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.
Important LM317T Characteristics and Options
An LM317 voltage regulator is a low-dropout (LDO) DC-DC converter that accepts and outputs over a broad range. The LM317T regulator has the following characteristics:
The design characteristics provide a quick overview of the LM317T that is helpful when choosing among the several variations, discussed below, that are available. Another important performance attribute for regulator circuits like the LM317T, is power conversion efficiency. 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.
LM317 Variants
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 at UL.
LM317 variants may include a variety of features, such as higher voltage input/output, thermal overheating protection, as shown in the LM317T datasheet, 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.
LM317 Regulation
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 for 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.
LM317T Datasheet Integration for PCB Design
To ensure accuracy of information and design data and models, you should institute component library best practices. Equally important is that your resource integrates with reliable industry distributors. 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.
Armed with accurate CAD models and the design information from the LM317T datasheet, enables you to make decisions such as what type of simulations to run. 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 or to analyze signal propagations, a specialized SPICE model to properly describe circuit behavior.
For sourcing components like the LM317T regulator at scale, depending on a comprehensive resource trusted throughout the PCB industry ensures you have access to manufacturer-vetted data and CAD models in vendor-specific and vendor-agnostic file formats, which can be quickly imported into these models into popular ECAD applications. If you’re looking for CAD models for common components or important design information on how to best use the LM317T datasheet, Ultra Librarian helps by compiling all your sourcing and CAD information in one place.
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