LTC4257CS8#PBF - IEEE 802.3af PoE PD Interface Controller | ADI
MPN: LTC4257CS8#PBF ✓ Active| Qty | Unit Price | Extended |
|---|---|---|
| 1 | $4.06 | $4.06 |
| 10 | $3.86 | $38.60 |
| 100 | $3.55 | $355.00 |
| 500 | $3.25 | $1,625.00 |
| 1,000 | $3.06 | $3,060.00 |
LTC4257CS8#PBF Overview
A PoE PD interface controller is the circuit block that lets a powered device, such as an IP phone or wireless access point, legally draw power from an Ethernet cable under IEEE 802.3af. It sits in the power-management IC hierarchy between the PoE PSE (power sourcing equipment) in the switch and the DC-DC converters that generate the device rails, providing detection, classification, and inrush control.
Key features include the complete 25k ohm detection signature resistor, a programmable classification current source, input current limit with thermal foldback, undervoltage lockout, and power good (PGOOD) signaling - all integrated in a single 8-pin package. The integrated 100V, 400mA MOSFET eliminates a discrete pass device, and PBF suffix parts are lead-free.
Technically, the LTC4257 manages the full PD handshake: it presents the signature resistance during PSE detection, sources the class-dependent current during classification, and then limits input current during power-up so inrush is controlled while the downstream converter starts. Thermal foldback reduces the current limit as junction temperature rises, protecting the internal MOSFET during sustained inrush or fault events.
Typical applications include IP phones, network routers and switches, Power over LAN endpoints, security cameras, and wireless access points - anywhere a PD must extract up to the 802.3af class power (13 W maximum for Class 3) from the Ethernet pair.
Design consideration: verify that the external classification resistor setting matches the power class your end equipment actually requires, since underspecifying prevents the PSE from allocating enough power and overclassing wastes budget on the switch.
This page synthesizes distributor pricing, verified drop-in alternatives, pinout guidance, and practical design notes not found in the manufacturer datasheet, giving engineers a complete single-page reference for the LTC4257CS8#PBF.
Drop-in alternatives for LTC4257CS8#PBF — same package, pin-to-pin compatible. Different-package parts requiring PCB rework are excluded.
Quick Comparison Tool — Select alternative parts for side-by-side comparison:
LTC4257IS8#PBF
✅ Drop-In📋 Reference alternative (not in catalog)
LTC4257CS8-1#PBF
✅ Drop-In📋 Reference alternative (not in catalog)
LTC4257IS8-1#PBF
✅ Drop-In ⚠️ Specs Unverified📋 Reference alternative (not in catalog)
LTC4257CS8#PBF Maximum Ratings & Electrical Characteristics
| Function | IEEE 802.3af PD Power over Ethernet Interface Controller |
| Compliance Standard | IEEE 802.3af (PoE) |
| Onboard MOSFET | 100 V, 400 mA power MOSFET integrated |
| Detection Signature | 25 k ohm onboard signature resistor |
| Classification | Programmable classification current source |
| Current Limit | Input current limit with thermal foldback |
| Undervoltage Lockout | Yes |
| Power Good Signaling | Yes (PGOOD) |
| Protection Features | Undervoltage, overcurrent, thermal protection |
| Max Output Power | 13 W |
| Package | 8-SOIC (0.154 in, 3.90 mm width) |
| Mounting Type | Surface Mount |
| Operating Temperature Grade | C grade (0C to +70C) |
| Packaging | Tube |
| Lead Finish | Lead-free (#PBF suffix) |
| RoHS Status | Compliant |
| Typical Applications | IP Phones, Power over LAN, Network Routers and Switches |
LTC4257CS8#PBF 8-soic (0.154 in, 3.90 mm width) Pin Configuration Guide
Pin configuration for LTC4257CS8#PBF (8-soic (0.154 in, 3.90 mm width) package). This power device features gate, drain, and source terminals. For non-polarized packages, refer to the manufacturer datasheet for exact pin 1 orientation and footprint details. Common applications include power supply design, motor driving, and load switching.
No detailed pinout data available for LTC4257CS8#PBF.
Refer to the datasheet for full pin configuration.
Typical Applications
LTC4257CS8#PBF is suitable for 6 applications: VoIP IP Phones, Network Routers and Switches, PoE Security Cameras, Wireless Access Points, Power over LAN Building Automation Endpoints, PoE Splitter and Midspan Adapter Modules.
VoIP IP Phones
Enterprise IP phones are the canonical IEEE 802.3af Powered Device: a single Ethernet cable delivers both data and up to 12.95 W of Class 2/3 power. The LTC4257CS8#PBF fits this role because it integrates the complete PD handshake - the 25 k detection signature resistor, classification current source, and undervoltage lockout - plus an onboard 100 V, 400 mA MOSFET that controls inrush when the switch applies 48 V to the pair. In a phone, the PGOOD output enables the downstream flyback or buck converter only after PoE power is stable, preventing erratic reset during cable insertion. The thermal-foldback current limit protects the internal MOSFET during the phone's converter startup, when bulk capacitor charging draws peak current.
Recommended
Network Routers and Switches
Stackable routers, midspan injectors, and management cards frequently need a small auxiliary PoE-powered section fed from the uplink port. The LTC4257CS8#PBF suits these endpoints because its classification current source lets the designer program the power class to match the auxiliary load exactly, avoiding wasted PSE power budget on a 24-port switch. The 100 V MOSFET rating comfortably withstands 80 V transients that appear on long or mismatched Ethernet runs, and the undervoltage lockout guarantees the interface does not gate power on until the PSE voltage is valid. Input current limit with thermal foldback keeps inrush bounded when multiple card-level bulk capacitors charge simultaneously.
Recommended
PoE Security Cameras
Fixed and PTZ network cameras are powered over the same Cat-5 cable that carries their video stream, typically drawing 5 W to 13 W depending on IR illuminators and heater options. The LTC4257CS8#PBF provides the front-end PD interface: during PSE detection it presents the mandated 25 k signature, during classification it sources the programmed class current so the switch allocates enough power for the camera's peak draw, and during power-up its 400 mA limited MOSFET charges the camera's input bulk capacitance without tripping the PSE. Because cameras see outdoor cable transients, the 100 V MOSFET rating adds margin well above the 57 V maximum 802.3af operating voltage.
Recommended
Wireless Access Points
802.11ac/ax ceiling access points commonly run on 802.3af Class 3 power (up to 12.95 W at the PD). The LTC4257CS8#PBF manages the powered-device interface while a downstream isolated converter generates the RF PA and digital rails. Its value here is sequencing discipline: PGOOD does not assert until PoE input is valid and inrush charging is complete, so the Wi-Fi radio never boots from a collapsing rail mid-association. The integrated signature and classification circuitry also reduces BOM count versus discrete 25 k resistor/current-source implementations, which matters in the tight ceiling-mount enclosure of an AP where board area is at a premium.
Recommended
Power over LAN Building Automation Endpoints
Door controllers, card readers, occupancy sensors, and HVAC actuators increasingly ride power over structured Ethernet cabling (Power over LAN) to avoid local electrician work. These endpoints draw a few watts and benefit from the LTC4257CS8#PBF's low component count: one SOIC-8 provides detection, classification, inrush limiting, and status signaling. The undervoltage lockout with built-in hysteresis prevents chatter when cable voltage sags under load, and the thermal foldback lets the part survive fault conditions - such as a jammed motor actuator - without board-level damage, since current is progressively reduced as the internal MOSFET heats.
Recommended
PoE Splitter and Midspan Adapter Modules
Standalone PoE splitter modules deliver isolated 5 V/12 V outputs to non-PoE equipment such as legacy cameras or point-of-sale terminals. The LTC4257CS8#PBF forms the module's powered-device front end, legitimately presenting the 25 k signature and class current to the PSE so the switch safely recognizes and powers the adapter. Its 400 mA current-limited internal MOSFET shapes the inrush profile when the module's input capacitors charge, meeting 802.3af timing windows without external hot-swap circuitry. PGOOD drives an indicator LED or enables the module's DC-DC stage only after the interface handshake completes, giving end users a trustworthy power-present signal.
Recommended
Recommended Products Summary
Engineering reference data for LTC4257CS8#PBF — comparison, design guidance, and compliance information.
Selection Guide
Comparison with Alternatives
| Parameter | This Product | LTC4257IS8#PBF | LTC4257CS8-1#PBF | LTC4257IS8-1#PBF |
|---|---|---|---|---|
| Package | SOIC-8 (3.90 mm width) | SOIC-8 - same | SOIC-8 - same | SOIC-8 - same |
| Brand | Analog Devices | Analog Devices | Analog Devices | Analog Devices |
| PoE Standard | IEEE 802.3af | IEEE 802.3af | IEEE 802.3af | IEEE 802.3af |
| Onboard MOSFET | 100 V, 400 mA | 100 V, 400 mA | 100 V, 400 mA | 100 V, 400 mA |
| Max Output Power | 13 W | 13 W | 13 W | 13 W |
| Operating Temperature | 0C to +70C (C grade) | -40C to +85C (I grade) | 0C to +70C (C grade) | -40C to +85C (I grade) |
| Current Limit | Single-level with thermal foldback | Single-level with thermal foldback | Precision dual-level | Precision dual-level |
| Lead Finish | Lead-free (#PBF) | Lead-free (#PBF) | Lead-free (#PBF) | Lead-free (#PBF) |
Key Differentiators
- Lowest integration in the family for standard inrush needs (vs LTC4257CS8-1#PBF)
- Commercial grade pricing (vs LTC4257IS8#PBF)
- Converter flexibility versus integrated solutions (vs LTC4267)
Design Notes
Do not hardwire the 25k detection signature externally - the LTC4257 already integrates it, and a parallel external resistor corrupts the PSE detection result. Similarly, verify the classification resistor against the true worst-case power of the end equipment: underclassing causes the PSE to deny or drop power at peak load, while overclassing wastes switch power budget. Ensure the downstream converter's input bulk capacitance stays within the limit the inrush current profile can charge within the 802.3af startup timing window.
The onboard 100 V, 400 mA MOSFET dissipates I x V during inrush and faults, and the thermal foldback feature exists precisely because the SOIC-8 has limited heat spreading. Estimated: charging a 100 uF bulk cap from 0 V to 48 V through a current limiter dissipates roughly 0.5 x C x V^2 = 0.5 x 100uF x 48^2 ≈ 115 mJ in the MOSFET per insertion event - acceptable intermittently, but repeated brownout cycling raises average dissipation. Provide a solid ground pour under the SOIC-8 and keep the classification/load within the programmed class.
Place the PoE input rectifier (diode bridge or ideal-diode bridge such as the LT4320), bulk input capacitor, and the LTC4257 close together on a compact input loop. Keep the node between the internal MOSFET and the downstream converter's inductor short to limit ringing from cable-induced transients - Ethernet runs routinely see spikes well above the nominal 48 V, which is why the MOSFET is rated 100 V. Follow the typical application layout in the manufacturer datasheet for the classification resistor routing.
Compliance Information
The #PBF suffix denotes Analog Devices lead-free, RoHS-compliant construction per distributor and manufacturer listings. REACH, halogen-free, and conflict-minerals status not stated in provided data.
