2 Detailed Description Using the TPS23754

Figure 3 shows that when the standard PoE voltage range feeds VDD bus after PoE handshaking or adapter plug-in, the internal constant current source will be triggered to start charging the external C_vc capacitor. The voltage on C_vc is a constant rising slope until reaching to 15V,typ. The charging current path to external C_vc is (1). Therefore, the controller starts sending PWM pulse to switch the external primary FET M1 while the internal current source of controller is turned off. Before the output voltage (Vout) achieves the target Vout, the external C_vc capacitor is providing the energy as discharging to Vc pin as the source of the internal gate driver. The current path is (2). After Vout achieves the target voltage, the AUX winding start providing the energy to the internal gate driver and recharging C_vc capacitor. The current path is (3) and the waveform is shown in Figure 4.

Figure 3. C_vc Current Path During the Startup

Figure 4. Startup waveform on C_vc