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LM1117: Output cap, ESR and frequency range

scott anderson
Prodigy 10 points
Part Number: LM1117

Hi,

when using the LM1117DTX-3.3 the datasheet specifies an output capacitor of ESR 0.3ohm to 22ohm. 

The ESR of capacitors change significantly with frequency.

Does this ESR apply to any specific frequency range?

The reasoning is

I see some electrolytic capacitors specifying an ESR at 100khz around 0.3 ohm, when i look at the ESR at 120hz using tan theta calculations the ESR shoots up to 210 ohm

Looking specifically, i am currently using the following cap and im not sure if it meets the ESR expectations.

RNE1C101MDS1PX

  • 0
    TI__Guru 50090 points

    Hi Scott,

    When we model the ESR of capacitors for stability, we use the values provided in the datasheet so the 0.3Ohm is the minimum specified at 100khz. However, as shown below:

    So the fact that you are using the recommended 100uF capacitor,  this relaxes the restriction a bit.  To guarantee stability over all operating conditions when using low ESR capacitors, a common recommendation is to add an external series resistor to make sure that the minimum ESR condition is met. 

    Note, we have released a next-generation improvement on a general-purpose wide VIN regulator in the TLV767. This is stable with any output capacitor dielectric and is a better performing device. 

    I hope this answers your question.