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LTC4257CS8-1 Datasheet(PDF) 7 Page - Linear Technology |
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LTC4257CS8-1 Datasheet(HTML) 7 Page - Linear Technology |
7 / 20 page LTC4257-1 7 42571fa summarized in Table 1. These various modes satisfy the requirements defined in the IEEE 802.3af specification. The input voltage is applied to the VIN pin and is with reference to the GND pin. This input voltage is always negative. To avoid confusion, voltages in this data sheet are always referred to in terms of absolute magnitude. Terms such as maximum negative voltage refer to the largest negative voltage and a rising negative voltage refers to a voltage that is becoming more negative. Refer- ences to electrical parameters in this applications section use the nominal value. Refer to the Electrical Characteris- tics section for the range of values a particular parameter will have. Table 1. LTC4257-1 Operational Mode as a Function of Input Voltage INPUT VOLTAGE (VIN with RESPECT to GND) LTC4257-1 MODE OF OPERATION 0V to – 1.4V Inactive – 1.5V to – 10V 25k Signature Resistor Detection – 11V to – 12.4V Classification Load Current Ramps up from 0% to 100% – 12.5V to UVLO* Classification Load Current Active UVLO* to –57V Power Applied to PD Load *UVLO includes hysteresis. Rising input threshold ≅ –36.0V Falling input threshold ≅ –30.5V Figure 2. PD Front End Using Diode Bridges On Main and Spare Inputs Series Diodes The IEEE 802.3af defined operating modes for a PD reference the input voltage at the RJ45 connector on the PD. However, PD circuitry must include diode bridges between the RJ45 connector and the LTC4257-1 (Figure 2). The LTC4257-1 takes this into account by compensat- ing for these diode drops in the threshold points for each range of operation. Since the voltage ranges specified in the LTC4257-1 electrical specifications are with respect to the IC pins, for both the signature and classification ranges, the LTC4257-1 lower end extends two diode drops below the IEEE 802.3af specification. A similar adjustment is made for the UVLO voltages. Detection During detection, the PSE will apply a voltage in the range of –2.8V to –10V on the cable and look for a 25k signature resistor. This identifies the device at the end of the cable as a PD. With the terminal voltage in this range, the LTC4257-1 connects an internal 25k resistor between GND and the VIN pins. This precision, temperature compensated resistor presents the proper characteristics to alert the Power Sourcing Equipment (PSE) at the other end of the cable that a PD is present and desires power to be applied. APPLICATIO S I FOR ATIO RX – 6 RX + 3 TX – 2 TX + RJ45 T1 POWERED DEVICE (PD) INTERFACE AS DEFINED BY IEEE 802.3af 42571 F02 1 7 8 5 4 SPARE – SPARE+ TO PHY BR2 BR1 GND 8 4 D3 LTC4257-1 VIN |
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