Electrical – Powering an Arduino Pro Mini 5V and a 3.3V nRF24L01+ Transceiver from One Small Battery

I think you're killing the tantalum capacitor across the input. In fact, it looks like it's bulged up in your photo.

Although the capacitor may be rated at 16V (I don't actually see a rating anywhere, but I'm inferring it from the size and input voltage spec) seasoned engineers will never use tantalum capacitors in this sort of situation. The input current needs to be limited by several ohms and/or the rated voltage should be something like three times the operating voltage. Your wall wart can probably supply several amperes.

Years ago, this information was anecdotal, but today you can find it buried deep within the manuals for such parts. They bury it, because it means you very seldom should use tantalum capacitors, and almost never across power supply rails unless the current is limited to much less than an ampere.

Many have been burned by this (perhaps literally). Often failures take place long after product has been shipped, and can cause localized charring, meaning (if your standards are high, as in aerospace or medical) the entire board must be scrapped.

The datasheet states that 2.2 uF is a minimum required for for stability. The typical output capacitance (Table 3) is stated as 10 uF. Adding more capacitance will smooth the output more and ensure better stability, but there are some side effects (below).

Just to make all of this clear as to why you need this capacitance, see the figure below. An LDO is simply a transistor (A) that is controlled by a feedback loop (B and C). The error amplifier is trying to make B equal to the internally generated Reference Voltage.

Now when a large load current is pulsed (like in a digital circuit), Vout will droop. This makes B too low, then C has to drive A on harder so it can compensate. This takes time. So if the pulses of load current are too fast, the loop will not be able to compensate quickly enough, and you can have oscillation.

The solution is to put a charge bank, i.e. a capacitor out there to source the quick pulses of current, and the slower job of controlling the average current is left to the regulator.

So why not go insanely big on this capacitance? There are a couple of reasons:

1. Cost. Big caps cost more.
2. Space. They take up more space.
3. Charge/discharge time. When you turn the circuit on, the capacitor has to charge up. This will cause the voltage on Vout to more slowly rise. Some circuits don't like to turn on slowly. Also, the charge has to be discharged when powering down. This is usually less important.

Finally, the input capacitance question. This is usually less clearly defined. The main point that is important is that you have a good enough source so that the LDO internal reference works well. A good idea is to look at what the datasheet test circuits use. This one shows a 0.1 uF capacitor placed on the input.