Analog Devices Inc. LTC4257CS8-1#TRPBF

Overview

LTC4257CS8-1#TRPBF from Analog Devices (formerly Linear Technology) is an IEEE 802.3af Power over Ethernet (PoE) Powered Device (PD) interface controller with integrated 100V/400mA power MOSFET, precision dual-level current limit (140mA low / 375mA high), onboard 25kΩ signature resistor, programmable classification (Class 0–3), and undervoltage lockout (UVLO turn-on at –36.0V). It enables robust PD front-end design for IP phones and wireless access points.

For engineers reviewing the LTC4257CS8-1#TRPBF datasheet, LTC4257CS8-1#TRPBF pinout, LTC4257CS8-1#TRPBF application, or LTC4257CS8-1#TRPBF equivalent, key selection considerations include PoE signature compliance, dual-level inrush control for legacy and IEEE-compliant PSEs, thermal shutdown at 140°C, power-good signaling referenced to VIN, and SO-8 package compatibility with board space-constrained PD designs.

Technical Context

The LTC4257CS8-1#TRPBF implements a negative-voltage PoE front end where all supply rails are referenced to GND but operate with –1.5V to –57V input on VIN. Its detection mode uses a temperature-compensated 25kΩ signature resistor between VIN and GND; classification applies a precise load current (±3.5% accuracy) via RCLASS pin with external resistor selection per IEEE Class 0–3.

Power delivery employs a hysteretic UVLO (–36.0V turn-on / –30.5V turn-off), dual-level current limiting (low-level 140mA for controlled C_LOAD charging; high-level 375mA for full-power operation), and thermal shutdown at 140°C. The PWRGD open-drain output signals power readiness when V_OUT–V_VIN falls below 1.5V (rising threshold) and releases above 3.0V (falling threshold).

Key Specifications

Pinout & Package

Package: 8-pin SO (Small Outline), 150°C/W θ_JA, lead-free RoHS-compliant #TRPBF tape-and-reel packaging.

Key Features

Applications

Equivalent & Alternatives

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

Compared with LT4275IMS#PBF and TPS23753APWPR, the LTC4257CS8-1#TRPBF uniquely combines IEEE 802.3af compliance, integrated MOSFET, dual-level current limiting, and signature disable in an 8-pin SO package-making it optimal for cost-sensitive, space-constrained, and feature-specific PD designs.

Availability

LTC4257CS8-1#TRPBF is available at Aetrix Electronics and suitable for IP phone power management, wireless access point PD interfaces, and telecom remote terminal power requiring stable component supply, long-term lifecycle support, and RoHS-compliant manufacturing.

Supply support for LTC4257CS8-1#TRPBF 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

Analog Devices acquired Linear Technology in 2017 and maintains its high-performance analog and power management product lines with rigorous automotive and industrial qualification.

The LTC4257CS8-1#TRPBF belongs to Linear's PoE PD controller family, designed specifically for simplified, reliable, and standards-compliant Powered Device front-end implementation in voice, data, and video networking equipment.

FAQ

What is the operating temperature range for the LTC4257CS8-1#TRPBF?

The LTC4257CS8-1#TRPBF is rated for 0°C to 70°C ambient operation (Commercial grade). Its absolute maximum junction temperature is 150°C, and thermal shutdown activates at 140°C to protect the internal MOSFET. This makes the LTC4257CS8-1#TRPBF suitable for indoor IP phones and access points but not extended-temperature industrial deployments unless derated.

How does the LTC4257CS8-1#TRPBF achieve IEEE 802.3af signature compliance with diode bridges?

The LTC4257CS8-1#TRPBF compensates for two series diode drops by offsetting its internal 25kΩ signature resistor to 23.25kΩ–26.00kΩ. This ensures the total PD resistance-including diode resistance-falls within the IEEE 802.3af 23.75kΩ–26.25kΩ specification when measured using the ∆V/∆I method at the RJ45 port. The LTC4257CS8-1#TRPBF achieves this without external components.

Can the LTC4257CS8-1#TRPBF be used with legacy Power Sourcing Equipment (PSE) that does not meet IEEE 802.3af inrush limits?

Yes. The LTC4257CS8-1#TRPBF's dual-level current limit starts at 140mA (low level) during initial capacitor charging, allowing safe startup with legacy PSEs having limited current capability. Once V_OUT–V_VIN drops below the power-good threshold, it switches to 375mA (high level) to support full 12.95W operation with compliant PSEs-enabling backward compatibility without design changes.

What is the purpose of the SIGDISA pin on the LTC4257CS8-1#TRPBF?

The SIGDISA pin on the LTC4257CS8-1#TRPBF enables dynamic control of PD presence: tying it to GND lowers the signature resistance to 9kΩ (invalid per IEEE 802.3af), preventing detection; leaving it open or tying it to VIN enables normal 25kΩ signature operation. This supports firmware-controlled hot-swap, multi-PD arbitration, and secure boot sequences in the LTC4257CS8-1#TRPBF.

Does the LTC4257CS8-1#TRPBF require external components for basic IEEE 802.3af compliance?

No. The LTC4257CS8-1#TRPBF integrates all core IEEE 802.3af PD functions-including 25kΩ signature resistor, classification current source, UVLO, dual-level current limit, thermal shutdown, and power-good signaling-requiring only an external RCLASS resistor for classification and a minimum 5µF output capacitor. No external MOSFET, sense resistor, or protection diodes are needed for baseline compliance.