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IRU3047CW Datasheet(PDF) 7 Page - International Rectifier |
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IRU3047CW Datasheet(HTML) 7 Page - International Rectifier |
7 / 19 page IRU3047 7 Rev. 1.0 09/09/02 www.irf.com For higher efficiency, a low ESR capacitor is recom- mended. For VIN1=12V, choose two Poscap from Sanyo 16TPB47M (16V, 47 mF, 70mV, 1.4A) For VIN2=5V, choose two 6TPB330M (6.3V, 330 mF, 40m V, 3A). 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 6TPB470M 470 mF, 6.3V has an ESR 40mV. Selecting two of these capacitors in parallel, 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. Where: VIN = Maximum Input Voltage VOUT = Output Voltage ∆i = Inductor Ripple Current fS = Switching Frequency ∆t = Turn On Time D = Duty Cycle For the buck converter, the inductor value for desired operating ripple current can be determined using the fol- lowing relation: For Di1=30% of I1, we get L3=5.46mH For Di2=30% of I2, we get: L4=3.47mH The Coilcraft DO5022HC series provides a range of in- ductors in different values and low profile for large cur- rents. For L3 choose DO5022P-602HC (6 mH, 7.5A) For L4 choose DO5022P-472HC (4.7 mH, 8.4A) Power MOSFET Selection The selections criteria to meet power transfer require- ments is based on maximum drain-source voltage (VDSS), gate-source drive voltage (VGS), maximum output cur- rent, On-resistance RDS(ON) and thermal management. The MOSFET must have a maximum operating voltage (VDSS) exceeding the maximum input voltage (VIN). The gate drive requirement is almost the same for both MOSFETs. Caution should be taken with devices at very low VGS to prevent undesired turn-on of the complemen- tary MOSFET, which results a shoot-through current. The total power dissipation for MOSFETs includes con- duction and switching losses. For the Buck converter the average inductor current is equal to the DC load cur- rent. The conduction loss is defined as: The total conduction loss is defined as: ESR [ ---(5) DVO DIO Where: DVO = Output Voltage Ripple DIO = Output Current DVO=100mV and DIO=5A, results to ESR=20mV VIN - VOUT = L 3 ; Dt = D3 ; D = 1 fS VOUT VIN Di Dt L = (VIN - VOUT) 3 ---(6) VOUT VIN 3Di3fS 2 2 PCOND(Upper Switch) = ILOAD 3RDS(ON)3D3q PCOND(Lower Switch) = ILOAD 3RDS(ON)3(1 - D)3q q = RDS(ON) Temperature Dependency PCON(TOTAL)=PCON(Upper Switch) q+PCON(LowerSwitch)q |
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