LTC4257 is a PoE PD power interface chip for IEEE 802.3af PoE system. It integrates 25k signature resistor, classification current source, input current limit with thermal foldback, undervoltage lockout and power good signalling, all in a single 8-pin package. It can be interfaced with a variety of LT DC/DC converter products to provide a cost effective power solution for IP phones, wireless access points and other PDs.
The LTC4257 is a Power over Ethernet (PoE) Powered Device (PD) interface controller that integrates a 25 kΩ signature resistor, classification current source, input‑current limit with thermal foldback, undervoltage lockout, and power‑good signalling in a single 8‑pin package. It simplifies PD design for IEEE 802.3af systems and can be paired with Linear Technology DC/DC converters for IP phones, wireless access points, and other PoE devices.
Key features include an onboard 100 V, 400 mA power MOSFET, programmable classification current (Class 0‑4), smart thermal protection, and support for both 8‑pin SO and low‑profile 3 mm × 3 mm DFN packages. The datasheet provides absolute maximum ratings, electrical characteristics, pin descriptions, and typical application circuits.
| Maximum Supply Voltage (Vin) | -57 V |
|---|---|
| Power Good Output Low Voltage | 1 mA |
| Thermal Shutdown Trip Temperature | 140 °C |
The LTC4257 can operate up to a maximum supply voltage of –57 V.