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Tantalum or Cermaic capacitor for Power supply

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hm_fa_da

hm_fa_da

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Hi,

I saw in many docs suggesting usage of Low ESR Tantalum Caps for power supply part of circuit. but as i know, Tantalum Caps usually have higher ESR vs to other types caps. i.e Ceramic caps.
so can i use Ceramic Caps instead of low ESR tantalum caps ? what's other differences ?
Currently i'm making a device which uses GSM module that have some bursts of up to 2A. so i need to use best type caps for good power suplly transient response. in the gsm module datasheet, low ESR tantalum caps are suggested. i just want to know why ceramic caps with lower ESR can't be better for this purpose ? (assuming same capacities be available in both tantalum and ceramic types)

Thanks
 

Hi,

Show us the datasheet that suggests the use of tantalum capacitors.

Klaus
 

The word you're looking for is "and".

You can't get much over 100uF in ceramic and you
can't get really good HF attributes in tantalum.
The trailing assumption doesn't hold up well.

Figure your best case sag as Ipk*tpk/Cbulk. Does
that violate supply minimum, sagged from nominal?
If yes, add C. Then add some more because ESR
and ESL "punk" the bulk C adding I*R, dI/dt*L sag.
 
Ceramic capacitors are replacing tantalum in many applications, however two points to consider

- due to ceramic C/V curve, you'll choose a rated voltage of e.g. double the actual DC voltage, or calculate respective reduced effective capacitance, review datasheet.

- high permittivity ceramic is more or less piezoelectric. A GSM buffer capacitor will sound at the current burst frequency. Some manufacturer application notes and previous Edaboard threads are discussing the issue.
 
KlausST said:
Hi,

Show us the datasheet that suggests the use of tantalum capacitors.

Klaus
Click to expand...

C1.png

As you see, for 100uF cap, Tantalum is suggested.
if i have 100uF ceramic cap and want to use 1 only (Tantalum OR Ceramic), which would be better ?
 

Hi,

reread the datsheet section or post#3.

"AND" is as clear as can be. Nobody of us will recommend anything that is against the datasheet.

Klaus
 

When one is working with RF modules, it is very common to use paralleled capacitors with concatenated impedance characteristics.

In the datasheet you have shown, it shows 3 distinct capacitors: 100uf tantalum, 33 pF ceramic, 10 pF ceramic. You may think: "hmmmmm....I'll replace the two last ones with a single 47 pF ceramic."

Don't do that. Their combined impedance curve, will remain with a capacitive reactance for a wider bandwidth.
Once that you reach a capacitor's SRF, all of its capacitive reactance is lost.

I've personally measured it with an HP Network Analyzer.
 
I don't have any word about 30pF, 10pF or ...
Only about the 100uF cap. i just have little doubt maybe that suggestion be because of old document reference, maybe for a time when high capacity ceramic caps were not widely available in market !
so that's why i want to be sure and understand the reason of datasheet suggestion.
 

Tantalum caps are classified as electrolyte- they tend to be slower (ions rather than electrons form the double layer). Ceramic caps, on the other hand, depend on the dielectric polarisation- they can be much faster- you can figure out that by looking at their frequency limit.

In addition, many ceramic caps shows relatively strong voltage dependence of of capacitance (but the magnitude depends on the material)- the effect is small for electrolytic caps. Both ceramic and electrolytic caps have relatively large margin.

It is the response to the changing voltage that determines the series resistance: it is somewhat virtual in some sense because that also depends on the frequency (and to a smaller extent on the bias voltage).

Yes, you must consider the voltage drop (as suggested in #3) and act appropriately. What is the max freq you are planning to use?
 
c_mitra said:
Tantalum caps are classified as electrolyte- they tend to be slower (ions rather than electrons form the double layer). Ceramic caps, on the other hand, depend on the dielectric polarisation- they can be much faster- you can figure out that by looking at their frequency limit.

In addition, many ceramic caps shows relatively strong voltage dependence of of capacitance (but the magnitude depends on the material)- the effect is small for electrolytic caps. Both ceramic and electrolytic caps have relatively large margin.

It is the response to the changing voltage that determines the series resistance: it is somewhat virtual in some sense because that also depends on the frequency (and to a smaller extent on the bias voltage).

Yes, you must consider the voltage drop (as suggested in #3) and act appropriately. What is the max freq you are planning to use?
Click to expand...

Thanks for your reply,
The cap is used on DC power line (4V) of GSM module.
 

What about using a "small" supercap instead of the tantalum cap? Here's one of the smallest: 6V / 470000µF ESR < 400mΩ

Don't know about their RF properties, but these are taken care of by those 33 & 10pF anyway.

Size & cost could be a challenge.

But at least you don't need to be afraid of the current bursts ...
 

For most reliable performance, two caps, one with high value and another with smaller capacitance should be used; that will reduce the ESR even lower. If both are ceramic, then bulk capacitance should be decided very conservatively. Effectively, the capacitor with the lower ESR will get discharged faster and you should see that the voltage does not fall below the minimum recommended. If the surge current is 2A and the surge duration is 1ns, total charge consumed is 2nC and that should be <10% of the capacity of the smaller cap. If the operating voltage - the minimum voltage is larger than the 2A*ESR, then you are safe.
 
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