onsemi LM317MSTT3

Overview

LM317MSTT3 from onsemi is a 500 mA adjustable positive voltage regulator in SOT-223 package, delivering 1.2 V to 37 V output with 1.25 V internal reference voltage, ±2% output voltage tolerance, and built-in thermal shutdown and current limiting. It serves as a local on-card linear regulator for industrial power supplies and embedded control boards requiring stable low-noise DC bias.

For engineers reviewing the LM317MSTT3 datasheet, pinout, applications, or equivalent options, this page provides verified technical context, real-world design meaning of key specs, validated pin functions, and two confirmed alternative parts for adjustable linear regulation in space-constrained PCB layouts.

Technical Context

The LM317MSTT3 operates as a floating three-terminal regulator, maintaining a precise 1.25 V reference between Vout and Adjust terminals. Its output voltage is set by external resistors (Vout = 1.25 V × (1 + R2/R1) + IAdj × R2), where IAdj is controlled to ≤100 µA - minimizing error contribution across load and input conditions.

It integrates safe-area compensation for the output transistor, internal short-circuit current limiting, and thermal overload protection at 180°C junction temperature. The SOT-223 package enables surface-mount implementation with heatsink connection via Pin 2 (Adjust), supporting continuous 500 mA operation under proper PCB copper area layout per Figure 26.

Key Specifications

Pinout & Package

SOT-223 (Case 318E) package with exposed heatsink pad connected internally to Pin 2 (Adjust). Requires minimum 2.0 oz copper pad (≥25 mm² recommended per Figure 26) for thermal performance at full 500 mA load.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar adjustable linear regulation applications.

Compared with LM317MSTT3, LM317LD-TR trades output current and thermal robustness for smaller footprint, while TLV1117-33CDCYR sacrifices adjustability and voltage range for higher current and lower dropout - neither offers pin compatibility or functional equivalence in programmable regulator roles.

Availability

LM317MSTT3 is available at Aetrix Electronics and suitable for industrial power supplies, microcontroller bias rails, programmable test equipment, and automotive body control modules requiring stable component supply with long-term lifecycle support.

Supply support for LM317MSTT3 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

onsemi (formerly ON Semiconductor) is a global semiconductor supplier specializing in energy-efficient power management, analog, and sensing solutions for automotive, industrial, and cloud infrastructure markets.

The LM317M series belongs to onsemi's legacy linear regulator product line, designed for robust, low-cost, and easy-to-implement adjustable voltage regulation in harsh environments and space-constrained PCBs.

FAQ

What is the maximum input-to-output voltage differential for LM317MSTT3?

The LM317MSTT3 supports a maximum input-output voltage differential of 40 Vdc. This rating allows operation with inputs up to 40 V above the regulated output voltage - for example, 45 V input regulating 5 V output. Exceeding this limit risks device failure due to internal transistor breakdown, as specified in the Absolute Maximum Ratings table on page 2 of the datasheet.

Can LM317MSTT3 be used as a constant-current source?

Yes, the LM317MSTT3 can be configured as a precision constant-current source by connecting a single resistor (R_SET) between the Adjust and output terminals. With the internal 1.25 V reference, output current equals 1.25 V / R_SET. For example, a 2.5 Ω resistor delivers 500 mA. This mode is explicitly documented in Figure 24 and Applications Information section of the LM317M datasheet.

What is the purpose of the heatsink pad on the LM317MSTT3 SOT-223 package?

The exposed heatsink pad on the LM317MSTT3 (SOT-223, Case 318E) is electrically connected to Pin 2 (Adjust) and serves as the primary thermal conduction path to the PCB. Per Figure 26, a minimum 25 mm² copper area with 2.0 oz thickness reduces thermal resistance to air (θ_JA) from 245°C/W to ~80°C/W, enabling reliable 500 mA operation at ambient temperatures up to 50°C.

Does LM317MSTT3 require an input bypass capacitor?

Yes, a 0.1 µF ceramic capacitor (C_in) is required between Vin and ground when the LM317MSTT3 is located more than a few centimeters from the main power supply filter. As noted in Figure 1 footnote and Applications Information section, this capacitor reduces sensitivity to input line impedance and prevents oscillation or instability caused by trace inductance.

How does the adjustment pin current (I_Adj) affect output voltage accuracy?

I_Adj (≤100 µA) flows through R2 in the standard resistor divider, adding an error term: V_out = 1.25 V × (1 + R2/R1) + I_Adj × R2. For R2 = 2.4 kΩ, this contributes ≤0.24 mV error - negligible vs. 1.25 V reference. However, using R2 > 10 kΩ increases error proportionally; datasheet recommends R1 = 240 Ω and R2 sizing to keep I_Adj error <0.1% of V_out.