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TPS659122YFFT INPUT/OUTPUT CAPACITANCE

Eli az
Intellectual 730 points

Hello,

I am currently designing a power supply module based on the TPS65922YFFT. The datasheet of the TPS includes the following recommended output capacitors table for the DC-DC converters:

47 µF / 6.3 V 0805 Murata GRM21BR60J476ME15 Ceramic X5R                                                                                                             22 µF / 6.3 V 0805 Murata GRM21BR60J226M Ceramic X5R                                                                                                                   10 µF / 10 V 0603 Murata GRM188R61A106ME69 Ceramic X5R                                                                                                           4.7 µF / 6.3 V 0603 Murata GRM188R60J475KE19 Ceramic X5R                                                                                                           4.7 µF / 6.3 V 0402 Murata GRM155R60J475ME87 Ceramic X5R

I noticed that the self-resonance frequency of the above capacitors is ranging from 800KHz to 4MHz. The switching frequency of the converters is around 3MHz. From a quick survey I have conducted, only typical ceramic capacitors with capacitance between 1uF-2.2uF have a self-resonance frequency of above 4Mhz and therefore still act as a capacitors in this frequency range.

My question is: Does the self-resonance frequency is an important parameter when coming to choose an input/output capacitors for the DC-DC converters of the TPS and why?

I will appreciate any help on this matter.

Thanks,

Eli

  • 0
    TI__Genius 10885 points
    Hi Eli,

    It is true that the self-resonance of the capacitors can impact the stability, so choosing a larger self-resonance would be recommended if your design allows for this.

    However, due to the internal loop filtering of these regulators, the self-resonant frequencies of the capacitors do not have to directly exceed the switching frequencies, evident by the capacitors tested within the datasheet, so there is some room for flexibility on this particular parameter.

    A good rule of thumb is to have the self-resonance larger than .5*Fsw (half the switching frequency). Additionally, the self-resonance can be increased by placing multiple capacitors in parallel to reduce their total inductance.
  • 0 in reply to Richard Solarski
    Intellectual 730 points
    Thank you very much Richard!