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LM117 output capacitor esr range

xingqiao liu
Prodigy 100 points
Other Parts Discussed in Thread: LM117

Hi,

What is the esr range for output capacitor of LM117? If a 10uF capacitor is connected  between adjust pin and ground, what influence would it have on stability of LM117?

best regards

liuxingqiao

  • 0
    TI__Intellectual 2010 points
    Xingqiao
    The interaction between the ESR of the load capacitor and the performance of your linear regulator is a matter of analyzing the stability of the feedback network used to control the regulator. LDO use high degree of feedback to control their loop. The interaction between the ESR of the load capacitor and the performance of your linear regulator is a matter of analyzing the stability of the feedback network used to control the regulator. You need to keep the phase response of your load within the range by appropriate ESR so the internal amplifier won't oscillate.

    The ESR of the output capacitor puts a zero in the loop gain which can be used to reduce excess negative phase shift. The frequency where the zero occurs is directly related to the value of the ESR and amount of output capacitance:

    FZERO = 1/(2π x COUT x ESR)

    For LM117 A 1 μF solid tantalum (or 25 μF aluminum electrolytic) on the output swamps this effect and insures stability. Any increase of the load capacitance larger than 10 μF will merely improve the loop stability and output impedance.

    Please refer to this document for more detail on loop stability with external capacitors.
    www.ti.com/.../snva020b.pdf