LM317CV Adjustable Positive Voltage Regulator 1.2–37V

The LM317CV is a three-terminal positive linear voltage regulator in a TO-220 package. It allows designers to set the output voltage with only two external resistors.

In addition, the regulator includes current limiting, thermal shutdown, and safe-operating-area protection. Therefore, it offers a practical solution for adjustable power supplies, current regulators, and electronic prototypes.

Key Features

• Adjustable positive output voltage
• Output range from approximately 1.2V to 37V
• Supports output currents up to 1.5A
• Requires only two resistors for voltage adjustment
• Provides good line and load regulation
• Includes internal current limiting
• Includes thermal shutdown protection
• Includes safe-operating-area protection
• Suitable for through-hole assembly
• Available in a TO-220 package

Technical Specifications

• Regulator Type: Adjustable Positive Linear Regulator
• Output Voltage Range: 1.2–37V
• Output Current: Up to 1.5A
• Reference Voltage: Typically 1.25V
• Maximum Input-to-Output Differential: 40V
• Typical Dropout Voltage: Approximately 2–3V
• Operating Junction Temperature: 0°C to +125°C
• Package: TO-220
• Number of Pins: 3
• Mounting Type: Through Hole

Pin Configuration

When viewing the printed face of the TO-220 package with the pins pointing downward:

1. ADJUST: Connects to the voltage-setting resistor network
2. OUTPUT: Provides the regulated output voltage
3. INPUT: Connects to the unregulated positive supply

The metal tab also connects electrically to the output pin. Therefore, use an insulating pad when mounting several devices on the same heatsink if their outputs differ.

Setting the Output Voltage

Use two resistors to set the output voltage:

VOUT = VREF × (1 + R2 ÷ R1) + IADJ × R2

For most basic calculations, you can use this simplified formula:

VOUT ≈ 1.25 × (1 + R2 ÷ R1)

A common design uses a 240Ω resistor for R1. Then, choose a fixed resistor or potentiometer for R2 to obtain the required output voltage.

Operating Requirements

The LM317CV is a linear regulator. Therefore, it cannot increase voltage, and its input must remain higher than the selected output.

For reliable regulation, provide approximately 2–3V of voltage headroom. For example, a regulated 9V output normally requires an input of about 11–12V.

The regulator converts excess voltage into heat. As a result, high current or a large voltage difference requires an adequate heatsink.

You can estimate the generated heat with this formula:

Power Dissipation = (VIN − VOUT) × Output Current

For example, regulating 12V down to 5V at 1A produces approximately 7W of heat. This operating condition requires substantial cooling.

Recommended External Components

For stable operation, use the components recommended for your circuit:

• 0.1µF input bypass capacitor
• 1µF tantalum or 25µF electrolytic output capacitor
• 10µF capacitor between ADJUST and ground for improved ripple rejection
• Protection diodes when using large output or adjustment capacitors
• Suitable heatsink for high-current applications

Applications

The LM317CV suits many electronic circuits, including:

• Adjustable bench power supplies
• Fixed-voltage regulators
• Current-limiting circuits
• Constant-current regulators
• LED current-control circuits
• Battery-charging circuits with suitable control and protection
• Arduino and microcontroller power supplies
• Educational electronics projects
• Prototyping and repair work

Important Notes

• Do not exceed the 40V input-to-output differential rating.
• The available output current depends on cooling and voltage difference.
• Use a heatsink when the regulator dissipates significant power.
• Check the pin configuration before connecting power.
• The regulator cannot boost the input voltage.
• Design battery chargers according to the battery chemistry and required protection.
• Always confirm the final output voltage before connecting sensitive equipment.

Package Includes

• 1× LM317CV Adjustable Positive Voltage Regulator in TO-220 Package