# How to set resistors R1 and R2 for the output voltage setting of uPC317?

Latest Updated:04/01/2007

## Question:

How should the resistors (R_{1} and R_{2}) for the output voltage setting circuit of the three-terminal positive adjustable regulator (uPC317) be set?

## Answer:

Basically, a 240 Ω resistor is recommended for R

Output voltage Vo is calculated by the following expression and is shown in the table below, where R

Vo = (1+R

If R

If R

If R

_{1}in the above standard connection.Output voltage Vo is calculated by the following expression and is shown in the table below, where R

_{1}is 240 Ω.Vo = (1+R

_{2}/R_{1}) × V_{REF}If R

_{1}is a value higher than 240 Ω, the following points must be noted.If R

_{1}is at 240 Ω, 1.25 V/240 Ω = 5.2 mA flows through R_{1}, even without load. In the meantime, the current (I_{ADJ}) flowing out from the ADJ pin is defined to be 100 uA (max.) in the electrical specifications, which is considerably lower than 5.2 mA and is negligible.If R

_{1}is increased to a value higher than 240 Ω (1 kΩ, for example), the value of the current flowing through R_{1}decreases. Consequently, the value of I_{ADJ}may no longer be negligible, resulting in deterioration of the output accuracy.**Suitable Products**

Bipolar Operational Amplifiers |

3 Terminal Regulators |