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Analog Devices Inc. LTC4257CDD-1#PBF
- Part No.:
- LTC4257CDD-1#PBF
- Manufacturer:
- Analog Devices Inc.
- Category:
- Power Over Ethernet (PoE) Controllers
- Package:
- 8-WFDFN Exposed Pad
- Datasheet:
-
LTC4257CDD-1#PBF.pdf
- Description:
- IC POE CNTRL 1 CHANNEL 8DFN
- Quantity:
- Payment:
- Shipping:
Inventory:375
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Product details
Overview
LTC4257CDD-1#PBF from Analog Devices (acquired 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), –36.0V UVLO turn-on threshold, and onboard 25kΩ signature resistor. It enables compliant PD designs for IP phones and wireless access points with minimal external components.
For engineers reviewing the LTC4257CDD-1#PBF datasheet, LTC4257CDD-1#PBF pinout, LTC4257CDD-1#PBF application, or LTC4257CDD-1#PBF equivalent, key selection considerations include its DFN (3mm × 3mm) package, negative-input voltage operation (–1.5V to –57V), programmable classification (Class 0–3 via RCLASS), thermal shutdown at 140°C, and power-good signaling referenced to VIN.
Technical Context
The LTC4257CDD-1#PBF implements a two-stage detection-to-power-up sequence: first presenting a 25kΩ signature resistance between –1.5V and –9.5V for PSE identification, then asserting programmable classification current (10.5–28mA) between –12.5V and –21V. Its dual-level current limit operates at 140mA during inrush (enabling large load capacitor charging) and switches to 375mA after VOUT–VIN falls below 1.5V.
It integrates undervoltage lockout with hysteresis (–36.0V turn-on / –30.5V turn-off), thermal overload protection that disables classification current above 140°C, and signature disable functionality via SIGDISA pin-pulling it to GND reduces signature resistance to 9kΩ, invalidating detection per IEEE 802.3af.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Supply Voltage Range | –1.5V to –57V (negative-referenced operation compatible with PoE diode bridge inputs) |
| Signature Resistance | 23.25kΩ to 26.00kΩ (IEEE 802.3af-compliant 2-point ∆V/∆I measurement, compensates for two series diodes) |
| Classification Current Accuracy | ±3.5% (ensures reliable Class 0–3 identification by PSE under –12.5V to –21V input) |
| Input Current Limit (Low/High) | 140mA / 375mA (dual-level control prevents inrush violation while enabling full 12.95W operation) |
| UVLO Thresholds | Turn-on: –36.0V ±1.2V; Turn-off: –30.5V ±1.2V (hysteresis prevents oscillation during marginal cable voltage drop) |
| Power MOSFET RON | 1.0Ω to 2.0Ω (low conduction loss at –48V input, supports continuous 350mA load current) |
| Thermal Shutdown | 140°C (protects internal MOSFET during sustained overload or repeated power cycling) |
Pinout & Package
Package: 8-lead (3mm × 3mm) plastic DFN with exposed pad soldered to electrically isolated PCB heatsink (θJA = 43°C/W, TJMAX = 125°C).
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| NC (Pin 1) | No internal connection | Must be left unconnected or tied to GND; no functional impact |
| RCLASS (Pin 2) | Classification current programming input | Resistor to VIN sets Class 0–3 current (e.g., 124Ω = Class 1, 10.5mA) |
| NC (Pin 3) | No internal connection | Must be left unconnected or tied to GND; no functional impact |
| VIN (Pin 4) | Negative power input | Accepts –1.5V to –57V from PoE diode bridge; all voltages referenced to GND |
| VOUT (Pin 5) | Regulated negative output | Delivers –48V to PD load via internal MOSFET; high-impedance until UVLO turn-on |
| PWRGD (Pin 6) | Open-drain power-good signal | Pulled low when VOUT–VIN < 1.5V (rising) or > 3.0V (falling); referenced to VIN |
| SIGDISA (Pin 7) | Signature disable control | Tie to GND to invalidate signature (9kΩ); float or tie to VIN to enable 25kΩ detection |
| GND (Pin 8) | System ground reference | Return path for signature, classification, and load current; connects to diode bridge cathode |
Key Features
| Feature | Design Value |
|---|---|
| Integrated 100V/400mA MOSFET | Eliminates external high-voltage switch, saves board space, and avoids signature corruption from leakage |
| Dual-level current limiting | Enables safe charging of large load capacitors (e.g., ≥5µF) while maintaining compatibility with legacy and IEEE 802.3af PSEs |
| Onboard 25kΩ signature resistor | Temperature-compensated and trimmed to meet IEEE 802.3af ∆V/∆I specification including diode drop compensation |
| Programmable classification (Class 0–3) | Single external resistor selects precise load current (10.5–28mA), enabling accurate power allocation by PSE |
| Thermal overload protection | Shuts down classification current and power MOSFET above 140°C, preventing damage during fault or repeated cycling |
Applications
| IP Phone Power Management | Wireless Access Point Interface |
|---|---|
Use Scenario: IP phone receives –48V PoE power through RJ45, requiring robust detection, classification, and inrush control before enabling VoIP processing circuitry. IC Role / Device Role / Timing Role: LTC4257CDD-1#PBF acts as the primary PD front-end controller, managing signature, classification, UVLO, and power delivery to downstream DC/DC converters. Use Value: Dual-level current limit prevents PSE overcurrent tripping during capacitor charging, while PWRGD ensures clean start-up sequencing for voice codec and Ethernet PHY. | Use Scenario: Dual-band 802.11ac access point draws up to 12.95W from PoE, requiring stable power delivery across varying cable lengths and legacy PSE compatibility. IC Role / Device Role / Timing Role: LTC4257CDD-1#PBF provides IEEE 802.3af-compliant interface with thermal protection and power-good signaling to coordinate RF and baseband subsystem enablement. Use Value: Onboard 100V MOSFET and low RON minimize conduction loss, extending operational margin in high-temperature enclosures without external heatsinking. |
| Telecom Remote Terminal Unit | Industrial IoT Edge Gateway |
Use Scenario: Outdoor telecom terminal uses PoE for backhaul connectivity and must survive voltage transients, thermal stress, and intermittent PSE availability. IC Role / Device Role / Timing Role: LTC4257CDD-1#PBF serves as hardened PD interface with UVLO hysteresis and thermal shutdown, ensuring reliable reconnection after brownouts. Use Value: Signature disable (SIGDISA) allows controlled power-down during firmware updates, preventing false PSE detection and unintended re-powering. | Use Scenario: Factory-floor edge gateway aggregates sensor data and requires PoE power with fail-safe behavior during network maintenance or PSE replacement. IC Role / Device Role / Timing Role: LTC4257CDD-1#PBF delivers regulated –48V with precise classification and power-good assertion to synchronize MCU boot and Ethernet initialization. Use Value: Programmable RCLASS resistor enables field-configurable power class (e.g., Class 2 for low-power mode), optimizing PSE resource allocation without hardware change. |
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 25.5W capability; includes I2C interface for status reporting and fault logging | Required for higher-power devices like PTZ cameras or multi-radio APs; adds system-level monitoring overhead | Select LT4275IMS#PBF when PoE+ compliance, digital diagnostics, or Class 4/5 support are mandatory |
| TPS23753APWPR | TI device with integrated DC/DC controller; supports flyback topology; no onboard MOSFET (external FET required) | Better suited for cost-sensitive, lower-volume designs where BOM flexibility outweighs integration benefits | Choose TPS23753APWPR if combining PD interface with isolated DC/DC conversion on single IC is preferred and external MOSFET layout is acceptable |
Compared with LT4275IMS#PBF and TPS23753APWPR, the LTC4257CDD-1#PBF offers superior integration for basic 802.3af systems-its monolithic MOSFET, fixed dual-current limit, and minimal external component count reduce design risk and PCB area, making it optimal for high-volume, space-constrained IP phones and access points.
Availability
LTC4257CDD-1#PBF is available at Aetrix Electronics and suitable for IP phone power management, wireless access point interface, and telecom remote terminal unit applications requiring stable component supply, long-term lifecycle support, and consistent parametric performance across production batches.
Supply support for LTC4257CDD-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, Inc. (ADI) is a global semiconductor leader specializing in high-performance analog, mixed-signal, and digital signal processing technologies.
The LTC4257CDD-1#PBF belongs to ADI's Power over Ethernet product line, designed specifically to simplify IEEE 802.3af Powered Device implementation with integrated power switching, signature, classification, and protection functions.
FAQ
What is the operating temperature range for the LTC4257CDD-1#PBF?
The LTC4257CDD-1#PBF is specified for commercial temperature operation from 0°C to 70°C. Its DFN package has a maximum junction temperature of 125°C and thermal resistance θJA of 43°C/W, enabling reliable performance in enclosed IP phone or access point enclosures without forced airflow. Derating guidelines apply above 70°C ambient.
How does the LTC4257CDD-1#PBF handle legacy Power Sourcing Equipment (PSE) that doesn't comply with IEEE 802.3af inrush limits?
The LTC4257CDD-1#PBF uses a dual-level current limit: 140mA during initial capacitor charging (low-level mode) and 375mA after VOUT stabilizes (high-level mode). This allows safe startup with legacy PSEs having limited current capacity while still delivering full 12.95W when connected to an 802.3af-compliant PSE. The LTC4257CDD-1#PBF automatically transitions modes based on VOUT–VIN voltage difference.
Can the LTC4257CDD-1#PBF be used with both polarity configurations of PoE (Mode A and Mode B)?
Yes-the LTC4257CDD-1#PBF is designed to operate with standard PoE diode bridges that accept either Mode A (pins 1/2 and 3/6) or Mode B (pins 4/5 and 7/8) polarity. Its VIN pin accepts negative voltage referenced to GND, and internal circuitry compensates for the voltage drop of two series diodes, ensuring correct signature, classification, and power delivery regardless of polarity.
What happens to the LTC4257CDD-1#PBF when the input voltage drops below the UVLO turn-off threshold?
When the input voltage at VIN falls below –30.5V (typical), the LTC4257CDD-1#PBF turns off its internal power MOSFET, places VOUT in high-impedance state, and re-enables the classification current source. PWRGD goes high-impedance, signaling power loss to downstream circuitry. The LTC4257CDD-1#PBF remains in this state until VIN rises above –36.0V (typical), at which point it resumes detection and power-up sequencing.
Is the LTC4257CDD-1#PBF pin-compatible with other packages in the LTC4257-1 family?
Yes-the LTC4257CDD-1#PBF shares identical pinout and electrical functionality with the SO-8 packaged LTC4257CS8-1#PBF. Both use the same 8-pin configuration (GND, SIGDISA, PWRGD, VOUT, NC, RCLASS, NC, VIN), enabling direct PCB footprint interchangeability between DFN and SO-8 variants without redesign.
LTC4257CDD-1#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-1#PBF FAQ
1.How can I place an order for LTC4257CDD-1#PBF through Aetrix?
Please submit a Request for Quotation (RFQ) for LTC4257CDD-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 LTC4257CDD-1#PBF reliable?
The price and inventory of LTC4257CDD-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 LTC4257CDD-1#PBF is usually 5 days.
3.What payment methods are accepted for LTC4257CDD-1#PBF?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for LTC4257CDD-1#PBF transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for LTC4257CDD-1#PBF?
LTC4257CDD-1#PBF orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your LTC4257CDD-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 LTC4257CDD-1#PBF?
For technical support, including LTC4257CDD-1#PBF datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your LTC4257CDD-1#PBF requirements.
6.How does Aetrix verify that LTC4257CDD-1#PBF is sourced from the original manufacturer or authorized distributors?
All LTC4257CDD-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 LTC4257CDD-1#PBF meets industry standards.
7.What is the process for return or replacement of LTC4257CDD-1#PBF?
All LTC4257CDD-1#PBF units undergo pre-shipment inspection (PSI). If there is an issue with LTC4257CDD-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 LTC4257CDD-1#PBF part is unused and in its original packaging.
Return procedure for LTC4257CDD-1#PBF:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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