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LT1640AH 数据表(PDF) 13 Page - Linear Technology |
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LT1640AH 数据表(HTML) 13 Page - Linear Technology |
13 / 34 page LTC4253/LTC4253A 13 425353afe OPERATION Interlock Conditions A start-up sequence commences once these “interlock” conditions are met: 1. The input voltage VIN exceeds VLKO (UVLO). 2. The voltage at UV > VUVHI (VUV for the LTC4253A). 3. The voltage at OV < VOVLO (VOV – VOVHST for the LTC4253A). 4. The input voltage at RESET < 0.8V. 5. The (SENSE – VEE) voltage < 50mV (VCB) 6. The voltage at SS is < 0.2V (20 • VOS) 7. The voltage on the TIMER capacitor (CT) is < 1V (VTMRL). 8. The voltage at GATE is < 0.5V (VGATEL) The first four conditions are continuously monitored and the latter four are checked prior to initial timing or GATE ramp-up. Upon exiting an OV condition, the TIMER pin voltage requirement is inhibited. Details are described in the Applications Information, Timing Waveforms section. If RESET < 0.8V occurs after the LTC4253/LTC4253A come out of UVLO (interlock condition 1) and undervoltage (in- terlock condition 2), GATE and SS are released without an initial TIMER cycle once the other interlock conditions are met (see Figure 13a). If not, TIMER begins the start-up sequence by sourcing 5μA into CT. If VIN, UV or OV falls out of range or RESET asserts, the start-up cycle stops and TIMER discharges CT to less than 1V, then waits until the aforementioned conditions are once again met. If CT successfully charges to 4V, TIMER pulls low and both SS and GATE pins are released. GATE sources 50μA (IGATE), charging the MOSFET gate and associated capacitance. The SS voltage ramp limits VSENSE to control the inrush current. PWRGD1 pulls active low when GATE is within 2.8V of VIN and DRAIN is lower than VDRNL. This sets off the power good sequence in which PWRGD2 and then PWRGD3 is subsequently pulled low after a delay, adjust- able through the SQTIMER capacitor CSQ or by external control inputs EN2 and EN3. In this way, external loads or power modules controlled by the three PWRGD signals are turned on in a controlled manner without overloading the power bus. Two modes of operation are possible during the time the MOSFET is first turned on, depending on the values of external components, MOSFET characteristics and nominal design current. One possibility is that the MOSFET will turn on gradually so that the inrush into the load capaci- tance remains a low value. The output will simply ramp to –48V and the LTC4253/LTC4253A will fully enhance the MOSFET. A second possibility is that the load current exceeds the soft-start current limit threshold of [VSS(t)/ 20 – VOS]/RS. In this case the LTC4253/LTC4253A ramp the output by sourcing soft-start limited current into the load capacitance. If the soft-start voltage is below 1.2V, the circuit breaker TIMER is held low. Above 1.2V, TIMER ramps up. It is important to set the timer delay so that, regardless of which start-up mode is used, the TIMER ramp is less than one circuit breaker delay time. If this condition is not met, the LTC4253/LTC4253A may shut down after one circuit breaker delay time. Board Removal When the board is withdrawn from the card cage, the UV/ OV divider is the first to lose connection. This shuts off the MOSFET and commutates the flow of current in the connector. When the power pins subsequently separate there is no arcing. Current Control Three levels of protection handle short-circuit and over- load conditions. Load current is monitored by SENSE and resistor RS. There are three distinct thresholds at SENSE: 50mV for a timed circuit breaker function; 100mV for an analog current limit loop (60mV for the LTC4253A); and 200mV for a fast, feedforward comparator which limits peak current in the event of a catastrophic short-circuit. If, due to an output overload, the voltage drop across RS exceeds 50mV, TIMER sources 200μA into CT.CTeventually charges to a 4V threshold and the LTC4253/LTC4253A shut off. If the overload goes away before CT reaches 4V and SENSE measures less than 50mV, CT slowly discharges (5μA). In this way the LTC4253/LTC4253A’s circuit breaker function responds to low duty cycle overloads, and ac- counts for the fast heating and slow cooling characteristic of the MOSFET. |
类似零件编号 - LT1640AH |
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类似说明 - LT1640AH |
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