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MCT8316A: Regarding the recommended value of the capacitor, is it necessary to consider the DC bias?

Shinichi Inoue
Guru 20486 points
Part Number: MCT8316A

Dear Specialists,

My customer is considering MCT8316A and has a question.

I would be grateful if you could advise.

---

(1)Recommended values are listed in the data sheet for the capacitance of capacitors such as Cbk and Ccp

The actual value of ceramic capacitors decreases due to DC bias characteristics.

Is it necessary to select a capacitor so that the capacitance value in the datasheet is the recommended value considering the reduction due to the DC bias characteristics?

Or is there no problem if we basically select a capacitor with the recommended value in the data sheet?

What capacitor size should I use?

(2) What kind of problems will occur if the capacitance of Cbk is less than the recommended value (22μF)?

Could you please let me know the minimum capacitance value.

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I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Expert 5577 points

    Hi Shinichi,

    Thanks for your question on the Motor Drives E2E Forum. 

    I will aim to provide an answer on this by the end of the week!

    Best,

    Robert

  • +1 in reply to Robert Wedan
    TI__Expert 5577 points

    Hi Shinichi,
    Thanks for your question on the Motor Drives E2E Forum. 


    1. The recommended values listed in the datasheet like CBK should be followed in order to know the typical and max electrical characteristics of certain features that rely on that capacitor value. An example of this could be IVMS or VBK.
    One can counter the effects of "de-rating" from temperature, DC bias, etc. by ensuring the capacitors are rated for proper voltage ratings (example: CCP = 16V) and proper thermal coefficients (example: X7R/X5R) as specified in the datasheet are followed.
    If engineers pick the correct rating and values as specified in the datasheet, they should have no problems in implementing our device reliably. Another thing customers can check is how our device is implemented in our EVM.


    2. In the specific case of CBK, the capacitor value has an impact on the whole buck regulator circuit and changing the value can actually destabilize the buck regulator circuit and cause output voltage ripples on VBK. This circuit is also fed back into the device through FB_BUCK to power internal circuitry in the device so we recommend always using the datasheet specified values.

    Is the customer using VBK to power any external circuitry? This may also be affected if you were to change the value of CBK.

    Best,

    Robert

  • 0 in reply to Robert Wedan
    Guru 20486 points

    Hi Robert,

    Thank you for your reply.

    For the rating of the ceramic capacitor, the rated voltage and temperature characteristics described in the data sheet should be selected 

    ON the other hand, ceramic capacitors can have different characteristics when their size changes.

    I’ll make the customer choose a capacitor that has the same or better characteristics than the capacitor used in the EVM.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi