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ACT4072 Datasheet(PDF) 5 Page - Active-Semi, Inc |
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ACT4072 Datasheet(HTML) 5 Page - Active-Semi, Inc |
5 / 10 page Innovative Products. Active Solutions. - 5 - www.active-semi.com Copyright © 2006 Active-Semi, Inc. ACT4072 Output Voltage Setting Figure 1 shows the connections for setting the out- put voltage. Select the proper ratio of the two feed- back resistors RFB1 and RFB2 based on the output voltage. Typically, use RFB2 ≈ 10kΩ and determine RFB1 from the output voltage: The inductor maintains a continuous current to the output load. This inductor current has a ripple that is dependent on the inductance value: higher induc- tance 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 ripple current requirement: VOUT 1.5V 1.8V 2.5V 3.3V 5V L 6.8 μH 6.8 μH 10 μH 15 μH 22 μH 1 V 222 . 1 V R R OUT FB2 1 FB - RIPPLE OUTMAX SW IN OUT IN OUT K I f V V V V L - 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 recom- mended. Since a 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 shortest possible traces. In the case of tantalum or electrolytic types, they can be further away if a small parallel 0.1µF ceramic ca- pacitor is placed right next to the IC. The output capacitor also needs to have low ESR to keep low output voltage ripple. The output ripple voltage is: where IOUTMAX is the maximum output current, KRIPPLE is the ripple factor, RESR is the ESR resistance of the output capacitor, fSW is the switching frequency, L is the inductor value, 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 type, typically choose a capacitance of about 22µF. In the case of tantalum or electrolytic type, the ripple is dominated by RESR multiplied by the ripple current. In that case, the output capacitor is chosen to have sufficiently low due to ESR, typically choose a ca- pacitor with less than 50m Ω ESR. Rectifier Diode Use a Schottky diode as the rectifier to conduct cur- rent when the High-Side Power Switch is off. The Schottky diode must have current rating higher than the maximum output current and the reverse volt- age rating higher than the maximum input voltage. Output Capacitor Input Capacitor APPLICATIONS INFORMATION 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 = 20% to 30% corresponding to the peak-to-peak ripple current being 20% to 30% of the maximum output current. With this inductor value (Table 1), the peak inductor current is IOUT × (1 + KRIPPLE / 2). Make sure that this peak inductor current is less that the 2.4A current limit. Finally, select the inductor core size so that it does not saturate at 2.4A. (1) (2) (3) Figure 1: Output Voltage Setting ESR RIPPLE OUTMAX RIPPLE R K L R OUT 2 SW IN LC f 28 V Table 1: Typical Inductor Values Inductor Selection |
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