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LTC4257 Datasheet(PDF) 9 Page - Linear Technology |
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LTC4257 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  LTC4257 9 4257fb probing exceeds 75ms, the LTC4257 may overheat. In this situation, the thermal protection circuit will engage and disable the classification current source in order to protect the part. The LTC4257 stays in classification mode until the input voltage rises above the UVLO turn-on voltage. Undervoltage Lockout IEEE 802.3af dictates a maximum turn-on voltage of 42V and a minimum turn-off voltage of 30V for the PD. In addition, the PD must maintain large on-off hysteresis to prevent resistive losses in the wiring between the PSE and the PD from causing start-up oscillation. The LTC4257 incorporates an undervoltage lockout (UVLO) circuit that monitors line voltage to determine when to apply power to the PD load (Figure 5). Before power is applied to the load, the VOUT pin is high impedance and at ground potential since there is no charge on capacitor C1. When the input voltage rises above the UVLO turn-on threshold, the LTC4257 removes the classification load current and turns on the internal power MOSFET. C1 charges up under LTC4257 current limit control and the VOUT pin transitions from 0V to VIN. This sequence is shown in Figure 1. The LTC4257 includes a hysteretic UVLO circuit that keeps power applied to the load until the input voltage falls below the UVLO turn-off threshold. Once the input voltage drops below –30V, the internal power MOSFET is turned off and the classification load current is re-enabled. C1 will discharge through the PD circuitry and the VOUT pin will go to a high impedance state. Input Current Limit IEEE 802.3af specifies a maximum inrush current and also specifies a minimum load capacitor between the GND and VOUT pins. To control turn-on surge current in the system, the LTC4257 integrates a current limit circuit with the onboard power MOSFET and sense resistor to provide a complete inrush control circuit without additional external components. The LTC4257 limits input current to less than the 400mA maximum specified by 802.3af, allowing the load capacitor to ramp up to the line voltage in a controlled manner. During this ramp up, a large amount of power is dissipated in the power MOSFET. The LTC4257 Figure 5. LTC4257 Undervoltage Lockout APPLICATIO S I FOR ATIO GND C1 5 µF MIN VIN 8 4 VOUT 5 LTC4257 4257 F05 TO PSE UNDERVOLTAGE LOCKOUT CIRCUIT PD LOAD CURRENT-LIMITED TURN ON + INPUT LTC4257 VOLTAGE POWER MOSFET 0V TO UVLO* OFF >UVLO* ON *UVLO INCLUDES HYSTERESIS RISING INPUT THRESHOLD ≅ –39.2V FALLING INPUT THRESHOLD ≅ –30.5V |
Similar Part No. - LTC4257_15 |
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Similar Description - LTC4257_15 |
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