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LTC2913 Datasheet(PDF) 11 Page - Linear Technology |
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LTC2913 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LT4363 11 4363fa APPLICATIONS INFORMATION The LT4363 limits the voltage and current delivered to the loadduringsupplytransientoroutputoverloadevents.The total fault timer period is set to ride through short-duration faults, while longer events cause the output to shut off and protect the MOSFET pass device from damage. The MOSFET provides a low resistance path from the input to the load during normal operation, while in fault conditions it operates as a series regulator. Overvoltage Fault The LT4363 limits the voltage at the output during an overvoltage at the input. An internal amplifier regulates the GATE pin to maintain 1.275V at the FB pin. During this interval the MOSFET is on and supplies current to the load. This allows uninterrupted operation during short overvoltage events. If the overvoltage condition persists, the timer causes the MOSFET to turn off. Overcurrent Fault The LT4363 features and adjustable current limit that pro- tectsagainstoutputshortcircuitsorexcessiveloadcurrent. During an overcurrent event, the GATE pin is regulated to limit the current sense voltage across the SNS and OUT pins to 50mV. In the case of a severe short at the output, where OUT is less than 2V, the current sense voltage is reduced to 25mV to further reduce power dissipation in the MOSFET. If the overcurrent condition persists, the timer causes the MOSFET to turn off. Fault Timer Overview Overvoltage and overcurrent conditions are limited in duration by an adjustable timer. A capacitor at the TMR pin sets the delay time before a fault condition is reported at the FLT pin as well as the overall delay before the MOSFET is turned off. The same capacitor also sets the cool down time before the MOSFET is allowed to turn back on. When either an overvoltage or overcurrent fault condition occurs, a current source charges the TMR pin capacitor. The exact current level varies as a function of the type of fault and the VDS voltage drop across the MOSFET. This scheme takes better advantage of the MOSFET’s available SafeOperatingArea(SOA)thanwouldafixedtimercurrent. The TMR pin is biased to 0.5V under normal operating conditions. In the presence of a fault the timer first charges to 1.275V, and then enters the early warning phase of operation. At this point the FLT pin pulls low and after charging to 1.375V, the timer shuts off the MOSFET. The warning phase is indicated by FLT low and gives time for the load to perform house-keeping chores such as data storage in anticipation of impending power loss. After faulting off, the timer enters the cool down phase. At the end of the cool down period the LT4363-1 remains off until reset, while the LT4363-2 automatically restarts. For the LT4363-2 retry is inhibited if the OV pin is greater than 1.275V. This prevents motorboating in the event there is a sustained input overvoltage condition. Fault Timer Operation in Overvoltage In the presence of an overvoltage condition when the LT4363 regulates the output voltage, the timer charges from 0.5V to 1.275V with a current that varies as a func- tion of VDS (see Figure 1). VDS is inferred from the drop across VCC and OUT. The timer current increases linearly from around 4µA with VDS ≤ 0.5V, to 50µA with VDS = 75V. Because VDS is measured indirectly, clamping or filtering at the VCC pin affects the timer current response. A graph of Overvoltage TMR Current vs (VCC – VOUT) is shown in the Typical Performance Characteristics. When TMR reaches 1.275V, the FLT pin is latched low as an early warning of impending shutdown. The timer cur- rent is cut to a fixed value of 6µA and continues to run until TMR reaches 1.375V, producing a fixed early warning period given by: CTMR = tWARNING • 6µA 100mV When TMR reaches 1.375V, the MOSFET is turned off and allowed to cool for an extended period. The total elapsed time between the onset of output regulation and turn-off is given by: tREG =CTMR • 0.775V ITMR + 100mV 6µA Because ITMR is a function of VCC – VOUT, the exact time in regulation depends upon the input waveform and the time required for the output voltage to come into regulation. |
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Similar Description - LTC2913 |
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