rated voltage.
    Therefore I requested to provide your recommended capacitance as effective capacitance.


    Best regards,
    Kazuki Kuramochi

  • 0 in reply to Kazuki Kuramochi
    TI__Mastermind 21805 points

    Hello Kuramochi,

    I would recommend at least 33nF effective capacitance (based on recommended capacitor range from the datasheet).

    Thanks,

    Matt

  • 0 in reply to Matt Hein
    TI__Genius 10545 points

    Hell Matt,

    I appreciate you swift reply.
    I understand.

    Best regards,
    Kazuki Kuramochi