The LM317T is an easy-to-use adjustable regulator for your next PCB.
That small black component that resembles a transistor might actually be an LM317T voltage regulator. This regulator is a simple component for use in low voltage or high voltage DC systems, including networking equipment, office equipment, rechargeable mobile devices, and much more. These regulators do not have the highest efficiency, but the small number of required external components and their small footprint makes them a great choice for power regulation.
If you need an LM317T schematic symbol, PCB footprint, and simulation model, you shouldn’t have to make these items by hand. An electronics search engine can give you access to an LM317T datasheet and CAD models for use in your ECAD application. Here’s why you should consider using the LM317T regulator in your next board and where you can access an LM317T datasheet.
Using an LM317T Voltage Regulator
An LM317 voltage regulator is a low-dropout (LDO) DC-DC converter that accepts and outputs over a broad range. Like other standard amplifiers, buffers, and linear regulator circuits, it comes in several variants that differ slightly in specifications and on-board features. The LM317T regulator has the following characteristics:
- Input and output voltage ranges: The allowed input voltage ranges from 3 to 40 V, and the output voltage ranges from 1.2 to 37 V.
- Current output: The maximum current output is in excess of 1.5 V. Due to feedback in an LM317T circuit, the typical minimum load current required to maintain regulation is 3.5 mA, depending on the manufacturer.
- Package style: The package style is 3-pin TO220. Note that variants of this component may use a different package style.
- Darlington pair topology: The integrated Darlington pair arrangement provides low impedance output with high capacitance for stable output power with 0.1% or lower line regulation. Because the Darlington pair acts like a capacitance multiplier, some LM317T datasheets will state an output capacitor is optional.
The maximum power conversion efficiency scales from 42% to 93% as the output voltage scales from minimum to maximum. In cases where high DC voltage is converted to low DC voltage, there will be significant heat dissipation during conversion, but the die-attached heat sink on the package helps keep the component cool by dumping heat into the substrate.
Because this is an LDO regulator, the input voltage should only be slightly higher than the output voltage to ensure high efficiency (i.e., low heat dissipation). If you need to convert high AC or DC voltage down to low DC voltage, it’s best to use a switching regulator on the intermediate stage to step down the input DC voltage. Simply set the input on the LM317T about 1 V above the desired output to ensure high efficiency.
The LM317T component is one of many variants on the standard LM317 voltage regulator, and the LM317T is arguably the most popular choice among these variants. Other variants on the LM317 provide the same or similar voltage/current output, line regulation, and topology. They differ in package styles, as shown in the component search engine results below. Common alternative package styles are SOT, WSON, DPAK, and D2PAK. If you want to build PCB footprints yourself, these package styles are highly standardized and can be easily made in your PCB design software.
LM317 variants available in component search engine results.
LM317 variants may include a variety of features, such as higher voltage input/output, thermal overheating protection (as shown in the LM317T datasheet from Texas Instruments), and overcurrent protection. Other variants use multiple Darlington pairs or a very complicated transistor-transistor topology. Be sure to check the circuit diagram in your datasheet to learn more about your LM317 variant.
The power output is adjusted by sourcing DC current into the adjust pin. This current then adjusts the bias on an integrated error amplifier, which is standard in LDO topologies. Normally, an LDO uses an integrated or external reference voltage source (e.g., silicon bandgap reference) to regulate the output voltage. These circuits are temperature-sensitive, which means the regulated output will vary with temperature.
By adjusting the DC current into the adjust pin, such as with a programmable current sense amplifier, the device can automatically compensate temperature variations. This would require a temperature measurement near the LM317, such as with a thermistor. If you don’t need manual or programmatic adjustment, regulation is still maintained with a feedback loop into the adjust pin, making the output power less sensitive to changes in temperature compared to regulators that use a temperature-sensitive reference.
Finding Your LM317T Datasheet and Components
When you need to find an LM317T datasheet and components, a component search engine can be a great resource to find popular components and LM317 variants. These tools provide users with a broad range of common and specialized variants alongside their specifications, datasheets, and CAD models for your PCB design software. You’ll also have access to sourcing information for components, including prices, stocks, lead times, and a distributor list.
Components like power regulators can be examined with a variety of simulation models. Using a phenomenological model is appropriate for a common topology like an LDO with a silicon bandgap reference circuit. However, more complicated variants need a specialized SPICE model to properly describe circuit behavior. When component simulation models are available in your search results, you can ensure your simulation results are accurate, and you won’t have to build a simulation model in your SPICE tool.
When you need to find an LM317T datasheet, or you need to source LM317T regulators at scale, try using the parts search features in Ultra Librarian. You’ll have access to verified LM317T CAD models in vendor-specific and vendor-neutral file formats, and you can quickly import these models into popular ECAD applications. You’ll also see up-to-date sourcing information from authorized worldwide distributors. All the component data you’ll find on Ultra Librarian can be accessed at no cost and is verified from component manufacturers.
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