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

Latest Updated:04/01/2007

Question:

How should the resistors (R1 and R2) 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 R1 in the above standard connection.
Output voltage Vo is calculated by the following expression and is shown in the table below, where R1 is 240 Ω.

Vo = (1+R2/R1) × VREF


If R1 is a value higher than 240 Ω, the following points must be noted.
If R1 is at 240 Ω, 1.25 V/240 Ω = 5.2 mA flows through R1, even without load. In the meantime, the current (IADJ) 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 R1 is increased to a value higher than 240 Ω (1 kΩ, for example), the value of the current flowing through R1 decreases. Consequently, the value of IADJ may no longer be negligible, resulting in deterioration of the output accuracy.
Suitable Products
Bipolar Operational Amplifiers
3 Terminal Regulators