MLCC vs Tantalum: For Decoupling, input to regulator and ripple reduction

Question

I require a 3.3uf capacitor in a 30 VDC environment for three different purposes:

1. decoupling of precision ADC or DAC;
2. input to a regulator (linear, switching, LDO); and
3. output of any regulator for the purpose of ripple reduction.

I can either use an MLCC (Multi-Layer Ceramic Capacitors) or Tantalum capacitor. I know the main drawback of MLCCs compared to tantalums is that the effective capacitance is reduced by applying DC voltage to them. So I set off to find suitable capacitors with voltage ratings of 50 VDC (to be on the safe side). I'm looking for 3.3uf Tantalum and MLCC whose effective capacitance is 3.3uf @ 30VDC.

Results:

• Tantalum: 3.3uf 50 VDC costs about $2.00

• MLCC 10uf 50 VDC X7R dielectric with effective capacitance of 3.3uf @30VDC costs about $0.70

Is there any reason that I might have missed which would warrant using the pricier Tantalum instead of the MLCC for aforementioned three purposes (exclude the case where a MLCC would make a regulator unstable due to the extremely low ESR) ?

Answer 1

The short answer is that if you need a lot of capacitance in a small space, then tantalums (or Niobium oxide for very low voltages) become attractive.

In this case, ceramics make sense.

I do not like using dry tantalums for a number of reasons; They are prone to failure simply due to reflow even when properly derated and with a low impedance source (which is what the power supply is) they can become spectacularly pyrotechnic. In addition, they have effectively zero capacitance above perhaps 400kHz (so if you need decoupling above this frequency, tantalums are no use anyway).

There are times to use tantalums, but I only use them if I must.

In the case of low ESR causing certain regulator instabilities (primarily LDO devices and current mode bucks), I would not trust the ESR of a tantalum to save me; the manufacturer will tell you the maximum ESR, but not the minimum, which is just as important.

In those cases, I use a ceramic with a series resistor to guarantee the correct effective ESR across temperature.

Answer 2

Yes, I know the question is old, but I thought this should be mentioned also.

While the use of ceramic caps for decoupling or output would be in general OK, their use as input caps, especially without the use of a resistor(as Peter Smith stated in a comment to his answer) can cause problems for some regulators in some configurations.

The following is a paraphase of the information from the Linear Tech App Note AN-88 "Ceramic Input Capacitors Can Cause Overvoltage Transients".

The problem is that a combination of lead inductance and output power filter inductors, output caps, the various caps and combined resistances(of the various ESRs and lead resistance) can form a series resonate tank circuit, which due to the ceramic caps very low ESR, is underdamped. This means that if the input power is already live when the regulator is turned on, it could see voltage transients of over twice the input voltage.

While this may not be a problem if the regulator in question can handle a large input(some automotive regulators can easily handle over 40V sustained and transients over 100V), now imagine using a 5V rated regulator with a 4V input, it could see spikes up to 9 or 10 volts. How often could it take this before failing...

The cure in this case is to damp the tank circuit in some form or fashion. A series 1ohm resistor could cure it, as could moving to a higher ESR cap if it can take the voltages involved.