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SS6639-33CXBG Datasheet(PDF) 10 Page - Silicon Standard Corp. |
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SS6639-33CXBG Datasheet(HTML) 10 Page - Silicon Standard Corp. |
10 / 12 page www.SiliconStandard.com 10 of 12 SS6639 Minimum and maximum input voltage, output voltage and output current must be established before an inductor can be selected. In discontinuous mode operation, at the end of the switch ON time, peak current and energy in the inductor build according to ( ) ON IN TON IN ON S ON S ON IN PK T * L V 2 x 1 * * L V ) T * L R R exp( 1 * R R V I ≅ − ≅ + − − + = (simple lossless equation), where L T ) R (R X ON S ON ∗ + = 2 PK I * L 2 1 EL = Power required from the inductor per cycle must be equal to, or greater than ) fsw 1 ( * ) (I * ) V V (V f P OUT IN D OUT SW L − + = in order for the converter to regulate the output. When the loading exceeds IOB, the PFM controller operates in continuous mode. Inductor peak current can be derived from − − + − − − + = 2 x 1 * T * 2L V V I * 2 x V V V V V I ON SW IN OUT SW IN SW D OUT PK Valley current (Iv) is − − − − − + = 2 x 1 T * 2L V V * I * 2 x V V V V V I * ON DE IN OUT SW IN SW D OUT V Table 1 Indicates resistance and height for each coil. Power Inductor Type Inductance ( µH ) Resistance ( Ω ) Rated Current (A) height (mm) 47 0.25 0.7 Sumida SMT Type CD54 100 0.50 0.5 4.5 47 0.25 0.7 Hold SMT Type PM54 100 0.50 0.5 4.5 Hold SMT Type PM75 33 0.11 1.2 5.0 Huan Feng PIN Type V0810 33 40m 2 10.0 CAPACITOR SELECTION A poor choice for an output capacitor can result in poor efficiency and high output ripple. Ordinary aluminum electrolytic capacitors, while inexpensive, may have unacceptably poor ESR and ESL. There are low ESR aluminum capacitors for switch mode DC-DC converters which work much better than general-purpose components. Tantalum capacitors provide still better performance but are more expensive. OS-CON capacitors have extremely low ESR in a small size. If the capacitance is reduced, the output ripple will increase. As most of the input supply is applied across the input bypass capacitor, the capacitor voltage rating should be at least 1.25 times greater than the maximum input voltage. DIODE SELECTION Speed, forward drop, and leakage current are three main considerations in selecting a rectifier diode. The best performance is obtained with a Schottky rectifier Rev.2.01 6/26/2003 |
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