LM317AEMPX/NOPB

The Texas Instruments LM317AEMPX/NOPB is a versatile, adjustable three-terminal positive voltage regulator capable of supplying more than 1.5A of load current over an output voltage range of 1.25V to 37V. It is part of the well-established LM317 family but offers improved performance, including better line and load regulation. The device is highly reliable, thermally protected, and ideal for a wide range of power supply applications, from general-purpose bench supplies to more demanding industrial or automotive circuits.

This specific model, LM317AEMPX/NOPB, is packaged in a compact SOT-223 surface-mount package, making it suitable for space-constrained applications where traditional TO-220 packages would be too large. The "A" variant of the LM317 provides improved output voltage tolerance compared to the standard LM317, enabling higher precision in regulated outputs.

### Key Features

* Adjustable Output Voltage: 1.25V to 37V using two external resistors.
* High Output Current Capability: Up to 1.5A continuous output with proper heat sinking.
* High Precision Reference: Output voltage typically maintained within ±1% for LM317A (compared to ±1.5% in standard LM317).
* Thermal Overload Protection: Integrated circuitry limits junction temperature by reducing output current or shutting down when thermal limits are exceeded.
* Short-Circuit Protection: Current limiting circuitry automatically limits output current in the event of a short.
* Safe Operating Area (SOA) Protection: Maintains device operation within safe limits under overload or fault conditions.
* Line Regulation: Typically 0.01%/V over the full input voltage range.
* Load Regulation: Typically 0.1% over a 10 mA to 1.5 A load range.
* Low Quiescent Current: Typical 50μA to 100μA, allowing efficient operation in lightly loaded conditions.
* Reverse Polarity Protection: Can be implemented externally with a protection diode if required.
* Surface-Mount Package: SOT-223 (TO-261) for reduced PCB footprint.

### Electrical Specifications

* Input Voltage Range: Up to 40V (recommended maximum 35V for thermal reliability)
* Output Voltage Range: 1.25V to 37V (set via external resistor divider)
* Output Current: 1.5A maximum (with adequate heat sinking)
* Reference Voltage (Vref): 1.25V ±1%
* Line Regulation: Typically 0.01%/V
* Load Regulation: Typically 0.1% for output currents up to 1.5A
* Temperature Coefficient: Typically 0.003%/°C
* Quiescent Current: Typically 50μA
* Thermal Shutdown Threshold: ~150°C junction temperature
* Operating Junction Temperature: –40°C to +125°C

### Functional Description

The LM317A operates as a linear regulator with an adjustable output voltage. Internally, it consists of a voltage reference (1.25V), an error amplifier, and a pass transistor. The output voltage is set by the ratio of two external resistors connected between the output and the adjust (ADJ) pin, and from ADJ to ground. The output voltage is given by the formula:

VOUT = VREF × (1 + R1/R2) + IADJ × R2

Where:

* VREF = 1.25V (internal reference)
* IADJ is typically 50μA (can often be neglected for simplicity)
* R1 and R2 set the output voltage

A typical configuration uses a fixed resistor (R1 = 240Ω) between OUT and ADJ, and a variable resistor or fixed divider (R2) from ADJ to GND to set the desired output.

### Applications

* General-Purpose Adjustable Voltage Regulation: In power supply units for lab equipment, chargers, and control circuits.
* Post-Regulation for Switching Supplies: Smooths out noise and ripple from switch-mode regulators.
* Current Regulators: Can be used with a sense resistor in a constant current configuration.
* Battery Chargers: Suitable for regulated charging of lead-acid, NiMH, or other battery chemistries.
* Embedded Systems: Regulates voltages for analog sensors, microcontrollers, and other logic circuitry.
* Industrial Control: Voltage scaling and regulation for control loops, signal conditioning circuits.

### Thermal and Package Considerations

* Package: SOT-223 (TO-261), ideal for surface-mount applications
* Thermal Resistance (θJA): ~53°C/W on standard 1 oz copper PCB with minimal copper area
* Power Dissipation: Depends on PCB layout and ambient temperature. For example, with 25°C ambient and good layout, the device can dissipate up to ~1.9W without additional cooling.
* PowerPAD Option: The thermal pad on the SOT-223 helps with heat dissipation when connected to a copper plane.

To maximize thermal performance, it is important to mount the SOT-223 package on a PCB area with sufficient copper connected to the pad beneath the device. Heat sinking, either through the PCB or additional airflow, is essential for maintaining output current levels close to the maximum 1.5A, especially at high input-to-output differentials.

### Example Application

Vin: 15V
Vout: 5V
Iout: 1A
R1: 240Ω
R2: 720Ω
Cout: 10μF tantalum or electrolytic capacitor
Cin: 1μF ceramic or electrolytic (improves transient response and stability)

A 1N4001 or similar diode is often added between output and input pins to protect against reverse input voltage or output short conditions, especially in battery-backed systems.

### Summary

The LM317AEMPX/NOPB from Texas Instruments is a highly reliable and precise adjustable voltage regulator. Its low line and load regulation, built-in thermal protection, and wide output range make it an ideal choice for both commercial and industrial power supply applications. Packaged in a compact SOT-223 form, it offers robust performance while minimizing PCB space, making it a go-to solution for engineers needing dependable and adjustable linear voltage regulation.