Trying to derive equation out of lm317 datasheet

• Trying to derive equation out of lm317 datasheet
  Posted by avogadro on 31 Oct, 2018 16:52
• 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.

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• #1 Reply
  Posted by spec on 31 Oct, 2018 20:07
• + avogadro
  
  Revised schematic with formula for output voltage attached.

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• #2 Reply
  Posted by westfw on 03 Nov, 2018 06:56
• 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")
  
• #3 Reply
  Posted by spec on 03 Nov, 2018 11:02
• Quote from: westfw on 03 Nov, 2018 06:56

  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")
  

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