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RT8509 Datasheet(PDF) 7 Page  Richtek Technology Corporation 

RT8509 Datasheet(HTML) 7 Page  Richtek Technology Corporation 
7 / 9 page RT8509 7 DS850901 March 2012 www.richtek.com © Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. where η is the efficiency of the converter, IIN(MAX) is the maximum input current, and IRIPPLE is the inductor ripple current. The input peak current can then be obtained by adding the maximum input current with half of the inductor ripple current as shown in the following equation : PEAK IN(MAX) I1.2 x I = Note that the saturated current of the inductor must be greater than IPEAK. The inductance can eventually be determined according to the following equation : 2 IN OUT IN 2 OUT OUT(MAX) OSC x (V ) x(V V ) L 0.4 x (V ) xI x f η − = where fosc is the switching frequency. For better system performance, a shielded inductor is preferred to avoid EMI problems. Diode Selection Schottky diodes are chosen for their low forward voltage drop and fast switching speed. When selecting a Schottky diode, important parameters such as power dissipation, reverse voltage rating, and pulsating peak current should all be taken into consideration. A suitable Schottky diode's reverse voltage rating must be greater than the maximum output voltage and its average current rating must exceed the average output current. Last of all, the chosen diode should have a sufficiently low leakage current level, since it will increase with temperature. Output Capacitor Selection The output ripple voltage is an important index for estimating chip performance. This portion consists of two parts. One is the product of the inductor current with the ESR of the output capacitor, while the other part is formed by the charging and discharging process of the output capacitor. As shown in Figure 1, ΔVOUT1 can be evaluated based on the ideal energy equalization. According to the definition of Q, the Q value can be calculated as the following equation : IN L OUT IN L OUT IN OUT OUT1 OUT OSC 11 1 Q x I I I I I I 22 2 V 1 x x C x V Vf ⎡⎤ ⎛⎞ ⎛ ⎞ =+ Δ − + − Δ − ⎜⎟ ⎜ ⎟ ⎢⎥ ⎝⎠ ⎝ ⎠ ⎣⎦ =Δ where fOSC is the switching frequency, and ΔIL is the inductor ripple current. Bring COUT to the left side to estimate the value of ΔVOUT1 according to the following equation : OUT OUT1 OUT OSC D x I V x C x f η Δ= where D is the duty cycle and η is the boost converter efficiency. Finally, taking ESR into account, the overall output ripple voltage can be determined by the following equation : OUT OUT IN OUT OSC D x I V I x ESR x C x f η Δ= + The output capacitor, COUT, should be selected accordingly. Time Time Inductor Current Output Current Output Ripple Voltage (ac) (1D)TS Δ VOUT1 Δ IL Input Current Figure 1. The Output Ripple Voltage without the Contribution of ESR Input Capacitor Selection Low ESR ceramic capacitors are recommended for input capacitor applications. Low ESR will effectively reduce the input voltage ripple caused by switching operation. A 10 μF capacitor is sufficient for most applications. Nevertheless, this value can be decreased for lower output current requirement. Another consideration is the voltage rating of the input capacitor which must be greater than the maximum input voltage. OUT OUT(MAX) IN(MAX) IN RIPPLE IN(MAX) V x I I = x V I = 0.4 x I η as an example, the inductor ripple current can be calculated according to the following equations : 
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