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LM5072MH-50 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM5072MH-50 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 24 page Modes of Operation Per the IEEE 802.3af specification, when a PD is connected to a PoE system it transitions through several operating modes in sequence including detection, classification (op- tional), turn on, normal operation, and power removal. Each operating mode corresponds to a specific PoE voltage range fed through the Ethernet cable. Figure 3 shows the IEEE 802.3af specified sequence of operating modes and the corresponding PD input voltages. Current steering diode-bridges are required for the PD inter- face to accept all allowable connections and polarities of PoE voltage from the RJ-45 connector (see the example application circuits in Figures 18, 19, 20 and 21). The bridge will cause some reduction of the input voltage sensed by the LM5072. To guarantee full compliance to the specification in all operating modes, the LM5072 takes into account the voltage drop across the bridge diodes and responds appro- priately to the voltage received from the PoE cable. Table 1 presents the response in each operating mode to voltages across the VIN and VEE pins. TABLE 1. Operating Modes With Respect To Input Voltage Mode of Operation Voltage from PoE Cable per IEEE 802.3af LM5072 Input Voltage (V IN pin to VEE pin) Detection (Signature) 2.7V to 10.0V 1.5V to 10.0V Classification 14.5V to 20.5V 12V to 23.5V Startup 42V max 38V (UVLO Release, V IN Rising) Normal Operation 57V to 36V 75V to 32V (UVLO, V IN Falling) 20184617 FIGURE 3. Sequence of PoE Operating Modes www.national.com 11 |
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