I am planning to use an AP1509-50SG converter in one of my projects. There is only a very small area to place components. Aluminum capacitors rated 300uF/16VDC or 330uF/25VDC have a large footprint area. I am thinking about using a tantalum capacitor instead of an aluminum one. Will that cause a problem? How should I choose tantalum capacitors?
I have an area of 12x15mm for the IC and the capacitors.
I would not use tantalum capacitors, especially for the input capacitor, they are very intolerant of current surges. If you do, it's recommended you limit the current (for example, by adding a few ohms in series, which may make them less useful) and use a voltage rating at least 3x working voltage to minimize the number of fires.
There is an ESR recommendation (along with a lot more information) you can find in the application note ANP013.
For the output capacitor, the ESR value is the most important parameter.
Eg. for a 3.3V supply with 1% ripple, and 100mA minimum load current, the ESR should be about 150mΩ.
The AP1509 is a part with a very low switching frequency by modern standards (150kHz). I would suggest replacing it with one that operates at a higher frequency to minimize the size of the capacitors and inductor, preferably to allow use of inexpensive, reliable and compact ceramic capacitors in both positions.
tl;dr: forget tantalum, update your regulator to a better one.
Diodes makes the AP63205 with a 1.1MHz switching frequency in a smaller (SOT23-6) package. You can use it with ceramic caps. Linky: https://www.digikey.com/en/products/detail/diodes-incorporated/ap63205wu-7/9858424
The benefits of using a faster switching frequency are less ripple (less capacitance needed) and lower value inductor (less I^R losses). Both serve to improve efficiency and footprint. And yes, efficiency is actually improved despite some increased parasitic loss due to the higher switching rate.
Your 5V regulator can use a couple of ceramic 22uF caps and 4.7uH inductor - resulting in a much, much smaller design than you could achieve with a 150kHz switcher.
As far as tantalum caps, besides their physical and electrical issues, there’s another reason to not use them: tantalum is a conflict mineral.
POSCAPs are a good option if you really need a large value in a small volume, which you don’t if you upgrade your switcher.
Finally, if you’re really hurting for space, MPS, TI and others make regulators with integrated inductors.
A tantalum capacitor will work fine. There are only a few, older ICs that cannot handle tantalum due to their low ESR (Effective Series Resistance). This is not one of them.
The lower ESR of a tantalum capacitor allows it to handle a ripple current better than the average electrolytic capacitor.
