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AOZ1605 Datasheet(PDF) 9 Page - Alpha & Omega Semiconductors |
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AOZ1605 Datasheet(HTML) 9 Page - Alpha & Omega Semiconductors |
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9 / 15 page  AOZ1605 Rev. 1.2 June 2012 www.aosmd.com Page 9 of 15 Inductor Selection There are two main considerations when choosing an inductor; the inductor should not saturate, and the inductor current ripple should be small enough to achieve the desire output voltage ripple. A 1 H inductor with a saturation current of at least 1 A is recommended for the AOZ1605 full load application. For maximum efficiency, the inductor’s resistance (DCR) should be as low as possible. For given input and output voltage, inductance and switching frequency together decide the inductor ripple current, which is, The peak inductor current is: High inductance gives low inductor ripple current but requires larger size inductor to avoid saturation. Low ripple current reduces inductor core losses. It also reduces RMS current through inductor and switches, which results in less conduction loss. Usually, peak to peak ripple current on inductor is designed to be 20% to 30% of output current. When selecting the inductor, make sure it is able to handle the peak current without saturation even at the highest operating temperature. The inductor takes the highest current in a buck circuit. The conduction loss on inductor need to be checked for thermal and efficiency requirements. Surface mount inductors in different shape and styles are available from Coilcraft, Elytone and Murata. Shielded inductors are small and radiate less EMI noise. But they cost more than unshielded inductors. The choice depends on EMI requirement, price and size. Input Capacitor The input capacitor must be connected to the VIN pin and PGND pin of AOZ1605 to maintain steady input voltage and filter out the pulsing input current. The voltage rating of input capacitor must be greater than maximum input voltage plus ripple voltage. For greater capacitor performance, the working capacitance voltage should be twice Vin. The input ripple voltage can be approximated by equation below: Since the input current is discontinuous in a buck converter, the current stress on the input capacitor is another concern when selecting the capacitor. For a buck circuit, the RMS value of input capacitor current can be calculated by: if we let m equal the conversion ratio: The relationship between the input capacitor RMS current and voltage conversion ratio is calculated and shown in Figure 1 below. It can be seen that when VO is half of VIN, CIN is under the worst current stress. The worst current stress on CIN is 0.5 x IO. Figure 3. ICIN vs. Voltage Conversion Ratio For reliable operation and best performance, the input capacitors must have current rating higher than ICIN_RMS at worst operating conditions. Ceramic capacitors are preferred for input capacitors because of their low ESR and high current rating. When selecting ceramic capacitors, X5R or X7R type dielectric ceramic capacitors should be used for their better temperature and voltage characteristics. Note that the ripple current rating from capacitor manufactures are based on certain amount of life time. Further de-rating may be necessary in practical design. I L VO fL ----------- 1 VO VIN --------- – = ILpeak IO I L 2 -------- + = V IN IO fCIN ----------------- 1 VO VIN --------- – VO VIN --------- = ICIN_RMS IO VO VIN --------- 1 VO VIN --------- – = VO VIN --------- m = 0 0.1 0.2 0.3 0.4 0.5 0 0.5 1 m I CIN_RMS(m) I O |
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