The problem of using Multi-Layer Ceramic Capacitors (MLCC) as output capacitor
Look at following picture carefully. There is a small problem in it.
Most of the time when there is a small area for components, we choose to use multi-layer ceramic capacitors (MLCC) as our output capacitor. They have good advantages over the other capacitors but It does not mean they can be used everywhere.
There are some common mistakes of using MLCCs. One of them is when you use MLCCs as output capacitors in some linear voltage regulators. These linear voltage regulators need some certain amount of Equivalent Series Resistance (ESR) in order to keep their loop stability. MLCC has been designed to have very small amount of ESR. So, putting them as the output capacitor may cause instability at the output voltage.
An important reference could be the datasheet of linear voltage regulators. As you probably have seen, the companies use following schematics most of the time as the minimum requirements.
According to the datasheet of LM317, the output capacitor in the schematic is some kind of polarized capacitor. Now, the datasheet explicitly expressed the output capacitor types in page 9:
“Although the LM317 is stable with no output capacitance, like any feedback circuit, certain values of external capacitance can cause excessive ringing. An output capacitance (CO) in the form of a 1.0 μF tantalum or 25 μF aluminum electrolytic capacitor on the output swamps this effect and insures stability.”
Another example is LFXX datasheet from ST. In page 5, datasheet shows that the output capacitor should be an electrolytic capacitor for test circuit.
Unlike the regulators above, other could use MLCCs as output capacitor. an example could be AP7311 from diodes. In page 10 of the datasheet, there is very good information about output capacitor.
“The AP7311 is stable with very small ceramic output capacitors. The recommended capacitance is from 1μF to 4.7μF, Equivalent Series Resistance (ESR) is from 10mΩ to 200mΩ, and temperature characteristic is X7R or X5R.”
As a result, remember to read linear voltage regulators datasheet. Use aluminum electrolytic capacitor if you have enough space and tantalum capacitor if you don’t. But, be careful when you’re using MLCCs.
References:
https://www.onsemi.com/pub/Collateral/LM317-D.PDF