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TPS2375 датащи(PDF) 10 Page - Texas Instruments |
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TPS2375 датащи(HTML) 10 Page - Texas Instruments |
10 / 22 page www.ti.com INTERNAL THRESHOLDS Detection Classification PD Powered Idle 1.4V V(CL_ON) V(CU_OFF) V(CU_H) V(UVLO_F) V(UVLO_R) V(VDD) DETECTION CLASSIFICATION UNDERVOLTAGE LOCKOUT (UVLO) TPS2376-H SLVS646A – SEPTEMBER 2006 – REVISED SEPTEMBER 2006 APPLICATION INFORMATION (continued) In order to implement the defined PoE functions shown in Figure 12, the TPS2376-H has a number of internal comparators with hysteresis for stable switching between the various states. Figure 13 relates the parameters in the Electrical Characteristics section to the PoE states. The mode labeled idle between classification and PD powered implies that the DET, CLASS, PG, and RTN pins are all high impedance. Figure 13. Threshold Voltages The 25 k Ω PD signature is measured by applying two voltages between 2.7 V to 10.1 V, that are at least 1 V apart, to the PD's PI and measuring the current. The resistance is calculated as a ∆V/∆I, with an acceptable range of 23.75 k Ω to 26.25 kΩ. The TPS2376-H is in detection mode whenever the supply voltage is below the lower classification threshold. The TPS2376-H draws a minimum of bias power in this condition, while PG and RTN are high impedance and the circuits associated with ILIM and CLASS are disabled. The DET pin is pulled to VSS during detection. Current flowing through R(DET) to VSS (Figure 1) produces the detection signature. The classification process applies a voltage between 14.5 V and 20.5 V, for a maximum of 75 ms, to the input of the PD, which in turn draws a fixed current set by R(CLASS). An 802.3af PSE measures the PD current to determine which of the five available classes (Table 1) that the PD is signaling. The total current drawn from the PSE during classification is the sum of bias currents and current through R(CLASS). The TPS2376-H disconnects R(CLASS) at voltages above the classification range to avoid excessive power dissipation (Figure 12 and Figure 13). A high power end-to-end system may choose to not implement classification, or redefine the power associated with each class. Low-voltage systems, for example 24 V, may not be able to use CLASS because the operational voltage may lie within the classification voltage range. This would cause the TPS2376-H classification circuits to dissipate power continuously. The power rating of the class resistor should be chosen so that it is not overstressed for the required 75-ms classification period, during which 10 V is applied. A higher wattage resistor might be required to withstand testing over longer time periods. The TPS2376-H incorporates an undervoltage lockout (UVLO) circuit that monitors line voltage to determine when to apply power to the downstream load and allow the PD to power up. The IEEE 802.3af specification dictates a maximum PD turn on voltage of 42 V and a minimum turn-off voltage of 30 V (Figure 13). The UVLO pin provides the flexibility to adjust the turn on and turn off to the IEEE 802.3af limits, or a custom set. Design the turn-on for 39.5 V if a design which uses the IEEE 802.3af limits is desired. 10 Submit Documentation Feedback |
Аналогичный номер детали - TPS2375 |
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Аналогичное описание - TPS2375 |
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