PoE power comes in the form of “48V at 350mA. If the PoE current is allowed to exceed 400mA, the standard calls for the PSE to break the circuit. This is a problem for devices that occasionally need a little more juice than PoE has to offer. Another problem is that “48V does not easily convert to commonly used positive voltage supply rails. You are practically forced to provide DC isolation along with the inverted down conversion to a more usable voltage. To meet these requirements, the LTC4267 used in Figure 1's circuit implements an input current limited DC isolated flyback converter, which provides a user-settable regulated low voltage.
The LTC4267 circuit in Figure 1 supplies 5V at 1.8A. 5V is a popular supply voltage to run logic, interface with other devices such as USB, and of primary concern in this application, sufficient to charge a single Li-ion cell to its maximum termination voltage of 4.2V.
Power Path and charger circuit
In Figure 1, the LTC4055 provides triple Power- Path control and Li-ion battery charging. Connecting an external Schottky diode to the LTC4055's OUT pin and the built-in wall adapter detection circuits creates one path. In this case, the “wall adapter” power comes from the LTC4267 5V power supply called 5VPOE. The second path is for USB power, not used in this application. The third path is the battery discharge path. When the 5VPOE power goes away, the LTC4055 detects the loss and automatically switches the battery power over to the OUT pin using its internal ideal diode circuit. There is no delay in the switchover, so power is never lost. —
Figure 1. Figure 1: The LTC4267 circuit supplies 5V at 1.8A to run logic and interface devices like USB or for charging Li-ion cells (lower left).
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