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WEBENCH® Tools/LMZ21701: what capacitor to use to replace obsolete GRM216R71E332KA01D?

Britton Kerin
Intellectual 310 points
Part Number: LMZ21701

Tool/software: WEBENCH® Design Tools

Web bench produce a design for an LMZ21701-based step-down converter which calls for a Murata GRM216R71E332KA01D 3.3 nF 1.0 mOhm ESR 25V IRMS 0.0 A 0805 (in) X7R capacitor. This part is obsolete and not available at at least some distributors. I'm trying to determine what part to replace it with. I notice that some possible replacements (e.g. Yaego CC0805KRX7R8BB332 and oters in this series) have either lower ESR at the operating frequency (2.537 MHz) or smaller size. Do I just want the lowest ESR possible, or is it important that it has the specified ESR given in the produced design document? Is it any problem to use a smaller replacement? In particular I'm trying to choose between these four Yaego parts: CC0201KRX7R8BB332 CC0402KRX7R8BB332 CC0603KRX7R8BB332 CC0805KRX7R8BB332. These have size 0201, 0402, 0603 and 0804 (in), and decreasing ESR. Will one of these work and which is best? Thanks
  • 0
    Intellectual 310 points
    I'd like to attach the datasheet for the proposed replacement caps and the produced design PDF but I don't see how to do it
  • 0 in reply to Britton Kerin
    Intellectual 310 points
    Since I can't attach the produced design, here are the design parameters: 6-15 Vin 1A 3.8 Vout The external components called for in the produced design are: C3216X6S1V106K160AC Series= X6S Cap= 10.0 uF ESR= 2.229 mOhm VDC= 35.0 V IRMS= 4.8593 A 3 C0805C106K8PACTU Series= X5R Cap= 10.0 uF ESR= 3.0 mOhm VDC= 10.0 V IRMS= 11.43 A 3 GRM216R71E332KA01D Series= X7R Cap= 3.3 nF ESR= 1.0 mOhm VDC= 25.0 V IRMS= 0.0 A 1 RC0603FR-07180KL Series= ? Res= 180.0 kOhm Power= 100.0 mW Tolerance= 1.0% 1 RC0201FR-07681KL Series= ? Res= 681.0 kOhm Power= 50.0 mW Tolerance= 1.0% 1 LMZ21701SILR Switcher 1
  • 0
    TI__Intellectual 1125 points

    I assume for this 3300pF that you're referring to the SS (soft start) capacitor.

    In this case, ESR is not of prime importance, just make sure to select a X7R or X5R capacitor, and high enough voltage rating to limit the effect of DC voltage bias on the effective capacitance.

    Regards

    Jean