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LP2985 Capacitor Selection

Don C
Prodigy 20 points
Other Parts Discussed in Thread: LP2985

Hi

I am trying to set up a design with the LP2985 LDO regulator and I'm having trouble selecting an output cap.  The datasheet indicates that the range of allowable ESR is roughly 6 to 800mohms.  I'm looking at small ceramic caps, and their ESR seems to drop by an order of magnitude per decade.  Over what frequency range should I worry about the ESR of my cap?

 

Thanks

  • 0
    TI__Guru**** 153186 points

    Hello Don

     

    I'm looking at small ceramic caps, and their ESR seems to drop by an order of magnitude per decade.

    Is that “per decade” of frequency? The capacitive reactance drops 10:1 per decade, but I’m not aware of ESR changing this much.

     

    Over what frequency range should I worry about the ESR of my cap?

    Frequencies near 100kHz is a good place to look for ESR value.

    With low ESR ceramic caps, the inductance can be a problem; the resonant frequency must be above 1 MHz; the higher the better.

     

  • 0 in reply to Ron Michallick
    Prodigy 20 points

    Hi Ron,

    Yes, I was referring to decade of frequency.  Kemet has a tool that will show you pretty plots of a capacitor's properties over temp/frequency/bias voltage.  It is a simulation, but it shows some big changes in ESR, as well as the reactance that you mention.

    Thanks for the info about frequency of interest.

  • 0
    TI__Expert 5640 points

    Typically, capacitor vendors specify the ESR at either 100Hz or 1kHz.  These are good typical ESR values that can be used in choosing a capacitor.  Always be sure to leave margin in the ESR value though.  The cap vendor will publish the worst case ESR value.  The actual measured ESR may be much lower and will vary with temperature. 

    The technically correct answer is the capacitor must meet the minimum ESR up to the cross over frequency of the control loop.  However, the cross over frequency is usually not known exactly.  Most linear regulators will have cross over frequencies below 100kHz so you can use the ESR numbers from this frequency or lower.