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LTC4257 Datasheet(PDF) 6 Page - Linear Technology |
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LTC4257 Datasheet(HTML) 6 Page - Linear Technology |
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6 / 20 page  LTC4257 6 4257fb The LTC4257 is intended for use as the front end of a Powered Device (PD) designed to IEEE 802.3af draft standard. The LTC4257 includes a trimmed 25k signature resistor, classification current source, and an inrush cur- rent limit circuit. With these functions integrated into the LTC4257, the signature and power interface for a PD that meets all the requirements of IEEE 802.3af can be built with a minimum of external components. Using an LTC4257 for the power and signature interface functions of a PD provides several advantages. The LTC4257 current limit circuit includes an onboard, 100V, 400mA power MOSFET with low leakage. This onboard low leakage MOSFET avoids the possibility of corrupting the 25k signature resistor while also saving board space and cost. In addition, the IEEE 802.3af inrush current limit requirement causes large transient power dissipation in the PD; the LTC4257 manages this turn-on sequence through the use of smart thermal protection circuitry. The LTC4257 is designed to allow multiple turn-on sequences without overheating the miniature 8-lead package. In the event of excessive power cycling, the LTC4257 provides thermally activated current-limit reduction to keep the onboard power MOSFET within its safe operating area. Operation The LTC4257 has several modes of operation depending on the applied input voltage as shown in Figure 1 and 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. DETECTION V1 CLASSIFICATION UVLO TURN-ON UVLO OFF POWER BAD UVLO OFF UVLO ON UVLO TURN-OFF τ = RLOAD C1 PWRGD TRACKS VIN DETECTION V2 –10 TIME –20 –30 –40 –50 –10 TIME –20 –30 –40 –50 –10 TIME –20 –30 –40 –50 ICLASS ILIMIT dV dt ILIMIT C1 = POWER BAD POWER GOOD DETECTION I1 CLASSIFICATION ICLASS DETECTION I2 LOAD CURRENT, ILOAD CURRENT LIMIT, ILIMIT 4257 F01 ICLASS DEPENDENT ON RCLASS SELECTION ILIMIT = 350mA (NOMINAL) I1 = V1 – 2 DIODE DROPS 25k Ω ILOAD = VIN RLOAD I2 = V2 – 2 DIODE DROPS 25k Ω GND 2 PSE IIN 8 6 5 R9 RLOAD RCLASS VOUT C1 GND 4 RCLASS PWRGD LTC4257 VOUT VIN VIN Figure 1. Output Voltage, PWRGD and PD Current as a Function of Input Voltage APPLICATIO S I FOR ATIO |
Similar Part No. - LTC4257_15 |
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Similar Description - LTC4257_15 |
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