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SC4603 Datasheet(PDF) 9 Page - Semtech Corporation |
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SC4603 Datasheet(HTML) 9 Page - Semtech Corporation |
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9 / 16 page  9 2008 Semtech Corp. www.semtech.com SC4603 POWER MANAGEMENT Applications Information - (Cont.) Power MOSFET Drivers The SC4603 has two drivers for external complemen- tary power MOSFETs. The driver block consists of one high side P-MOSFET driver, PDRV, and one low side N- MOSFET driver, NDRV, which are optimized for driving external power MOSFETs in a synchronous buck con- verter. The output drivers also have gate drive non-over- lap mechanism that gives a dead time between PDRV and NDRV transitions to avoid potential shoot through problems in the external MOSFETs. By using the proper design and the appropriate MOSFETs, a 6A converter can be achieved. As shown in Figure 2, t d1, the delay from the P-MOSFET off to the N-MOSFET on is adaptive by detecting the voltage of the phase node. t d2, the delay from the N-MOSFET off to the P-MOSFET on is fixed, is 50ns for the SC4603. This control scheme guarantees avoiding the cross conduction or shoot through between two MOSFETs and minimizes the conduction loss in the bottom diode for high efficiency applications. NMOSFET Gate Drive PMOSFET Gate Drive Phase node Ground td1 td2 Figure 2. Timing Waveforms for Gate Drives and Phase Node Inductor Selection The factors for selecting the inductor include its cost, efficiency, size and EMI. For a typical SC4603 applica- tion, the inductor selection is mainly based on its value, saturation current and DC resistance. Increasing the in- ductor value will decrease the ripple level of the output voltage while the output transient response will be de- graded. Low value inductors offer small size and fast tran- sient responses while they cause large ripple currents, poor efficiencies and more output capacitance to smooth out the large ripple currents. The inductor should be able to handle the peak current without saturating and its copper resistance in the winding should be as low as possible to minimize its resistive power loss. A good trade- off among its size, loss and cost is to set the inductor ripple current to be within 15% to 30% of the maximum output current. The inductor value can be determined according to its operating point and the switching frequency as follows: OMAX s in out in out I I f V ) V V ( V L ⋅ ∆ ⋅ ⋅ − ⋅ = Where: f s = switching frequency and ∆I = ratio of the peak to peak inductor current to the maximum output load current. The peak to peak inductor current is: OMAX p p I I I • ∆ = − After the required inductor value is selected, the proper selection of the core material is based on the peak in- ductor current and efficiency requirements. The core must be able to handle the peak inductor current I PEAK without saturation and produce low core loss during the high frequency operation. 2 I I I p p OMAX PEAK − + = The power loss for the inductor includes its core loss and copper loss. If possible, the winding resistance should be minimized to reduce inductor’s copper loss. The core loss can be found in the manufacturer’s datasheet. The inductor’s copper loss can be estimated as follows: WINDING LRMS 2 COPPER R I P ⋅ = Where: I LRMS is the RMS current in the inductor. This current can be calculated as follows: 2 OMAX LRMS I 3 1 1 I I ∆ ⋅ + ⋅ = Output Capacitor Selection Basically there are two major factors to consider in se- lecting the type and quantity of the output capacitors. The first one is the required ESR (Equivalent Series Re- sistance) which should be low enough to reduce the volt- age deviation from its nominal one during its load changes. The second one is the required capacitance, which should be high enough to hold up the output voltage. Before the SC4603 regulates the inductor current to a new value |
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