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LMZ23605 Cin 150uF Cap?

Paul Rudloff
Prodigy 10 points
Other Parts Discussed in Thread: LMZ23605

The datasheet of the LMZ23605 stated on page 17:

"Additional bulk capacitance with higher ESR may be required to damp any resonant effects of the input
capacitance and parasitic inductance of the incoming supply lines. The LMZ23605 typical applications schematic
and evaluation board include a 150 μF 50V aluminum capacitor for this function. There are many situations
where this capacitor is not necessary."

Is this 150uF cap really necessary? Can you list some applications where this will be required? I may have a height restriction for the recommended Aluminum capacitor. If this required, is there an alternate capacitor type that will perform similar to the 150uF aluminum?

Thanks.

  • 0
    TI__Genius 10095 points

    Whether or not the higher ESR capacitor is necessary or not depends on your system.

    A buck converter has a negative input impedance.  When you have an inductance (your trace wires if you aren't using a filter) and capacitance ( your input capacitors) and a negative impedance (your buck converter) you create an oscillator and the stability of your buck converter is affected. 

    To damp the oscillations you place a capacitor with high esr. in parallel with your input capacitors.

    The capacitor does not need to be electrolytic it can be another chemistry as long as the capacitance and esr are in the right values.  Or it can be ceramic with the resistance added externally.

    Take a look at Simple Success with conducted EMI page 6 for a description and calculation for this damping capacitor.

    www.ti.com/lit/an/snva489c/snva489c.pdf

    ‎
    Regards,
    Marc