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Trying to derive equation out of lm317 datasheet
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avogadro:
I was just trying to derive the equation for output voltage of lm317 for fun and I cant get it. Dont know what Im doing wrong. I attached the image of what I was trying to derive. The closest I get is V0 = Vref((R2 + R3)/(R1)) - 10.
spec:
+ avogadro

Revised schematic with formula for output voltage attached.
westfw:
In your second picture: R3, D1 (a zener) and C2 essentially form a conventional -1.25 zener voltage regulator, with the -1.25V output at the D1/R3 junction.
R1/R2 and the LM317 are the normal lm317 regulator, with the "adjust" input biased to -1.25V instead of ground. offsetting the normal 1.25V regulation point of the 317.  (which tries to make sure that Vout is "Adjust+1.25V")
spec:

--- Quote from: westfw on November 03, 2018, 06:56:26 am ---In your second picture: R3, D1 (a zener) and C2 essentially form a conventional -1.25 zener voltage regulator, with the -1.25V output at the D1/R3 junction.
R1/R2 and the LM317 are the normal lm317 regulator, with the "adjust" input biased to -1.25V instead of ground. offsetting the normal 1.25V regulation point of the 317.  (which tries to make sure that Vout is "Adjust+1.25V")

--- End quote ---

Yeah, that is right. Just standard stuff, straight from the datasheet. :)
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