LM317 is an adjustable voltage regulator widely used in electronic circuits. It can stabilize the input voltage to a user-specified output voltage. LM317 is a 3-terminal adjustable positive voltage regulator with an adjustable voltage range from 1.25V to 37V. It can provide a current greater than 1.5A at the output end. In most applications, due to load irregularities, the resulting output voltage fluctuates, leading to load damage. Hence the use of a voltage regulator.

LM317 is one of the most widely used power integrated circuits. It not only has the simplest form of a fixed three-terminal voltage regulator circuit, but also has the characteristics of adjustable output voltage. In addition, it also has the advantages of wide voltage regulation range, good voltage stabilization performance, low noise, and high ripple suppression ratio.

Ⅰ.The function of LM317

1.Provide stable output voltage: LM317 can maintain a fixed output voltage and is not affected by input voltage changes or load changes. This is crucial for many electronic devices and circuits as it ensures that other components in the circuit are able to operate at a stable voltage.
2.Current Limit: The LM317 has an internal current limit feature that prevents the current from exceeding its rating. This helps protect circuits and connected equipment.
3.Adjustable output voltage: LM317 allows users to adjust the output voltage by connecting an external resistor divider network to adapt it to the needs of various applications. This makes it ideal for electronic circuits requiring different voltage requirements.
4.Low dropout voltage: The input voltage of LM317 can be higher than the required output voltage, which means that its efficiency is relatively high. This is useful for battery-powered applications as it extends battery life. ,
5.Overheating protection: LM317 usually has built-in overheating protection, which can shut down the chip when its temperature is too high to prevent damage.

Ⅱ.Main parameters of LM317

•Output current:5mA-1.5A
•Output voltage:1.25-37V DC
•Minimum input-output voltage difference:3V DC
•Maximum input-output voltage difference:40V DC
•Using ambient temperature:-10-+85℃
•Storage environment temperature:-65-+150℃
•Rated output current:100mA-1.5A
•Ripple suppression:80dB
•Load regulation:0.1%
•Line regulation rate:0.01%/V

Ⅲ.Working principle of LM317

1.LM317 is a common three-terminal voltage regulator chip with three main pins: input (Vin), output (Vout) and adjustment (ADJ). Here is a basic description of these pins:
•Input (Vin): This is the power input pin, usually connected to a power supply that requires a stable voltage.
•Output (Vout): This is a stable voltage output pin that can be connected to a circuit or device that requires such stable voltage.
•Adjustment (ADJ): This pin is mainly used to adjust the output voltage. By adjusting the voltage of this pin, the output voltage can be changed.

2.Feedback control: Return a part of the output voltage to the ADJ pin of LM317 through an external resistor divider network. There is a comparator inside the chip that compares the feedback voltage on the ADJ pin (generated by resistor voltage division) with the internal reference voltage. Based on this comparison result, the chip automatically adjusts its output voltage to maintain a stable ADJ voltage, thereby keeping the output voltage unchanged.

3.Reference voltage source: LM317 contains a stable reference voltage source (usually 1.25 volts) internally, which is the key to its operation. This reference voltage source is generated internally within the chip and is used to compare and adjust the output voltage.

4.Voltage adjustment: By changing the resistance value in the resistor divider network, the ADJ voltage can be adjusted, thereby adjusting the output voltage of LM317. This makes the LM317 ideal for applications requiring different output voltages.

5.Overheating protection: LM317 has overheating protection function. In circuit design, overheating protection is a very important safety mechanism that can prevent equipment from being damaged due to overheating or dangerous situations such as fire. In the LM317 chip, the overheating protection function is implemented through the internal temperature sensor and related circuits. When the temperature of the chip exceeds a certain value, these circuits will trigger the overheating protection mechanism and automatically reduce the output voltage, thereby reducing the power consumption of the chip and preventing further increase in temperature.

6.Current Limit: The LM317 usually has an internal current limiter to prevent the output current from exceeding its rating. If the load current exceeds its capability, it automatically reduces the output voltage to limit the current.

Ⅳ.MAXIMUM RATINGS OF LM317

Ⅴ.ELECTRICAL CHARACTERISTICS OF LM317

Ⅵ.Application fields of LM317

1.Power supply equipment: LM317 is usually used to manufacture a variety of power supply equipment, including fixed and adjustable. The system can provide stable power supply voltage for a variety of electronic devices and circuits.

2.Experimental power supply: LM317 is often used to build laboratory power supplies, and the power supply is used for electronic experiments and prototype development. Its adjustable voltage feature makes it suitable for a variety of different types of circuits.

3.LED driver: LM317 can be used to drive LED lights to provide constant current or constant voltage drive to ensure stable LED brightness. This is very useful in lighting applications.

4.Battery Charger: LM317 can be used to build chargers to charge various types of batteries, including lithium-ion batteries, nickel-cadmium batteries, etc. It allows the user to adjust the output voltage to suit a specific type of battery.

5.Electronic equipment and instruments: LM317 can be used in various electronic equipment and instruments to provide stable power supply voltage, such as test equipment, measuring instruments and communication equipment.

6.Solar battery charging control: In solar battery charging systems, LM317 can be used to control and stabilize the output voltage of solar panels to adapt to battery charging needs.

7.Automotive electronics: LM317 can be used in automotive electronic circuits to provide a stable power supply voltage for sensors and other electronic components while adapting to voltage changes in the automotive battery.

8.Wearable devices and portable electronics: LM317 can be used to provide stable power supply to wearable devices and portable electronics to ensure their normal operation.

Ⅶ.Efficiency of LM317

The efficiency of the LM317 depends on several factors, including input voltage, output voltage, output current, and chip temperature. Overall, the LM317 generally exhibits relatively high efficiency, but is not an absolutely efficient regulator.

1.Efficiency calculation method of LM317:
Efficiency (η) = (output voltage / input voltage) × 100%

2.The efficiency of LM317 is mainly affected by the following two factors:

•Output current: The output current also affects the efficiency of the LM317. Higher output current will result in greater power dissipation, thereby reducing efficiency. Therefore, when selecting the LM317, the required output current needs to be considered.
•Input-output voltage differential: Input-output voltage differential is the difference between the input voltage and the desired output voltage. If the difference between input voltage and output voltage is smaller, the efficiency is higher. The minimum input-to-output voltage difference of the LM317 is typically about 2.5V. Therefore, efficiency is higher when the input voltage is very close to the output voltage.

Ⅷ.Alternate model of LM317

1.LM337: LM337 is the negative regulator version of LM317, which produces a negative output voltage. It has similar characteristics and working principles to the LM317.

2.LM7805: LM7805 is a fixed 5V output voltage regulator. It does not have the adjustability of LM317, but it is suitable for applications that require a stable 5V power supply.

3.LM1117: The LM1117 is another adjustable voltage regulator, similar to the LM317, but has a lower voltage drop and therefore is more effective when the input-output voltage difference is smaller.

4.AMS1117: This is an affordable adjustable voltage regulator, similar to LM317, commonly used in small power supply applications.

5.LM2940/LM2941: These voltage regulators are low-voltage versions of the LM317 and are typically used in electronic devices that require lower output voltages, such as 3.3V or 2.5V.

Frequently Asked Questions

1.What are the voltage stabilizing characteristics of LM317?
The LM317 is a linear voltage regulator that maintains a stable output voltage by adjusting the output voltage to offset input voltage and load changes. This means that it can provide a relatively stable output voltage with respect to changes in input voltage and load.

2.Can LM317 sink current?
It can draw a constant current of 100mA, 200mA, 500mA or 1A, selected via a rotary switch. The official minimum input voltage is 4V, but in practice the current remains constant at input voltages as low as 3.6V. As long as adequate cooling is provided for the LM317, the maximum input voltage is 35V.

3.How to control LM317?
This signal can be controlled using a microcontroller or any other digital circuit. The same microcontroller can be used to dynamically monitor the output and adjust the LM317T. Using an RC low-pass filter and an op amp, you can convert the PWM signal to a DC level, thereby regulating the voltage output of the LM317.

4.How to determine the maximum current output of LM317?
Under high output current conditions, the LM317 will generate some heat. Ensure appropriate cooling measures, such as using a heat sink, to maintain the chip within a safe temperature range. Excessive temperatures may cause the thermal protection feature to trigger and reduce output current.