LM350 – A Complete Guide on Voltage Regulation

Introduction to the LM350 Adjustable Voltage Regulator

The LM350 is a versatile and widely used adjustable voltage regulator IC. It provides an adjustable output voltage ranging from 1.2V to 33V with a current capability of up to 3A. The LM350 offers several key features that make it an excellent choice for various voltage regulation applications:

Adjustable output voltage from 1.2V to 33V
Output current capability up to 3A
Built-in thermal overload protection
Short-circuit protection
Requires only two external resistors to set the output voltage
Available in TO-220, TO-263, and TO-3 packages

In this comprehensive guide, we will dive into the details of the LM350 voltage regulator, its specifications, applications, and how to use it effectively in your projects.

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LM350 Pinout and Package Options

The LM350 is available in three different package options to cater to various design requirements:

TO-220 (3-pin)
TO-263 (3-pin)
TO-3 (2-pin)

The pinout for the LM350 is as follows:

Pin	Function
1	Adjustment (ADJ)
2	Output (VOUT)
3	Input (VIN)

In the TO-3 package, the tab is connected to the output pin.

Specifications and Electrical Characteristics

Here are some key specifications and electrical characteristics of the LM350:

Parameter	Value
Input Voltage Range	3V to 35V
Output Voltage Range	1.2V to 33V
Output Current (Continuous)	3A
Line Regulation	0.01% / V
Load Regulation	0.1%
Dropout Voltage	1.5V @ 3A
Quiescent Current	5mA

For detailed specifications, refer to the LM350 datasheet provided by the manufacturer.

Setting the Output Voltage

To set the output voltage of the LM350, you need to use two external resistors, R1 and R2. The output voltage is determined by the following equation:

VOUT = VREF * (1 + R2 / R1) + IADJ * R2

Where:
– VREF = 1.25V (reference voltage)
– IADJ = 50μA (typical adjustment pin current)

A common choice for R1 is 240Ω, which simplifies the equation to:

VOUT = 1.25V * (1 + R2 / 240Ω)

To calculate the value of R2 for a desired output voltage, use this formula:

R2 = (VOUT / 1.25V - 1) * 240Ω

For example, to set the output voltage to 5V, the value of R2 would be:

R2 = (5V / 1.25V - 1) * 240Ω = 720Ω

Typical Application Circuit

Here is a typical application circuit for the LM350 voltage regulator:

          +--------------+
          |              |
 IN ------|VIN        OUT|------- OUT
          |              |
          |        ADJ   |
          +----+-----|---+
               |      |
               |     +-+
               |     | | R1
               |     | | 240Ω
               |     +-+
               |      |
               |     +-+
               +-----| | R2
                     | |
                     +-+
                      |
                     GND

In this circuit:
– C1 and C2 are input and output capacitors for stability and noise reduction (typical values: 0.1μF ceramic)
– D1 is a protection diode to prevent reverse voltage (optional)
– R1 and R2 set the output voltage as per the formula mentioned earlier

Thermal Considerations and Heatsinking

The LM350 can dissipate a significant amount of power, especially when used with high output currents. To ensure proper operation and prevent damage, it is essential to consider the thermal aspects of your design.

The power dissipation in the LM350 can be calculated using the following formula:

PD = (VIN - VOUT) * IOUT

Where:
– PD = Power dissipation
– VIN = Input voltage
– VOUT = Output voltage
– IOUT = Output current

If the power dissipation exceeds the package’s thermal limits, a heatsink must be used to maintain the junction temperature within the safe operating range. The required heatsink thermal resistance can be calculated using this formula:

θ(HS) = (TJ(max) - TA) / PD - θ(JC)

Where:
– θ(HS) = Heatsink thermal resistance
– TJ(max) = Maximum junction temperature (typically 125°C)
– TA = Ambient temperature
– PD = Power dissipation
– θ(JC) = Junction-to-case thermal resistance (depends on the package)

Select a heatsink with a thermal resistance lower than the calculated θ(HS) value to ensure proper cooling.

Protection Features

The LM350 includes built-in protection features to safeguard the device and the connected load:

Thermal overload protection: The LM350 has an internal thermal shutdown circuit that turns off the device when the junction temperature exceeds a safe limit (typically 175°C). This protects the device from damage due to excessive power dissipation.
Short-circuit protection: The LM350 has a built-in current limit that protects the device in case of a short circuit at the output. When the output current exceeds the current limit (typically 3.5A), the output voltage will drop to maintain a safe current level.

These protection features make the LM350 a robust and reliable choice for voltage regulation applications.

Applications of the LM350

The LM350 voltage regulator finds use in a wide range of applications, such as:

Power supplies for electronic circuits
Battery Chargers
Voltage-controlled current sources
Adjustable voltage references
LED drivers
Motor speed control
Solar panel voltage regulation

Its adjustable output voltage and high current capability make the LM350 a versatile choice for many projects.

Troubleshooting Tips

If you encounter issues while using the LM350, here are some troubleshooting tips:

Check the input voltage: Ensure that the input voltage is within the specified range (3V to 35V) and has sufficient headroom for the desired output voltage and dropout.
Verify the output voltage setting: Double-check the values of R1 and R2 and make sure they are correctly calculated and connected.
Check the load current: Make sure the load current does not exceed the maximum rated current of the LM350 (3A continuous).
Inspect the board for shorts or open connections: Carefully examine the PCB for any shorts, open connections, or damaged components that may affect the LM350’s operation.
Monitor the temperature: If the LM350 is running hot, check the power dissipation and thermal management. Use a heatsink if necessary.

Frequently Asked Questions (FAQ)

Q: Can I use the LM350 with an input voltage higher than 35V?
A: No, the maximum input voltage for the LM350 is 35V. Exceeding this limit may damage the device.
Q: What is the minimum output voltage I can set with the LM350?
A: The minimum output voltage for the LM350 is 1.2V, which is determined by the internal reference voltage.
Q: Do I need to use a heatsink with the LM350?
A: It depends on the power dissipation and ambient temperature. If the power dissipation exceeds the package’s thermal limits, a heatsink is necessary to prevent overheating and damage.
Q: Can I parallel multiple LM350s for higher current output?
A: While it is possible to parallel LM350s, it is not recommended as it can lead to current hogging and uneven load sharing. It is better to use a single voltage regulator with a higher current rating or a different topology for high-current applications.
Q: What is the purpose of the adjustment pin on the LM350?
A: The adjustment pin is used to set the output voltage of the LM350 using an external voltage divider network (R1 and R2). It also provides a means for remote voltage sensing and adjustment.

Conclusion

The LM350 is a popular and versatile adjustable voltage regulator that offers a wide output voltage range, high current capability, and built-in protection features. By understanding its specifications, application circuits, and thermal considerations, you can effectively use the LM350 in your projects for reliable voltage regulation.

Remember to carefully select the appropriate package, calculate the output voltage setting resistors, and ensure proper thermal management for optimal performance and longevity.

With its ease of use and robust design, the LM350 remains a go-to choice for many engineers and hobbyists in a variety of voltage regulation applications.