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LTC4257CS8-1 Datasheet(PDF) 10 Page - Linear Technology |
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LTC4257CS8-1 Datasheet(HTML) 10 Page - Linear Technology |
10 / 20 page LTC4257-1 10 42571fa Undervoltage Lockout The IEEE specification dictates a maximum turn-on volt- age of 42V and a minimum turn-off voltage of 30V for the PD. In addition, the PD must maintain large on-off hyster- esis to prevent resistive losses in the wiring between the PSE and the PD from causing start-up oscillation. The LTC4257-1 incorporates an undervoltage lockout (UVLO) circuit that monitors line voltage to determine when to apply power to the PD load (Figure 6). Before power is applied to the load, the VOUT pin is high impedance and sitting at ground potential since there is no charge on capacitor C1. When the input voltage rises above the UVLO turn-on threshold, the LTC4257-1 removes the classifica- tion load current and turns on the internal power MOSFET. C1 charges up under LTC4257-1 current limit control and the VOUT pin transitions from 0V to VIN. This sequence is shown in Figure 1. The LTC4257-1 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 LTC4257-1 integrates a dual level current limit circuit with an onboard power MOSFET and sense resistor to provide a complete inrush control circuit without additional external components. At turn on, the LTC4257-1 will limit input current to the low level, allowing the load capacitor to ramp up to the line voltage in a controlled manner. The LTC4257-1 has been specifically designed to interface with legacy PSEs which do not meet the inrush current requirement of the IEEE 802.3af specification. This is APPLICATIO S I FOR ATIO GND C1 5 µF MIN VIN 8 4 VOUT 5 LTC4257-1 42571 F06 TO PSE UNDERVOLTAGE LOCKOUT CIRCUIT PD LOAD CURRENT-LIMITED TURN ON + INPUT LTC4257-1 VOLTAGE POWER MOSFET 0V TO UVLO* OFF >UVLO* ON *UVLO INCLUDES HYSTERESIS RISING INPUT THRESHOLD ≅ –36V FALLING INPUT THRESHOLD ≅ –30.5V Figure 6. LTC4257-1 Undervoltage Lockout |
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