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TPS63020: Maximum allowable ESR for CIN and COUT?

Jonathan Bender
Expert 7895 points
Part Number: TPS63020

Hello,

I have a TPS63020 design (VIN: 3V to 3.6V, VOUT: 3.3V, IOUT(max): 2A) originally using ceramic CIN and COUT (68 uF each). Due to some availability issues, I am forced to consider choosing a different capacitor. Most of the availability in the case size and capacitance I need is tantalum or other chemistries with higher ESR (70 mohm+). Based on a failed Webench attempt and this post, it looks like 70 mohm+ ESR CIN/COUT won't work at all.

Is there a maximum usable ESR for TPS63020 above what you can typically expect for a MLCC (5 mohm or less)?

I also have a 25 mV ripple pk-pk max specification, so past a certain ESR, that might be the limiting factor, not the converter. Please also let me know if there is a different converter that has a higher allowable CIN/COUT ESR to meet my design requirements.

Thanks,

Jonathan

  • 0
    TI__Expert 7925 points
    Hi Jonathan,

    All our buck-boost converters were designed to be used with low ESR ceramic capacitors, but there is no clear maximum ESR value. Otherwise you can affect the converter stability, increase the ripple and decrease the efficiency. In your case the device works in buck-boost mode, so both input and output capacitor are critical. Do you expect large load transients, or is this 25 mV limit just for the switching ripple?

    It might be possible to split the capacitor into two parts - one ceramic and one tantalum, but this is not yet tested. We are aware of the worldwide shortage of ceramic capacitors and are looking into it.

    Best regards,
    Milos
  • 0 in reply to Milos
    TI__Expert 7895 points

    Hi Milos,

    Thanks for the reply. I don't expect significant load transients, and efficiency isn't too important for me either. 25 mVpp is the DC ripple specification.

    Unfortunately I am limited to a single 1206 on CIN and COUT due to PCBs already being created. On a future board spin, either having multiple small MLCCs to meet the total capacitance requirements, or at least an 0402 MLCC next to the IC plus a larger ESR bulk capacitor, will certainly be considered.The lowest ESR 1206s I could find with some stock are 35-45 mohm, such as these: http://www.vishay.com/docs/40174/t55.pdf.

    I also have some concerns about the potentially higher ESL of non-MLCC CIN/COUT and the subsequent switching voltage stress placed on the IC. I'm going to configure the TPS63020EVM-487 to match my use case and test the ripple, stability, and transient performance across VIN conditions, along with checking the peak voltages at VIN, VOUT, and the SW nodes.

    Please let me know if you're able to share any other insight on non-MLCC effects (stability, ripple, etc) with TPS63020 or similar devices.

    Thanks,

    Jonathan

  • 0 in reply to Jonathan Bender
    TI__Expert 7925 points
    Hi Jonathan,

    If there is room only for one capacitor, we can only recommend the ceramic type.

    We plan to test few devices (TPS63020 among them) with ceramic and tantalum capacitors mixed on the output, but this will take a few weeks. If you plan to use poscaps, keep in mind that they have large leakage current, if that is important. And it would be great if you can share the results of your tests here.

    Thanks,
    Milos