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LTC4263CDE-TRPBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC4263CDE-TRPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LTC4263 11 4263fd APPLICATIONS INFORMATION LTC4263 OPERATION Signature Detection The IEEE 802.3af specification defines a specific pair-to- pair signature resistance used to identify a device that can accept power via its Ethernet connection. When the port voltage is below 10V, an IEEE 802.3af compliant PD will have an input resistance of approximately 25kΩ. Figure 2 illustrates the relationship between the PD sig- nature resistance and the required resistance ranges the PSE must accept and reject. According to the IEEE 802.3af specification, the PSE must accept PDs with signatures between 19kΩ and 26.5kΩ and may or may not accept resistances in the two ranges of 15kΩ to 19kΩ and 26.5kΩ to 33kΩ. The black box in Figure 2 represents the typical 150Ω pair-to-pair termination used in Ethernet devices like a computer’s network interface card (NIC) that cannot accept power. The LTC4263 checks for the signature resistance by forcing two test currents on the port in sequence and measuring the resulting voltages. It then subtracts the two V-I points to determine the resistive slope while removing voltage offset caused by any series diodes or current offset caused by leakage at the port (see Figure 3). The LTC4263 will typically accept any PD resistance between 17kΩ and 29.7kΩ as a valid PD. Values outside this range (exclud- ing open and short-circuits) are reported to the user by a code flashed via the LED pin. The LTC4263 uses a force-current detection method in order to reduce noise sensitivity and provide a more robust detection algorithm. The first test point is taken by forcing a test current into the port, waiting a short time to allow the line to settle and measuring the resulting voltage. This result is stored and the second current is applied to the port, allowed to settle and the voltage measured. The LTC4263 will not power the port if the PD has more than 5μF in parallel with its signature resistor unless legacy mode is enabled. The LTC4263 autonomously tests for a valid PD connected to the port. It repeatedly queries the port every 580ms, or every 3.2s if midspan backoff mode is active (see below). If detection is successful, it performs classification and power management and then powers up the port. Midspan Backoff IEEE 802.3af requires the midspan PSE to wait two seconds after a failed detection before attempting to detect again unless the port resistance is greater than 500kΩ. This requirement is to prevent the condition of an endpoint PSE and a midspan PSE, connected to the same PD at the same time, from each corrupting the PD signature and prevent- ing power-on. After the first corrupted detection cycle, the midspan PSE waits while the endpoint PSE completes detection and turns the port on. If the midspan mode of the LTC4263 is enabled by connecting the MIDSPAN pin to VDD5, a 3.2 second delay occurs after every failed detect cycle unless the result is an open circuit. Figure 2. IEEE 802.3af Signature Resistance Ranges Figure 3. PD 2-Point Detection RESISTANCE PD PSE 0Ω 10k 15k 4263 F02 19k 26.5k 26.25k 23.75k 150Ω (NIC) 20k 30k 33k REJECT ACCEPT REJECT VDD48 – VOUT 0 60 50 40 30 20 10 0 510 15 20 4263 F04 25 TYPICAL CLASS 3 PD LOAD LINE 48mA 33mA PSE LOAD LINE 23mA 14.5mA 6.5mA CLASS 4 CLASS 2 CLASS 1 CLASS 0 CLASS 3 OVER CURRENT |
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