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APW7098 Datasheet(PDF) 23 Page  Anpec Electronics Coropration 

APW7098 Datasheet(HTML) 23 Page  Anpec Electronics Coropration 
23 / 30 page Copyright © ANPEC Electronics Corp. Rev. A.6  Oct., 2009 APW7098 www.anpec.com.tw 23 Application Information (Cont.) Output Inductor Selection (Cont.) IN OUT V V D = Where F SW is the switching frequency of the regulator. Although the inductor value and frequency are increased and the ripple current and voltage are reduced, a tradeoff exists between the inductor’s ripple current and the regu lator load transient response time. A smaller inductor will give the regulator a faster load tran sient response at the expense of higher ripple current. Increasing the switching frequency (F SW) also r e d u c e s the ripple current and voltage, but it will increase the switching loss of the MOSFETs and the power dissipa tion of the converter. The maximum ripple current oc curs at the maximum input voltage. A good starting point is to choose the ripple current to be approximately 30% of the maximum output current. Once the inductance value has been chosen, select an inductor that is capable of carrying the required peak current without going into saturation. In some types of inductors, especially core that is made of ferrite, the ripple current will increase abruptly when it saturates. This results in a larger out put ripple voltage. For twophase converter, the inductor value (L) determines the sum of the two inductor ripple currents, ∆I PP, and af fects the load transient reponse. Higher inductor value reduces the output capacitors’ripple current and induces lower output ripple voltage. The ripple current can be approxminated by: Output Capacitor Selection Output voltage ripple and the transient voltage de viation are factors that have to be taken into con sideration when selecting output capacitors. Higher capacitor value and lower ESR reduce the output ripple and the load transient drop. Therefore, selecting high performance low ESR capacitors is recommended for switching regulator applications. In addition to high fre quency noise related to MOSFET turnon and turnoff, the output voltage ripple includes the capacitance voltage drop ∆V COUT and ESR voltage drop ∆VESR caused by the AC peaktopeak sum of the inductor’s current. The ripple voltage of output capacitors can be represented by: These two components constitute a large portion of the total output voltage ripple. In some applications, multiple capacitors have to be paralleled to achieve the desired ESR value. If the output of the converter has to support another load with high pulsating current, more capaci tors are needed in order to reduce the equivalent ESR and suppress the voltage ripple to a tolerable level. A small decoupling capacitor in parallel for bypassing the noise is also recommended, and the voltage rating of the output capacitors are also must be considered. To support a load transient that is faster than the switching frequency, more capacitors are needed for reducing the voltage excursion during load step change. For getting same load transient response, the output capacitance of twophase converter only needs around half of output capacitance of singlephase converter. Another aspect of the capacitor selection is that the total AC current going through the capacitors has to be less than the rated RMS current specified on the ca pacitors in order to prevent the capacitor from over heating. Input Capacitor Selection Use small ceramic capacitors for high frequency decoupling and bulk capacitors to supply the surge cur rent needed each time highside MOSFET turns on. Place the small ceramic capacitors physically close to the MOSFETs and between the drain of highside MOSFET and the source of lowside MOSFET. The important parameters for the bulk input capacitor are the voltage rating and the RMS current rating. For reliable operation, select the bulk capacitor with voltage and cur rent ratings above the maximum input voltage and larg est RMS current required by the circuit. The capacitor volt age rating should be at least 1.25 times greater than the maximum input voltage and a voltage rating of 1.5 times is a conservative guideline. For twophase converter, the IN OUT SW OUT IN P  P V V L F 2V  V I × × = ∆ ESR P P ESR SW OUT P P COUT R I V F C 8 I V × − × − ∆ = ∆ × ∆ = ∆ 
