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Original question:

LM1085 - STABILITY with load/decoupling ceramic capacitor

LM1085: Recommended capacitance of ceramic capacitor and minimum required ESR value

Y.Ottey
Mastermind 6145 points
Part Number: LM1085

Dear all

I woud like to ask about output capacitor of LM1085.

After reviewing the LM1085 data sheet, the data sheet recommends that the output capacitor be 10uF tantalum or 50uF aluminum electrolytic.
After confirming the following application notes, we are currently considering connecting an ESR resistor in series with a ceramic capacitor as the output capacitor.
Please answer the following questions:

http://www.ti.com/lit/an/slva214a/slva214a.pdf


1． Is it recommended that the recommended capacitance when using a ceramic capacitor is 10μF or more?

2. From the formula (1 / ESR × C) in the following post, the minimum ESR required was confirmed to be at least 0.2Ω when the capacitance is 10μF.
Is there no problem with the above method?

https://e2e.ti.com/support/power-management/f/196/t/862307?tisearch=e2e-sitesearch&keymatch=LM1085
If not, what is the minimum required ESR value?
(In addition, please tell me the ESR resistance required when using a low ESR ceramic capacitor.)

Best Regards,

Y.Ottey

  • 0
    TI__Mastermind 34345 points

    Hello Y.Ottey,

    The main concern is the added zero for stability.
    Typically you want excellent load transient response, which at high frequencies is dominated by the capacitor (before the LDO feedback loop can react).
    In that sense you also want low ESR in the output capacitor.

    The datasheet suggests the ESR zero must be placed around 500 kHz.
    Increasing the effective output capacitance would allow for reduced ESR as you can maintain the same zero frequency.

    It is recommended to use 10uF minimum to maintain transient performance.
    The post you linked to calculated radians, not frequency, unfortunately.
    This is good news however as the minimum ESR actually decreases.
    To calculate the ESR zero frequency: Freq = 1 / [ (2*pi) * ESR * C ]

    50uF at 500 kHz equates to 6.3 miliohms of ESR.

    22uF at 500 kHz equates to 14.5 miliohms of ESR.

    10uF at 500 kHz equates to 32 miliohms of ESR.

    These values are too high for ceramic capacitors, so adding a small series resistance is a good idea if you need to use a ceramic capacitor.

    The following App Note was written around the time this LDO was released, and may be useful to you.
    http://www.ti.com/lit/an/snva020b/snva020b.pdf

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Mastermind 6145 points

    Dear Stephen

    Thank you for your reply.

    In other words, if a ceramic capacitor of 10μF is used as the output capacitor for the LM1085 (fixed output type), is it acceptable to recognize that the operation can be stabilized by connecting a resistance value of 32mΩ in combination with the ESR of the capacitor in series? 

    Regards,

    Y.Ottey

  • 0 in reply to Y.Ottey
    TI__Mastermind 34345 points

    Hi Y.Ottey,

    Yes this is correct.

    Thanks,

    - Stephen