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LMZ21701 - Stable with >200 µF output Capacitor?

Markus Rau
Prodigy 60 points
Other Parts Discussed in Thread: LMZ21701, LMZ22005

Hello,


during a redesign we replaced a LMZ22005 with a LMZ21701 module. We currently use an 270 µF (8 mOhm ESR) elektrolytic Cap in parallel with an 470 nF ceramic output Capacitor, an 2.2 µH Inductor in series, and another pair 270 µF and 470 nF caps. We use the module to create a 5V rail from a 12V supply for several analog chips, so good output filtering is a requirement.

The capacitor values are above the datasheets recommended output capacitor range of 10 µF - 200 µF. Can the excessive output capacitors cause instability?

Regards,

Markus Rau

  • 0
    TI__Guru 57555 points

    It is possible that large capacitance may cause instability.  Also, there may be a problem with the device starting-up correctly.

    We suggest that you carefully test this application before committing it to production. 

    Of course TI can not condone violating the data sheet recommendations.

  • 0 in reply to Frank De Stasi
    Prodigy 60 points

    Thanks for the feedback. We will redisign the capacitors to ensure we stay within the recommended capacitance range.


    Can a C-L-C pi filter for filtering the output ripple negatively affect the power module? Assume the capacitor before the inductor is larger than the minimum required capacitance value, and the total capacitance is lower than the maximum value.

  • 0 in reply to Markus Rau
    TI__Guru 57555 points

    We have not tested this device with an output filter.  However it should work.  Be sure to close the feedback loop before the filter.