Problem understanding dropout voltage of LM317T

Hi everyone, I am relatively new to electronics and am having some trouble resolving an inconsistency (at least in my mind) of the dropout voltage of the LM317T voltage regulator. I would like to use this chip as a constant current source that drives a laser diode ( $I_{op} = 25 \, mA, \, V_{op} = 2.2 \, V $ ) and am trying to determine whether I can use the 5V pin on a Raspberry Pi as the power source.

The problem is that I have seen multiple references online that state that the LM317T has a dropout voltage of around 3 V (see for example this CircuitLab forum thread or this blog post). CircuitLab simulations and basic calculations indicate that a 5 V input with a 3 V dropout will not be enough to provide the 25 mA of current required. However, when I look at the spec sheet for the LM317 on page 4, this suggests that the dropout voltage at 25 deg C and 25 mA load should be only be about 1.5 V. Simulations with a dropout voltage of 1.5 V in CircuitLab indicate that the 5 V output of the Raspberry Pi will successfully provide 25 mA of current.

Could someone please explain to me why I see 3 V quoted so often for the LM317T when the spec sheet does not even indicate a dropout voltage higher than 2.5 V? Am I misinterpreting the spec sheet?

Thanks a bunch!

by kmdouglass
July 15, 2017

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