LM323: Everything You Need to Know

What is the LM323?

The LM323 is a popular 3-terminal adjustable voltage regulator IC capable of supplying over 3A of output current. It is a member of the LM series of linear voltage regulators originally developed by National Semiconductor (now part of Texas Instruments).

The LM323 offers several advantages:
– High output current capability (>3A)
– Wide input voltage range (up to 40V)
– Adjustable output voltage via two external resistors
– Built-in overload and thermal shutdown protection
– Low quiescent current
– Simple implementation requiring minimal external components

These features make the LM323 well-suited for a variety of voltage regulation applications where high current output is needed, such as powering electronics, motors, servos, LED lighting, battery charging, etc.

Request PCB Manufacturing & Assembly Quote Now

LM323 Specifications

Key specifications for the LM323 include:

Parameter	Value
Output Current	>3A
Input Voltage Range	3V to 40V
Output Voltage Range	1.2V to 37V
Quiescent Current	5mA (typical)
Line Regulation	0.01%/V (typical)
Load Regulation	0.1% (typical)
Ripple Rejection	80 dB (typical)
Operating Temperature Range	0°C to 125°C

The wide 3-40V input range accommodates a variety of unregulated input sources. The output voltage can be set to any value between 1.2V and 37V via two external resistors, allowing configuration for common voltages like 3.3V, 5V, 12V, etc.

Line and load regulation are both very good, minimizing output voltage variations due to input voltage changes or load current demands. The high 80 dB ripple rejection filters out AC line noise.

How the LM323 Works

The LM323 uses a bandgap voltage reference and error amplifier to regulate the output voltage. The bandgap reference provides a stable 1.25V reference voltage.

The error amplifier compares a sample of the output voltage, taken from the adjustment pin, to this 1.25V reference. If the sampled voltage is too low, the error amp increases the current through the series pass transistor, raising the output voltage. If the sampled voltage is too high, the error amp decreases the pass transistor current to lower the output.

The two external resistors (R1, R2) connected to the adjustment pin set the regulated output voltage based on the equation:

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

Where Iadj is a small 50-100μA quiescent current from the ADJ pin that can usually be ignored.

Overload protection works by limiting the pass transistor current to a safe value if the output is shorted to ground. Thermal shutdown turns off the pass transistor if the die temperature exceeds a 150°C threshold, allowing the device to cool. Normal operation automatically resumes once the temperature falls about 20°C below the shutdown threshold.

Applications

Some common applications for the LM323 include:

Bench power supply – With a suitable transformer, rectifier, and filter capacitors, the LM323 can regulate the output voltage for a adjustable bench power supply capable of supplying >3A.
Battery charger – The LM323 can be configured as a constant voltage/constant current (CV/CC) battery charger for various battery chemistries like lithium-ion, lead-acid, NiCd, NiMH, etc.
Motor voltage regulator – For powering DC motors, the LM323 can regulate the voltage to control the speed. The high current capability is suitable for driving larger motors.
LED constant current supply – By placing a sensing resistor in series with the LED load, the LM323 can be used as a constant current source for driving high-power LEDs or LED strings.
Voltage step-down (buck) converter – When powered from a higher voltage, the LM323 can step down the voltage with high efficiency to power lower voltage circuitry.
Raspberry Pi/Arduino power supply – The LM323 can be used to make a 5V, >3A power supply well-suited for powering a Raspberry Pi, Arduino, or similar Embedded Computers from a higher input voltage.

LM323 Circuit

Here is a typical LM323 voltage regulator circuit:

The key components are:

C1 (input capacitor) – Filters the input supply and provides stability. A 0.1μF ceramic in parallel with a larger 1-10μF electrolytic is common. Place close to the IC.
C2 (output capacitor) – Improves transient response and filters output noise. Use a low-ESR 1-10μF electrolytic or tantalum. Place close to the IC.
D1 (protection diode) – Protects the IC from reverse polarity of the input supply. A 1N4001 or similar diode works well.
R1, R2 (adjustment resistors) – Set the output voltage based on the ratio of these two resistors. Use the equation Vout = 1.25 * (1 + R2/R1) to calculate values.

Additional capacitors C3 (adjust pin bypass) and C4 (input bypass) can be added to improve noise rejection but are optional. Use a heat sink for full 3A output current to keep the IC junction temperature below 125°C.

Calculating Resistor Values

To set the output voltage, the values of adjustment resistors R1 and R2 must be calculated. A common approach is to pick a value for R1 (100-1000Ω) then calculate R2 based on the desired output voltage.

Rearranging the output voltage equation to solve for R2 gives:

R2 = R1 * (Vout/1.25 - 1)

For example, to get a 5V output with R1 = 240Ω:

R2 = 240 * (5/1.25 - 1) 
   = 240 * 3
   = 720Ω

So R1 = 240Ω and R2 = 720Ω would give a 5V regulated output.

Here are resistor values for some standard output voltages with R1 = 240Ω:

Vout	R1	R2
3.3V	240Ω	384Ω
5V	240Ω	720Ω
12V	240Ω	2160Ω

FAQ

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

A: The absolute maximum input voltage is 40V. In practice, keep the input at least 2-3V above the desired output voltage for proper regulation.

Q: How much current can the LM323 supply?

A: The LM323 can typically supply over 3A of continuous output current with a suitable heat sink. The exact current limit depends on the input-output voltage differential and the thermal conditions.

Q: What happens if the output is shorted to ground?

A: The LM323 has built-in short circuit protection that limits the output current to a safe value. The device will not be damaged by a sustained output short but it will get hot, so a heat sink is still recommended.

Q: Can I parallel multiple LM323s for more output current?

A: Yes, you can parallel LM323s to increase the current capability. Add a small 0.1-0.2Ω Ballast Resistor in series with each output to help balance the load between units. Make sure each IC has its own heat sink.

Q: Does the LM323 need a heat sink?

A: For output currents under 1A, the LM323 may not need a heat sink if the input-output voltage differential is small and there is good air flow. For higher currents, a heat sink is required to prevent thermal shutdown. Use a thermal resistance calculator to determine the heat sink size.

I hope this comprehensive LM323 Guide is helpful! Let me know if you have any other questions.