LM350 – A Complete Guide on Voltage Regulation

What is the LM350 and How Does It Work?

The LM350 is a popular adjustable Voltage Regulator IC that provides a simple way to regulate voltage in electronic circuits. It can supply up to 3A of current and has an adjustable output voltage range of 1.2V to 33V. The LM350 operates by using a bandgap reference voltage and error amplifier to maintain a constant output voltage across a range of input voltages and load currents.

The LM350 has just three terminals:
1. Input (VIN) – Connects to the unregulated DC input voltage
2. Output (VOUT) – Provides the regulated output voltage
3. Adjustment (ADJ) – Sets the regulated output voltage using a resistor divider

Here is the basic circuit diagram for using an LM350 voltage regulator:

      ┌────────┐
 ─────┤VIN VOUT├─────┬────── +VOUT
      │    ADJ │     │  
 ─────┴────────┴─────┴────── GND
          │
          ├─────┬────────────
          │     │
         R1    R2
          │     │
          └─────┴────────────

By selecting appropriate values for resistors R1 and R2, you can set the desired output voltage using this formula:

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

Some key specifications of the LM350:
– Input voltage range: Up to 35V
– Adjustable output voltage range: 1.2V to 33V
– Output current: Up to 3A
– Line regulation: 0.01% typical
– Load regulation: 0.1% typical
– Dropout voltage: 1.5V at 3A

Advantages of the LM350

Some of the main benefits of using an LM350 for voltage regulation include:

1. Simple to use – Requires few external components
2. Adjustable output – Can set desired voltage with resistors
3. High current capability – Up to 3A output current
4. Built-in protection – Has thermal overload and short-circuit protection
5. Low cost – Inexpensive linear regulator IC

Disadvantages of the LM350

The LM350 also has some limitations compared to other types of voltage regulators:

1. Inefficient at high voltages – Significant power is dissipated as heat
2. Large dropout voltage – Requires 1.5V headroom at full load
3. No built-in heatsinking – May require external heatsink for high power

Calculating Output Voltage and Resistor Values

To use the LM350, you need to calculate the output voltage setting resistors R1 and R2 based on your desired regulated voltage.

The output voltage is determined by this formula:

VOUT = VREF * (1 + R2 / R1)
where VREF = 1.25V for the LM350

By rearranging the equation, we can calculate R2 based on R1 and desired VOUT:

R2 = R1 * (VOUT / VREF – 1)

A common value used for R1 is 240 ohms. Plugging that in:

R2 = 240 * (VOUT / 1.25 – 1)

So for example, to get a 5V output:
R2 = 240 * (5.0 / 1.25 – 1) = 720 ohms

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

VOUT	R1	R2
1.5V	240Ω	48Ω
3.3V	240Ω	384Ω
5.0V	240Ω	720Ω
12V	240Ω	2.16kΩ

Note: Use resistors with tight 1% tolerance for best accuracy.

Adjusting Output Voltage

One convenient feature of the LM350 is that you can adjust the output voltage at any time by changing the values of R1 and R2. This allows you to fine-tune the voltage without modifying the circuit.

To increase VOUT, either:
– Decrease R1
– Increase R2

To decrease VOUT, either:
– Increase R1
– Decrease R2

Just be sure to use the equation to calculate the right resistances to avoid exceeding the input voltage or current limits.

Input and Output Capacitors

For optimal stability, the LM350 datasheet recommends using input and output capacitors, especially for output voltages above 5V or in high-current applications.

The input capacitor helps reduce input ripple and noise. Use a 0.1uF ceramic capacitor close to the VIN and GND pins.

IN ───┬─── LM350 VIN
     ─┴─ 0.1uF
      │ 
     GND

The output capacitor improves load regulation and transient response. Use at least a 1uF tantalum or 10uF aluminum electrolytic capacitor between VOUT and GND.

       1uF
VOUT ─┬───┴─ GND  
      │
    LM350 

For outputs greater than 10V or 1A current, increase COUT to 22uF or more. Locate the capacitor close to the IC.

Thermal Considerations and Heatsinking

The LM350 can handle an impressive 3A of output current, but this comes with a catch – the more current it supplies, the hotter the IC will get due to power dissipation. At high power levels, the LM350 will need additional heatsinking to avoid overheating and thermal shutdown.

The maximum power the LM350 can dissipate depends on the ambient temperature and its thermal resistance from junction to air (θJA). Without a heatsink, the θJA of the TO-220 package is 60°C/W.

Use this equation to calculate the power dissipation:

PD = (VIN – VOUT) * IOUT

And to calculate the junction temperature rise:

TJ rise = PD * θJA

For example, with VIN=15V, VOUT=5V, and IOUT=1A:

PD = (15V – 5V) * 1A = 10W

Assuming a 50°C max ambient temp, the junction temp would be:

TJ = 50°C + (10W * 60°C/W) = 650°C !

Since the max rated junction temp is 125°C, this would cause the LM350 to go into thermal shutdown. To fix this, we need to add a heatsink to lower θJA.

The size of heatsink needed depends on the power dissipated. Here are some common TO-220 heatsink thermal resistance values:

Heatsink	θCS (°C/W)
None	60
Small	30
Medium	12
Large	6

A medium heatsink would lower the junction temp to a safe level:

TJ = 50°C + (10W * 12°C/W) = 170°C

Be sure to use thermal paste between the LM350 and heatsink for optimal transfer. Also allow adequate airflow around the heatsink.

By properly managing Thermal dissipation through smart component selection and heatsinking, you can unleash the full power potential of the LM350 regulator in your designs.

Protection Features

The LM350 has several built-in protection features that help prevent damage from common fault conditions:

Thermal Overload Protection

If the internal junction temperature exceeds 125°C, the LM350 will temporarily shut down until it cools off. This prevents damage from sustained overheating.

Short-Circuit Protection

The LM350 is designed to survive a continuous short from output to ground. The output current is limited to a safe level.

Safe Operating Area

The LM350 can handle a wide range of output voltages and currents within its safe operating area (SOA). Consult the datasheet graphs to ensure your design stays within the SOA limits.

Applications and Circuit Examples

The LM350 is a versatile regulator suited for many applications, such as:

• Battery Chargers
• Constant current sources
• Programmable power supplies
• Voltage reference for ADCs/DACs
• LED drivers
• Motor speed control

Here are a few example LM350 circuits for common applications:

Constant Current Source

By connecting a resistor RS from the ADJ pin to ground, the LM350 can supply a constant current to a load, set by:

IOUT = 1.25V / RS

      ┌────────┐
 ─────┤VIN VOUT├─────┬────── +VOUT
      │    ADJ │     │  
 ─────┴────┬───┴─────┴────── GND
           │
           RS
           │
           └────────────────── 

For example, a 12.5 ohm resistor will set the current to 100mA.

Programmable Power Supply

Combining a potentiometer with the ADJ pin resistors allows you to create an adjustable voltage power supply:

      ┌────────┐
 ─────┤VIN VOUT├─────┬────── +VOUT
      │    ADJ │     │  
 ─────┴────┬───┴─────┴────── GND
           │
         ┌─┴─┐
        R1  R2 
         └─┬─┘
           │
           ↓

Turning the pot will vary the output from 1.2V up to the input voltage. Use a multiturn pot for fine control.

Battery Charger

An LM350 makes a simple constant-voltage battery charger when connected to a current-limiting resistor:

                   RS
      ┌────────┐   ┌─┴─┐
 ─────┤VIN VOUT├───┤   ├─┬─ +VBAT
      │    ADJ │   └───┘ │  
 ─────┴────┬───┴─────────┴─ -VBAT
           │
         ┌─┴─┐
        R1  R2 
         └───┘

RS limits the charging current while R1/R2 set the float voltage. Ensure VBAT never exceeds VIN.

PCB Layout Tips

When laying out an LM350 circuit on a PCB, follow these guidelines for best performance:

1. Place input and output caps close to IC pins
2. Use wide traces for high current paths
3. Provide large ground plane for heatsinking
4. Keep ADJ pin away from noise sources
5. Route feedback traces away from inductor fields

A good PCB layout is essential to get the most out of your LM350 voltage regulator design.

Frequently Asked Questions

Q: Can I parallel multiple LM350s for more current?

A: Yes, you can connect two or more LM350s in parallel to increase the current capacity. Tie all the VIN, ADJ, and GND pins together. Connect Ballast Resistors (~0.1 ohm) in series with each VOUT pin to ensure even current sharing.

Q: How do I reduce output ripple and noise?

A: To minimize output ripple and noise:
– Use low-ESR output capacitors
– Add a feedforward capacitor across top feedback resistor
– Keep ADJ pin away from switching noise
– Use RC snubber on ADJ if needed

Q: What happens if the input voltage goes above 35V?

A: Applying more than 35V to the LM350 input pin may damage the device. Use a zener diode or voltage suppressor to clamp VIN to a safe level if there is a risk of overvoltage.

Q: Can I get a negative output voltage?

A: No, the LM350 only regulates positive voltages with respect to ground. For negative outputs, consider using an LM337 negative regulator or an inverting switching regulator topology.

Q: What’s the minimum output current required for stability?

A: The LM350 is designed to be stable with no load. However, at very light loads (< 1mA), it may oscillate without an output capacitor. Always use a 1uF or greater capacitor across VOUT and GND.