1
\$\begingroup\$

I am searching an suitable input and output capacitor (for example MLCC) for following Low Dropout Regulator (LDO): Analog ADP151.

The datasheet says that the ESR of the caps should be between 0.001 and 0.2 Ω.

I know the ESR vs. Frequency graphs on the capacitor manufacturer pages, but at which frequency do I have to look to find a suitable capacitor?

\$\endgroup\$
4
  • 3
    \$\begingroup\$ Possible duplicate of Choosing capacitors for a linear voltage regulator \$\endgroup\$
    – Artūras Jonkus
    Commented Jan 12, 2017 at 21:02
  • 1
    \$\begingroup\$ @ArtūrasJonkus - I don't see anything on that page that answers OP's question. They have similar titles yes, but they are very different questions if you bother to read them. \$\endgroup\$
    – I. Wolfe
    Commented Jan 12, 2017 at 21:10
  • \$\begingroup\$ They are similar - person wants to choose capacitor. Only this time he thinks that ESR is kind of crucial. Anyway, if people don't think I'm right they won't flag it \$\endgroup\$
    – Artūras Jonkus
    Commented Jan 12, 2017 at 21:20
  • \$\begingroup\$ When ESR is too low, the linear amp has too low phase margin with a capacitive load. When ESR is too high step loads causes excessive ripple due to insufficient gain to correct voltage error at max slew rate. <1mohm is possible but hard to achieve except in metal film.(plastic). but they are $$$ . So 1uF X7R or X5R as stated (in spec) is ideal \$\endgroup\$
    – D.A.S.
    Commented Jan 12, 2017 at 22:35

2 Answers 2

Reset to default
5
\$\begingroup\$

Specification says, p.12

"A minimum of 1 µF capacitance with an ESR of 1 Ω or less is recommended to ensure the stability of the ADP151"

and

"Any good quality ceramic capacitor can be used with the ADP151, as long as it meets the minimum capacitance and maximum ESR requirements"

You will be very-very hard pressed to find a 1uF small ceramic cap with 1mOhm ESR, so you can safely ignore the contradictory statements from ADP151 specifications.

So the answer is: any ceramic cap will do the job, just as spec said.

ADDENDUM: There is no universal answer to a question at which frequency of the complex ESR(f) dependence one should look. The ESR itself doesn't define whether the LDO will be stable or not, the entire transfer function of feedback loop versus gain needs to be simulated to get proper component. As a simple answer one should look at the frequency where the LDO feedback amplifier starts to rotate signal phase substantially. The PSRR (Power Supply Rejection Ratio) chart might serve as a good indication where this area is, where the chart has some singularity. For ADP151 this area seems to be in 1-2 MHz area.

\$\endgroup\$
5
  • \$\begingroup\$ beaten to the post, +1 for agility \$\endgroup\$
    – Neil_UK
    Commented Jan 12, 2017 at 21:39
  • \$\begingroup\$ Any 1uF ceramic cap except Y5V and Z5U !! and not electrolytic unless ultra low ESR type and definitely not plastic. (too low ESR) \$\endgroup\$
    – D.A.S.
    Commented Jan 12, 2017 at 22:28
  • \$\begingroup\$ And how about the max. rating of 0.2 Ω? Is this a problem with a ceramic X7R or X5R? Can I find those values in the datasheets of the cap or somewhere else? \$\endgroup\$
    – I0BitMore
    Commented Jan 14, 2017 at 10:04
  • \$\begingroup\$ @I0BitMore: Yes, every reputable manufacturer (Murata, Kemet, TDK, Yageo, Panasonic, Vishay, etc.) do have ESR data charts for each capacitor over a range of frequencies. Example: psearch.en.murata.com/capacitor/product/… More data can be found here, mouser.com/ds/2/281/c02e-2905.pdf \$\endgroup\$
    – Ale..chenski
    Commented Jan 14, 2017 at 17:04
  • \$\begingroup\$ I wonder why this answer has been accepted. The original question was "I know the ESR vs. Frequency graphs on the capacitor manufacturer pages, but at which frequency do I have to look to find a suitable capacitor ?" and this question has not been answered in my opinion. For example looking at this spec sheet of a 1206 1uF capacitor, there's an ESR-vs-frequency graph. ESR ranges from 20 Ohm (@0.1Hz) to approx. 10 mOhm (@10kHz). What is the ESR for this capacitor? passivecomponent.com/wp-content/uploads/ecd/… \$\endgroup\$
    – Daniel K.
    Commented Feb 11, 2020 at 9:20
1
\$\begingroup\$

ADP151 is an excellent regulator. Low noise, high PSRR...

I did transient response and impedance tests to check for output caps:

  • 1µF MLCC X7R will work fine, like datasheet says.
  • 10µF MLCC does offer improved transient response at minimal extra cost.
  • Going higher is only necessary for paranoid audiophiles.

This regulator isn't picky. FYI it has about 0.1 ohm output impedance and it begins to rise above 10kHz.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.