I know that ratio R1 to R2 determine the output voltage of LM317. E.g. R1 = 200, R2=330 ohms will produce about 3.3V. My question is, what if I use 2K and 3.3K for R1 and R2? What is the impact of increasing the resistors values but keeping the ratio the same?
Answer
The output voltage is determined not by the ratio of R1 to R2. It's given by the following equation:
\$ V_{OUT} = 1.25 \left(1+\frac{R_1}{R_2}\right) + I_{ADJ}R_2\$
For ordinary purposes, the \$I_{ADJ}R_2\$ term can be discarded, because \$I_{ADJ}\$ is on the order of \$100\mbox{ }{\mu}A\$.
You've multiplied your resistors by 10, so this error term will also be multiplied by 10, going from 33 mV to 330 mV, or 0.33 V.
No comments:
Post a Comment