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WEBENCH® Tools/LM61460: Input / Output Capacitor Selection

Kazuki Itoh
Genius 10670 points
Part Number: LM61460

Tool/software: WEBENCH® Design Tools

Hi team,

I wonder how to  interpret webench recommended input / output ceramic capacitor value.

When 24V/9V 5.0A simulation is performed, recommended output caps are:

Cap = 47 µF Total Derated Cap = 57 µF VDC = 16 V ESR = 3.04 mΩ Package = 1210 QTY=3

When I try to pick up other capacitor outside of webench for actual design, is it better to use total derated cap value 57uF @ 9V as a reference?

Is this also applicable to input capacitor value?

I'd like to learn how webench nicely picks up input/output cap when the input/output voltage differs from datasheet example.

Regards,

Itoh

  • 0
    TI__Guru 50120 points

    Hi Itoh,

    Can you share the WEBENCH design that you generated for my own reference?

    At higher output voltages, generally you could lower the amount of output caps. The datasheet recommendations for output capacitors are for 3.3Vout and 5Vout on Table 4.

    One thing to note is that at higher output voltage the output capacitors will experience more DC bias effects which lowers the effective output capacitance. This also applies to the input capacitors. Also note that the output capacitors are picked based on load transient requirement. If you expect the application to experience large transients (ie. 500mA - 5A load swings), it is recommended to increase the output capacitance.

    Usually the capacitors are picked to be common available capacitors and have roughly double the voltage rating of the expected DC voltage that will be applied to the capacitor. In the case case for 9Vout condition, the nearest eligible capacitor voltage rating would be 16V and 25V rated caps of which the 16V is picked. 

    Regard,

    Jimmy 

  • 0 in reply to Jimmy Hua
    TI__Genius 10670 points

    Hello Jimmy-san,

    Thank you for your explanation.

    When I want to pick up input / output capacitor with different voltage rating, should it be OK if I pick up the capacitor so that the derated cap value is kept at least webench recommended value and also load transient is OK?

    Regards,

    Itoh

  • 0 in reply to Kazuki Itoh
    TI__Guru 50120 points

    Hi Itoh-san,

    That will be fine if you picked input/output caps such that the derated cap value is close to the WEBENCH recommended value.

    I'd like to point out that the datasheet states "Table 4 and Table 5 have been generated assuming typical derating for 16-V X7R caps.

    If you are using 3.3V or 5V output condition, you can simply refer to Table 4 and Table 5 for the appropriate capacitor values. 

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    TI__Genius 10670 points

    Hello Jimmy-san,

    Thank you for your suggestion.

    My customer wants to learn more about how to pick up 100V input cap instead of 32V input cap suggested from webench.

    He needs to know how much we can rely on the suggested total derated cap value from webnech for this selection as the suggested derated input cap value is as small as 1.6uF.

    Could you please comment on this?

    Best regards,

    Itoh

  • 0 in reply to Kazuki Itoh
    TI__Guru 50120 points

    Hi Itoh-san,

    One thing I'd like to point out is the LM61460EVM also has roughly 10uF of input capacitance which is the recommended minimum 10uF capacitance detailed in the datasheet (Section 9.2.2.5). The EVM is using a total of (2 x 4.7uF + 2 x 2.2uF) of input capacitance with 1210 package and is capable up to 36V input. User's guide link (here).

    You can also use Murata's online SimSurf tool to simulate what the effective capacitance will be after derating.

    For example,

    given a X5R | 10uF | 50V rated | 1206 package capacitor derated at 36Vin will result in the following capacitance:

     

    given a X7S | 10uF | 50V rated | 1210 package capacitor derated at 36Vin will result in the following capacitance

    Clearly the larger package capacitor will have less derating. The choice of 1206 vs 1210 is based on your customer's size constraint and input source. If the input source is already stable going into the converter, you may not need that large of an effective capacitance. However for a more robust design assuming customer is okay with 1210 package, I would suggest picking 1210 size capacitor. 

    I believe Webench is designing with 1206 package and 10uF which would result in a derated capacitance of around 1uF-2uF depending on the specific component. I think this is fine, but again for better robustness, if the customer is okay with 1210 caps, then I'd go with 10uF | 50V+ rated| 1210 capacitor. 

    Regards,

    Jimmy