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APW7060K Datasheet(PDF) 11 Page - Anpec Electronics Coropration |
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APW7060K Datasheet(HTML) 11 Page - Anpec Electronics Coropration |
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11 / 16 page  Copyright ANPEC Electronics Corp. Rev. A.3 - Oct., 2003 APW7060 www.anpec.com.tw 11 P UPPER = Iout 2 (1+ TC)(R DS(ON))D + (0.5)(Iout)(VIN)(tsw)FS P LOWER = Iout 2 (1+ TC)(R DS(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 include an additional transition loss.The switching internal, t sw, is a function of the reverse transfer capacitance C RSS. Figure 4 illustrates the switching waveform of the MOSFET. The (1+TC) term is to factor in the temperature depen- dency of the R DS(ON) and can be extracted from the “R DS(ON) vs Temperature” curve of the power MOSFET. Linear Regulator Input/Output Capacitor Selec- tion The input capacitor is chosen based on its voltage rating. Under load transient condition, the input ca- pacitor will momentarily supply the required transient current. A 1uF ceramic capacitor will be sufficient in most applications. The output capacitor for the linear regulator is chosen to minimize any droop during load transient condition. In addition, the capacitor is chosen based on its volt- age rating. Linear Regulator MOSFET Selection In addition to choosing the pass MOSFET for its abil- ity to sustain the load current requirement (see Maxi- mum Output Voltage of Linear Controller), another cri- teria is its efficiency of heat removal. The power dissi- pated by the MOSFET is given by: Pdiss = Iout * (V IN - VOUT2) Inductor Selection The inductance of the inductor is determined by the output voltage requirement. The larger the inductance, the lower the inductor’s current ripple. This will trans- late into lower output ripple voltage. The ripple current and ripple voltage can be approximated by: I RIPPLE = L Fs V V OUT - IN × ∆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 transient response at the expense of higher ripple cur- rent and vice versa. The maximum ripple current oc- curs at the maximum input voltage. A good starting point is to choose the ripple current to be approxi- mately 30% of the maximum output current. Once the inductance value has been chosen, select an inductor that is capable of carrying the required peak current without going into saturation. In some type 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. PWM Regulator MOSFET Selection The selection of the N-channel power MOSFETs are determined by the R DS(ON), reverse transfer capacitance (C RSS) and maximum output current requirement.The losses in the MOSFETs have two components: con- duction loss and transition loss. For the upper and lower MOSFET, the losses are approximately given by the following : Application Information IN OUT V V × |
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