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APW7057 Datasheet(PDF) 10 Page - Anpec Electronics Coropration |
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APW7057 Datasheet(HTML) 10 Page - Anpec Electronics Coropration |
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10 / 15 page  Copyright © ANPEC Electronics Corp. Rev. A.5 - Jun., 2008 APW7057 www.anpec.com.tw 10 Application Information 2 2 Component Selection Guidelines Output Capacitor Selection The selection of C OUT is determined by the required effec- tive series resistance (ESR) and voltage rating rather than the actual capacitance requirement. Therefore, select high performance low ESR capacitors that are intended for switching regulator applications. In some applications, multiple capacitors have to be paralled to achieve the desired ESR value. If tantalum capacitors are used, make sure they are surge tested by the manufactures. If in doubt, consult the capacitors manufacturer. Input Capacitor Selection The input capacitor is chosen based on the voltage rating and the RMS current rating. For reliable operation, select the capacitor voltage rating to be at least 1.3 times higher than the maximum input voltage. The maximum RMS current rating requirement is approximately I OUT/2 , where I OUT is the load current. During power up, the input capaci- tors have to handle large amount of surge current. If tanta- lum capacitors are used, make sure they are surge tested by the manufactures. If in doubt, consult the capacitors manufacturer. For high frequency decoupling, a ceramic capacitor be- tween 0.1 µF to 1µF can be connected between V CC and ground pin. Inductor Selection The inductance of the inductor is determined by the out- put voltage requirement. The larger the inductance, the lower the inductor’s current ripple. This will translate into lower output ripple voltage. The ripple current and ripple voltage can be approximated by: I RIPPLE = V IN - VOUT Fs x L V OUT V IN x ∆V OUT = IRIPPLE x ESR where Fs is the switching frequency of the regulator. There is a tradeoff exists between the inductor’s ripple current and the regulator load transient response time. A smaller inductor will give the regulator a faster load tran- sient response at the expense of higher ripple current and vice versa. The maximum ripple current occurs at the maximum in- put voltage. A good starting point is to choose the ripple current to be approximately 30% of the maximum output current. Once the inductance value has been chosen, select an inductor that is capable of carrying the required peak cur- rent without going into saturation. In some types of inductors, especially core that is make of ferrite, the ripple current will increase abruptly when it saturates. This will result in a larger output ripple voltage. MOSFET Selection The selection of the N-channel power MOSFETs are de- termined by the R DS(ON), reverse transfer capacitance (CRSS) and maximum output current requirement.The losses in the MOSFETs have two components: conduction loss and transition loss. For the upper and lower MOSFET, the losses are approximately given by the following equations: P UPPER = Iout (1+ TC)(RDS(ON))D + (0.5)(Iout)(VIN)(tsw)FS P LOWER = Iout (1+ TC)(RDS(ON))(1-D) where I OUT is the load current TC is the temperature dependency of R DS(ON) F S is the switching frequency t sw is the switching interval D is the duty cycle Note that both MOSFETs have conduction losses while the upper MOSFET includes an additional transition loss. The switching internal, t sw, is the function of the reverse transfer capacitance C RSS. Figure 3 illustrates the switch- ing waveform internal of the MOSFET. Layout Consideration In high power switching regulator, a correct layout is im- portant to ensure proper operation of the regulator. In general, interconnecting impedances should be mini- mized by using short, wide printed circuit traces. Signal and power grounds are to be kept separate and finally combined using ground plane construction or single point grounding. Figure 4 illustrates the layout, with bold lines indicating high current paths. Components along the bold lines should be placed close together. |
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