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MAX4370ESA Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX4370ESA Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 16 page Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection ______________________________________________________________________________________ 11 __________Applications Information Component Selection N-Channel MOSFET Select the external N-channel MOSFET according to the application’s current level. The MOSFET’s RDS(ON) should be chosen low enough to have a minimum volt- age drop at full load to limit the MOSFET power dissipa- tion. High RDS(ON) can cause output ripple if the board has pulsing loads, or it can trigger an external under- voltage reset monitor at full load. Determine the device’s power rating requirement to accommodate a short-circuit condition on the board during start-up (see MOSFET Thermal Considerations). MOSFETs can typically withstand single-shot pulses with higher dissipation than the specified package rat- ing. Also, since part of the inrush current limiting is achieved by limiting the gate dV/dt, it is not necessary to use a MOSFET with low gate capacitance. Table 2 lists some recommended manufacturers and compo- nents. Sense Resistor The slow comparator threshold voltage is set at 50mV. Select a sense resistor that causes a 50mV voltage drop at a current level above the maximum normal operating current. Typically, set the overload current at 1.2 to 1.5 times the nominal load current. The fast com- parator threshold is set at 200mV. This sets the fault current limit at four times the overload current limit. Choose the sense-resistor power rating to accommo- date the overload current (Table 3): PSENSE = (IOVERLOAD)2 · RSENSE Start-Up Timing Capacitor (CTIM) The start-up period (tSTART) is determined by the capaci- tor connected at CTIM. This determines the maximum time allowed to completely turn on the MOSFET. The default value for tSTART is chosen by leaving CTIM floating and is approximately 5.5µs. This is also the minimum value (not controlled and dependent on stray capacitance). Longer timings are determined by the value of the capacitor, according to Figure 6, and can be determined as follows: tSTART (ms) = 0.31 · CTIM (nF) Set the tSTART timer to allow the MOSFET to be enhanced and the load capacitor to be completely charged. There are two methods of completing the start-up sequences. Case A describes a start-up sequence that does not use the current-limiting feature and slowly turns on the MOSFET by limiting the gate dV/dt. Case B uses the current-limiting feature and turns on the MOSFET as fast as possible while still preventing high inrush current. 0.01 0.1 1 10 100 1000 CAPACITANCE (nF) 1000 0.1 0.01 0.001 1 10 100 Figure 6. Start-Up Period vs. CTIM Table 3. Current Levels vs. RSENSE 1 50 0.5 100 RSENSE (m Ω) 5 10 OVERLOAD THRESHOLD SET BY SLOW COMPARATOR (A) 4 2 20 FAULT CURRENT THRESHOLD SET BY FAST COMPARATOR (A) Table 2. Component Manufacturers 704-264-8861 888-522-5372 402-564-3131 PHONE www.irctt.co www.fairchildsemi.com www.vishay.com INTERNET 310-322-3331 IRC Sense Resistors 602-244-3576 www.irf.com www.mot-sps.com/ppd/ International Rectifier Motorola Fairchild COMPONENT Dale-Vishay MANUFACTURER MOSFETs |
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