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Analog Devices Inc. LTC4257IDD#PBF
- Part No.:
- LTC4257IDD#PBF
- Manufacturer:
- Analog Devices Inc.
- Category:
- Power Over Ethernet (PoE) Controllers
- Package:
- 8-WFDFN Exposed Pad
- Datasheet:
-
LTC4257IDD#PBF.pdf
- Description:
- IC POE CNTRL 1 CHANNEL 8DFN
- Quantity:
- Payment:
- Shipping:
Inventory:1,842
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Product details
Overview
LTC4257IDD#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, 25kΩ signature resistor, programmable classification current (Class 0–4), precision input current limit (350mA typ), and undervoltage lockout (UVLO turn-on at –39.2V). It enables compact, cost-effective PD front-end design for IP phones and wireless access points.
For engineers reviewing the LTC4257IDD#PBF datasheet, LTC4257IDD#PBF pinout, LTC4257IDD#PBF application, or LTC4257IDD#PBF equivalent, key selection considerations include negative-input PoE voltage handling (–1.5V to –57V), thermal foldback protection (120°C trip), power good signaling referenced to VIN, and DFN (3mm × 3mm) package compatibility with high-density board layouts.
Technical Context
The LTC4257IDD#PBF implements a complete IEEE 802.3af PD detection, classification, and power application sequence using internal circuitry for 25kΩ signature resistance measurement (compensated for two series diodes), programmable classification current via external RCLASS resistor, and UVLO with hysteresis (–39.2V turn-on / –30.5V turn-off). Its architecture integrates all critical functions-signature, classification, current limiting, and thermal management-into a single monolithic IC.
It features a smart thermal protection system that dynamically reduces input current limit to 188mA (50%) at 120°C and shuts down output at 140°C, while maintaining full functionality during inrush events. The onboard 100V MOSFET provides low on-resistance (1.6Ω typ) and minimal leakage (150µA VOUT leakage), enabling direct connection to –48V PSE supplies without external FETs or discrete current-limiting components.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VIN Operating Range | –1.5V to –57V: Supports full IEEE 802.3af PoE input range with polarity-agnostic diode bridge interface. |
| Signature Resistance | 23.25kΩ to 26.00kΩ: Precision trimmed and temperature-compensated 25kΩ path meets IEEE ∆V/∆I measurement requirement with diode drop compensation. |
| Classification Current Accuracy | ±3.5%: Ensures reliable Class 0–4 identification by PSE under –12.5V to –21V input conditions. |
| Input Current Limit | 300mA to 400mA: Programmable hard current limit prevents inrush damage and satisfies IEEE 802.3af 400mA max specification. |
| Power Good Thresholds | Rising: 2.7–3.3V; Falling: 1.3–1.7V (VIN-referenced): Provides robust enable signal for downstream DC/DC converters after safe VOUT ramp-up. |
| Thermal Protection | 120°C overtemp trip, 140°C shutdown: Enables repeated hot-plug cycles without latch-up or permanent damage in compact DFN package. |
| On-Resistance (RON) | 1.0Ω to 2.0Ω: Low conduction loss supports >90% efficiency in typical –48V PD applications with 350mA load. |
Pinout & Package
Package: 8-lead (3mm × 3mm) plastic DFN with exposed pad (electrically isolated, must be soldered to PCB heatsink). Thermal resistance θJA = 43°C/W.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| Pin 1 (NC) | No Connect | Unused; must be left floating or tied to GND per layout best practice - no internal connection. |
| Pin 2 (RCLASS) | Classification Current Set Input | Connects external resistor to VIN to program Class 0–4 current (e.g., 124Ω for Class 1); sets ICLASS = 1.237V/RCLASS. |
| Pin 3 (NC) | No Connect | Unused; no internal function - leave unconnected. |
| Pin 4 (VIN) | Negative Power Input | Accepts –48V from PSE via diode bridge; referenced to GND; absolute max –100V. |
| Pin 5 (VOUT) | Regulated Negative Output | Delivers current-limited –48V to PD load via internal MOSFET; high-impedance until UVLO threshold crossed. |
| Pin 6 (PWRGD) | Open-Drain Power Good Signal | Sinks up to 1mA when active; referenced to VIN; asserts low impedance only when VOUT is stable and MOSFET fully on. |
| Pin 7 (NC) | No Connect | Unused; no internal connection - leave unconnected. |
| Pin 8 (GND) | Ground Reference | System ground return path; ties to diode bridge cathode and logic ground; forms reference for all internal circuits. |
Key Features
| Feature | Design Value |
|---|---|
| Integrated 100V/400mA Power MOSFET | Eliminates need for external high-voltage switch, reducing BOM count and PCB area by >30% vs discrete solutions. |
| Onboard 25kΩ Signature Resistor | Temperature-compensated and diode-drop-adjusted to ensure IEEE 802.3af compliance without calibration or trimming. |
| Programmable Classification (Class 0–4) | Enables precise PSE power allocation via single external RCLASS resistor - supports interoperability across multi-vendor PSEs. |
| Smart Thermal Foldback | Dynamically scales current limit (100% → 50% → 0%) based on die temperature, allowing safe operation in sealed enclosures or high-ambient environments. |
| VIN-Referenced Power Good Output | Provides timing-accurate enable signal for downstream DC/DC converters, synchronized to VOUT stabilization-not input voltage level. |
Applications
| IP Phone Power Management | Wireless Access Point (WAP) |
|---|---|
|
Use Scenario: Power delivery to VoIP handsets over standard Cat5e Ethernet cabling in enterprise offices. IC Role / Device Role / Timing Role: PD interface controller managing detection, classification, and controlled power ramp-up before enabling voice codec and DSP subsystems. Use Value: Enables single-cable deployment with guaranteed 15.4W PoE budget allocation and automatic fallback to Class 0 if classification fails. |
Use Scenario: Powering dual-band 802.11ac access points mounted on ceilings or walls where AC outlets are unavailable. IC Role / Device Role / Timing Role: Front-end PoE controller coordinating with DC/DC converters to supply regulated 3.3V/5V/12V rails for RF transceivers and baseband processors. Use Value: Delivers stable –48V input conditioning with thermal foldback, preventing brownouts during simultaneous 2.4GHz/5GHz transmission bursts. |
| Telecom Remote Terminal | Industrial IoT Gateway |
|
Use Scenario: Outdoor telecom cabinets housing DSLAM line cards powered via PoE from central office switches. IC Role / Device Role / Timing Role: Robust PD interface ensuring reliable startup under cold-start (-40°C) and high-humidity conditions with sustained 350mA load current. Use Value: Exposed-pad DFN package dissipates heat efficiently in confined spaces; UVLO hysteresis prevents oscillation on long cable runs (>100m). |
Use Scenario: Edge gateway aggregating sensor data from Modbus/RS485 field devices in factory automation systems. IC Role / Device Role / Timing Role: Primary PoE power manager enabling hot-swap capability and graceful shutdown during PSE power cycling or firmware updates. Use Value: PWRGD signal synchronizes microcontroller boot sequence with VOUT stability, eliminating race conditions in safety-critical control loops. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar PoE PD interface applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| TPS23753APWPR | TI device uses external MOSFET; higher quiescent current (1.2mA vs 0.5mA); no integrated signature resistor - requires external 25kΩ. | Requires additional pass transistor and RC network; better suited for high-current (>600mA) or custom-classification designs. | Select TPS23753APWPR only when needing >400mA continuous current or dual-event classification support not offered by LTC4257IDD#PBF. |
| MAX5941AESA+ | Maxim device offers adjustable UVLO thresholds and separate enable pin; lacks onboard MOSFET - needs external FET and gate driver. | Greater flexibility in startup sequencing but increases component count and layout complexity for basic 802.3af compliance. | Choose MAX5941AESA+ for designs requiring programmable UVLO hysteresis or independent shutdown control beyond LTC4257IDD#PBF's fixed thresholds. |
Compared with TPS23753APWPR and MAX5941AESA+, the LTC4257IDD#PBF delivers lowest bill-of-materials count and smallest footprint for standard IEEE 802.3af Class 0–4 PDs, trading configurability for integration density and thermal robustness in space-constrained applications.
Availability
LTC4257IDD#PBF is available at Aetrix Electronics and suitable for IP phone power management, wireless access point deployments, and telecom remote terminal applications requiring stable component supply, long-term lifecycle support, and RoHS-compliant packaging.
Supply support for LTC4257IDD#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 legacy portfolio of high-performance analog and power management ICs with rigorous reliability testing and automotive-grade qualification paths.
The LTC4257IDD#PBF belongs to Linear's PoE PD controller product line, designed specifically to simplify IEEE 802.3af-compliant powered device implementation with minimal external components and robust thermal management in compact packages.
FAQ
What is the operating temperature range for the LTC4257IDD#PBF?
The LTC4257IDD#PBF is rated for industrial temperature operation from –40°C to +85°C ambient. Its DFN package features an exposed thermal pad that must be soldered to an electrically isolated PCB heatsink to achieve the specified θJA of 43°C/W and sustain full 400mA current capability across this range.
Does the LTC4257IDD#PBF support IEEE 802.3at (PoE+) or 802.3bt?
No, the LTC4257IDD#PBF is strictly compliant with IEEE 802.3af (Type 1) only. It does not support higher-power classifications, autoclassification, or multi-event detection required for 802.3at or 802.3bt. For PoE+ applications, consider the LTC4271 or newer Analog Devices PD controllers with extended voltage and current ratings.
How is the classification current set on the LTC4257IDD#PBF?
The LTC4257IDD#PBF uses an external resistor (RCLASS) connected between Pin 2 (RCLASS) and Pin 4 (VIN) to set classification current. The internal 1.237V reference generates ICLASS = 1.237V / RCLASS - e.g., 124Ω yields ~10mA for Class 1. Table 2 in the datasheet lists exact resistor values for Classes 0–4.
What is the purpose of the PWRGD pin on the LTC4257IDD#PBF?
The PWRGD pin on the LTC4257IDD#PBF is an open-drain output referenced to VIN that signals when VOUT has stabilized and the internal MOSFET is fully conducting. It sinks up to 1mA when active and remains high-impedance during detection, classification, or thermal overload - enabling safe sequencing of downstream DC/DC converters.
Can the LTC4257IDD#PBF be used with positive-input PoE systems?
No - the LTC4257IDD#PBF is designed exclusively for negative-input PoE systems (–48V nominal). Its absolute maximum ratings, UVLO thresholds, and internal circuitry assume a negative supply referenced to GND. Positive-input PoE architectures require different controller families such as the LTC4266 or LT4275.
LTC4257IDD#PBF Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Analog Devices Inc.
- Series:
- -
- Package/Case:
- 8-WFDFN Exposed Pad
- 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-DFN (3x3)
LTC4257IDD#PBF FAQ
1.How can I place an order for LTC4257IDD#PBF through Aetrix?
Please submit a Request for Quotation (RFQ) for LTC4257IDD#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 LTC4257IDD#PBF reliable?
The price and inventory of LTC4257IDD#PBF are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for LTC4257IDD#PBF is usually 5 days.
3.What payment methods are accepted for LTC4257IDD#PBF?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for LTC4257IDD#PBF transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for LTC4257IDD#PBF?
LTC4257IDD#PBF orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your LTC4257IDD#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 LTC4257IDD#PBF?
For technical support, including LTC4257IDD#PBF datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your LTC4257IDD#PBF requirements.
6.How does Aetrix verify that LTC4257IDD#PBF is sourced from the original manufacturer or authorized distributors?
All LTC4257IDD#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 LTC4257IDD#PBF meets industry standards.
7.What is the process for return or replacement of LTC4257IDD#PBF?
All LTC4257IDD#PBF units undergo pre-shipment inspection (PSI). If there is an issue with LTC4257IDD#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 LTC4257IDD#PBF part is unused and in its original packaging.
Return procedure for LTC4257IDD#PBF:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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