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LTC4265IDE-PBF Datasheet(PDF) 7 Page - Linear Technology |
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LTC4265IDE-PBF Datasheet(HTML) 7 Page - Linear Technology |
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7 / 20 page  LTC4265 7 4265f APPLICATIONS INFORMATION These modes satisfy the requirements defined in the IEEE 802.3af/at specification. Table 1. LTC4265 Modes of Operation as a Function of Input Voltage GND (V) LTC4265 MODES OF OPERATION 0V to 1.4V Inactive (Reset After 1st Classification Event) 1.5V to 9.8V (5.4V to 9.8V) 25k Signature Resistor Detection Before 1st Classification Event (Mark, 11k Signature Corrupt After 1st Classification Event) 12.5V to ON/UVLO* Classification Load Current Active ON/UVLO* to 60V Inrush and Power Applied To PD Load >71V Overvoltage Lockout, 4265 Operations are Disabled *ON/UVLO includes hysteresis. Rising input threshold, 37.2V Max. Falling input threshold, 30V Min. INPUT DIODE BRIDGE In the IEEE 802.3af/at standard, the modes of operation reference the input voltage at the PD’s RJ45 connector. Since the PD must handle power received in either polarity from either the data or the spare pair, input diode bridges BR1 and BR2 are connected between the RJ45 connector and the LTC4265 (Figure 2). RX– 6 RX+ 3 TX– 2 TX+ RJ45 T1 POWERED DEVICE (PD) INPUT 4265 F02 1 7 8 5 4 SPARE– SPARE+ TO PHY BR2 0.1μF 100V BR1 GND D3 LTC4265 VIN Figure 2. PD Front End Using Diode Bridges on Main and Spare Inputs The input diode bridge introduces a voltage drop that affects the range for each mode of operation. The LTC4265 compensates for these voltage drops so that a PD built with the LTC4265 meets the IEEE 802.3af/at-established voltage ranges. Note that the Electrical Specifications reference with respect to the LTC4265 package pins. DETECTION During detection, the PSE looks for a 25k signature resis- tor which identifies the device as a PD. The PSE will apply two voltages in the range of 2.8V to 10V and measures the corresponding currents. Figure 1 shows the detection voltages V1 and V2 and the corresponding PD current. The PSE calculates the signature resistance using the ΔV/ΔI measurement technique. The LTC4265 presents its precision, temperature-compen- sated 25k resistor between the GND and VIN pins, alerting the PSE that a PD is present and requests power to be applied. The LTC4265 signature resistor also compensates for the additional series resistance introduced by the input diode bridge. Thus a PD built with the LTC4265 conforms to the IEEE 802.3af/at detection specifications. |
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