# LM317 equation help

So I have this voltage regulator circuit. Applying the voltage divider,

then

Why every site shows Vout = Vadj * (1 + R2/R1)?

Am I doing something wrong?

• You are confusing Vadj with Vref Mar 19 '18 at 20:01

You are confusing Vadj with Vref.

The device setps up Vref between the right output pin and the adjustment pin.

As such the current through R1 is

$I_{R1} = V_{REF}/R_1$

So, ignoring the small adjustment current through the device for now..

$V_{OUT} = V_{REF} + I_{R_1} * R_2$

$= V_{REF} + V_{REF} * R_2 /R_1$

$= V_{REF}(1 + R_2 /R_1)$

Of course in reality you need to add in $I_{ADJ} * R_2$ to that.

• Thanks! I found where I started to confuse things, but I didn't fully understand. Is the current through R1 passing R2 also? Isn't there a current node where it splits throug adj and R2? Mar 20 '18 at 18:57
• So I did a little bit more research after your answer gave me something new to search. After one hour of researching I opened the LM317 datasheet, and at basic circuit operation everything was explained. I wrote down Kirchhoff's law on the output and the result is exactly the equation that I didn't understand in the first place. Thank you very much for your help! Mar 20 '18 at 19:31