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Analog Devices Inc. LTC4257IS8-1#PBF
- Part No.:
- LTC4257IS8-1#PBF
- Manufacturer:
- Analog Devices Inc.
- Category:
- Power Over Ethernet (PoE) Controllers
- Package:
- 8-SOIC (0.154", 3.90mm Width)
- Datasheet:
-
LTC4257IS8-1#PBF.pdf
- Description:
- IC POE CNTRL 1 CHANNEL 8SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:139
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Product details
Overview
LTC4257IS8-1#PBF from Analog Devices (formerly Linear Technology) is an IEEE 802.3af Power over Ethernet (PoE) Powered Device (PD) interface controller in an 8-pin SOIC package. It integrates a 100V/400mA onboard power MOSFET, precision dual-level current limit (140mA low / 375mA high), 25kΩ signature resistor with disable, programmable classification (Class 0–3), undervoltage lockout (UVLO turn-on at –36.0V), and open-drain PWRGD output - enabling robust, space-efficient PD front-end design for IP phones and wireless access points.
For engineers reviewing the LTC4257IS8-1#PBF datasheet, LTC4257IS8-1#PBF pinout, LTC4257IS8-1#PBF application, or LTC4257IS8-1#PBF equivalent, key selection considerations include its negative-voltage operation (–1.5V to –57V), thermal shutdown at 140°C, VIN-referenced SIGDISA control, dual-current-limit sequencing for legacy and IEEE-compliant PSEs, and compatibility with 3mm × 3mm DFN and SOIC-8 footprints.
Technical Context
The LTC4257IS8-1#PBF operates exclusively with negative input voltage referenced to GND (–1.5V to –57V), using internal hysteresis in UVLO (–36.0V turn-on / –30.5V turn-off) to prevent oscillation during marginal line conditions. Its dual-level current limit circuit autonomously transitions from 140mA (low) during C1 charging to 375mA (high) once VOUT–VIN falls below 1.5V, satisfying both IEEE 802.3af inrush limits and legacy PSE compatibility.
Signature detection occurs between –1.5V and –9.5V via a trimmed 25kΩ resistor (23.25–26.00kΩ, IEEE 2-point ∆V/∆I compliant); classification uses RCLASS-programmed current (±3.5% accuracy) across –12.5V to –21V. The SIGDISA pin disables signature by pulling it to GND (yielding 9–11.8kΩ invalid resistance) or driving high relative to VIN, while PWRGD asserts low when VOUT–VIN ≤ 1.5V and releases above 3.0V.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VIN Operating Range | –1.5V to –57V (negative supply referenced to GND; supports standard PoE polarity) |
| Signature Resistance | 23.25kΩ to 26.00kΩ (IEEE 802.3af-compliant 2-point measurement; compensates for two series diodes) |
| Classification Accuracy | ±3.5% (measured ICLASS = 1.237V / RCLASS; excludes RCLASS tolerance) |
| Current Limit Levels | Low: 140mA (inrush control for large C1); High: 375mA (steady-state power delivery) |
| UVLO Thresholds | Turn-on: –36.0V ±1.2V; Turn-off: –30.5V ±1.2V (hysteresis prevents cycling on resistive cable drops) |
| Power MOSFET RON | 1.0Ω to 2.0Ω (100V-rated, 400mA max; integrated to eliminate external FET and reduce BOM count) |
| PWRGD Trip Points | Rising: 2.7–3.3V (VOUT–VIN); Falling: 1.3–1.7V (enables reliable DC/DC enable timing) |
Pinout & Package
Package: 8-lead plastic SOIC (S8), 3.9mm × 4.9mm body, 1.27mm pitch, exposed pad not present. Rated for –40°C to +85°C ambient operation (I-grade).
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| Pin 1 (NC) | No internal connection | Must be left unconnected; no routing or grounding required |
| Pin 2 (RCLASS) | Classification current programming input | Connects to VIN via resistor (e.g., 124Ω for Class 1); sets ICLASS = 1.237V / RCLASS |
| Pin 3 (NC) | No internal connection | Must be left unconnected; no routing or grounding required |
| Pin 4 (VIN) | Negative power input | Accepts –1.5V to –57V from PSE via diode bridge; all voltages referenced to GND |
| Pin 5 (VOUT) | Negative power output | Delivers regulated –48V to load through internal MOSFET; high-impedance until UVLO turn-on |
| Pin 6 (PWRGD) | Open-drain power-good signal | Referenced to VIN; pulls low when VOUT–VIN ≤ 1.5V (enables downstream DC/DC startup) |
| Pin 7 (SIGDISA) | Signature disable control | Tie to GND to invalidate signature (9–11.8kΩ); float or tie to VIN to enable 25kΩ detection |
| Pin 8 (GND) | Ground reference | System return path; connects to diode bridge cathode and PCB ground plane |
Key Features
| Feature | Design Value |
|---|---|
| Integrated 100V/400mA power MOSFET | Eliminates external high-voltage switch, reduces board area by >30%, and avoids signature corruption from leakage |
| Dual-level input current limit | 140mA (low) enables safe charging of arbitrary load capacitance; 375mA (high) delivers up to 12.95W per IEEE 802.3af |
| 25kΩ signature resistor with disable | Trimmed, temperature-compensated resistance meets IEEE 802.3af detection; SIGDISA pin allows dynamic PD presence masking |
| Programmable classification (Class 0–3) | Resistor-selectable current source (±3.5% accuracy) enables precise power allocation matching PD load requirements |
| Thermal overload protection | Shuts down MOSFET at 140°C junction temperature; prevents damage during repeated power cycling or prolonged classification |
Applications
| IP Phone Power Management | Wireless Access Point (WAP) |
|---|---|
Use Scenario: IEEE 802.3af-powered VoIP handset with integrated audio codec and Ethernet PHY requiring stable –48V rail and controlled power-up sequencing. IC Role / Device Role / Timing Role: PD interface controller handling detection, classification, inrush limiting, and PWRGD signaling to enable DC/DC converter startup after capacitor charge. Use Value: Dual current limit prevents PSE overcurrent tripping during cold start; integrated MOSFET eliminates discrete high-voltage FET and saves 22 mm² PCB area. |
Use Scenario: Dual-band 802.11ac access point deployed in ceiling-mounted enclosures where thermal margin and compact front-end design are critical. IC Role / Device Role / Timing Role: Primary PoE interface managing signature, classification, and thermal-safe power delivery to multiple DC/DC rails (2.5V, 3.3V, 5V). Use Value: Thermal shutdown at 140°C ensures reliability in confined spaces; PWRGD hysteresis (1.5V–3.0V) prevents false DC/DC resets during transient line disturbances. |
| Legacy PSE-Compatible Surveillance Camera | Industrial IoT Gateway |
Use Scenario: Outdoor PTZ camera powered via PoE but connected to non-802.3af-compliant midspan injectors with limited inrush current capability. IC Role / Device Role / Timing Role: Enables interoperability by holding low-level current limit (140mA) until full charge, then switching to high limit only when line voltage exceeds UVLO threshold. Use Value: Eliminates need for external inrush limiters or PSE-specific firmware; supports plug-and-play deployment across mixed PSE infrastructure. |
Use Scenario: DIN-rail mounted edge gateway aggregating Modbus, CAN, and sensor data, requiring PoE power with fail-safe signature disable during firmware update. IC Role / Device Role / Timing Role: Provides secure PD presence control via SIGDISA pin - pulled to GND during update to prevent accidental PSE detection and power application. Use Value: Prevents unsafe power application during reconfiguration; invalid 9–11.8kΩ signature ensures PSE remains in idle state per IEEE 802.3af clause 15.3.2. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar PoE PD interface applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| TPS23753APWR | Single-level 400mA current limit; no programmable classification; requires external MOSFET for >100V operation | Best suited for cost-sensitive, fixed-class PDs without legacy PSE support needs | Select if dual-level current limiting and integrated 100V MOSFET are not required; verify external FET selection for –48V operation |
| MAX5941BASA+ | Fixed Class 3 only; 100V MOSFET with 1.8Ω RON; no SIGDISA pin or signature disable function | Targeted at simple, high-volume PDs where classification flexibility and dynamic signature control are unnecessary | Choose when design requires minimal feature set and lower unit cost; confirm absence of SIGDISA does not impact system security requirements |
Compared with TPS23753APWR and MAX5941BASA+, the LTC4257IS8-1#PBF uniquely combines dual-level current limiting, integrated 100V/400mA MOSFET, and SIGDISA-controlled signature disable - making it optimal for PDs needing interoperability with legacy PSEs, thermal resilience in enclosed environments, and secure power-state management.
Availability
LTC4257IS8-1#PBF is available at Aetrix Electronics and suitable for IP phone power management, wireless access point designs, industrial IoT gateways, legacy PSE-compatible surveillance systems, and telecom power control applications requiring stable component supply, long-term lifecycle assurance, and traceable sourcing.
Supply support for LTC4257IS8-1#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 its portfolio of high-performance analog and power management ICs. The company focuses on precision signal conditioning, power conversion, and interface solutions for demanding industrial, automotive, and communications applications.
The LTC4257IS8-1#PBF belongs to Linear's Power over Ethernet (PoE) PD controller product line, designed specifically to simplify IEEE 802.3af-compliant Powered Device implementation by integrating signature, classification, current limiting, and power-switching functions into a single compact package.
FAQ
What is the operating temperature range for the LTC4257IS8-1#PBF?
The LTC4257IS8-1#PBF is rated for an industrial ambient temperature range of –40°C to +85°C. This I-grade specification is confirmed by the "I" in the part number (LTC4257IS8-1#PBF) and aligns with the device's thermal shutdown threshold of 140°C junction temperature. The SOIC-8 package has a θJA of 150°C/W, requiring appropriate PCB copper area for sustained high-current operation.
How does the LTC4257IS8-1#PBF handle signature disable functionality?
The LTC4257IS8-1#PBF uses the SIGDISA pin (Pin 7) to dynamically invalidate its signature resistor. When SIGDISA is tied to GND (±0.1V), the internal signature resistance drops to 9–11.8kΩ - outside the IEEE 802.3af 23.75–26.25kΩ valid range - preventing PSE detection. If unused, SIGDISA must be tied to VIN to enable normal 25kΩ operation; floating is not allowed.
Can the LTC4257IS8-1#PBF be used with both legacy and IEEE 802.3af-compliant PSEs?
Yes, the LTC4257IS8-1#PBF is explicitly designed for interoperability. Its dual-level current limit (140mA low / 375mA high) allows safe startup with legacy PSEs lacking inrush current compliance, while the high-level limit delivers full 12.95W power when connected to IEEE 802.3af PSEs. This behavior is automatic and requires no external configuration.
What is the purpose of the PWRGD pin on the LTC4257IS8-1#PBF?
The PWRGD pin (Pin 6) is an open-drain output referenced to VIN that signals when the internal power MOSFET is fully on and VOUT is stable. It asserts low when VOUT–VIN ≤ 1.5V (rising threshold: 2.7–3.3V), providing a clean enable signal to downstream DC/DC converters. During detection, classification, or thermal shutdown, PWRGD remains high-impedance - ensuring safe power sequencing in the LTC4257IS8-1#PBF-based PD.
Does the LTC4257IS8-1#PBF require external components for basic IEEE 802.3af compliance?
The LTC4257IS8-1#PBF achieves full IEEE 802.3af PD compliance with minimal external parts: only an input diode bridge, RCLASS resistor (for classification), and output bulk capacitor (≥5µF). No external MOSFET, current-sense resistor, or UVLO circuitry is needed - the 100V/400mA integrated FET, dual current limit, and hysteresis-controlled UVLO are all on-die.
LTC4257IS8-1#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:
- -40°C ~ 85°C
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 8-SO
LTC4257IS8-1#PBF FAQ
1.How can I place an order for LTC4257IS8-1#PBF through Aetrix?
Please submit a Request for Quotation (RFQ) for LTC4257IS8-1#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 LTC4257IS8-1#PBF reliable?
The price and inventory of LTC4257IS8-1#PBF are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for LTC4257IS8-1#PBF is usually 5 days.
3.What payment methods are accepted for LTC4257IS8-1#PBF?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for LTC4257IS8-1#PBF transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for LTC4257IS8-1#PBF?
LTC4257IS8-1#PBF orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your LTC4257IS8-1#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 LTC4257IS8-1#PBF?
For technical support, including LTC4257IS8-1#PBF datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your LTC4257IS8-1#PBF requirements.
6.How does Aetrix verify that LTC4257IS8-1#PBF is sourced from the original manufacturer or authorized distributors?
All LTC4257IS8-1#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 LTC4257IS8-1#PBF meets industry standards.
7.What is the process for return or replacement of LTC4257IS8-1#PBF?
All LTC4257IS8-1#PBF units undergo pre-shipment inspection (PSI). If there is an issue with LTC4257IS8-1#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 LTC4257IS8-1#PBF part is unused and in its original packaging.
Return procedure for LTC4257IS8-1#PBF:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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