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WEBENCH® Tools/LMZM33603: Cout for LMZM33603 looks wrong

Stephen Fenwick
Intellectual 260 points
Part Number: LMZM33603

Tool/software: WEBENCH® Design Tools

For a design with the LMZM33603 in Web Bench, 24V in, 12V out, 2A, the tool is recommending the TDK C2012X5R1V226M125AC, claiming that the total derated capacitance for two is about 29uF. See screenshot

However, when I put the same part into TDK's SEAT tool, it tells me that at 12V bias, that part has only about 3uF:

What am I missing here? Is this an error in your Webbench data about this part?

  • 0
    TI__Guru 50120 points

    Hi Stephen,

    Thank you for bringing this up. The derating for that suggested capacitor at 12Vout does not satisfy the recommended output capacitance based on the datasheet. I will voice this to the Webench modeling team.

    I'd recommend using the 2 x 22uF capacitor (GRT32ER61E226KE13) like the LMZM33603EVM. The larger size allows for less derating effects. This way with the effective output capacitance will be well above the recommended output capacitance for 12Vout based on Table 5

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    Intellectual 260 points

    Hmm. That puts the effective capacitance right at the bottom of the range. Is that ok?

  • 0 in reply to Stephen Fenwick
    TI__Guru 50120 points
    I don't think I understand the question. The recommended output capacitance for a 12V output is 15uF. If you are using capacitor that I suggested, you will easily have 22uF which is still within the ballpark range of the recommended output capacitance. If you are referring to the effective capacitance curve being near the bottom of the range, then yes it is but that wouldn't affect the operation of the capacitor.
  • 0 in reply to Jimmy Hua
    Intellectual 260 points

    In the LMZM33603 spec, table 1, for Vout of 12V, the required output cap range is given as 22uF to 80uF, and there's a footnote about requiring that to be taken after accounting for DC bias derating.

  • 0 in reply to Stephen Fenwick
    TI__Guru 50120 points

    Hi Stephen,

    I apologize for the confusion since I was referencing the wrong datasheet. Like you've mentioned the 22uF would be on the lower end of the range based on the LMZM33603 datasheet Table 1. You can increase it to 3 x 22uF to have an effective capacitance of 33uF at 12Vout.  Additionally Table 6 in the datasheet provides guidance on the recommended output capacitors that you can use as well. You would need to account for derating as you've mentioned when picking from this list of capacitor.

    Regards,

    Jimmy