POE PSE
Power sourcing equipment supplies a compatible Ethernet powered device. Its controller detects the load, manages the applicable classification process and controls delivery within the negotiated power budget. Supported types, port counts and monitoring features depend on the controller.
The PSE checks the PD signature before enabling normal power. Classification establishes the supported power level, and monitoring supports current limiting and disconnect handling. The LTC4292/LTC4291 example uses external MOSFETs and sense resistors with transformer-isolated communication between its analog and digital controllers.
In plain terms
Like a power utility's smart meter at every house — it asks the home for its load profile before energising, then watches the draw moment by moment to disconnect on a fault.
Why designers use it
- Managing compatible downstream PoE loads.
- Monitoring ports and coordinating their power budgets.
- Implementing the selected PoE controller's detection, classification and fault-handling functions.
Best for
- Managed switches
- Midspans
- Industrial PoE
Key specifications
- LTC4292/LTC4291 example: 4 PSE ports, 2 power channels per port
- Supported PSE types: IEEE 802.3bt Type 3 and 4 in the cited chipset
- PD delivered power: Programmable up to 71.3 W in the cited chipset (Not a universal per-port output for every PSE.)
- Control interface: I²C up to 1 MHz in the cited chipset
- Power path: External MOSFETs and per-channel sense resistors
When not to use it
- When the controller's supported PoE types, channel count or power budget do not suit the installation.
- As a substitute for the required power supply, isolation, protection or complete-system compliance work.
Common mistakes
- Pairing controller names from different chipsets: LTC4292/LTC4291 is the cited four-port IEEE 802.3bt pair, not LTC4291/LTC4271.
- Confusing power delivered to the PD with power available at the application after conversion.
- Assuming every controller integrates the port MOSFETs or uses the same sense circuit.
- Copying a fixed maintain-power current or timing rule across different PoE types without checking the exact specification.
Where you will find it
- An Ethernet switch or midspan injector can use a PSE controller to power compatible downstream devices. The supply, cabling and port budget must support the simultaneous loads.
- The LTC4292/LTC4291 is a documented four-port chipset; its datasheet is the basis for the example here, not a claimed teardown of a named commercial switch.
A short history
ADI documents different PSE chipsets for different generations and requirements. The LTC4290/LTC4271 example in its overview and the LTC4292/LTC4291 IEEE 802.3bt example are separate pairings. Similar part numbers do not establish interchangeability.
Good to know
- A port's negotiated power and the switch's total power budget are separate constraints.
- Power at the PD and usable power after its converter are not the same quantity.