Uhm, sorry .....
The LM317 is a VARIABLE voltage regulator, that you can also use as current regulator using a resistor that gives you the needed voltage drop (1,25V) for the current that you want (if you use the fixed voltage models, no way for have a current regulation) ..... so, for the ohm law, 10 ohm gives you 125mA, 12,5 ohm gives you 100mA, and so on .....
But you must keep in mind that, in this mode, you have a double voltage dropout (one from the regulator, and one from the resistor).
I never said this clearly before, cause i always considered it obvious, but i see that in a lot of posts, peoples talks always about regulators dropouts, and never considerate the resistor one, so i just wanted to point this
Anyway, keep in mind, calculating the power supply, that using the LM317, you have a total dropout of 3,75V (2,5 on the regulator, and 1,25 on the resistor) ..... when you need a dropout a bit low, you can use the LM1117 or AS1117 (NOT 117, but 1117), that have a lower internal dropout ..... using it, you have a total dropout of 2,4V (1,15V on the regulator and 1,25V on the resistor)
The only way for have a minor dropout with linear regulators is to use current sources with a current loopback, like the LT3080 ..... with it, you can use a 0,2 ohm sense resistor, and with a 20 Kohm trimmer regulate the current from 0 to 1,1A with a total dropout of 1,3V plus the dropout on the resistor, that depend from the current ..... as example, for 500mA the dropout on a 0,2 ohm resistor is 100mV, so the total dropout becomes 1,4V (don't be fooled from the datasheet that say dropout 400mV ..... this is true,
but only if the control voltage is higher than the power voltage ..... using it with a single power source, you need to join the 2 inputs, so the dropout is the one of the control voltage, that is around 1,3V)
At the moment, afaik, the only better solution is using boost converters ..... but who know, manufacturers are always searching for better products
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