Introduction to the LM323 Voltage Regulator

The LM323 is a versatile and reliable 3-terminal adjustable voltage regulator capable of supplying over 3A of output current. Its robustness and ease-of-use have made it a popular choice for a wide range of power supply applications for over 40 years. In this comprehensive guide, we’ll cover everything you need to know about the LM323, including its key features, electrical characteristics, applications, and design considerations.

Key Features of the LM323

The LM323 boasts an impressive set of features that contribute to its widespread use in power supply designs:

1. High Output Current: The LM323 can deliver a continuous output current of over 3A, making it suitable for powering demanding loads.
2. Adjustable Output Voltage: The output voltage can be set to any value between 1.2V and 37V using just two external resistors.
3. Low Dropout Voltage: With a typical dropout voltage of 1.5V at full load, the LM323 can maintain regulation even with input voltages close to the output voltage.
4. Thermal Overload Protection: Built-in thermal shutdown protects the device from damage due to excessive power dissipation.
5. Short Circuit Protection: The LM323 can withstand indefinite short circuits without damage.
6. Wide Operating Temperature Range: The device is specified to operate from 0°C to 125°C.

Electrical Characteristics

The following table summarizes the key electrical characteristics of the LM323:

Parameter	Min	Typ	Max	Unit
Output Voltage Range	1.2	–	37	V
Line Regulation	–	0.01	0.04	%/V
Load Regulation	–	0.1	0.5	%
Quiescent Current	–	5	10	mA
Output Current Limit	–	3.5	–	A
Dropout Voltage (@ 3A)	–	1.5	1.8	V
Operating Temperature Range	0	–	125	°C

Applications

The LM323’s high output current and adjustable output voltage make it suitable for a wide range of applications, including:

1. Bench Power Supplies: The LM323 is often used in adjustable benchtop power supplies for electronics labs and workshops.
2. Battery Chargers: With its high current capability, the LM323 can be used to build robust battery chargers for various battery chemistries.
3. Automotive Electronics: The wide input voltage range and high output current of the LM323 make it well-suited for automotive power supply applications.
4. Industrial Control Systems: The LM323 can provide reliable power for sensors, actuators, and other industrial control components.
5. Consumer Electronics: From LED drivers to audio amplifiers, the LM323 finds use in a variety of consumer electronic devices.

Designing with the LM323

Basic Circuit Configuration

The basic circuit configuration for the LM323 consists of the regulator IC, input and output capacitors, and two resistors for setting the output voltage. The following schematic shows a typical application circuit:

[Insert schematic image here]

The output voltage is set by the ratio of resistors R1 and R2 according to the equation:

Vout = 1.25V * (1 + R2/R1)

Capacitors C1 and C2 provide input and output filtering, respectively. Typical values for C1 and C2 are 0.33μF and 1μF, but these may be adjusted based on the specific application requirements.

Thermal Considerations

The LM323 can dissipate a significant amount of power, especially when operating with high input-to-output voltage differentials and high load currents. To ensure reliable operation, it’s important to properly manage the device’s thermal performance.

The power dissipated by the regulator can be calculated as:

Pdiss = (Vin – Vout) * Iout

Where Pdiss is the power dissipation, Vin is the input voltage, Vout is the output voltage, and Iout is the load current.

The junction-to-ambient thermal resistance (θJA) of the LM323 depends on the package type and PCB layout. For the TO-3 package, θJA is typically around 4°C/W with proper heatsinking. The maximum allowable junction temperature is 125°C.

To ensure the device operates within its thermal limits, the following condition must be met:

Pdiss * θJA ≤ 125°C – Ta

Where Ta is the ambient temperature.

If the calculated junction temperature exceeds the maximum allowable value, a larger heatsink or improved PCB layout may be necessary.

Protection Diodes

Although the LM323 includes built-in protection against reverse input polarity and output short circuits, it’s still recommended to include external protection diodes in the design.

A reverse polarity protection diode should be placed in series with the input to prevent damage in case the input voltage is accidentally reversed. A Schottky diode with a current rating exceeding the maximum load current is recommended to minimize the forward voltage drop.

An output protection diode, also known as a “catch” diode, should be placed between the output and ground to provide a safe discharge path for inductive loads. This diode should have a reverse voltage rating greater than the maximum input voltage and a forward current rating exceeding the maximum load current.

PCB Layout Considerations

Proper PCB layout is essential for achieving optimal performance and reliability with the LM323. Some key layout considerations include:

1. Minimizing trace inductance: Keep the traces connecting the input capacitor, output capacitor, and LM323 as short and wide as possible to minimize parasitic inductance.
2. Proper grounding: Use a solid ground plane and keep the ground connections for the input and output capacitors close to the LM323’s ground pin.
3. Heatsinking: If using a surface-mount package, provide sufficient copper area around the device for heat dissipation. If using the TO-3 package, ensure proper heatsink mounting and thermal interface material application.
4. Input/output filtering: Place the input and output capacitors as close to the LM323 as possible to maximize their effectiveness in reducing noise and ripple.

FAQ

Q1: What is the maximum input voltage for the LM323?

A1: The absolute maximum input voltage for the LM323 is 40V. However, to ensure reliable operation, it’s recommended to keep the input voltage below 37V.

Q2: Can the LM323 be used as a constant current regulator?

A2: Yes, the LM323 can be configured as a constant current regulator by connecting a sense resistor in series with the load and using the voltage across this resistor to control the regulator’s output voltage.

Q3: What is the typical efficiency of the LM323?

A3: The efficiency of the LM323 depends on the input voltage, output voltage, and load current. In general, efficiency ranges from 40% to 60% under typical operating conditions.

Q4: Can the LM323 be paralleled for higher output current?

A4: While it’s possible to parallel LM323 regulators for increased output current, it’s not recommended due to the difficulty in ensuring equal current sharing between devices. It’s better to use a single regulator with a higher current rating.

Q5: What is the purpose of the adjustment pin on the LM323?

A5: The adjustment pin is used to set the output voltage of the LM323. By connecting a resistor divider between the output, adjustment pin, and ground, the output voltage can be set to any value between 1.2V and 37V.

Conclusion

The LM323 is a robust and versatile 3-terminal adjustable voltage regulator that has stood the test of time. Its high output current, wide input voltage range, and built-in protection features make it a popular choice for a variety of power supply applications. By understanding the device’s electrical characteristics, thermal considerations, and design guidelines, engineers can effectively incorporate the LM323 into their designs to create reliable and efficient power solutions.