0
\$\begingroup\$

In this TI datasheet of TPS62180, it was mentioned that the LC filter combination should be selected from a set of predefined values. I understand this is usually the case for an internally compensated converter. enter image description here

  1. Why are the capacitor values specified in nominal value since effective capacitance is what matters finally? In the feedback loop design, we need to consider effective capacitance at the operating voltage, right?

  2. Why is the manufacturer so specific about x 47uF, as long as effective capacitance is met we can choose any value, right?

\$\endgroup\$
3
  • \$\begingroup\$ I am sure they mean nominal. By and large, capacitors with similar ratings have roughly similar de-ratings also. TI has tools that will allow you to analyze the loop stability with specific part numbers. Maybe check TI webbench. \$\endgroup\$
    – user57037
    Commented Sep 2, 2021 at 3:19
  • \$\begingroup\$ This is because X7R caps have this tolerance with DC voltage decrease +/- initial tolerance so use much higher rated V \$\endgroup\$
    – D.A.S.
    Commented Sep 2, 2021 at 3:20
  • \$\begingroup\$ There is a big difference between 8 * 47uF capacitors, and one 376uF (390uF) capacitor, when you consider ESR. lead inductance, and PCB layout, in fast switching circuits. The datasheet is warning you that the internal design assumes the configuration shown : deviate from that and performance may be affected. \$\endgroup\$
    – user16324
    Commented Sep 2, 2021 at 11:33

1 Answer 1

Reset to default
0
\$\begingroup\$

Not all 47 uF ceramic Caps are the same, yet X series most common in this size of MLCC are +/-20% and drop 60% with rated DC voltage. This is normal.

Some are better.

ESR can also be low , soft termination for crack prevention and damped for low piezo microphonic/speaker effects.

enter image description here enter image description here

Also for C=2x47uF and L = 1uH the SRF series resonant frequency is 16 kHz

  • which rises to 24 kHz if C drops 60%.

  • So you do not want switching frequency ripple in your output

  • final Rs & SRF are both important for damping AC to DC in choosing L, DCR , C and ESR along with RdsOn

  • these affect final Q factor, control-loop step-load stability, ripple power dissipation in the cap. and temp rise, MTBF, & efficiency.

This was not intended as an endorsement that Samsung are best, just an example.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.