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APW8855 ๋ฐ์ดํฐ์ํธ(HTML) 36 Page  Anpec Electronics Coropration 

APW8855 ๋ฐ์ดํฐ์ํธ(HTML) 36 Page  Anpec Electronics Coropration 
36 / 43 page Copyright ยฉ ANPEC Electronics Corp. Rev. A.1  Dec., 2015 APW8855 www.anpec.com.tw 36 16.Application Information Output Voltage Selection For PWM converter, the inductor value (L) determines the sum of the inductor ripple currents ฮ I PP, and affects the load d transient response. Higher inductor value reduces the output capacitors ยกยฆ ripple current and induces lower output ripple voltage. The ripple current can be approximated by: The duty cycle (D) of a buck converter is the function of the input voltage and output voltage. Once an output voltage is fixed, it can be written as: 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 ยกยฆs ripple current and the regulator load transient response time. A smaller inductor will give the regulator a faster load transient response at the expense of higher ripple current. Increasing the switching frequency (F SW) also reduces the ripple current and voltage, but it will increase the switching loss of the MOSFETs and the power dissipation of the converter. The maximum ripple current occurs 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 output ripple voltage. Output Capacitor Selection Output voltage ripple and the transient voltage deviation are factors that have to be taken into consideration 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 frequency noise related to MOSFET turnon and turnoff, the output voltage ripple includes the capaci tance voltage drop V COUT and ESR voltage drop V ESR caused by the AC peaktopeak sum of the inductorโs current. The ripple voltage of output capacitors can be represented by: IN OUT V V D = IN OUT Sw OUT IN P  P V V L F V V ฮI ร ร โ = ฮ ฮ Sw OUT P P COUT F C 8 ฮV ฮI ร ร = โ ESR P P ESR R ฮI ฮV ร โ = 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 capacitors 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, 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 capacitors in order to prevent the capacitor from overheating. 
