Introduction to the LM317 Adjustable Voltage Regulator
The LM317 is a popular adjustable positive voltage regulator capable of supplying over 1.5A of output current over an output voltage range of 1.25V to 37V. It is exceptionally easy to use and requires only two external resistors to set the output voltage. The lm317 Datasheet provides all the key specifications and application information needed to incorporate this versatile regulator into a wide variety of power supply designs.
Key Features and Benefits of the LM317
The LM317 offers several advantages over fixed output voltage regulators:
- Adjustable output voltage from 1.25V to 37V
- Output current capability exceeding 1.5A
- Line regulation typically 0.01%/V
- Load regulation typically 0.1%
- Thermal overload protection
- Short circuit protection
- Output transistor safe operating area protection
These features make the LM317 suitable for use in applications requiring a regulated voltage that may need to be adjusted, high output current, and/or robust protection against fault conditions. Its wide operating voltage range allows it to be used as a simple adjustable benchtop power supply, battery charger, constant current source, and more.
Understanding the LM317 Datasheet Specifications
To use the LM317 effectively in a circuit design, it’s important to understand the key specifications provided in the datasheet. Here are some of the most important parameters:
Output Voltage Range
The LM317 can provide an adjustable output voltage (VOUT) from 1.25V up to 37V. The output voltage is set by two external resistors, R1 and R2, according to the formula:
VOUT = 1.25V * (1 + R2/R1) + IADJ * R2
Where IADJ is the LM317’s adjustment pin current, typically 50μA. Note that the minimum output voltage of 1.25V is set by the reference voltage between the output and adjustment pins.
Output Current
The LM317 has an output current capability exceeding 1.5A, provided adequate heat sinking is used. The available output current is dependent on the input-to-output voltage differential and the thermal resistance from junction to ambient. These relationships are illustrated in the datasheet’s “Current Limit” graph and “Thermal Regulation” curves.
Line and Load Regulation
Line regulation refers to the change in output voltage due to a change in input voltage, while load regulation refers to the change due to a change in load current. The LM317 datasheet specifies typical values of 0.01%/V for line regulation and 0.1% for load regulation. This means the output will remain stable over a wide range of operating conditions.
Protection Features
The LM317 includes several built-in protection features:
– Thermal overload protection shuts down the regulator if the junction temperature exceeds a safe limit.
– Short circuit protection limits the output current to a safe value during a short.
– Safe operating area protection prevents damage to the output transistor under high current and high voltage stress.
The datasheet SOA graph shows the allowable combinations of output voltage and current. Operation outside this region may activate one of the protection features or cause permanent damage.
Basic LM317 Adjustable Regulator Circuit
Here is the basic circuit for using the LM317 as an adjustable voltage regulator:
+---------+
IN | | OUT
+------| LM317 |------+
GND | | |
+--| |--+ +-+--+
| | | |
| +-------| | R2
| | |
+--+--+ +-+--+
| | |
| R1 | |
| | |
+-----+ -----
-
R1 and R2 set the output voltage according to:
VOUT = 1.25V * (1 + R2/R1)
Typical values would be R1 = 240Ω and R2 up to 5kΩ for an adjustable output voltage range of 1.25V to 22V. The input voltage should be at least a few volts higher than the desired output to ensure regulation. An input and output capacitor, both in the 1μF range, should also be included close to the LM317 pins for stability.
LM317 as Constant Current Source
The LM317 can also be configured as a constant current source by connecting the load between the OUT and ADJ pins:
+---------+
IN | | OUT
+------| LM317 |------+
GND | | |
+--| |--+ |
| | |
| +---------|
| |
| +-+
| |
| | RSET
| |
| +-+
+--+--+ |
| | |
|RLOAD| |
| | |
+-----+ -----
-
In this configuration, the LM317 will attempt to maintain a constant 1.25V across RSET, resulting in a constant current of:
IOUT = 1.25V / RSET
RSET is typically a low value under 10Ω for currents in the hundreds of mA range. RLOAD represents the load being driven. The voltage compliance depends on the input voltage and thermal conditions.
LM317 Thermal Considerations and Heatsinking
Like any linear regulator, the LM317 dissipates a significant amount of power as heat, especially when the input voltage is much higher than the output or when driving high load currents. The power dissipated is given by:
PD = (VIN – VOUT) * IOUT
This heat must be properly managed to prevent the LM317 from entering thermal shutdown or sustaining damage. In many cases, this requires attaching the LM317 package to a suitable heatsink.
The maximum allowable junction temperature rise is determined by:
TJ(max) = TA(max) + (PD * ΘJA)
Where TA is the maximum ambient temperature, PD is the power dissipated, and ΘJA is the junction-to-ambient thermal resistance. ΘJA depends on factors like the package type, PCB copper area, airflow, and any heatsink used.
The datasheet provides typical values of ΘJA for different package types and PCB conditions. For example, in still air with a minimum recommended PCB copper area, ΘJA is 60°C/W for the TO-220 package. So with a maximum TA of 50°C and PD of 10W, the junction temperature would reach an absolute maximum of:
TJ = 50°C + (10W * 60°C/W) = 650°C
Clearly this exceeds the safe limit of 125°C! In this case, a heatsink must be added to reduce ΘJA. The required heatsink thermal resistance ΘSA can be calculated from:
ΘSA ≤ (TJ(max) – TA(max)) / PD – ΘJC – ΘCS
Where ΘJC is the junction-to-case thermal resistance (5°C/W for TO-220) and ΘCS is the case-to-heatsink thermal resistance (usually 1-2°C/W). Continuing the example:
ΘSA ≤ (125°C – 50°C) / 10W – 5°C/W – 1°C/W ≤ 1.5°C/W
So a heatsink with a thermal resistance of 1.5°C/W or lower would be adequate for this scenario. The heatsink datasheet should be consulted for dimensions and mounting requirements to achieve the rated performance.
FAQ on LM317 Voltage Regulator
What is the minimum output voltage of the LM317?
The minimum output voltage is 1.25V, set by the reference voltage between the OUT and ADJ pins.
How much output current can the LM317 supply?
The LM317 is capable of supplying in excess of 1.5A output current, provided the package is adequately heatsinked. The exact current limit depends on the input-output voltage differential and thermal conditions.
What is the purpose of the adjustment (ADJ) pin on the LM317?
The ADJ pin is used to set the regulated output voltage. A voltage divider connected between OUT, ADJ, and GND will program the output to:
VOUT = 1.25V * (1 + R2/R1)
Where R1 is connected between ADJ and GND, and R2 is connected between OUT and ADJ.
How can I use the LM317 as a constant current source?
By connecting the load between OUT and ADJ (instead of OUT and GND), the LM317 will regulate the current through the load to:
IOUT = 1.25V / RSET
Where RSET is a low value resistor connected between ADJ and GND. The load sees a constant current regardless of its terminal voltage (within the compliance range set by the input voltage and thermal conditions).
What is thermal shutdown and how does it protect the LM317?
Thermal shutdown is a built-in protection feature that shuts down the LM317 output if the junction temperature exceeds a safe level around 125°C. This prevents permanent damage from overheating. The LM317 will automatically recover once it has cooled a few degrees below the shutdown threshold. Proper heatsinking should be used to prevent unintended thermal cycling.
Conclusion
The LM317 adjustable voltage regulator is a versatile building block for linear power supplies requiring voltages from 1.25V to 37V at currents over 1.5A. Its ease of use, robust protection features, and availability in several packages make it a popular choice for both hobbyists and professional designers.
By understanding the specifications and graphs provided in the LM317 datasheet, an appropriate regulator circuit can be designed for a wide range of applications. Care must be taken to manage the potentially high power dissipation through proper heatsinking.
While newer switchmode regulators offer higher efficiency, the LM317 remains an excellent solution where simplicity and low noise are paramount. Studying its datasheet provides a foundation in practical linear regulator design.
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