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TPS62813-Q1: Output capacitor

Attila Molnár
Prodigy 80 points
Part Number: TPS62813-Q1
Other Parts Discussed in Thread: TPS62810EVM-015

Tool/software:

Hi TI Expert,

I have a question regarding the output capacitor for the TPS62813-Q1.
The TPS6281x-Q1 datasheet (https://www.ti.com/lit/ds/slvsdu1j/slvsdu1j.pdf) shows an example schematic (see Section 8.1, page 9) that is very similar to our intended circuit.

The switching frequency is set to 2.25 MHz using an 8.06 kΩ resistor connected to the COMP/FSET pin.
The output capacitor bank comprises two 22 µF, 10 V Murata GCM31CR71A226KE02L capacitors and one 10 µF, 6.3 V Murata GCM188D70J106ME36 capacitor.

Refer to the Murata website for capacitor specifications (https://www.murata.com/en-eu/products/productdetail?partno=GCM31CR71A226KE02%23).

Given that the switching frequency surpasses the capacitor's self-resonant frequency (determined by the equivalent series capacitance, ESR, and ESL), the capacitor's dominant behavior changes from capacitive to inductive.

See the Murata GCM188D70J106ME36 datasheet (https://www.murata.com/en-eu/products/productdetail?partno=GCM188D70J106ME36%23)


The resonance frequency of the GCM188D70J106ME36 is: about 2 MHz
The resonance frequency and switching frequency are roughly the same.

The TPS62810EVM-015 evaluation module (https://www.ti.com/lit/ug/slvubg0a/slvubg0a.pdf) utilizes two 22 µF Murata GCM31CR71A226KE02L capacitors, as previously mentioned.

Since the output capacitors behave inductively at the switching frequency, how do they effectively suppress output ripple? Are these capacitors suitable, or should we select capacitors with a resonant frequency above 10 MHz?

I look forward to your response.

Best regards,
Attila

  • 0
    TI__Mastermind 20630 points

    Hello Attila,

    When you use multiple capacitors in parallal, it somehow flattens the impedance curve so there will be no ESL behavior .

    But yes, if you want to filter out the switching freqeuncy noise them, it is good to choose capacitors with SRF above the switching freqyency of the device.

    Thanks,

    Farheen

  • 0 in reply to Farheen
    Prodigy 80 points

    Hi Farheen,

    Thank you for your quick response.

    Could you please help me calculate the resultant self-resonant frequency of parallel capacitors? I'd like to know how to do this for both multiple identical capacitors and multiple different capacitors.

    Thank you for your assistance,

    Attila

  • 0 in reply to Attila Molnár
    TI__Mastermind 20630 points

    Hello Attila,

    Best way can be to simulate it. I hope this link can help you.

    https://youtu.be/QpWPTolhfEo

    As you capacitor models and ESR and ESL, so make sure to download the full model of capacitor and connect them in parallel and then simulate for impedance.

    Thanks,

    Farheen