LM317 regulator introduction, pin diagram, parameters, working principle and application circuit diagram
The LM317 is a widely used, three-terminal adjustable positive voltage regulator IC developed by National Semiconductor. It offers an output voltage range from 1.2V to 37V and can supply up to 1.5A of current. This makes it highly versatile for a variety of power supply applications. The simplicity of the LM317 lies in its requirement of only two external resistors to set the desired output voltage, making it user-friendly and cost-effective.
One of the key advantages of the LM317 is its superior line and load regulation compared to standard fixed-voltage regulators. It also includes built-in protection features such as overcurrent, overheat, and short-circuit protection, ensuring safe operation under various conditions. Additionally, it has safety zone protection that prevents damage due to excessive power dissipation. While the LM317 typically does not require an external capacitor, it is recommended when the input filter capacitor is more than 6 inches away from the IC. Output capacitors can improve transient response, while input capacitors enhance ripple rejection.
The LM317 can be used in special configurations as well. For example, by connecting the adjustment terminal to a higher voltage, it can regulate voltages beyond its normal range, provided the input-output voltage difference remains within limits. It can also be used in programmable power supplies by connecting the adjustment pin to a variable voltage source. However, care must be taken to avoid shorting the output, which could damage the device.
**Key Features:**
- Adjustable output voltage as low as 1.2V
- Guaranteed 1.5A output current
- Typical line regulation of 0.01%
- Typical load regulation of 0.1%
- 80dB ripple rejection ratio
- Built-in short-circuit, overcurrent, and overheat protection
- Safety area protection for the pass transistor
- Standard three-terminal package
**Voltage Range:**
The LM317 allows continuous adjustment from 1.25V to 37V, making it suitable for a wide range of applications.
**Pin Configuration:**
The LM317 has three pins: Input (IN), Output (OUT), and Adjustment (ADJ). The input receives the unregulated voltage, the output provides the regulated voltage, and the adjustment pin is connected to a resistor network to set the output voltage.
**Working Principle:**
The LM317 operates as a series regulator. The internal circuitry maintains a constant 1.25V reference between the output and adjustment terminals. By using a resistor divider between the output and the adjustment pin, the output voltage can be adjusted. A small current flows through the resistor network, and the voltage at the adjustment pin controls the output voltage accordingly.
**Internal Schematic:**
The internal structure of the LM317 includes a reference voltage source, error amplifier, and pass transistor. These components work together to maintain a stable output voltage despite variations in input or load conditions.
**Application Circuits:**
There are multiple common application circuits for the LM317:
1. **Standard Regulated Power Supply Circuit:** This configuration uses two resistors to set the output voltage and includes input and output capacitors for stability.
2. **Adjustable Current Limit and Voltage Circuit:** This circuit adds a current limit feature, allowing control over both voltage and current output.
3. **5.0V Electronic Shutdown Regulator:** This design includes a shutdown feature to turn off the output when not needed.
4. **Current Regulator Circuit:** This setup regulates the output current rather than the voltage.
5. **Adjustable Current Limiter:** This circuit protects the load from excessive current.
6. **Soft Start Circuit:** This helps prevent inrush current during startup, protecting the regulator and connected components.
In all cases, it’s important to ensure proper heat dissipation, especially when operating at high currents. Without a heatsink, the LM317 can dissipate up to 2W, but with a heatsink, this can increase to 15W. Capacitors should be placed close to the IC inputs and outputs to reduce noise and improve performance. Diodes like IN4002 are often added for protection against reverse polarity or short circuits.
Overall, the LM317 is a reliable and flexible component that has been widely used in power supply designs for decades. Its ease of use, robustness, and versatility make it a popular choice among engineers and hobbyists alike.
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