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Analog Devices Inc. LTC4257CS8-1#PBF
- Part No.:
- LTC4257CS8-1#PBF
- Manufacturer:
- Analog Devices Inc.
- Category:
- Power Over Ethernet (PoE) Controllers
- Package:
- 8-SOIC (0.154", 3.90mm Width)
- Datasheet:
-
LTC4257CS8-1#PBF.pdf
- Description:
- IC POE CNTRL 1 CHANNEL 8SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:4,617
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Product details
Overview
LTC4257CS8-1#PBF 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), 25kΩ signature resistor, and programmable classification (Class 0–3). It enables robust PD front-end design for IP phones and wireless access points operating from –48V PSE supplies.
For engineers reviewing the LTC4257CS8-1#PBF datasheet, LTC4257CS8-1#PBF pinout, LTC4257CS8-1#PBF application, or LTC4257CS8-1#PBF equivalent, key selection criteria include UVLO thresholds (–36.0V turn-on / –30.5V turn-off), thermal shutdown at 140°C, power-good trip points (1.5V fall / 3.0V rise), on-resistance (1.6Ω typ), and compatibility with legacy and IEEE-compliant PSEs.
Technical Context
The LTC4257CS8-1#PBF implements a negative-voltage PoE interface where all supply rails are referenced to GND, with VIN operating from –1.5V to –57V. Its dual-level current limit operates in sequence: low-level (140mA) during initial capacitor charging, then automatic transition to high-level (375mA) once VOUT–VIN falls below the 1.5V power-good threshold.
Signature disable (SIGDISA) forces invalid 9kΩ resistance when pulled to GND, while undervoltage lockout incorporates hysteresis (≈5.5V) to prevent oscillation. The onboard MOSFET's 1.6Ω RON and 150µA VOUT leakage are specified at –48V, supporting stable operation under full IEEE 802.3af load conditions up to 12.95W.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VIN Operating Range | –1.5V to –57V: Supports standard –48V PoE with margin for diode drops and line loss. |
| UVLO Turn-On / Turn-Off | –36.0V / –30.5V: Hysteretic lockout prevents cycling during marginal PSE voltage conditions. |
| Dual Current Limit | 140mA (low) / 375mA (high): Enables safe inrush control for large load capacitors and legacy PSE compatibility. |
| Signature Resistance | 23.25kΩ–26.00kΩ: Precision 25kΩ ±5% trimmed resistor meets IEEE 802.3af two-point ∆V/∆I measurement. |
| RON (MOSFET) | 1.6Ω typical at –48V: Minimizes conduction loss and thermal stress during sustained 375mA operation. |
| Power Good Threshold | Fall: 1.5V, Rise: 3.0V (VOUT–VIN): Provides clean enable signal to downstream DC/DC converters after capacitor charge completion. |
| Thermal Shutdown | 140°C: Protects internal MOSFET during extended overload or repeated power cycling events. |
Pinout & Package
Package: 8-pin SO (Small Outline), 3.9mm × 4.9mm body, lead-free (Pb-free) finish per #PBF suffix. Thermal resistance θJA = 150°C/W.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| Pin 1 (NC) | No internal connection | Unused; must be left floating or tied to GND per layout best practice. |
| Pin 2 (RCLASS) | Classification current select | Connects external resistor to VIN to set Class 0–3 current (e.g., 124Ω for Class 1, 68.1Ω for Class 2). |
| Pin 3 (NC) | No internal connection | Unused; no internal bond wire; electrically isolated. |
| Pin 4 (VIN) | Negative power input | Accepts –48V from PSE via diode bridge; absolute max –100V. |
| Pin 5 (VOUT) | Negative power output | Delivers regulated –48V to PD load through internal MOSFET; high-impedance until UVLO turn-on. |
| Pin 6 (PWRGD) | Open-drain power-good indicator | Pulled low when VOUT–VIN < 1.5V; signals DC/DC converter it may safely start. |
| Pin 7 (SIGDISA) | Signature disable control | Pull to GND to force invalid 9kΩ signature; float or tie to VIN to enable normal 25kΩ detection. |
| Pin 8 (GND) | System ground reference | Return path for signature, classification, and load current; connects to RJ45 shield and diode bridge cathodes. |
Key Features
| Feature | Design Value |
|---|---|
| Integrated 100V/400mA MOSFET | Eliminates external high-voltage switch, reduces BOM count, and avoids signature corruption from discrete FET leakage. |
| Dual-level current limiting | Enables reliable startup with arbitrarily large input capacitors while meeting IEEE 802.3af 50ms inrush window. |
| Programmable IEEE 802.3af classification | Supports Class 0–3 via single RCLASS resistor; excludes reserved Class 4 per specification. |
| Thermal overload protection | Shuts down MOSFET and classification current if junction exceeds 140°C, preventing permanent damage during fault conditions. |
| Signature disable with invalid resistance | Allows controlled PD isolation by forcing 9kΩ signature-invalid per IEEE 802.3af-without external circuitry. |
Applications
| IP Phone Power Interface | Wireless Access Point PD Front-End |
|---|---|
Use Scenario: Standalone VoIP handset drawing up to 12.95W from 802.3af PSE, requiring cold-start capability and legacy PSE interoperability. IC Role / Device Role / Timing Role: Primary PD interface controller managing signature, classification, inrush, and power delivery sequencing. Use Value: Dual current limit ensures safe capacitor charging across PSE generations; integrated MOSFET saves 30mm² PCB area vs discrete solution. |
Use Scenario: Dual-band 802.11ac AP with high peak current demands, deployed in mixed PSE environments including older midspans. IC Role / Device Role / Timing Role: Negative-voltage PoE front-end providing power-good signaling to isolated DC/DC converters. Use Value: 1.6Ω RON limits conduction loss to <1.2W at 375mA; thermal shutdown protects against overheating during firmware update surges. |
| Telecom Remote Terminal Unit | Industrial PoE-Powered Sensor Hub |
Use Scenario: Outdoor telecom node operating in –40°C to 85°C ambient, powered via long cable runs with significant IR drop. IC Role / Device Role / Timing Role: Robust PD controller with wide UVLO hysteresis to tolerate voltage sag during PSE brownouts. Use Value: –36.0V turn-on threshold ensures reliable activation even with 6V diode + cable loss; 150µA VOUT leakage preserves signature integrity. |
Use Scenario: DIN-rail mounted sensor aggregator with multiple 4–20mA loops and Ethernet backhaul, requiring fail-safe power management. IC Role / Device Role / Timing Role: Fault-tolerant PoE interface with SIGDISA-controlled signature disable for remote maintenance mode. Use Value: Pulling SIGDISA to GND forces invalid signature, allowing hot-swap isolation without PSE reboot or network reconfiguration. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar PoE PD interface applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| LTC4257IS8-1#PBF | Same functionality, extended temperature range (–40°C to 85°C) vs. LTC4257CS8-1#PBF (0°C to 70°C); identical electrical specs and pinout. | Suitable for industrial or outdoor deployments requiring wider ambient tolerance. | Select LTC4257IS8-1#PBF when operating ambient exceeds 70°C or requires automotive-grade reliability screening. |
| MAX5984DETE+T | Single-level 400mA current limit; no dual-mode inrush control; supports IEEE 802.3at (PoE+) with higher power classes. | Targeted at next-gen PoE+ systems requiring >12.95W; lacks legacy PSE inrush optimization. | Choose MAX5984DETE+T only when upgrading to PoE+ infrastructure and abandoning legacy midspan support. |
Compared with LTC4257IS8-1#PBF, the LTC4257CS8-1#PBF trades extended temperature rating for lower cost in commercial environments; versus MAX5984DETE+T, it provides superior inrush control for legacy PSEs but lacks PoE+ classification support.
Availability
LTC4257CS8-1#PBF is available at Aetrix Electronics and suitable for IP phone power management, wireless access point front-ends, and telecom remote terminal units requiring stable component supply across production lifecycles.
Supply support for LTC4257CS8-1#PBF 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 full product support, documentation, and manufacturing continuity for all Linear-branded ICs.
The LTC4257CS8-1#PBF belongs to Linear's PoE PD controller family, designed specifically to simplify IEEE 802.3af compliance with minimal external components and robust thermal management.
FAQ
What is the maximum input voltage rating for the LTC4257CS8-1#PBF?
The LTC4257CS8-1#PBF has an absolute maximum VIN rating of –100V with respect to GND. Its operational range is –1.5V to –57V, accommodating standard –48V PoE supplies plus margin for diode drops and line loss. Exceeding –100V risks permanent damage per Absolute Maximum Ratings.
How does the LTC4257CS8-1#PBF implement dual-level current limiting?
The LTC4257CS8-1#PBF starts in low-level current limit (140mA) during initial VOUT ramp-up to charge bulk capacitors safely. Once VOUT–VIN falls below 1.5V, it automatically switches to high-level limit (375mA) for steady-state operation. This sequence meets IEEE 802.3af inrush requirements while enabling full power delivery.
Can the LTC4257CS8-1#PBF be used with non-IEEE 802.3af Power Sourcing Equipment?
Yes-the LTC4257CS8-1#PBF is explicitly designed for interoperability with legacy PSEs that lack strict inrush current control. Its low-level current limit allows gradual capacitor charging, preventing PSE shutdown during startup, while maintaining full compatibility with IEEE 802.3af-compliant equipment.
What happens when the SIGDISA pin is pulled to GND on the LTC4257CS8-1#PBF?
Pulling SIGDISA to GND forces the LTC4257CS8-1#PBF into invalid signature mode, presenting 9kΩ resistance (outside IEEE 802.3af 23.75kΩ–26.25kΩ range) and disabling all internal functions-including classification current, MOSFET drive, and PWRGD. This isolates the PD without requiring PSE reset.
Does the LTC4257CS8-1#PBF require external components for basic IEEE 802.3af compliance?
The LTC4257CS8-1#PBF requires only three external components for core functionality: an RCLASS resistor (to set classification class), a minimum 5µF output capacitor (C1), and input diode bridge (not integrated). All signature, current limit, UVLO, thermal protection, and power-good functions are fully internal.
LTC4257CS8-1#PBF Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Analog Devices Inc.
- Series:
- -
- Package/Case:
- 8-SOIC (0.154", 3.90mm Width)
- Packaging:
- Tube
- Product Status:
- Active
- Type:
- Controller (PD)
- Number of Channels:
- 1
- Power - Max:
- 12.95 W
- Internal Switch(s):
- Yes
- Auxiliary Sense:
- No
- Standards:
- 802.3af (PoE)
- Voltage - Supply:
- 57V (Max)
- Current - Supply:
- 3mA
- Operating Temperature:
- 0°C ~ 70°C
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 8-SO
LTC4257CS8-1#PBF FAQ
1.How can I place an order for LTC4257CS8-1#PBF through Aetrix?
Please submit a Request for Quotation (RFQ) for LTC4257CS8-1#PBF on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.
2.Are the price and stock information for LTC4257CS8-1#PBF reliable?
The price and inventory of LTC4257CS8-1#PBF are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for LTC4257CS8-1#PBF is usually 5 days.
3.What payment methods are accepted for LTC4257CS8-1#PBF?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for LTC4257CS8-1#PBF transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for LTC4257CS8-1#PBF?
LTC4257CS8-1#PBF orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your LTC4257CS8-1#PBF order is processed, you will receive an email with the shipment details and tracking number.
Note: Tracking information may take up to 24 hours to appear. Express delivery typically takes 3–5 business days.
5.How can I obtain technical support or documentation for LTC4257CS8-1#PBF?
For technical support, including LTC4257CS8-1#PBF datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your LTC4257CS8-1#PBF requirements.
6.How does Aetrix verify that LTC4257CS8-1#PBF is sourced from the original manufacturer or authorized distributors?
All LTC4257CS8-1#PBF products on Aetrix are procured from qualified distributors and authorized channels. Our dedicated quality assurance team conducts strict verification, including traceability checks and, if necessary, third-party testing. This ensures that LTC4257CS8-1#PBF meets industry standards.
7.What is the process for return or replacement of LTC4257CS8-1#PBF?
All LTC4257CS8-1#PBF units undergo pre-shipment inspection (PSI). If there is an issue with LTC4257CS8-1#PBF, returns or replacements are accepted under the following conditions:
1.Quantity discrepancies, incorrect items, or visible external defects (such as breakage or corrosion), acknowledged by Aetrix.
2.The issue is reported within 90 days of delivery.
3.The LTC4257CS8-1#PBF part is unused and in its original packaging.
Return procedure for LTC4257CS8-1#PBF:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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