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Analog Devices Inc. LTC4257CDD#PBF
- Part No.:
- LTC4257CDD#PBF
- Manufacturer:
- Analog Devices Inc.
- Category:
- Power Over Ethernet (PoE) Controllers
- Package:
- 8-WFDFN Exposed Pad
- Datasheet:
-
LTC4257CDD#PBF.pdf
- Description:
- IC POE CNTRL 1 CHANNEL 8DFN
- Quantity:
- Payment:
- Shipping:
Inventory:167
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Product details
Overview
LTC4257CDD#PBF from Analog Devices (formerly Linear Technology) is an IEEE 802.3af Power over Ethernet (PoE) Powered Device (PD) interface controller in a 3mm × 3mm DFN-8 package. It integrates a 100V/400mA onboard power MOSFET, precision 25kΩ signature resistor, programmable classification current source (Class 0–4), input current limit (350mA typ), and undervoltage lockout (UVLO turn-on at –39.2V). It enables compact, thermally robust PD front-end design for IP phones and wireless access points.
For engineers reviewing the LTC4257CDD#PBF datasheet, LTC4257CDD#PBF pinout, LTC4257CDD#PBF application, or LTC4257CDD#PBF equivalent, key selection considerations include its negative-input voltage operation (–57V max), smart thermal foldback (120°C trip), PWRGD open-drain output referenced to VIN, RCLASS-programmable classification, and compatibility with dual-diode bridge input architectures per IEEE 802.3af.
Technical Context
The LTC4257CDD#PBF operates with a negative supply referenced to GND, supporting detection (–1.5V to –9.5V), classification (–12.5V to –21V), and powered operation (UVLO turn-on at –39.2V). Its internal 25kΩ signature resistor is trimmed to compensate for two series diode drops, ensuring IEEE 802.3af compliance when used with standard PD diode bridges.
It implements smart thermal protection: at 120°C, input current limit reduces to 188mA (50% of nominal); at 140°C, it shuts down completely. The PWRGD signal asserts low when VDS across the internal MOSFET falls below 1.5V (rising threshold) and deasserts when VDS exceeds 3.0V (falling threshold), providing reliable power-good indication for downstream DC/DC converters.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VIN Operating Range | –1.5V to –57V (supports full IEEE 802.3af PD input range with diode bridge compensation) |
| Signature Resistance | 23.25kΩ to 26.00kΩ (2-point ∆V/∆I method; offset to meet IEEE spec with two series diodes) |
| Classification Current Accuracy | ±3.5% (ensures reliable Class 0–4 identification under –12.5V ≤ VIN ≤ –21V) |
| Input Current Limit | 300mA to 400mA (thermally regulated; 350mA typical for controlled inrush into load capacitor) |
| On-Resistance (RON) | 1.0Ω to 2.0Ω (low-loss power path; enables <1.5V drop at 350mA for efficient PoE delivery) |
| UVLO Turn-On/Off | –39.2V / –30.5V (hysteretic thresholds prevent oscillation during line impedance drop) |
| PWRGD Thresholds | Rising: 2.7–3.3V; Falling: 1.3–1.7V (VDS monitoring ensures stable enable timing for DC/DC startup) |
Pinout & Package
Package: 8-lead, 3mm × 3mm plastic DFN with exposed pad (θJA = 43°C/W; requires soldering exposed pad to electrically isolated PCB heatsink).
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| Pin 1 (NC) | No Connect | Unused; must be left floating or tied to GND per layout guidelines |
| Pin 2 (RCLASS) | Classification Current Set Input | Connect external resistor to VIN to program Class 0–4 current (e.g., 124Ω for Class 1) |
| Pin 3 (NC) | No Connect | Unused; must be left floating or tied to GND |
| Pin 4 (VIN) | Negative Power Input | Accepts –48V (nominal) from PSE via diode bridge; referenced to GND |
| Pin 5 (VOUT) | Negative Power Output | Delivers regulated –48V to PD load through internal MOSFET; high-impedance until UVLO turn-on |
| Pin 6 (PWRGD) | Power Good Open-Drain Output | Sinks 1mA when active; referenced to VIN; signals VDS < 1.5V (MOSFET fully on) |
| Pin 7 (NC) | No Connect | Unused; must be left floating or tied to GND |
| Pin 8 (GND) | Ground Reference | System ground return path; ties to diode bridge cathode and logic ground |
Key Features
| Feature | Design Value |
|---|---|
| Integrated 100V/400mA Power MOSFET | Eliminates external high-voltage switch; reduces BOM count and board area while maintaining <2Ω RON |
| Trimmed 25kΩ Signature Resistor | Offset to 23.25–26.00kΩ ensures IEEE 802.3af compliance with two-series-diode bridge losses |
| Programmable Classification (Class 0–4) | Single external RCLASS resistor sets load current per IEEE table; supports power-aware PSE allocation |
| Smart Thermal Foldback | Reduces current limit to 188mA at 120°C and shuts off at 140°C-prevents damage during repeated inrush or high-ambient operation |
| UVLO with Hysteresis | Turn-on at –39.2V, turn-off at –30.5V; prevents cycling due to cable IR drop under load |
| PWRGD Referenced to VIN | Enables direct interfacing with VIN-referenced monitoring circuits; avoids level-shifting for fault detection |
Applications
| IP Phone Power Interface | Wireless Access Point (WAP) Front End |
|---|---|
|
Use Scenario: IEEE 802.3af-compliant VoIP phone receiving –48V power and data over single Ethernet cable. IC Role / Device Role / Timing Role: PD interface controller handling detection, classification, inrush limiting, and power-good signaling before enabling internal DC/DC converter. Use Value: Enables single-chip PoE front end with integrated MOSFET and thermal protection-reduces component count by ≥4 vs discrete solutions and sustains 350mA continuous load without derating at 70°C ambient. |
Use Scenario: Dual-band 802.11ac WAP deployed in ceiling-mounted enclosures with limited airflow and variable cable lengths. IC Role / Device Role / Timing Role: Primary power interface managing diode-bridge input, classification handshake, and controlled VOUT ramp-up into large bulk capacitance. Use Value: Smart thermal foldback prevents shutdown during repeated power-cycling events; DFN-8 exposed pad dissipates heat efficiently in confined spaces where SO-8 would exceed TJMAX. |
| Telecom Remote Terminal Unit | Industrial IoT Edge Gateway |
|
Use Scenario: Outdoor telecom cabinet housing multiple PDs connected to centralized PSE via long (>100m) CAT5e runs. IC Role / Device Role / Timing Role: Robust PD controller tolerating high line impedance, voltage sag, and ambient temperatures up to 70°C. Use Value: UVLO hysteresis (8.7V gap) prevents false turn-off during transient IR drops; 25kΩ signature tolerance (±5%) ensures reliable detection even with aging diodes. |
Use Scenario: DIN-rail mounted gateway aggregating sensor data over PoE-powered field nodes in factory automation. IC Role / Device Role / Timing Role: Safety-critical PD interface delivering clean, monitored –48V to isolated RS-485 and CAN transceivers. Use Value: PWRGD output provides deterministic enable timing for downstream isolators; leakage-limited VOUT (150µA) prevents unintended biasing of unpowered circuitry. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar PoE PD interface applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| LT4275IMS#PBF | Supports IEEE 802.3at (PoE+) with 600mA current limit; includes I2C interface for dynamic classification and fault reporting. | Required for >12.95W PDs (e.g., PTZ cameras); adds microcontroller dependency for configuration. | Select LT4275IMS#PBF only if PoE+ power budget or telemetry is needed; LTC4257CDD#PBF remains optimal for cost-sensitive 802.3af-only designs. |
| TPS23753APWPR | TI device with integrated DC/DC controller; supports flyback topology; no onboard MOSFET-requires external FET. | Reduces total solution size for integrated power conversion but increases layout complexity and BOM count. | Choose TPS23753APWPR when combining PD interface and primary-side regulation is preferred; LTC4257CDD#PBF excels when separating interface and DC/DC stages simplifies thermal management. |
Compared with LT4275IMS#PBF and TPS23753APWPR, the LTC4257CDD#PBF delivers lowest component count and simplest layout for 802.3af PDs, leveraging its monolithic MOSFET and zero-external-component current limiting-ideal for space-constrained, high-volume IP phones and WAPs where PoE+ capability is unnecessary.
Availability
LTC4257CDD#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, long-term lifecycle support, and RoHS-compliant packaging.
Supply support for LTC4257CDD#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 PoE portfolio with full technical documentation, reference designs, and long-term manufacturing commitment.
The LTC4257CDD#PBF belongs to Linear's IEEE 802.3af PD controller family, designed specifically to simplify front-end implementation for cost-sensitive, thermally constrained PoE-powered endpoints such as voice and wireless infrastructure equipment.
FAQ
What is the operating temperature range for the LTC4257CDD#PBF?
The LTC4257CDD#PBF is rated for 0°C to 70°C ambient operation (C-grade). Its DFN-8 package features an exposed thermal pad with θJA = 43°C/W, enabling reliable performance in enclosed IP phone or WAP enclosures without forced airflow. The internal thermal shutdown activates at 140°C junction temperature, providing robust overtemperature protection during inrush or sustained overload conditions.
How does the LTC4257CDD#PBF handle diode bridge voltage drops in PoE input paths?
The LTC4257CDD#PBF compensates for two series diode drops in its signature and classification thresholds. Its 25kΩ internal resistor is trimmed to 23.25–26.00kΩ so that, combined with typical diode resistance (~1.25kΩ total), the overall PD signature meets IEEE 802.3af's 23.75–26.25kΩ requirement. Similarly, UVLO and classification voltage ranges are offset downward to maintain correct functional boundaries at the RJ45 connector.
Can the LTC4257CDD#PBF be used with IEEE 802.3at (PoE+) systems?
No-the LTC4257CDD#PBF is strictly compliant with IEEE 802.3af (up to 12.95W). It lacks the higher current limit (600mA), extended classification protocol, and auxiliary power discovery required for 802.3at. For PoE+ applications, Analog Devices recommends the LT4275IMS#PBF or LT4276AIDE#PBF, which support both standards and provide backward compatibility with 802.3af PSEs.
What is the purpose of the PWRGD pin on the LTC4257CDD#PBF, and how should it be interfaced?
The PWRGD pin on the LTC4257CDD#PBF is an open-drain output referenced to VIN that asserts low when the internal MOSFET's VDS falls below 1.5V-indicating stable power delivery to the load. It must be pulled up externally (e.g., to VOUT or a local rail) and interfaced with level-shifting or clamping (e.g., diode to VOUT) if driving logic referenced to VOUT or GND, since its voltage swing is relative to VIN (e.g., –48V + 0.5V when active).
Does the LTC4257CDD#PBF require external components for basic IEEE 802.3af compliance?
Yes-basic operation requires only three external components: an RCLASS resistor (to set classification class), a minimum 5µF output capacitor (C1) between VOUT and GND, and input diode bridge (typically two S1B or DF01SA diodes). No external current-sense resistors, gate drivers, or thermal sensors are needed-the LTC4257CDD#PBF integrates all core PD interface functions including signature, classification, current limiting, UVLO, and power-good signaling.
LTC4257CDD#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:
- 0°C ~ 70°C
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 8-DFN (3x3)
LTC4257CDD#PBF FAQ
1.How can I place an order for LTC4257CDD#PBF through Aetrix?
Please submit a Request for Quotation (RFQ) for LTC4257CDD#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 LTC4257CDD#PBF reliable?
The price and inventory of LTC4257CDD#PBF are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for LTC4257CDD#PBF is usually 5 days.
3.What payment methods are accepted for LTC4257CDD#PBF?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for LTC4257CDD#PBF transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for LTC4257CDD#PBF?
LTC4257CDD#PBF orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your LTC4257CDD#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 LTC4257CDD#PBF?
For technical support, including LTC4257CDD#PBF datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your LTC4257CDD#PBF requirements.
6.How does Aetrix verify that LTC4257CDD#PBF is sourced from the original manufacturer or authorized distributors?
All LTC4257CDD#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 LTC4257CDD#PBF meets industry standards.
7.What is the process for return or replacement of LTC4257CDD#PBF?
All LTC4257CDD#PBF units undergo pre-shipment inspection (PSI). If there is an issue with LTC4257CDD#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 LTC4257CDD#PBF part is unused and in its original packaging.
Return procedure for LTC4257CDD#PBF:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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