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LTC4251 Datasheet(PDF) 10 Page - Linear Technology |
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LTC4251 Datasheet(HTML) 10 Page - Linear Technology |
10 / 20 page LTC4251/LTC4251-1/ LTC4251-2 10 425112fa APPLICATIO S I FOR ATIO OPERATIO Higher overloads are handled by an analog current limit loop. If the drop across RS reaches 100mV, the current limiting loop servos the MOSFET gate and maintains a constant output current of 100mV/RS. Note that because SENSE > 50mV, TIMER charges CT during this time and the LTC4251/LTC4251-1/LTC4251-2 will eventually shut down. Low impedance failures on the load side of the LTC4251/ LTC4251-1/LTC4251-2 coupled with 48V or more driving potential can produce current slew rates well in excess of 50A/ µs. Under these conditions, overshoot is inevitable. A fast SENSE comparator with a threshold of 200mV detects overshoot and pulls GATE low much harder and hence much faster than can the weaker current limit loop. The 100mV/RS current limit loop then takes over, and servos the current as previously described. As before, TIMER runs and latches the LTC4251/LTC4251-1/LTC4251-2 off when CT reaches 4V. The LTC4251/LTC4251-1/LTC4251-2 circuit breaker latch is reset by either pulling UV/OV momentarily low, or drop- ping the input voltage VINbelowtheinternalUVLOthreshold of 8.2V. Although short-circuits are the most obvious fault type, several operating conditions may invoke overcurrent pro- tection. Noise spikes from the backplane or load, input steps caused by the connection of a second, higher voltage supply, transient currents caused by faults on adjacent circuit boards sharing the same power bus, or the insertion of non-hot swappable products could cause higher than anticipated input current and temporary de- tection of an overcurrent condition. The action of TIMER and CT rejects these events allowing the LTC4251/ LTC4251-1/LTC4251-2 to “ride out” temporary overloads and disturbances that would trip a simple current com- parator and in some cases, blow a fuse. SHUNT REGULATOR A fast responding regulator shunts the LTC4251/ LTC4251-1/LTC4251-2 VIN pin. Power is derived from –48RTN by an external current limiting resistor. The shunt regulator clamps VIN to 13V (VZ). A 1µF decoupling capacitor at VIN filters supply transients and contributes a short delay at start-up. A 10k 1/2W (RIN) resistor can be two 5k 1/4W resistors in series. INTERNAL UNDERVOLTAGE LOCKOUT (UVLO) Internal circuitry monitors VIN for undervoltage. The exact thresholds are defined by VLKO and its hysteresis, VLKH. When VIN rises above 9.2V (VLKO) the chip is enabled; below 8.2V (VLKO-VLKH) it is disabled and GATE is pulled low. The UVLO function at VIN should not be confused with the UV/OV pin. These are completely separate functions. UV/OV COMPARATORS Two hysteretic comparators for detecting under- and overvoltage conditions, with the following thresholds, monitor the dual function UV/OV pin: UV turning on at VUVHI UV turning off at VUVLO OV turning off at VOVHI OV turning on at VOVLO The UV and OV trip point ratio for LTC4251 is designed to match the standard telecom operating range of 43V to 75V. The LTC4251-2 implements a UV threshold of 43V only. A divider (R1, R2) is used to scale the supply voltage. Using R1 = 402k and R2 = 32.4k gives a typical operating range of 43.2V to 74.4V. The under- and overvoltage shutdown thresholds are then 39.2V and 82.5V. 1% divider resistors are recommended to preserve threshold accuracy. The same resistor values can be used for the LTC4251-2. (Refer to Block Diagram) |
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