LM340T5 Voltage Regulator Everything You Need to Know

The LM340T5 is a three-terminal positive linear voltage regulator that converts a higher DC input voltage into a fixed and regulated 5 V output. Its three pins are Input, Ground, and Output, making it simple to connect and use. This article covers its specifications, pinout, working principle, application circuits, power dissipation, protection features, applications, comparisons, and possible replacements.

Catalog

LM340T5 Features and Specifications

Specification	LM340T5 Value
Regulator type	Positive linear voltage regulator
Output type	Fixed
Number of outputs	One
Nominal output voltage	5 V
Output voltage at 25°C	4.8 V to 5.2 V
Output voltage over operating conditions	4.75 V to 5.25 V
Rated test load	Specifications provided at up to 1 A
Maximum practical output current	Depends on input voltage, heat sinking, and operating temperature
Peak output current	2.4 A typical at 25°C
Short-circuit current	2.1 A typical at 25°C
Recommended input voltage	Approximately 7.5 V to 20 V for full specified regulation
Absolute maximum DC input voltage	35 V
Dropout voltage	2 V typical at 1 A and 25°C
Quiescent current	8 mA typical at 25°C
Maximum quiescent current	8.5 mA over the operating temperature range
Line regulation	3 mV typical, 50 mV maximum at 25°C under the specified test conditions
Load regulation	10 mV typical, 50 mV maximum at 25°C for a 5 mA to 1.5 A load change
Ripple rejection	62 dB minimum, 80 dB typical at 120 Hz
Output noise voltage	40 µV typical from 10 Hz to 100 kHz
Output resistance	8 mΩ typical at 1 kHz
Output temperature coefficient	−0.6 mV/°C typical
Minimum load used for full regulation specifications	5 mA
Operating temperature range	0°C to +125°C
Maximum junction temperature	150°C
Storage temperature range	−65°C to +150°C
Internal power dissipation	Internally limited
Junction-to-ambient thermal resistance	54°C/W for the TO-220 package without a heat sink
Junction-to-case thermal resistance	4°C/W for the TO-220 package
Thermal overload protection	Built in
Short-circuit current limiting	Built in
Safe-operating-area protection	Built in
ESD susceptibility	2 kV
Package type	Three-lead TO-220
Mounting type	Through-hole
Pin configuration	Pin 1 Input, Pin 2 Ground, Pin 3 Output
Output capacitor	Not required for basic stability, but 0.1 µF can improve transient response

LM340T5 Pinout & Pin Functions

Pin Number	Pin Name	Pin Function
1	Input	Connects to the positive unregulated DC input supply. The input voltage must remain high enough above 5 V to maintain proper regulation.
2	Ground	Connects to the circuit ground and serves as the reference point for the input and output voltages. The metal tab is also electrically connected to ground.
3	Output	Provides the regulated positive 5 V DC output to the load. An optional output capacitor can improve transient response.

LM340T5 Working Principle

The LM340T5 converts a higher DC input voltage into a fixed 5 V output. In the diagram, the internal reference circuit produces a stable voltage. The error amplifier continuously compares this reference with a sample of the output voltage.

When the output falls below 5 V, the regulator increases the drive to the pass transistor, allowing more current to reach the load. When the output rises, it reduces the transistor drive. This feedback process keeps the output close to 5 V despite changes in the input voltage or load.

The internal circuit also provides current limiting, safe-area protection, and thermal shutdown. Since excess voltage is released as heat, the LM340T5 may require a heat sink when operating with a high input voltage or load current.

LM340T5 Typical Application Circuit

Three typical LM340T5 circuits: a fixed-output regulator, an adjustable-output regulator, and a current regulator.

In the fixed-output circuit, the LM340T5 converts a higher DC input into a regulated 5 V output. The 0.22 µF input capacitor helps reduce input noise and is especially important when the regulator is far from the power-supply filter. The optional output capacitor improves transient response when the load changes quickly.

The adjustable-output circuit uses resistors R1 and R2 to raise the output above 5 V. The LM340T5 maintains about 5 V between its output and ground pins, while the resistor network shifts the ground reference. This circuit cannot regulate below the device’s fixed 5 V level.

The current-regulator circuit uses resistor R1 to set a nearly constant output current. The LM340T5 maintains about 5 V across R1, so the approximate current is:

The resistor must have a suitable power rating for the required current.

LM340T5 Input Voltage Requirements

The LM340T5 requires an input voltage higher than its fixed 5 V output. It typically needs about 2 V of voltage headroom, but approximately 7.5 V is required to maintain specified line regulation at loads up to 1 A. If the input voltage falls too low because of supply ripple or battery discharge, the regulator enters dropout and the output may fall below 5 V.

For proper operation, the input should normally remain within the datasheet’s specified range of approximately 7.5 V to 20 V. The device has an absolute maximum DC input rating of 35 V, but this is a damage limit, not a recommended operating voltage. A higher input voltage also creates more heat, so using 35 V continuously would be impractical for most 5 V applications.

LM340T5 Output Current and Load Capacity

The LM340T5 is fully specified at a load current of up to 1 A and may deliver more than 1 A when adequate cooling is provided. However, its actual load capacity depends on the input voltage, ambient temperature, heat sink, and available airflow. The regulator cannot deliver its highest current continuously if excessive power must be released as heat.

For example, supplying 1 A from a 12 V input requires the regulator to drop 7 V and dissipate about 7 W. Without a suitable heat sink, thermal shutdown may occur before the load receives the required current. The connected load must also maintain an input voltage high enough for regulation and remain within the regulator’s safe thermal limits.

LM340T5 Power Dissipation and Heat-Sink Calculation

The LM340T5 converts the difference between its input and output voltages into heat. Its approximate power dissipation is calculated using:

PD≈ (V_IN−V_OUT) × I_OUT

For a 12 V input, 5 V output, and 1 A load:

PD= (12V−5V) × 1A =7W

The approximate junction temperature without a heat sink is:

TJ=TA+(PD×θJA)

For the TO-220 package, the datasheet gives a junction-to-ambient thermal resistance of approximately 54°C/W without a heat sink. At 7 W, the calculated temperature rise would be about 378°C, which is far beyond the safe limit. A heat sink is therefore essential in this example.

The maximum allowable heat-sink thermal resistance can be estimated using:

Here, is approximately 4°C/W for the TO-220 package, while represents the thermal resistance between the case and heat sink. For reliable operation, use a design temperature below the absolute maximum junction temperature and include a safety margin.

LM340T5 Protection and Safety Features

The LM340T5 includes internal current limiting to reduce output current during an overload or short circuit. Its safe-operating-area protection also limits the pass transistor under conditions where high input voltage and output current occur together. These protections reduce the risk of immediate device damage.

Thermal shutdown activates when the internal temperature becomes excessive. The regulator reduces or interrupts its output until the temperature falls. However, repeated thermal shutdown is a sign of poor thermal design and can reduce long-term reliability. These internal features do not replace correct heat sinking, input protection, proper grounding, and operation within the datasheet limits.

LM340T5 Applications

• Providing stable 5 V power for microcontroller boards

• Regulating the supply voltage for sensor modules

• Powering controllers and peripherals in embedded systems

• Supplying control sections in industrial equipment

• Creating a fixed 5 V rail in consumer electronics

• Delivering low-noise power for audio equipment

• Supporting low-voltage sections in telecommunications equipment

• Reducing automotive DC voltage to a regulated 5 V level

• Producing a stable output in laboratory power supplies

• Powering logic devices and interfaces on development boards

• Supplying local 5 V power in test and measurement equipment

• Replacing fixed 5 V regulators in older electronic equipment

LM340T5 vs 7805 Voltage Regulator

Feature	LM340T5	7805 Voltage Regulator
Regulator type	Standard positive linear regulator	Standard positive linear regulator
Output voltage	Fixed 5 V	Fixed 5 V
Output current	Fully specified at 1 A and may provide more with proper cooling	Common versions provide 1 A to 1.5 A with proper cooling
Typical dropout voltage	About 2 V at 1 A	Commonly about 2 V, depending on the model
Recommended input	Approximately 7.5 V or higher for full regulation	Commonly 7 V to 8 V or higher
Maximum input voltage	35 V absolute maximum	Commonly 30 V to 35 V, depending on the manufacturer
Quiescent current	About 8 mA typical	Commonly several milliamperes
Main package	Three-pin TO-220	Commonly TO-220, with other packages available
Pin arrangement	Input, Ground, Output	Usually Input, Ground, Output, but verify the datasheet
Protection features	Current limiting, thermal shutdown, and safe-area protection	Usually current limiting and thermal protection
Product availability	Original LM340T-5.0 is obsolete	Many 7805 versions remain available
Best use	Existing LM340T5 designs and legacy equipment	General-purpose fixed 5 V regulation

LM340T5 vs LDO Regulator

Feature	LM340T5	LDO Regulator
Regulator type	Standard linear regulator	Low-dropout linear regulator
Output voltage	Fixed 5 V	Fixed or adjustable, depending on the model
Dropout voltage	About 2 V at 1 A	Usually much lower, often below 1 V
Required input voltage	Approximately 7.5 V or higher for full regulation	Some 5 V LDOs can operate from inputs only slightly above 5 V
Efficiency	Mainly determined by the ratio of output voltage to input voltage	Also determined by the voltage ratio, but lower dropout improves low-headroom operation
Heat generation	High when the input is much greater than 5 V	Lower when operating with a small input-to-output difference
Quiescent current	About 8 mA typical	Can range from microamperes to several milliamperes
Capacitor requirements	Generally simple and tolerant	Capacitor value and ESR requirements vary by model
Output noise	Low compared with many switching regulators	Low-noise versions are available
Battery operation	Less suitable because of its dropout and quiescent current	Often better for battery-powered equipment

LM340T5 vs Buck Converter

Feature	LM340T5	Buck Converter
Conversion method	Linear regulation	High-frequency switching
Output voltage	Fixed 5 V	Fixed or adjustable
Efficiency	Approximately (VOUT/VIN)	Commonly much higher, depending on the converter and load
Power loss	Excess voltage is released as heat	Energy is transferred through switching components
Heat generation	Can be high at large voltage drops or load currents	Usually lower for the same conversion
External components	Requires few components	Requires an inductor, capacitors, and sometimes an external diode
Circuit complexity	Simple	More complex
Output noise	Generally low	Higher ripple and switching noise
Electromagnetic interference	Very low switching interference	Requires careful layout and filtering
Input-voltage headroom	Needs about 2 V above the 5 V output	Some models operate with a smaller input-output difference, but not all
Heat-sink requirement	May require a heat sink	Often does not require one at the same power level
Battery life	Lower when the input is much higher than 5 V	Usually longer because of better efficiency

Equivalents and Replacements

Replacement	Manufacturer	Output	Current Capability	Compatibility
LM340AT-5.0/NOPB	Texas Instruments	Fixed 5 V	Up to 1.5 A	Direct replacement
LM7805CT/NOPB	Texas Instruments	Fixed 5 V	Up to 1.5 A	Direct or close replacement
UA7805CKCS	Texas Instruments	Fixed 5 V	Up to 1.5 A	Direct replacement
L7805CV	STMicroelectronics	Fixed 5 V	Up to 1.5 A	Close replacement
L7805ABV	STMicroelectronics	Fixed 5 V	Up to 1.5 A	Close replacement
MC7805CTG	onsemi	Fixed 5 V	Up to 1 A	Close replacement
MC7805ACTG	onsemi	Fixed 5 V	Up to 1 A	Close replacement
LM2940CT-5.0	Texas Instruments	Fixed 5 V	Up to 1 A	Alternative, not always direct
LM317T	Various	Adjustable	Up to 1.5 A	Not a direct replacement
5 V switching regulator module	Various	Fixed or adjustable 5 V	Model-dependent	Functional alternative

Mechanical Dimensions

Manufacturer

National Semiconductor originally manufactured the LM340T5 using its established analog semiconductor design, wafer fabrication, assembly, packaging, and electrical testing capabilities. The company produced the regulator in a three-pin TO-220 package and incorporated internal current limiting, thermal shutdown, and safe-operating-area protection to support reliable operation. Its manufacturing facilities included sites in the United States, Scotland, and Malaysia, supported by international design and sales operations. Texas Instruments completed its acquisition of National Semiconductor in September 2011, adding National’s analog products, technical expertise, and manufacturing capacity to TI’s Analog business.

Frequently Asked Questions [FAQ]

1. Can the LM340T5 be connected directly to an AC power source?

No. The LM340T5 accepts DC input only. An AC source must first pass through a transformer or suitable AC-DC supply, rectifier, and filter capacitor before reaching the regulator.

2. What capacitors should be placed around the LM340T5?

A 0.22 µF input capacitor is recommended when the regulator is far from the power-supply filter. A 0.1 µF output capacitor is optional for basic stability but can improve transient response. Larger capacitors may help reduce low-frequency ripple.

3. Why does the LM340T5 output contain ripple or noise?

Ripple may result from poor input filtering, long wires, weak grounding, rapid load changes, or incorrect capacitor placement. Place bypass capacitors close to the regulator pins and use short, low-resistance ground connections.

4. Can two LM340T5 regulators be connected in parallel for more current?

Direct parallel connection is not recommended because small output-voltage differences can cause one regulator to carry most of the load. Proper current-sharing resistors or an external pass-transistor design would be required.

5. Does the LM340T5 need a protection diode?

A protection diode may be needed when a large output capacitor is used or when the input can become shorted while the output remains charged. The diode provides a safe discharge path and helps prevent reverse current through the regulator.

6. Can the LM340T5 power an Arduino or another 5 V development board?

It can provide regulated 5 V if the board’s total current remains within the regulator’s electrical and thermal capacity. The regulated output should be connected to the correct 5 V input point, not to a higher-voltage input intended for an onboard regulator.