Introduction to LM350 Voltage Regulator

The LM350 is a popular adjustable voltage regulator IC capable of supplying over 3A of output current. It is an easy-to-use, reliable linear regulator that provides excellent regulation and ripple rejection. The LM350 is widely used in various applications, including power supplies, Battery Chargers, and voltage-controlLED Circuits.

Key Features of LM350

• Adjustable output voltage from 1.2V to 33V
• Output current up to 3A
• Excellent line and load regulation
• Low dropout voltage
• Built-in thermal overload protection
• Short-circuit protection
• Available in TO-220, TO-263, and TO-3 packages

How Does the LM350 Work?

The LM350 is a three-terminal adjustable positive voltage regulator. It consists of a reference voltage source, an error amplifier, a pass transistor, and a feedback network. The reference voltage is set to 1.25V, and the output voltage is adjusted by setting the ratio of two external resistors connected to the adjustment pin.

Basic Circuit Configuration

To use the LM350, you need to connect three external components:

1. Input capacitor (Cin): A 0.1µF ceramic capacitor connected between the input pin and ground to improve transient response and noise rejection.
2. Output capacitor (Cout): A 1µF tantalum or 10µF aluminum electrolytic capacitor connected between the output pin and ground to improve stability and transient response.
3. Adjustment resistors (R1 and R2): Two resistors that set the output voltage according to the equation:

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

where Iadj is the adjustment pin current, typically around 50µA.

Here’s a basic circuit diagram for the LM350:

        ┌───────────────────┐
   ───┬─┤IN             OUT├──┬────── Vout
      │ │                   │  │
    Cin │                   │ Cout
      │ │                   │  │
   ───┴─┤GND            ADJ├──┴─┬────── GND
        └─────────┬─────────┘    │
                  │              │
                 R1              │
                  │              │
                 ───            R2
                  │              │
                 GND            GND

Choosing the Adjustment Resistors

To set the desired output voltage, you need to choose appropriate values for R1 and R2. A common recommendation is to choose R1 between 240Ω and 1.5kΩ, and then calculate R2 using the equation:

R2 = R1 × (Vout / 1.25V – 1)

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

R2 = 240Ω × (5V / 1.25V – 1) = 720Ω

You can use the following table as a quick reference for common output voltages:

Vout	R1	R2
3.3V	240Ω	384Ω
5V	240Ω	720Ω
12V	240Ω	2.16kΩ
24V	240Ω	4.56kΩ

Protections and Limitations

The LM350 has several built-in protection features to ensure safe and reliable operation:

Thermal Overload Protection

The LM350 has an internal thermal shutdown circuit that turns off the pass transistor when the junction temperature exceeds approximately 175°C. This protects the device from damage due to excessive heat dissipation. The thermal shutdown is automatically reset when the temperature drops below the threshold.

Short-Circuit Protection

The LM350 has a current limit circuit that protects the device from damage due to short-circuits or excessive load currents. When the output current exceeds the limit (typically around 3.5A), the pass transistor is turned off, and the output voltage drops to zero. The current limit is automatically reset when the short-circuit condition is removed.

Dropout Voltage

The dropout voltage is the minimum input-to-output voltage difference required for the regulator to maintain regulation. For the LM350, the typical dropout voltage is around 2V at full load current. This means that the input voltage must be at least 2V higher than the desired output voltage for proper regulation.

Power Dissipation

The maximum power dissipation of the LM350 depends on the package and the ambient temperature. To ensure safe operation, you need to calculate the power dissipation and ensure that it does not exceed the maximum rating. The power dissipation can be calculated using the equation:

Pd = (Vin – Vout) × Iout

For example, with Vin = 24V, Vout = 5V, and Iout = 1A, the power dissipation is:

Pd = (24V – 5V) × 1A = 19W

If the power dissipation exceeds the maximum rating, you may need to use a heat sink or a different package to ensure proper thermal management.

Applications of LM350

The LM350 is a versatile voltage regulator that can be used in various applications, such as:

1. Fixed voltage power supplies
2. Adjustable bench power supplies
3. Battery chargers
4. Voltage-controlled circuits
5. Constant current sources
6. Motor speed controls
7. Lighting control systems

Frequently Asked Questions (FAQ)

1. What is the maximum input voltage for the LM350?

The maximum input voltage for the LM350 is 35V. However, to ensure proper operation and avoid exceeding the maximum power dissipation, it is recommended to keep the input voltage below 30V.

2. Can I parallel multiple LM350s to increase the output current?

Yes, you can parallel multiple LM350s to increase the output current. However, you need to ensure proper load sharing by using small Ballast Resistors (typically 0.1Ω to 0.5Ω) in series with each regulator’s output. This helps to balance the current among the regulators and prevent one device from taking more than its share of the load current.

3. How do I adjust the output voltage of the LM350?

To adjust the output voltage of the LM350, you need to change the values of the adjustment resistors R1 and R2. Increase R2 to increase the output voltage, or decrease R2 to decrease the output voltage. You can also use a potentiometer for R2 to make the output voltage adjustable.

4. What is the typical ripple rejection of the LM350?

The typical ripple rejection of the LM350 is around 75 dB at 120 Hz, with a 10µF output capacitor. This means that the regulator can effectively suppress input voltage ripple and provide a clean, stable output voltage.

5. Can I use the LM350 as a negative voltage regulator?

No, the LM350 is designed as a positive voltage regulator. If you need a negative voltage regulator, you can use the LM337, which is the negative counterpart of the LM350.

Conclusion

The LM350 is a reliable, easy-to-use adjustable voltage regulator that offers excellent performance and protection features. By understanding its basic operation, choosing the right external components, and considering its limitations, you can effectively use the LM350 in a wide range of applications. Whether you need a fixed voltage power supply, an adjustable bench power supply, or a voltage-controlled circuit, the LM350 is a solid choice for your voltage regulation needs.