I am trying to figure out the significance of the two capacitors recommended for use in an LM317 DC-DC converter circuit.
Recommended circuit diagrams show that I should be using a 0.1uf Ceramic Capacitor and a 1uf Electrolytic capacitor.
Why the difference in capacitors (size and makeup)? Can I use two capacitors that are the same type and composition, assuming they are of the larger size? For example, can I use two 1uf electrolytic capacitors instead? Will this just make the circuit slower to level out?
I understand how capacitors work (roughly) but I don't understand their purpose in this circuit.
The reason I ask is because I took a look at Other Voltage Regulators and they seem to have the exact same capacitors.
Best Answer
The two capacitors used in the LM317 typical application are described on page five of this datasheet. An identical schematic to the one you provided is given:
Along with a helpful note about why the capacitors are needed:
The best practice, for either capacitor, is to always include them. Specifically for \$C_i\$, place it as close to the input pin on the LM317 as reasonable.
If you have determined that you know what you're doing, you can decide to omit or alter the suggested schematic. In determining that, for \$C_i\$, an appreciable distance in on the order of centimeters, but you should also factor in the level of noise you expect on the input supply and how much the load the LM317 will be changing. For \$C_O\$, you can likely omit it if you have decoupling capacitors at the inputs of any ICs and relatively high current draw devices, anywhere things are going to be changing how much current they're drawing in a short amount of time.
Do check out the related questions and answers regarding how to use decoupling capacitors and how to select the type of capacitors to use.