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APU3048 データシート(PDF) 6 Page - Advanced Power Electronics Corp. |
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APU3048 データシート(HTML) 6 Page - Advanced Power Electronics Corp. |
6 / 15 page 6/15 APU3048 Output Capacitor Selection The criteria to select the output capacitor is normally based on the value of the Effective Series Resistance (ESR). In general, the output capacitor must have low enough ESR to meet output ripple and load transient requirements, yet have high enough ESR to satisfy sta- bility requirements. The ESR of the output capacitor is calculated by the following relationship: The Sanyo TPC series, PosCap capacitor is a good choice. The 6TPC150M 150 mF, 6.3V has an ESR 40mV. Selecting two of these capacitors in parallel for each output, results to an ESR of ≅ 20mV which achieves our low ESR goal. The capacitor value must be high enough to absorb the inductor's ripple current. The larger the value of capaci- tor, the lower will be the output ripple voltage. The resulting output ripple current is smaller then each channel ripple current due to the 180 8 phase shift. These currents cancel each other. The cancellation is not the maximum because of the different duty cycle for each channel. Inductor Selection The inductor is selected based on output power, operat- ing frequency and efficiency requirements. Low induc- tor value causes large ripple current, resulting in the smaller size, but poor efficiency and high output noise. Generally, the selection of inductor value can be reduced to desired maximum ripple current in the inductor ( ∆i); the optimum point is usually found between 20% and 50% ripple of the output current. For the buck converter, the inductor value for desired operating ripple current can be determined using the fol- lowing relation: If the high value feedback resistors are used, the input bias current of the Fb pin could cause a slight increase in output voltage. The output voltage set point can be more accurate by using precision resistor. Soft-Start Programming The soft-start timing can be programmed by selecting the soft start capacitance value. The start up time of the converter can be calculated by using: Where: CSS is the soft-start capacitor ( mF) For a start-up time of 7.5ms, the soft-start capacitor will be 0.1 mF. Choose a ceramic capacitor at 0.1mF. Boost Supply Vc To drive the high-side switch it is necessary to supply a gate voltage at least 4V greater than the bus voltage. This is achieved by using a charge pump configuration as shown in Figure 9. The capacitor is charged up to approximately twice the bus voltage. A capacitor in the range of 0.1 mF to 1mF is generally adequate for most applications. Input Capacitor selection The input filter capacitor should be based on how much ripple the supply can tolerate on the DC input line. The ripple current generated during the on time of control MOSFET should be provided by input capacitor. The RMS value of this ripple is expressed by: Where: D is the Duty Cycle, simply D=VOUT/VIN. IRMS is the RMS value of the input capacitor current. IOUT is the output current for each channel. For VIN1=12V, IOUT1=4A and D1=0.275 Results to: IRMS1=1.78A And for VIN2=5V, IOUT2=4A and D2=0.36 Results to: IRMS2 =1.92A For higher efficiency, a low ESR capacitor is recom- mended. For VIN1=12V, choose one Poscap from Sanyo 16TPB47M (16V, 47 mF, 70mV, 1.4A) For VIN2=5V, choose one 6TPC150M (6.3V, 150 mF, 40m V, 1.9A). tSTART = 75 3 Css (ms) ---(2) IRMS = IOUT D 3(1-D) ---(3) ESR [ ---(4) DVO DIO Where: DVO = Output Voltage Ripple DIO = Output Current DVO=75mV and DIO=3A, results to: ESR=25mV VIN - VOUT = L 3 ; Dt = D3 ; D = 1 fS VOUT VIN Di Dt L = (VIN - VOUT) 3 ---(5) VOUT VIN 3Di3fS Where: VIN = Max Input Voltage VOUT = Output Voltage ∆i = Inductor Ripple Current fS = Switching Frequency ∆t = Turn On Time D = Duty Cycle |
同様の部品番号 - APU3048 |
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同様の説明 - APU3048 |
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