Texas Instruments LM5071MTX-50

Overview

LM5071MTX-50 from Texas Instruments is a Power over Ethernet (PoE) Powered Device (PD) controller with integrated 80 V, 1 Ω, 400 mA internal MOSFET and current-mode PWM control for flyback/forward DC-DC converters. It supports IEEE 802.3af compliance, auxiliary power interface (AUX pin), programmable classification (RCLASS), and 50% maximum duty cycle limiting - enabling robust PoE-powered IP phones, VoIP endpoints, and networked surveillance cameras.

For engineers reviewing the LM5071MTX-50 datasheet, LM5071MTX-50 pinout, LM5071MTX-50 application, or LM5071MTX-50 equivalent, key selection considerations include its -50 suffix duty-cycle limit (no slope compensation), TSSOP-16 package compatibility, PoE signature/classification timing behavior, and auxiliary power enable functionality for dual-source (PoE + AC adapter) operation.

Technical Context

The LM5071MTX-50 implements a dedicated IEEE 802.3af PD interface with detection (1.8–10 V), classification (12.5–25 V), and UVLO-controlled power-up sequencing. Its internal 80 V MOSFET handles inrush current limiting (100 mA) and steady-state DC current limit (390 mA), while the PWM section provides cycle-by-cycle current sensing via CS pin (0.5 V threshold) and soft-start ramping via SS pin (10 µA current source).

This variant uses a fixed 50% maximum duty cycle limit without slope compensation - distinguishing it from the -80 variant - and operates with oscillator frequencies of 175–225 kHz (RT = 30.3 kΩ) or 505–665 kHz (RT = 10.5 kΩ). The AUX pin enables forced startup below UVLO threshold (2.5 V threshold, 0.5 V hysteresis), supporting wall-adapter backup power integration.

Key Specifications

Pinout & Package

TSSOP-16 package with exposed thermal pad (PW suffix); 4.4 mm × 5.0 mm footprint, 0.65 mm pitch, JEDEC MO-153 compliant.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar PoE PD controller applications.

Compared with LM5071MTX-80 and LT4275IMS#PBF, the LM5071MTX-50 offers lowest-cost, minimal-component PoE PD implementation for Class 0–3 devices with strict duty-cycle constraints, while avoiding the complexity of external MOSFETs or digital interfaces.

Availability

LM5071MTX-50 is available at Aetrix Electronics and suitable for VoIP endpoints, network cameras, wireless access points, and IoT gateways requiring stable component supply across industrial temperature ranges and multi-year production cycles.

Supply support for LM5071MTX-50 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 delivering analog and embedded processing solutions, with deep expertise in power management, signal chain, and connectivity ICs for industrial, automotive, and communications markets.

The LM5071 product line delivers integrated Power over Ethernet Powered Device controllers targeting cost-sensitive, space-constrained networking equipment requiring IEEE 802.3af compliance and auxiliary power flexibility.

FAQ

What is the key functional difference between LM5071MTX-50 and LM5071MTX-80?

The LM5071MTX-50 implements a 50% maximum duty cycle limit without slope compensation, whereas the LM5071MTX-80 provides an 80% limit with integrated slope compensation. This makes the -50 variant suitable for topologies where duty cycle must be strictly capped below 50% - such as certain flyback designs with wide input ranges - while the -80 variant supports higher-duty applications requiring subharmonic suppression. Both share identical pinout, PoE interface logic, and auxiliary power features.

Does LM5071MTX-50 require an external MOSFET for PoE power switching?

No. The LM5071MTX-50 integrates an 80 V, 1 Ω, 400 mA rated internal MOSFET for PoE input switching and inrush current limiting. This eliminates the need for an external high-voltage switch in standard Class 0–3 implementations. The internal FET handles both detection/classification current sourcing and post-UVLO bulk charging of the SMPS input capacitor.

How does the AUX pin enable auxiliary power support in LM5071MTX-50?

The AUX pin on LM5071MTX-50 has a 2.5 V rising threshold with 0.5 V hysteresis. When driven above this level via a resistor divider from a 12 V AC adapter, it forces UVLO release regardless of VIN voltage - enabling immediate startup even if PoE voltage is absent or below 25 V. This provides true dual-source redundancy without external OR-ing logic or microcontroller intervention.

What oscillator frequency options does LM5071MTX-50 support?

The LM5071MTX-50 supports two primary oscillator frequency ranges: 175–225 kHz (with RT = 30.3 kΩ) and 505–665 kHz (with RT = 10.5 kΩ). Frequency is set by a single resistor from RT to ARTN. The RT/SYNC pin also accepts narrow ac-coupled synchronization pulses from an external clock, enabling phase-aligned operation in multi-rail systems.

Can LM5071MTX-50 be used in isolated DC-DC converter designs?

Yes. While the LM5071MTX-50's internal error amplifier (1.25 V reference, FB/COMP pins) is optimized for non-isolated buck-derived topologies, it supports isolated designs via optocoupler feedback: the COMP pin includes a 5 kΩ internal pull-up to 5 V, allowing direct connection to an optocoupler transistor collector. In such cases, the secondary-side TL431 or shunt regulator drives the optocoupler LED, closing the regulation loop across the isolation barrier.