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LM317: Vout formula question

Bryan Huang
Intellectual 445 points
Part Number: LM317

Hi Team:

About Vout formula question, 

I calculate with this formula , the result is strange.

But I calculate with below formula,the result is right, compare Tina simulation .

So is it right?  The formula for battery charger circuit .

  • 0
    TI__Guru 55940 points

    Hi Bryan,

    Due to the topology of LM317, the internal reference voltage is across the top resistor (R1) in the resistor divider.  Most modern linear regulators and LDOs have the reference across the bottom resistor (R2).  

    As such the equation to calculate Vout for LM317 is different from the equation seen in most modern linear regulator datasheets.  The correct equation for LM317 as stated in the datasheet is:

    Vout = Vref x (1 + (R2 / R1)) + (Iadj x R2)

    This equation is sometimes simplified to Vout = Vref x (1 + (R2 / R1)) since Iadj is 50 uA typically and therefore negligible in many applications.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Intellectual 445 points

    Hi Ryan:

    sorry, I have concern is another formula,as below.

  • 0 in reply to Bryan Huang
    TI__Guru 55940 points

    Hi Bryan,

    It appears, that in the latest revision of the datasheet this section was reformatted.  Unfortunately the equations 4 and 6 are incorrect.  Thank you for pointing this out, we will correct it on the next revision of the datasheet.  Here are the correct equations for equations 4 and 6:

    Equation 4:  Vout = 1.25 V x (1 + (R2 / R1))  Please note that this is Vout = Vref x (1 + (R2 / R1)) when substituting in Vref.

    Equation 6: Output impedance = RS x (1 + (R2 / R1))  Below is the original section of the datasheet.

    Very Respectfully,

    Ryan