The TPS2378 is an IEEE802.3at (Power over Ethernet) Powered Device controller, featuring internal pass MOSFET for loads up to 25.5W, Type 1 (a.k.a. 802.3af) compatibility and auxiliary power source support.
The IC is normally used together with a DC-DC step down regulator to power a network device (the PD) from a PoE compliant switch or injector (the PSE). A proper 802.3at device requires an isolated power supply with some safety characteristics that makes it not trivial to implement, and there are many DC-DC ICs with integrated PoE controller to make it easier, but as I wasn’t really interested in that part I just went for an easier project with just the PoE controller and some ballast… And what better ballast than some high power white LEDs!
This project is a small PoE flashlight, that can be powered by any 802.3af or 802.3at compliant injector or switch. It can be used as a PoE tester, or if you get trapped in a dark datacenter at night!
The TPS2378 is a feature rich device, and can be used in different configurations depending on product needs. The typical application (from the device official datasheet) shows the device with some basic external elements, including the circuit for an alternative power source.
The CDB pin, not used here, is actually very important and should be connected to the enable/slow-start pin of the DC/DC converter in order to make it start only after Cbulk is loaded.
The 802.3at standard specifies some ranges of detection currents to implement the PoE signature, used by the PSE to calculate if a new PD is within its power budget. The detection and signature currents are controlled by the biasing resistors attached to DEN e CLS, and should use the specified value depending on the required class.
FOr More Details: Power over Ethernet Flashlight