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ISL6310 Datasheet(PDF) 17 Page  Intersil Corporation 

ISL6310 Datasheet(HTML) 17 Page  Intersil Corporation 
17 / 27 page 17 FN9209.3 December 12, 2006 proportional to the output current, the overcurrent trip level, IMAX, can be set by selecting the proper value for ROCSET, as shown in Equation 13. Once the output current exceeds the overcurrent trip level, VDROOP will exceed VOCSET, and a comparator will trigger the converter to begin overcurrent protection procedures. At the beginning of overcurrent shutdown, the controller turns off both upper and lower MOSFETs. The system remains in this state for a period of 4096 switching cycles. If the controller is still enabled at the end of this wait period, it will attempt a softstart (as shown in Figure 14). If the fault remains, the tripretry cycles will continue indefinitely until either the controller is disabled or the fault is cleared. Note that the energy delivered during tripretry cycling is much less than during fullload operation, so there is no thermal hazard. General Design Guide This design guide is intended to provide a highlevel explanation of the steps necessary to create a multiphase power converter. It is assumed that the reader is familiar with many of the basic skills and techniques referenced below. In addition to this guide, Intersil provides complete reference designs that include schematics, bills of materials, and example board layouts for many applications. Power Stages The first step in designing a multiphase converter is to determine the number of phases. This determination depends heavily on the cost analysis which in turn depends on system constraints that differ from one design to the next. Principally, the designer will be concerned with whether components can be mounted on both sides of the circuit board, whether throughhole components are permitted, the total board space available for powersupply circuitry, and the maximum amount of load current. Generally speaking, the most economical solutions are those in which each phase handles between 25 and 30A. All surfacemount designs will tend toward the lower end of this current range. If throughhole MOSFETs and inductors can be used, higher perphase currents are possible. In cases where board space is the limiting constraint, current can be pushed as high as 40A per phase, but these designs require heat sinks and forced air to cool the MOSFETs, inductors and heat dissipating surfaces. MOSFETs The choice of MOSFETs depends on the current each MOSFET will be required to conduct, the switching frequency, the capability of the MOSFETs to dissipate heat, and the availability and nature of heat sinking and air flow. Lower MOSFET Power Calculation The calculation for the approximate power loss in the lower MOSFET can be simplified, since virtually all of the loss in the lower MOSFET is due to current conducted through the channel resistance (rDS(ON)). In Equation 14, IM is the maximum continuous output current, IPP is the peaktopeak inductor current (see Equation 1), and d is the duty cycle (VOUT/VIN). An additional term can be added to the lowerMOSFET loss equation to account for additional loss accrued during the dead time when inductor current is flowing through the lowerMOSFET body diode. This term is dependent on the diode forward voltage at IM, VD(ON), the switching frequency, FSW, and the length of dead times, td1 and td2, at the beginning and the end of the lowerMOSFET conduction interval respectively. The total maximum power dissipated in each lower MOSFET is approximated by the summation of PLOW,1 and PLOW,2. Upper MOSFET Power Calculation In addition to rDS(ON) losses, a large portion of the upper MOSFET losses are due to currents conducted across the input voltage (VIN) during switching. Since a substantially higher portion of the upperMOSFET losses are dependent on switching frequency, the power calculation is more complex. Upper MOSFET losses can be divided into separate components involving the upperMOSFET switching times, the lowerMOSFET bodydiode reverse recovery charge, Qrr, and the upper MOSFET rDS(ON) conduction loss. R OCSET I MAX RCOMP DCR ⋅⋅ 100 μAR S ⋅  = (EQ. 13) 0A 0V OUTPUT CURRENT FIGURE 14. OVERCURRENT BEHAVIOR IN HICCUP MODE OUTPUT VOLTAGE P LOW 1 , r DS ON () · I M N  ⎝⎠ ⎜⎟ ⎛⎞ 2 1d – () ⋅ I LPP , 2 1d – () ⋅ 12  + ⋅ = (EQ. 14) P LOW 2 , V DON () FSW I M N  I PP 2  + ⎝⎠ ⎛⎞ t d1 ⋅ I M N  I PP 2  – ⎝⎠ ⎛⎞ t d2 ⋅ + ⋅⋅ = (EQ. 15) ISL6310 
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