LM317 VOLTAGE REGULATOR

The LM317 is a very popular, adjustable three-terminal positive voltage regulator capable of supplying more than 1.5A over an output voltage range of 1.25V to 37V.

It's exceptionally easy to use, requiring only two external resistors to set the desired output voltage. It also has built-in features that make it robust and reliable.

Key Features and Specifications

• Adjustable Output Voltage Range:1.25V to 37V.

• Maximum Output Current: Typically more than 1.5A.

• Minimal External Components: Requires only two external resistors (R1 and R2) for its basic operation as an adjustable regulator.

• Internal Protection: It includes thermal overload protection and internal short-circuit current limiting to prevent damage.

• Regulation Performance: It offers good performance with typical line regulation of 0.01%V and typical load regulation of 0.1%.

Output Voltage Calculation

The LM317 works by maintaining a constant reference voltage of V_ref = 1.25V between its Output and Adjustment terminals. This reference voltage is dropped across the fixed resistor, R_1, to create a constant current flow that sets the output voltage.

Where:

• V_Ref is the constant reference voltage, which is 1.25V.

• R_1 is the fixed resistor (commonly set to 240).

• R_2 is the second resistor, which can be fixed or a potentiometer to make the output voltage variable.

In practical circuits, a small current (Adjustment pin current, I_ADJ) flows through the Adjustment pin, which adds a small error term to the formula:

However, since I_ADJ is very small (typically 50uA, the simplified formula is accurate enough for most basic applications.

Main Applications

The versatility of the LM317 makes it suitable for a wide variety of electronic projects. Its primary uses are:

1. Adjustable Voltage Regulator: This is the most common application, where it is used in variable voltage power supplies to provide any positive DC voltage within its range.

2. Fixed Voltage Regulator: By choosing the two external resistors (R_1 and R_2) for a specific ratio, it can be used to provide a constant, fixed output voltage not available from standard fixed-voltage regulators (like the 78xx series).

3. Precision Current Regulator/Limiter: By placing the current-setting resistor between the Output and Adjustment pins, the LM317 can be configured to supply a constant output current regardless of the load's impedance, which is useful for applications like battery chargers or driving high-power LEDs.

4. Local and On-Card Regulation: It's frequently used to provide stable power to specific components on a larger circuit board (on-card regulation).