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LTC4257 Datasheet(PDF) 11 Page - Linear Technology |
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LTC4257 Datasheet(HTML) 11 Page - Linear Technology |
11 / 20 page LTC4257 11 4257fb Thermal Protection The LTC4257 includes smart thermal protection in order to provide full device functionality in a miniature package while maintaining safe operating temperatures. Several factors create the possibility for tremendous power dissipation within the LTC4257. IEEE 802.3af mandates that inrush current be limited to less than 400mA while standard telecom power can be as high as 57V. At turn on, before the load capacitor has charged up, the instanta- neous power dissipated by the LTC4257 can be over 20W. As the load capacitor charges up, the power dissipation in the LTC4257 will decrease until it reaches a steady-state value dependent on the DC load current. The size of the load capacitor determines how fast the power dissipation in the LTC4257 subsides. At room temperature, the LTC4257 can handle load capacitors as large as 800 µF without going into thermal shutdown. With a large load capacitor like this, the LTC4257 die temperature will increase by about 50 °C during a single turn-on sequence. If for some reason power were removed from the part and then quickly reapplied so that the LTC4257 had to charge up the load capacitor again, the temperature rise would be excessive if safety precautions were not implemented. The LTC4257 protects itself from thermal damage by monitoring the die temperature. If the die temperature exceeds the overtemperature trip point, the part switches to a half-power mode where the current limit is set to 50% of its normal level. This reduces power dissipation and helps prevent further heating. If the part continues to heat up and reaches the shutdown temperature, the current is reduced to zero and very little power is dissipated in the part until it cools below the overtemperature set point. The LTC4257 current limit will continue switching between 0%, 50% and 100% current levels (Figure 7) until the load capacitor is fully charged. If the PD is designed to operate at a high ambient tempera- ture and with the maximum allowable supply (57V), there will be a limit to the size load capacitor that can be charged up before the LTC4257 reaches the overtemperature trip point. Hitting the overtemperature trip point intermittently does not harm the LTC4257, but it will delay completion of capacitor charging. Capacitors up to 200 µFcanbecharged without a problem. During classification, excessive heating of the LTC4257 can occur if the PSE violates the 75ms probing time limit. To protect the LTC4257, the thermal protection circuitry will disable classification current if the die temperature exceeds the overtemperature trip point. When the die cools down below the trip point, classification current is re-enabled. Once the LTC4257 has charged up to the load capacitor and the PD is powered and running, there will be some residual heating due to the DC load current of the PD flowing through the internal MOSFET. In some applica- tions, the LTC4257 power dissipation may be significant and if dissipated in the S8 package, excessive package heating could occur. This problem can be solved with the use of the DD package which has superior thermal perfor- mance. The DD package includes an exposed pad that should be soldered to an isolated heatsink on the printed circuit board. Figure 7. Smart Thermal Protection State Diagram APPLICATIO S I FOR ATIO 4257 F07 T < 120 °C T < 120 °C T > 120 °C T > 140 °C 100% CURRENT UVLO TURN ON 50% CURRENT 0% CURRENT |
Similar Part No. - LTC4257_15 |
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Similar Description - LTC4257_15 |
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