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TPS2376DRG4 Datasheet(PDF) 11 Page - Texas Instruments |
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TPS2376DRG4 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 25 page 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 data sheet TPS2375 TPS2376 TPS2377 www.ti.com............................................................................................................................................................ SLVS525B – APRIL 2004 – REVISED APRIL 2008 Figure 19. Threshold Voltages This feature of IEEE 802.3af eliminates powering and potentially damaging Ethernet devices not intended for application of 48 V. When a voltage in the range of 2.7 V to 10.1 V is applied to the PI, an incremental resistance of 25 k Ω signals the PSE that the PD is capable of accepting power. A PD that is capable of accepting power, but is not ready, may present an incorrect signature intentionally. The incremental resistance is measured by applying two different voltages to the PI and measuring the current it draws. These two test voltages must be within the specified range and be at least 1 V apart. The incremental resistance equals the difference between the voltages divided by the difference between the currents. The allowed range of resistance is 23.75 k Ω to 26.25 k Ω. The TPS2375 is in detection mode whenever the supply voltage is below the lower classification threshold. The TPS2375 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 ground during detection. Current flowing through R(DET) to VSS (Figure 1) produces the detection signature. For most applications, a 24.9-kΩ, 1%, resistor is recommended. R(DET) can be a small, low-power resistor because it only sees a stress of about 5 mW. When the input voltage rises above the 11.3 V lower classification comparator threshold, the DET pin goes to an open-drain condition to conserve power. The input diode bridge incremental resistance can be hundreds of ohms at the low currents seen at 2.7 V on the PI. The bridge resistance is in series with R(DET) and increases the total resistance seen by the PSE. This varys with the type of diode selected by the designer, and it is not usually specified on the diode data sheet. The value of R(DET) may be adjusted downwards to accommodate a particular diode type. Once the PSE has detected a PD, it may optionally classify the PD. This process allows a PSE to determine the PD power requirements in order to allot only as much power as necessary from its fixed input power source. This allows the PSE to power the maximum number of PDs from a particular power budget. This step is optional because some PSEs can afford to allot the full power to every powered port. The classification process applies a voltage between 14.5 V and 20.5 V to the input of the PD, which in turn draws a fixed current set by R(CLASS). The PSE measures the PD current to determine which of the five available classes (Table 1) that the PD is signalling. The total current drawn from the PSE during classification is the sum of bias currents and current through R(CLASS). The TPS2375 disconnects R(CLASS) at voltages above the classification range to avoid excessive power dissipation (Figure 18 and Figure 19). The value of R(CLASS) should be chosen from the values listed in Table 1 based on the average power requirements of the PD. The power rating of this resistor should be chosen so that it is not overstressed for the required 75-ms classification period, during which 10 V is applied. The PD could be in classification for extended periods during bench test conditions, or if an auxiliary power source with voltage within the classification range is connected to the PD front end. Thermal protection may activate and turn classification off if it continues for more than 75 ms, but the design must not rely on this function to protect the resistor. Copyright © 2004–2008, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Link(s): TPS2375 TPS2376 TPS2377 |
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