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AND8079 Datasheet(PDF) 4 Page  ON Semiconductor 

4 page AND8079/D http://onsemi.com 4 Component Selection Input Inductor The input inductor (LIN) is used to isolate the input power supply from the switching portion of the buck regulator. LIN also limits the inrush current into the bulk input capacitors and limits the input current slew rate that results from the transient load. The inductor blocks the ripple current and transfers the transient current requirement to the bulk input capacitor bank. The design equations for LIN are listed below and for connivance an inductance of 1.0 mH is chosen. The cutoff frequency of the second order LC filter provides adequate attenuation for the 200 kHz switching frequency of the NCP1571. LIN + DV (dI dt)Max + 5V * 2.5 V 10 A 5 ms + 1.25 mH f * 3db + 1 2 p LIN CIN + 1 2 p 1 mH 5400 mF + 216 Hz where: LIN = input inductor CIN = bulk input capacitor(s) dI/dt = 10 A in 5.0 ms Input Capacitors The input filter capacitors provide a charge reservoir that minimizes the supply voltage variations due to the pulsating current through the MOSFETs. The input capacitors are chosen primarily to meet the ripple current rating of the capacitors. The design equation is listed below. ICin(RMS) + D (1 * D) Iout2 + .5 (1 * .5) 102 + 5A where: D = duty cycle = VOUT/VIN = 2.5 V/5.0 V = 0.5 IOUT = maximum output current The Rubycon 10 V 1800 mf capacitors have a ripple current rating of 2.55 A. Thus only 2 of the capacitors are needed to meet the ripple requirements; however, 3 capacitors were chosen to be conservative. Output Inductance The main criterion in selecting the output filter inductance (LOUT) is to provide a satisfactory response to the load transients. The inductance affects the output voltage ripple by limiting the rate at which the current can either increase or decrease. The design equation used for selecting LOUT is listed below. A 2.2 mH inductor was chosen for the design. LOUT + (VIN * VOUT) tr DI + (5 V * 2.5 V) 10 ms 10 A + 2.5 mH where: tr = output transient load time Output Capacitors The output capacitors are selected to meet the desired output ripple requirements. The key specifications for the capacitors are their ESR (Equivalent Series Resistance) and ESL (Equivalent Series Inductance). In order to obtain a good transient response, a combination of low value/high frequency ceramic capacitors and bulk electrolytic capacitors are placed as close to the load as possible. The voltage change during the load current transient is: DVOUT + DIOUT ESL Dt ) ESR ) tr COUT ^ DIOUT ESR Empirical data indicates that most of the output voltage change that results from the load current transients is determined by the capacitor ESR; therefore, the maximum allowable ESR can be approximated from the following equation. ESR max ^ DVOUT DIOUT + 75 mV 10 A + 7.5 mW The number of capacitors is calculated by using the equation listed below. Number of capacitors + ESRCAP ESR max + 19 mW 7.5 m W + 2.5 The ESR of the Rubycon 6.3 V 1800 mF capacitors is specified at 19 m W; therefore, 3 capacitors are used in the design. MOSFET Selection The output switch MOSFETs are chosen based on the gate charge/gatesource threshold voltage, gate capacitance, on resistance, current rating and the thermal capacity of the package. In this DDR design, the MOSFETs were chosen for economical reasons and have a current and power rating that is much better than needed for this design. In addition, the MOSFETs selected were verified by measuring the thermal characteristics of the devices on the PCB. The power dissipation design equation for selecting the MOSFETs is given below. P + IMAX2 RDS(ON) D ) IMAX VDS Tr FS 2 ) IMAX VDS Tf FS 2 where: Tr = rise time or turnon time of MOSFET Tf = fall time or turnoff time of MOSFET FS = switching frequency 
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