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Capacittor of LP3876 and LP3882

Takahiro Nishizawa
Genius 5575 points
Other Parts Discussed in Thread: LP3882, LP3876

I have questions about Cout ESR in LP3876 and LP3882.

We try to use ceramic capacitor.

In datasheet of LP3876 p.10 Figure.22,

In datasheet of LP3882 p.8 Figure.20,

We want to know about STABLE REGION.

Must we follow the STABLE REGION when we use ceramic capacitor?

Is there anything wrong if we don't follow the STABLE REGION?

In datasheet of LP3876 p.10,

: this is not an issue with ceramics, as their ESR is extremely low.

Do not you have to follow STABLE REGION if we use ceramic capacitor about LP3876 and LP3882?

Best regards,

Takahiro Nishizawa

  • 0
    TI__Guru 55940 points

    Hi Takahiro,

    The regulators that you selected are older devices before ceramic capacitors became common in industry.  For stable operation, the output capacitor must provide the ESR and capacitance as stated in the datasheet.  If you wish to use ceramic capacitors, you will still need to meet the ESR requirements.  This can be accomplished by placing a series resistor with the capacitor.  Per the ESR curves you referenced, a 1 Ohm resistor in series with a ceramic capacitor should be adequate.

    You can learn more about how ESR impacts a linear regulator in the following:

    The statement that you reference about ceramic capacitors is specifically discussing how ESR changes over temperature.  

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Genius 5575 points
    Thank you for your answer!


    We have questions about ESR STABLE REGION.

    Please tell us the requency range about ESR STABLE REGION in LP3876 datasheet Figure 22.

    Please tell us the requency range about ESR STABLE REGION in LP3882 datasheet Figure 20.



    Best regards,


    Takahiro Nishizawa
  • 0 in reply to Takahiro Nishizawa
    TI__Guru 55940 points
    Hi Takahiro,

    You should use the ESR at the resonant frequency for the capacitor. This will be the minimum point on the impedance curve of the capacitor.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Genius 5575 points
    Thank you for your answer!

    You should use the ESR at the resonant frequency for the capacitor. This will be the minimum point on the impedance curve of the capacitor.

    STABLE REGION is the frequency lower than resonant frequency about using capacitor.
    Is this understanding correct?

    Best regards,

    Takahiro Nishizawa
  • 0 in reply to Takahiro Nishizawa
    TI__Guru 55940 points

    Hi Takahiro,

    The ESR requirement for a linear regulator is for the ESR resonant frequency of the selected capacitor.  As long as the ESR at the resonant frequency of the capacitor is within the stable region of the regulator, the ESR requirements are met.  For example, if the ESR of your capacitor was 1 Ohm at the resonant frequency, you would satisfy the ESR requirements of the regulator across all load conditions.

    Very Respectfully,

    Ryan