Voltage rating and tantalum capacitors

Question

I was originally considering using a 10 µF 6.3 V 0603 ceramic capacitor for stabilising the onboard voltage regulator on my microcontrollers. However, it may not be capable because of temperature coefficients, tolerances and applied voltage (losing up to 60% of the rating). Therefore, I am considering moving to using tantalum capacitors.

The microcontroller LDOs output 2.25 V - 2.75 V for powering the internal core. I am using a PIC24FJ64GA004 and dsPIC33FJ128GP804. Would it be fine to use a 4 V capacitor here? I have always used 2-3x rating for electrolytic capacitors, but maybe the same problems with aluminum electrolytic capacitors as for tantalum capacitors do not apply (with lifetime decreasing over applied voltage considerably.)

Ideally I'd like to reach a 50,000-100,000 hr MTTF. If tantalum capacitors aren't an option I'm considering using a 22 µF 6.3 V 0805 ceramic capacitor in place (I might have to resize the footprint), because even with a loss of 60% it stays within the minimum requirement of 4.7 µF. Any tips? Has anyone used tantalum capacitors before? What about ceramic capacitors for microcontroller LDOs?

Answer 1

It depends heavily on the temperature you expect in your application. Recommendations say that 80 % of the rated voltage should never be exceeded, and at 85 °C, you should not apply more than 50 % of the rated voltage. The catalog in this link, for example, has many derating diagrams for various types of tantalum capacitors. From this source, with annotations:

Further Reading: Surge in Solid Tantalum Capacitors, John Gill, AVX Ltd, Tantalum Division, Paignton, England, Lit. Nr. S-HDF10M295-N

Answer 2

4 V tantalum capacitors will be just fine, but personally I love the ceramic ones so much. Also, each µF of a ceramic capacitor is "better" than a tantalum one due to lower ESR.

So before going for tantalum, I would check the price of ceramic capacitors with a better dielectric which still gives you 4.7 µF at 85 °C (185 °F).

You should check your LDO specification on ESR limits. Some might oscillate with too-low ESR. And if it's stable with a 1 µF ceramic one at 85 °C - I would personally stay with ceramic. Just more reliability.

Answer 3

10 µF 0603 will probably be Y5V dielectric which has a poor temperature coefficient and voltage dependence. X7R dielectric (will be a larger case) should be fine.