Texas Instruments LM317LZ/LFT3

Overview

LM317LZ/LFT3 from Texas Instruments is a 3-terminal, adjustable positive-voltage linear regulator delivering up to 100mA output current with 1.25V to 37V programmable output voltage range. It uses floating topology, sees only input-to-output differential voltage (max 45V), and requires no output capacitor for stability. It serves as a precision voltage or current regulator in industrial power supplies and analog signal conditioning circuits.

For engineers reviewing the LM317LZ/LFT3 datasheet, LM317LZ/LFT3 pinout, LM317LZ/LFT3 application, or LM317LZ/LFT3 equivalent, key selection considerations include its 100mA output capability, ±3000V HBM ESD rating, 1.25V reference voltage accuracy, thermal shutdown protection, and compatibility with TO-92 package for through-hole prototyping and low-cost production.

Technical Context

The LM317LZ/LFT3 implements an NPN Darlington output stage with internal 1.25V bandgap reference and feedback amplifier referenced to the OUTPUT pin-not ground-enabling floating operation and high-voltage regulation (e.g., >100V input when VO = 37V). Its bias current flows into the OUTPUT pin, requiring ≥1.28mA minimum load current to maintain regulation.

It integrates overcurrent and thermal-overload protection that remains active even if the ADJUST pin is disconnected. Ripple rejection reaches 71.5dB at 120Hz with optional 10µF ADJ-to-ground capacitor, and line regulation is typically 0.007%/V for the RFB-fab chip variant.

Key Specifications

Pinout & Package

LM317LZ/LFT3 is packaged in TO-92 (LP) - a 3-pin through-hole plastic package measuring 4.8mm × 3.68mm with standard lead spacing. Pin 1 is INPUT, Pin 2 is OUTPUT, Pin 3 is ADJUSTMENT. All OUTPUT pins are internally connected; no NC pins present in this variant.

Key Features

Applications

Equivalent & Alternatives

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

Compared with LM317LZ/LFT3, LM317LDR2G offers SMT compatibility but reduced thermal performance, while TLV1117-33CDCYR trades adjustability for higher current and lower dropout-making it unsuitable for floating, high-VI–VO, or current-regulation use cases.

Availability

LM317LZ/LFT3 is available at Aetrix Electronics and suitable for solar energy charge controllers, HVAC sensor biasing, optical network terminal units, and energy storage system monitoring requiring stable component supply with long-term industrial lifecycle support.

Supply support for LM317LZ/LFT3 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

Texas Instruments is a global semiconductor leader specializing in analog, embedded processing, and power management technologies, with decades of heritage in precision linear regulators and industrial-grade components.

The LM317L series was designed for rugged, cost-sensitive industrial and telecom power applications-emphasizing floating operation, wide input differential tolerance, and minimal external component count for reliable voltage or current regulation.

FAQ

What is the maximum input-to-output differential voltage supported by LM317LZ/LFT3?

The LM317LZ/LFT3 supports a maximum input-to-output differential voltage of 45V for the CSO:RFB fabrication variant. This rating enables safe operation with high-input sources such as rectified AC or multi-cell battery strings, provided power dissipation (VI–VO) × IO remains within thermal limits defined by the TO-92 package's RθJA of 139.5°C/W.

Does LM317LZ/LFT3 require an output capacitor for stability?

No, LM317LZ/LFT3 does not require an output capacitor for stability due to its internal compensation and floating architecture. An optional 1µF ceramic or tantalum capacitor may be added between OUTPUT and ground to improve transient response-especially under dynamic load changes-but introduces no risk of oscillation or phase-margin degradation.

How is output voltage calculated for LM317LZ/LFT3 in standard configuration?

LM317LZ/LFT3 output voltage is calculated as VO = 1.256V × (1 + R2/R1) + IADJ × R2, where IADJ is 44–57µA (CSO:RFB). For most designs using R2 ≤ 10kΩ, the IADJ × R2 term contributes <0.6V error and can be neglected; thus VO ≈ 1.256V × (1 + R2/R1) defines practical setting accuracy.

Can LM317LZ/LFT3 be used as a constant-current source?

Yes, LM317LZ/LFT3 can function as a precision constant-current source by connecting a single resistor R between ADJUST and OUTPUT terminals. The output current is ILIMIT = 1.256V / R. For example, a 25Ω resistor sets 50.2mA-ideal for LED biasing or battery trickle charging without additional sensing components.

What thermal considerations apply to LM317LZ/LFT3 in TO-92 package at 100mA?

In TO-92 (LP) package, LM317LZ/LFT3 has RθJA = 139.5°C/W. At 100mA and 10V VI–VO, power dissipation is 1W, resulting in ~140°C junction rise above ambient. To stay within 125°C TJ max, ambient must remain below –15°C unless heatsinking or airflow is applied-thus derating to ≤50mA is recommended for enclosed industrial enclosures at 25°C ambient.