Functional Description

The LM317T functions by maintaining a constant reference voltage of 1.25V between its output and adjustment (ADJ) terminal. By configuring the regulator with an external voltage divider (two resistors), the output voltage is set according to the following formula:

$$V_{\text{OUT}}=V_{\text{REF}}(1+\frac{R_2}{R_1})+I_{\text{ADJ}}\cdot R_2$$

VOUT​=VREF​(1+R1​R2​​)+IADJ​⋅R2​

• $V_{\text{REF}}\approx 1.25V$VREF​≈1.25V

• $R_1$R1​ and $R_2$R2​ are external resistors

• $I_{\text{ADJ}}$IADJ​ is the adjustment pin current (typically ~50µA, usually negligible in most designs)

---

Application Circuit

A typical application includes:

R1 & R2: Set the output voltage using the formula

$$V_{\text{OUT}}=1.25(1+\frac{R2}{R1})$$

VOUT​=1.25(1+R1R2​)

C1_in: Input capacitor (e.g., 0.1µF to 1µF) — stabilizes input voltage.

C2_out: Output capacitor (e.g., 1µF to 10µF) — improves transient response.

---

Thermal and Protection Features

• Thermal Overload Protection: Shuts down the regulator if the junction temperature exceeds safe levels (~125–150°C).

• Current Limiting: Internal circuitry limits the output current to protect the regulator and connected components.

• Safe Area Protection: Prevents operation outside the safe operating region of the power transistor.

---

Applications

• Adjustable power supplies

• Battery chargers

• Constant current sources

• Precision current regulators

• Post-regulation for switching power supplies

---

Notes on Use

• Proper heat sinking is essential for full 1.5A operation; the TO-220 package can dissipate significant power if mounted correctly.

• A minimum load current (~10mA) is often required for proper regulation.

• When used in high-output voltage configurations, the differential voltage between IN and OUT should be managed to avoid excessive power dissipation.