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How to choose input and output capacitor for non-isolated DC/DC converter?

Jerry Tang
Intellectual 440 points
Other Parts Discussed in Thread: TPS54160

Hi,

When using a non-isolated DC/DC converter,input and output capacitor are very important for the device operate correctly.

In tihs case,what parameters must we care?How to get the capacitor value in certain condition?

Thanks!

  • 0
    TI__Guru**** 191445 points
    It varies for different devices. Usually the individual datasheets will have specific recommendations. In most cases you will need some ceramic input bypass capacitance directly across the VIN and GND pins as close to the IC as possible. Usually in the range of 2.2uF to 10uF minimum, but once again it depends on the specific IC. For output capacitor, it is to specifically related to the actual IC and your intended application. If you can tell me which IC and your application I can make a suggestion.
  • 0 in reply to JohnTucker
    Intellectual 440 points

    Hi John,

    Thanks for your suggestion.I want to use TPS54160 for the design.The input is 36V,the expected output is 5V/1A.

    How to choose the input and output capacitor correctly?Would you please offer more details for me?

  • 0 in reply to Jerry Tang
    TI__Guru**** 191445 points

    For input capacitance see the datasheet section 9.2.2.5.  TPS54160 requires at least 3 uF effective input bypass capacitance.  it should be X5R or X7R.  You have to take DC bias into account.  2 x 2.2 uF or 1 x 4.7 should be sufficient if the voltage rating is high enough.  If your circuit is located a long way from your input voltage source or if a long cable is used , you may need some bulk electrolytic as well.  The value is not critical, 47 or 100 uF is what I typically use.

    For the output, see section 9.2.2.3.  The transient requirement is usually the most stringent.  I typically do not design with less than 22 uF of output capacitance.  I am usually more conservative and use 2 x 22 uF or more.  The reference circuit uses 47 uF,  That is probably a good choice for many applications.

    Alternately, you can use the online Webench tool to create your design.  Webench tends to pick lower values of output capacitance than I prefer, but you can always change that within Webench.

  • 0 in reply to JohnTucker
    Intellectual 440 points

    Hi John,

    If no input capacitor,what will happen when using the DC/DC converter?

  • 0 in reply to Jerry Tang
    TI__Guru**** 191445 points
    Most likely it will not work at all or have intermittent operation. I have never actually tried it. I have seen many instances where the IC would not start up due to the input capacitor too far from the IC (maybe 1 cm is too far).
  • 0 in reply to Jerry Tang
    TI__Mastermind 40080 points

    Terry,

    For the Boost converter, input capacitance is not so critical, but the  output capacictance is very critical, put it as near to the IC as possible.

    For the BUCK converter, input capacitor is very very critical, put it as near to the IC's Vin pin  as possible, output cap is not so critical.

    But both input and output need to put certain ammount of capacitors based on the output power and output ripple requirement. Or there will be stability issue or even can't startup.

    BR,Helen