LTC4265CDETRPBF

The LTC4265CDETRPBF is a fully IEEE 802.3af compliant Power over Ethernet (PoE) Powered Device (PD) interface controller from Analog Devices. This device provides all the necessary interface functions between the Ethernet cable and the PD’s local power supply, including detection, classification, inrush current limiting, and power switch control.

In a PoE system, the Power Sourcing Equipment (PSE) detects and classifies PDs before applying full power. The LTC4265 handles all the required handshaking: it presents a 25kΩ signature resistance during detection so the PSE can identify it as a valid PD, and it provides a programmable classification current so the PSE knows how much power the device needs (Class 0 through Class 4, up to 12.95W at the PD).

A key feature of the LTC4265 is its integrated 0.45Ω power MOSFET. Unlike older PD controllers that require an external series MOSFET, the LTC4265 integrates the main power switch, reducing component count, board space, and cost. The integrated MOSFET also ensures precise current limiting and thermal protection.

The device includes undervoltage lockout (UVLO) with hysteresis to ensure that power is only applied to the load when the input voltage is above the minimum threshold. Inrush current limiting controls the rate at which the load capacitance is charged, preventing voltage drops on the Ethernet cable that could affect other devices. A power good (PGD) output signals when the output is ready, enabling the downstream DC-DC converter to start up.

The LTC4265 operates as a PoE PD interface with the following functional phases:

1. Detection Phase: When the Ethernet cable is first plugged in, the PSE applies a low voltage (2.7V to 10.1V) and measures the current to detect the PD signature. The LTC4265 presents a 25kΩ resistance (with some capacitance) during this phase, identifying itself as a valid PoE device.

2. Classification Phase: After successful detection, the PSE applies a higher voltage (15.5V to 20.5V) and measures the classification current. The LTC4265 sources a classification current determined by the resistor on the CLASS pin. Different current levels correspond to different power classes (Class 0 = 15.4W at PSE, Class 1 = 4W, Class 2 = 7W, Class 3 = 15.4W, Class 4 = reserved).

3. Power-Up Phase: After classification, the PSE applies full voltage (44V to 57V). The LTC4265 begins the power-up sequence when VDD exceeds the UVLO rising threshold (~36V). The internal power MOSFET is gradually turned on, charging the output capacitor with a controlled inrush current (limited to ≤150mA per 802.3af).

4. Normal Operation: Once the output capacitor is charged and the MOSFET is fully on, the device enters normal operation. The PGD (power good) output goes high, signaling that power is good and the downstream DC-DC converter can start up. In normal mode, the LTC4265 acts as a low-resistance switch between the input (VDD) and output (RTN).

5. Protection Features: The LTC4265 includes thermal shutdown that turns off the MOSFET if the die temperature exceeds safe limits. It also includes overcurrent protection and undervoltage lockout with hysteresis to prevent operation at abnormally low voltages.

6. Mark/Event Detection: The LTC4265 supports the optional Mark event (maintain power signature) as specified in 802.3af, allowing the PSE to briefly reduce voltage for signaling purposes without causing the PD to power down.

• VoIP phones and video conference endpoints
• Wi-Fi access points and wireless routers
• IP security cameras and NVR systems
• Industrial Ethernet sensors and IoT gateways
• IPTV set-top boxes and digital signage
• Thin clients and networked printers