How do I calculate the values needed for the feedback resistors of adjustable linear regulators, such as the LM317?
Two resistors are used to set the value of the output voltage for adjustable regulators. For adjustable standard linear regulators, such as LM317 and LM337, R1 is the resistor between Vout and ADJ, while R2 goes from ADJ to ground. For adjustable low-dropout regulators such as LM2941 and LP2951, as well as the SIMPLE SWITCHER converters, R1 is the resistor between ADJ and ground, while R2 goes from ADJ to Vout. (The difference in these configurations is due to variation in how the regulator”s internal reference and error amplifier are arranged.) Check the datasheet for your specific product for the arrangement of its feedback resistors. The values of the resistors are determined from this formula: Vout = Vref (1 + R2 / R1). R1 is generally specified on the datasheet, and you must solve for R2. A more precise form of the equation allows for adjust pin bias current: Vout = Vref (1 + R2 / R1) + (Iadj * R2). Vref usually is 1.25V for linear (LM317/337 type) regulators and 1.23V for SIMPLE
