LM317 circuit capacitor question

by ntgCleaner   Last Updated November 17, 2018 02:25 AM

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.

Answers 4

If I recall it rightly, the 0.1uf capacitor filters high frequency noise in the LM317 input and the 1uf capacitor is used to stabilize output.

The circuit will work without any of those, but will amplificate input noise and the output will not be very stable.

If you can, keep them as they are, you can use aluminium or tantalum instead of traditional electrolytic ones for the 1uf capacitor, and, for the 0.1uf, polystyrene ones give a good result also.

June 10, 2014 21:45 PM

The capacitor on the input side is stated in the data sheet to be recommended, period. It improves transient response, according to some data sheets I have seen before and cannot now find.

The capacitor in the output side is stated in the data sheet to stabilize the output: specifically, to reduce ringing, reduce output impedance, and improve stability. Without it, the output impedance looks inductive to the load.

June 10, 2014 23:10 PM

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:

lm317 application

Along with a helpful note about why the capacitors are needed:

Note: 3. \$C_i\$ is required when the regulator is located an appreciable distance from power supply filter. \$C_O\$ is not needed for stability; however, it does improve transient response. Since \$I_{ADJ}\$ is controlled to less than 100 μA, the error associated with this term is negligible in most applications.

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.

June 10, 2014 23:45 PM

I just finished a good two months trying to figure this out. Here is what I know:

1) the larger capacitor on the left side of the Olympic 31-7 or lm337 between the input voltage and ground is the filter capacitor that helps remove any input voltage that is coming from your power supply so that doesn't fluctuate or rather the output doesn't pass any of the input ripple voltage voltage.

The filter capacitor is a low pass filter and it has to be pretty large depending on how large you are replicas and if your getting your supply voltages from a Transformer that is stepping down the AC mains voltage you're going to need probably at least a thousand microphone instead of one microphone. I think your circuit is assuming that the input supply voltage is already DC and doesn't have much ripple voltage so it doesn't need to filter that much.

It basically keeps the input voltage at a constant value and you can calculate it using: C=I*dT/dV(peak to peak ripple)

This could be a tantalum capacitor in your circuit or you could use an electrolytic capacitor. I have never used to tantalum capacitor that hasn't exploded in my face so I wouldn't recommend that one and the aluminum electrolytic only explodes maybe 5% of the time so I do recommend that. Basically just need to be polarized. Tantalum capacitors take up about 5% as much space so I recommend using tantulum if you know how to not make them explode.

  1. Capacitor on the right side of the voltage regulator or on the output, more accurately, serve the same purpose except for its filtering any how to put triple that may have passed through the voltage regulator and into your load.

It also supplies current to the load when your input voltage is Bellow your input filter capacitors voltage.

The output capacitor should be a multi-layered ceramic capacitor and it shouldn't be polarized because you are load May cause it be at a different polarity during transient voltage spikes on your output.

I know that my mistake when making this circuit, or rather my attempt at making these circuit similar to yours, was trying to figure out the value of the output capacitor. Even though it's a pretty small capacitance it will have in equivalent series resistance that will mess with your load regulation because it's a parallel resistance with the load and your voltage divider to the adjust pin.

The output voltage capacitor value needs to be small enough so that it's corner frequency is larger than the frequency of any changes in load current due to changing the output load with a fast switch, mosfet or transistor. If it has a lower corner frequency than it will not discharge the output capacitor fast enough in the output voltage could go into the lm316 Darlington npn transistor output stage.

Danny Sebahar
Danny Sebahar
November 17, 2018 02:10 AM

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