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

Vadj=R2/(R1+R2) * Vout

then

Vout = Vadj * (R1+R2)/R2 = Vadj * (1 + R1/R2).

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

Am I doing something wrong?

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  • \$\begingroup\$ You are confusing Vadj with Vref \$\endgroup\$ – Trevor_G Mar 19 '18 at 20:01
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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.

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  • \$\begingroup\$ 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? \$\endgroup\$ – SsMikke Mar 20 '18 at 18:57
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    \$\begingroup\$ 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! \$\endgroup\$ – SsMikke Mar 20 '18 at 19:31

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