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Are electrolytic caps necessary?

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Pommie

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I'm currently designing a project using smt parts. Looking for 1206 size capacitors produces 22uF 10V ceramic capacitors. Can these be used instead of the normally called for electrolytics? Will they also be suitable as bypass capacitors or is there something inherently better about the 100nF ceramics normally used? Having not come across these small high value ceramics before I'm hoping someone here has experience of them.

Thanks,

Mike.
 

JLNY

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I suppose that it depends on the application, but certainly there is no hard rule stating that one material has to be used over another as long as the values, voltage ratings, etc. are suitable for the application and won't be a significantly higher cost.

In some ways ceramic capacitors may even have advantages over electrolytics because they are non-polar, and ceramic caps often have low ESR at high frequency. I think that 100nF ceramic caps are often used in conjunction with higher-value electrolytics for their low ESR at RF frequencies where a high-value electrolytic would be too inductive at RF to provide good suppression.

If your only reason for using ceramic caps is to use SMD components, though, there are certainly SMD electrolytic caps:

http://au.element14.com/c/passive-c...apacitors?capacitance=22uf&voltage-rating=10v
 
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jpanhalt

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Here are two references I use:
TI
http://www.ti.com/lit/an/sloa069/sloa069.pdf
See particularly pages: 3 and 8
Good reading like many TI application notes, IMHO.

Murata
http://www.murata.com/~/media/webre...atalog/products/emc/emifil/c39e.ashx?la=en-us
Heavy reading, but some ideas, e.g., use a variety of different capacitances.

In brief, electrolytic capacitors turn into inductors at a lower frequency than do MLCC capacitors. If you are working at low frequency with a switching power supply, electrolytic capacitors may give higher capacitance in a smaller package. There are few disadvantages (other than size) to ceramic capacitors. Less important, there may be an advantage to using decoupling capacitors of different values around the board instead of slavishly using just 100 nF versions.

John
 

crutschow

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Typically a 100nF ceramic cap is used across power and ground for each IC on a PCB to decouple the high frequency noise, with one or more large(10-100μF) electrolytic caps on each board to suppress the lower frequency ripple and noise.
 

ronsimpson

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Can these be used instead of the normally called for electrolytics?
Some times I need to build a board that is only 0.25 inches thick. Ele. Caps are too tall.

Many times I build switching power supplies running at 1mhz. I need capacitors very close to the IC+MOSFETs. At 1 and 2mhz the lead/trace length to get to a ele. cap is too much. Most of the ceramic capacitors are good at high current. (can't use "tant")
 

Pommie

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Thanks all for the info. The circuit in question is a micro USB to a Lipo charger followed by a stepup regulator to 5V. Here's the schematic,charger.png
The battery connects to the central connector. Looking through the datasheets suggests that ceramic caps are to be preferred. Can anyone see any problems with this circuit?

ronsimpson Why can't tantalums be used?

Thanks,

Mike.
Datasheets attached for anyone interested.
 

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ronsimpson

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Why can't tantalums be used?
Tantalums can not handle high current. (like on the input/output of a switching power supply)
I use ceramic near the PWM and then might use tantalums for "bulk" energy storage.
>0.1uF ceramic + 10uF ceramic at the PWM then 220uF tantalums or Ele then at the load(s) 0.1uF on each IC.
 

jpanhalt

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I think there is some disagreement about use of tantalum for filtering power supplies. It used to be that ripple current was a major disadvantage. Today, it appears that view is changing (particularly with low ESR caps) to just staying within design limits.

See: http://www.vishay.com/docs/40031/apprippl.pdf
Vishay said:
Solid tantalum capacitors are preferred for filtering applications in small power supplies and DC/DC converters in a broad range of military, industrial and commercial systems including computers, telecommunications, instruments and controls and automotive equipment. Solid tantalum capacitors are preferred for their high reliability, long life, extended shelf life, exceptional stability with temperature and their small size.
John
 

Nigel Goodwin

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I think there is some disagreement about use of tantalum for filtering power supplies.
As I've mentioned before, historically tantalum capacitors were EXTREMELY unreliable - far more so than electrolitics.

And every time I've mentioned it previously people pop up and say that was because their voltage rating were highly imaginary - and that the claimed voltage specification of the capacitor needed to be much higher than the rail they were used on.

I can't comment on modern ones, as TV manufacturers stopped using them due to the huge problems.

Still it made repairs fairly easy - just look for a tantalum capacitor and check if it's S/C. Only problem was large numbers of them on the same voltage rail, spread all over a large PCB.
 

jpanhalt

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And every time I've mentioned it previously people pop up and say that was because their voltage rating were highly imaginary - and that the claimed voltage specification of the capacitor needed to be much higher than the rail they were used on.
It may be worthy to note that the application note I cited is titled:
"Application Notes AC Ripple Current
Calculations Solid Tantalum Capacitors"

The thrust of that note, which is dated April, 2006, is about calculating power dissipation by tantalum capacitors and the need to stay within manufacturer's specifications. It does provide a table for maximum RMS voltage compared to maximum DC voltage:
upload_2017-6-26_12-47-21.png
But that is not about imaginary ratings, but rather power dissipation. In fact, it does not use the term "derating," as according to the note, that is not what the limitation is; although, some people like to use that term for simplicity of communication.

John
 

DerStrom8

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Bypass capacitors nowadays are almost exclusively a ceramic type due to their low ESR and ESL. Electrolytics tend to have higher parasitic inductance and capacitance and are not suitable for bypass, but instead are often used across the bus (not across the IC supply) to help filter out any additional noise from the power supply. It entirely depends on your application and the requirements set forth by the upstream and downstream components' datasheets.

Many device datasheets specify what type of capacitor is recommended for a certain application. For example, I recently designed a basic power supply with a number of regulators, some passives, some control logic, and a micro. The datasheets for the regulators recommended tantalum capacitors specifically for the output. This was because the regulators relied on a certain range of ESR for stability (don't ask my why the manufacturer designs around a relatively uncontrolled number -- I have no idea), and electrolytic capacitors tend to have a much higher ESR than tantalums (too high to keep the output properly stable). You also often can't find high capacitance values in ceramic packages, and this is where tantalums can be useful.
 

ronsimpson

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Please state our goal here.
How much current will you pull from the USB source? (500mA)
How much current do you want to charge the LIPO with? (500mA)
How much current for the load? What are you powering? (100mA)
What size of LIPO?
What voltage will you put on the LIPO? (3.7v)
How long will the lipo support the load?
Will you be "charging" and powering the load at the same time?
 

Pommie

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The LIPO is a 1000mAh single cell and will be charged at 1/2C (500mA). See here.
The chip above stops charging at 4.2
The load is a pic driving one or more vibrators - normally 30mA - occasionally (once per hour max) around 300mA for 0.5 Sec.
It may be accidentally left on whilst charging but I don't think this is a problem.
The off state will be the chip in sleep mode drawing a few 10s of nA.
I'm hoping the LIPO will last at least 1 day with constant use - 12 hours would suffice.
If a problem with battery life I can switch to the 2Ah one stocked by Sparkfun.

Mike.
 

ronsimpson

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The load is a pic driving one or more vibrators - normally 30mA - occasionally (once per hour max) around 300mA for 0.5 Sec.
I don't think you can get 300mA out of the 61220. Probably more like 150 to 200mA. See page 6.
Note both the battery charging current and the 61220 input current come from the input regulator.

The first IC is a linear regulator. 500mA in 500mA out.


The last IC is PWM. Power in = power out - some loss. So input current and output current have a non 1:1 relationship.
The last IC is a boost power supply. If the battery was at 2.5V and the output is at 5V and 100mA; then it will bake 25omA of input current.
Output 5V 0.1A 0.5watts
Input 2.5V 0.2A 0.5watts + 10 to 15% more current for efficiency.
 

Pommie

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Sorry Ron, for some reason I had it in my head I had 500mA available. I can sort that so it's never more than 100mA and then only for less than 1 second.
When (accidentally) powered whilst charging it will never take more than 30mA. Do you (or anyone else) have experience with LIPOs and capacity. In theory I have 3.7Wh and I'm using ~150mW - is it reasonable to expect at least 12 hours (1.8Wh) from this setup?

Thanks all,

Edit, is it showing that I'm a software guy? :)

Mike.
 

ronsimpson

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Sorry Ron, for some reason I had it in my head I had 500mA available.
The datasheet for most boost PWM might say "500mA switch" or transistor but that is the peak and only for a small percent of time. It is easy to mis-read the data sheet.

You understand now. I think you have moved events in time so only one buzzer is on at one time. I think 100mA peak and 30mA all the time is good.
3.7Wh is like 370mWh for 10 hours. or 185mWh for 20 hours.

Look for the minimum discharge voltage. (3.0V??) It is the voltage where you should stop pulling power from the battery. Your current is low so there is no problem there. Watch for max charge voltage and min discharge voltage.
 
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