Electronic – Choosing capacitors for a linear voltage regulator


I'm trying to use an LM1117 linear voltage regulator to convert to 3.3v (input voltage will be 9 or 5 volts; not yet decided). The datasheet suggests using 10uF tantalum capacitors on the input and output. While I could just go with the suggestion, I find most of the tantalum capacitors that are available are considerably more expensive than other capacitors, and in a SMT form factor (I'd prefer through-hole). I'm thinking of using aluminum or ceramic caps instead.

This excerpt from the datasheet specifies what ESR ranges are acceptable: Output Capacitor
The output capacitor is critical in maintaining regulator stability, and must meet the required conditions for both
minimum amount of capacitance and equivalent series resistance (ESR). The minimum output capacitance
required by the LM1117 is 10 µF, if a tantalum capacitor is used. Any increase of the output capacitance will
merely improve the loop stability and transient response. The ESR of the output capacitor should range between
0.3 Ω to 22 Ω. In the case of the adjustable regulator, when the CADJ is used, a larger output capacitance (22-µF
tantalum) is required.

As I understand it, ceramic caps have a very low ESR. Could I just add a 0.3 Ω to 22 Ω resistor in series with a ceramic capacitor? I've read what ESR is and isn't, and I don't understand it one bit, so right now it's just a range of numbers I'm trying to match. However, ceramic caps tend to have smaller capacitance, so it might not be as easy to get 10uF ceramic caps.

If I recall correctly, aluminum caps have a much higher ESR (though exactly what "higher" means I'm not sure). Additionally, it can be difficult to get a datasheet for a capacitor simply because they're such common components (I'm in China and purchasing online, but don't read Chinese). So I'm not sure how to make sure I'm within the ESR range specified.

Finally I should note that I don't have any of these components yet, but I'll probably get an assortment of capacitors when I do order.

So my question: What capacitors can be used in this situation, and with what value resistors (if any)? Note that I'm interested in the theory, not just this single situation, but not being experienced with capacitors some specific examples could be helpful.

Related: ESR and CSR of Capacitor

Best Answer

Yes, you can simply put a resistor in series with a ceramic capacitor. The lower the better from the point of view of bypassing, so I would aim at 0.5 to 1 ohm. If you have lots of space, the electrolytic is fine (in fact you can parallel the two), and they are cheap. There are low ESR electrolytics and ones that are not-so-low, read the datasheet. If no datasheet, no buy.

You should be able to read the numbers on the datasheets even if some of it is in a foreign language. If you're going to the market and picking shiny parts off of vendors displays without looking at datasheets you will get bitten. I've always been able to get answers to questions such as the load capacitance of a crystal without being the most amazing linguist around.

The x1117-3.3 is extremely cheap and very available in China so I don't see any reason not to use it. If you don't need the power dissipation there are better choices in SOT-23.