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FSP3126 Datasheet(PDF) 4 Page - FOSLINK SEMICONDUCTOR CO.,LTD |
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FSP3126 Datasheet(HTML) 4 Page - FOSLINK SEMICONDUCTOR CO.,LTD |
4 / 9 page WIDE INPUT 2A STEP DOWN CONVERTER 2007-5-28 4/9 FSP3126 APPLICATION INFORMATION Output Voltage Setting Figure1. Output Voltage Setting Figure 1 shows the connections for setting the output voltage. Select the proper ratio of the two feedback resistors RFB1 and RFB2 based on the output voltage. Typically, use RFB2 ≈ 10kΩ and determine RFB1 from the following equation: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − = 1 V 293 . 1 V R R OUT 2 FB 1 FB (1) Inductor Selection The inductor maintains a continuous current to the output load. This inductor current has a ripple that is dependent on the inductance value: higher inductance reduces the peak-to-peak ripple current. The trade off for high inductance value is the increase in inductor core size and series resistance, and the reduction in current handling capability. In general, select an inductance value L based on the ripple current requirement: RIPPLE OUTMAX SW IN OUT IN OUT K I f V ) V V ( V L − • = (2) where VIN is the input voltage, VOUT is the output voltage, fSW is the switching frequency, IOUTMAX is the maximum output current, and KRIPPLE is the ripple factor. Typically, choose KRIPPLE = 30% to correspond to the peak-to-peak ripple current being 30% of the maximum output current. With this inductor value, the peak inductor current is IOUT • (1 + KRIPPLE / 2). Make sure that this peak inductor current is less that the 3A current limit. Finally, select the inductor core size so that it does not saturate at 3A. Typical inductor values for various output voltages are shown in Table 1. VOUT 1.5V 1.8V 2.5V 3.3V 5V L 6.8µH6.8µH10µH15µH22µH Table 1. Typical Inductor Values Input Capacitor The input capacitor needs to be carefully selected to maintain sufficiently low ripple at the supply input of the converter. A low ESR capacitor is highly recommended. Since large current flows in and out of this capacitor during switching, its ESR also affects efficiency. The input capacitance needs to be higher than 10µF. The best choice is the ceramic type; however, low ESR tantalum or electrolytic types may also be used provided that the RMS ripple current rating is higher than 50% of the output current. The input capacitor should be placed close to the IN and G pins of the IC, with the shortest traces possible. In the case of tantalum or electrolytic types, they can be further away if a small parallel 0.1µF ceramic capacitor is placed right next to the IC. Output Capacitor The output capacitor also needs to have low ESR to keep low output voltage ripple. The output ripple voltage is: ESR RIPPLE OUTMAX RIPPLE R K I V = OUT 2 SW IN LC f 28 V • + (3) where IOUTMAX is the maximum output current, KRIPPLE is the ripple factor, RESR is the ESR of the output capacitor, fSW is the switching frequency, L is the inductor value, and COUT is the output capacitance. In the case of ceramic output capacitors, RESR is very small and does not contribute to the ripple. Therefore, a lower capacitance value can be used for ceramic capacitors. In the case of tantalum or electrolytic capacitors, the ripple is dominated by RESR multiplied by the ripple current. In that case, the output capacitor is chosen to have sufficiently low ESR. |
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