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Old 14th February 2004, 10:39 PM   #1
Prune is offline Prune  Canada
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Default Regulator adjustment

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.
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Old 15th February 2004, 03:12 AM   #2
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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.
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Old 15th February 2004, 07:16 PM   #3
maylar is offline maylar  United States
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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.
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Old 15th February 2004, 08:54 PM   #4
Prune is offline Prune  Canada
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Thanks dude.
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Old 15th February 2004, 10:02 PM   #5
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Quote:
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.
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Old 21st May 2004, 08:28 AM   #6
s2kov is offline s2kov  Canada
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Is it possible to use the trimmer in replacement for the R2 to get the exact output voltage i want?
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Old 16th June 2010, 03:53 AM   #7
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Quote:
Originally Posted by jackinnj View Post
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?
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