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LTC4264 Datasheet(PDF) 11 Page - Linear Technology |
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LTC4264 Datasheet(HTML) 11 Page - Linear Technology |
11 / 24 page LTC4264 11 4264f APPLICATIONS INFORMATION During classification, the PSE presents a fixed voltage between –15.5V and –20.5V to the PD (Figure 6). With the input voltage in this range, the LTC4264 asserts a load current from the GND pin through the RCLASS resistor. The magnitude of the load current is set with the selection of the RCLASS resistor. The resistor value associated with each class is shown in Table 2. Table 2. Summary of IEEE 802.3af Power Classifications and LTC4264 RCLASS Resistor Selection CLASS USAGE MAXIMUM POWER LEVELS AT INPUT OF PD (W) NOMINAL CLASSIFICATION LOAD CURRENT (mA) LTC4264 RCLASS RESISTOR ( Ω, 1%) 0 Default 0.44 to 12.95 <5 Open 1 Optional 0.44 to 3.84 10.5 124 2 Optional 3.84 to 6.49 18.5 69.8 3 Optional 6.49 to 12.95 28 45.3 4 Reserved by IEEE. See Apps 40 30.9 5 Undefined IEEE. See Apps 56 22.1 A substantial amount of power is dissipated in the LTC4264 during classification. The IEEE 802.3af specification limits the classification time to 75ms in order avoid excessive heating. The LTC4264 is designed to handle the power dissipation during the probe period. If the PSE probing exceeds 75ms, the LTC4264 may overheat. In this situa- tion, the thermal protection circuit will engage and disable the classification current source, protecting the LTC4264 from damage. When the die cools, classification is auto- matically resumed. Classification presents a challenging stability problem for the PSE due to the wide range of loads possible. The LTC4264 has been designed to avoid PSE interoperability problems by maintaining a positive I-V slope throughout the signature and classification ranges up to UVLO turn- on as shown in Figure 6b. The positive I-V slope avoids areas of negative resistance and helps prevent the PSE from power cycling or getting “stuck” during signature or classification probing. In the event a PSE overshoots beyond the classification voltage range, the available load current aids in returning the PD back into the classification voltage range. (The PD input may otherwise be “trapped” by a reverse-biased diode bridge and the voltage held by the 0.1µF capacitor.) By gently ramping the classification current on and maintaining a positive I-V slope until UVLO turn-on, the LTC4264 provides a well behaved load, as- suring interoperability with any PSE. GND RCLASS VIN LTC4264 CONSTANT LOAD CURRENT INTERNAL TO LTC4264 4264 F06a RCLASS CURRENT PATH V PD PSE PSE CURRENT MONITOR PSE PROBING VOLTAGE SOURCE –15.5V TO –20.5V INPUT VOLTAGE (V) 0 –40 4264 F06b –10 –20 –30 Figure 6a. PSE Probing PD During Classification Figure 6b. LTC4264 Positive I-V Slope |
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