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Replacing Tantalum Bypass Capacitors with Multi-Layer Ceramic Devices

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Rick_Hille
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Re: ESR matching is another important consideration when replacing tantalums with MLCCs
Rick_Hille   11/29/2016 3:53:06 PM
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I find I remember design tips like these better when there's a bit of humor associated with them.  Although there's nothing funny about an LDO going unstable due to ESR,  I recall reading an app note or datasheet long ago (can't remember where) where the LDO's ESR stability plot with the widening at one end was described as "the Horn of Doom"  Any readers remember that one, and where?

Rcurl
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Re: ESR matching is another important consideration when replacing tantalums with MLCCs
Rcurl   11/26/2016 7:08:34 AM
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@elizabethsimon: "Most newer parts are fine with low ESR but some of the older parts are still in common use so it pays to check your datahseets."

About a year ago I was having lunch with a semiconductor sales rep and the topic of bypass capacitors came up. I asked why many of the spec sheets for regulators specifically called for tantalums when it looked like ceramics would be more cost effective.  He told me that in most cases it was because the spec sheets were written when tantalums were cheap and plentiful and that ceramics would always be a good alternative.  Based on the information that has come to light in this thread it's obvious his advice was wrong!   

Rcurl
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Re: Some MLCC App Note Examples
Rcurl   11/26/2016 6:58:29 AM
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Thanks, 2Torr for those links.  There's some really valuable information there. I downloaded all of those app notes (the one from your other post too) and will keep local copies in my "Interesting documents" file.  AN-1482 was particularly informative.  

I learned some important stuff here.

jlwayt
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MLCC cap issues
jlwayt   11/25/2016 7:49:02 PM
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Another issue with large value MLCC's is leakage. They can tend to leak internally due to porosity through their dielectric ( pinholes formed during the screenprinting process ) and internal cracks and defects. This may not be noticeable with line-powered circuits but battery-operated low power circuits will die prematurely and finding the root cause can be a real bear. For battery-powered circuits I still prefer tantalum for larger value caps.

2Torr
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Capacitors are key to voltage regulator design
2Torr   11/23/2016 6:48:24 PM
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Here's another good overview App Note:

Engineers note: Capacitors are key to voltage regulator design

http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=snoa842

 

2Torr
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Some MLCC App Note Examples
2Torr   11/23/2016 6:16:06 PM
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I agree with elizabethsimon. It's always good to follow the datasheet recommendations. Most of the stability concerns are with using ultra-low ESR MLCCs, but even regular MLCCs with low ESR can cause issues.

Here's a link to a TI App Note that details stability concerns with LDO regulators using output MLCCs:

AN-1482 LDO Regulator Stability Using Ceramic Output
Capacitors

http://www.ti.com/lit/an/snva167a/snva167a.pdf

 

And for switching regulators, an App Note from Rohm may be helpful:

The Important Points of Multi-layer Ceramic
Capacitor Used in Buck Converter circuit

http://rohmfs.rohm.com/en/products/databook/applinote/ic/power/switching_regulator/cera_cap_appli-e.pdf

 

and for MLCCs used on the input of DC-DC converters:

Ceramic Input Capacitors Can Cause Overvoltage Transients

http://www.linear.com/docs/24956

 

 

elizabethsimon
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Re: ESR matching is another important consideration when replacing tantalums with MLCCs
elizabethsimon   11/23/2016 5:18:06 PM
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Low ESR is generally not a problem for linear regulators but some of the older switching regulators rely on the capacitor ESR to damp out the ringing that might occur between the inductor and capacitor. Also some early LDOs might have a problem as well. Most newer parts are fine with low ESR but some of the older parts are still in common use so it pays to check your datahseets.

 

Rcurl
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Re: ESR matching is another important consideration when replacing tantalums with MLCCs
Rcurl   11/23/2016 4:18:37 PM
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@2Torr: "Matching the ESRs can also be very important, depending on the application circuit."

That's very interesting. I definitely need to dig deeper into this. I just went back and checked the several types of linear regulators we use and they generally say something like "Make sure to use a low ESR capacitor placed close to the input and to the output using the shortest trace possible".  None of them say anything about the ESR being too low- but since you've got me thinking about it I think I do remember an article that cautioned against using ceramic caps for a specific device because it could cause overshoot and ringing.

At the time I took that with a grain of salt, but now I need to go back and try to find that information. Cany you point me to any information about this? 

   

Rcurl
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Re: Conflict minerals
Rcurl   11/23/2016 3:10:19 PM
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Hi Ed- Thanks for the Conflict-free sourcing link.  Since we no longer use tantalum capacitors that's one less thing to worry with.  I'll take a closer look at that website to see if any of our other components might be affected.

2Torr
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ESR matching is another important consideration when replacing tantalums with MLCCs
2Torr   11/23/2016 2:31:25 PM
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When replacing tantalum caps with MLCCs, one should consider more than just matching the capacitor value. Matching the ESRs can also be very important, depending on the application circuit. For example, many voltage regulators require output caps with a finite ESR to ensure stability; some even forbit the use of MLCCs. MLCCs have much lower ESRs than tantalums, so when using MLCCs, additional series resistors may be required for stability.

Also, power supplies that use MLCCs for decoupling are much more prone to resonances and anti-resonances (for multiple, different values caps in parallel) that can cause detrimental power supply oscillations. Again, careful consideration of the power supply distribution network (PDN) impedance is required for proper decoupling capacitor selection.


On a side note, if a MLCC ever drops on the floor or table, it's best to simply throw it out. The mechancial stress of impact can cause MLCC microfractures and internal stresses that will only get worse during high-temp reflow soldering and also make the MLCCs more sensitive to future stresses, decreasing their reliability.

 

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