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LP2985-N: LP2985 design guidelines & capacitor constraints

Leo Chow
Expert 3120 points
Part Number: LP2985-N
Other Parts Discussed in Thread: LP2985,

Hi,

i have a question with regards to LP2985 recommended design guideline which relates to capacitors to use on my design. I'm now looking into downsizing the passive ceramic capacitor that are designed in surrounding LP2985 family of LDO.

For this family of LDO it requires a specific minimum capacitance at input and output across the loading and temperature. If we are downsizing the capacitor, the total capacitance may drop accordingly. Instead of applying a single 0603/0805 size capacitor that meets the requirement, assuming i'm replacing it with multiple number of capacitors in order to meet the total capacitance, (such as add 2x 0402 parts or 4x 0201 parts) will that impact the LDO performance?

How about for those switching regulator filtering capacitor, usually required about 22uF (LC filtering). Will this cause any stability and radiation issue? What is the recommended approach if i were to shrink down caps?

Regards,

Leo

  • 0
    TI__Guru 55940 points
    Hi Leo,

    While I do not believe this will impact your application, please keep in mind that LP2985-N and LP2985 are different products. LP2985-N was released by National Semiconductor before the acquisition. LP2985 was released by TI to compete with the National device; however, it is a different design and has slightly different performance.

    In general for any linear regulator, the capacitor requirements given within the datasheet for capacitor/capacitance value and ESR should be met. The capacitors are used to compensate the LDO control loop. By paralleling multiple capacitors you will be able to meet the capacitance requirement even by using small capacitor values; however, keep in mind that the ESR requirements must be met as well.

    Ultimately the capacitors are used for stability. Perhaps the quickest and easiest method to check the stability of the regulator under your specific application conditions is to perform load transients while monitoring the output voltage on a scope. The following application report discusses this process:

    www.ti.com/.../slva381b.pdf

    You can learn more about the difference between capacitor and capacitance in chapter 2 of our LDO Basics E-Book:

    www.ti.com/.../slyy151.pdf

    Very Respectfully,
    Ryan