Regulator adjustment

[Prune]

In the LM317 and LM337 datasheets, output is adjusted by changing the value of R2, for a given R1. However, choosing R1 (between Vout and Adj) is not explained. So how do I choose R1 for best performance (not efficiency, but in terms of noise). In some schematics I see R1 to be 120, in others, 1K.

[jackinnj]

i use 270R or 243R, there is a little adjustment for the Adjust Pin current specified in the term sheet. at any rate, I always wind up using a pot to set the exact voltage, then wire in the right resistor.

if you're worried about noise and still wanna "plug and play", try the LT1963A -- 50uV noise, about 200X better than the LM317.

[maylar]

They pick 5 ma as a nominal reference current so as not to be affected by the regulator's own internal current. That'd be 250 ohms and 240 is a "close enough" standard value.

[Prune]

Thanks dude.

[jackinnj]

Originally posted by maylar
They pick 5 ma as a nominal reference current so as not to be affected by the regulator's own internal current. That'd be 250 ohms and 240 is a "close enough" standard value.

there's a minimum current for the LM317 to regulate, too little current and it won't regulate properly -- doesn't seem to be a problem with the LM317LZ, however.

[s2kov]

Is it possible to use the trimmer in replacement for the R2 to get the exact output voltage i want?

[Andersonix]

Originally Posted by jackinnj

if you're worried about noise and still wanna "plug and play", try the LT1963A -- 50uV noise, about 200X better than the LM317.

Sorry for the grave digging, but I'm just now catching up about voltage regulators. How do you make it PnP? For +5V?