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Questions about using ceramic capacitors at LT8708 input & output circuit

Se-woong
on Jan 30, 2020

Hello,

My customer wants to use LT8708 & LT8708-1 in their product which is a fuel cell Dron application.

But as the size issue of PCB, they wnat to use ceramic capacitors at input & output of LT8708 instead of recommended tantal capaciotrs in your EVM design.

1) Input Circuit in EVM

--> Capacity of input capacitance:  270uF *3 ea

2) Ouput Circuit in EVM

--> Capacity of output capacitances : 220uF * 4ea

Q1) Would you recommend the quantity of ceramic capacitors ?

       1-1)Recommened  No. of Ceramic Capaitor (MLCC, 4.7uF) at input circuit

       1-2) Recommened  No. of Ceramic Capaitor (MLCC, 4.7uF) at output circuit

Q2) Are there any issues when  they change designed tantal capacitors to ceramic capacitors?

      If you have any concerns about this request, please advise me.

Regards,

Se-woong.

  • 0 on Feb 6, 2020 5:41 AM over 1 year ago

    Hello,

    Would you let me know your status about this request?

    My customer is still waiting your advices in a week.

    Regards,

    Se-woong

  • 0
    •  Analog Employees 
    on Feb 11, 2020 8:02 AM over 1 year ago in reply to Se-woong

    Hi Se-woong, 

    If you are going to replace the input and the output caps of DC2719A, then the cap must be able to withstand the IRMS rating that you will get from the equations given in page 37 and 38 of the datasheet. Using the input and output parameters of DC2719A, we get an IRMS rating of 20A. Since there are 9 caps in the input, then each cap must be able to withstand an RMS rating of 2.222A. The 3*270uF input capacitors were selected to withstand this IRMS rating. 

    Going back to your questions,

    (1) Keep in mind that the value of the bulk input and output capacitor depends on the allowable ripple voltage of your intended application. You can choose a value for the input capacitor by using the equation in page 38 for the ripple voltage. Also take note of the Irms rating of the capacitor that you are going to use. You can put several caps in parallel to meet this rating.

    (2) There will be no issue with the design if you use either a tantalum or a ceramic capacitor. Normally, we use the former capacitor to achieve a high voltage, large value capacitor. This is usually not achievable with ceramic capacitors. If you use ceramic caps in the input of DC2719, then you would need 8pcs of 100uF to replace the 3pcs of 270uF. It defeated the purpose of a smaller PCB size using the tantalum capacitor. 

    Regards,