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NX2305CSTR Datasheet(PDF) 8 Page - Microsemi Corporation |
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NX2305CSTR Datasheet(HTML) 8 Page - Microsemi Corporation |
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8 / 19 page  NX2305 8 Rev.5.0 08/19/08 APPLICATION INFORMATION Symbol Used In Application Information: VIN - Input voltage VOUT - Output voltage IOUT - Output current DVRIPPLE - Output voltage ripple FS - Switching frequency DIRIPPLE - Inductor current ripple Design Example Power stage design requirements: VIN=12V VOUT=1.8V IOUT =10A DVRIPPLE <=20mV DVTRAN<=100mV @ 10A step FS=300kHz Output Inductor Selection The selection of inductor value is based on induc- tor ripple current, power rating, working frequency and efficiency. Larger inductor value normally means smaller ripple current. However if the inductance is chosen too large, it brings slow response and lower efficiency. Usu- ally the ripple current ranges from 20% to 40% of the output current. This is a design freedom which can be decided by design engineer according to various appli- cation requirements. The inductor value can be calcu- lated by using the following equations: IN OUT OUT OUT RIPPLE INS RIPPLE OUTPUT V -VV 1 L= I VF I =kI ×× × ...(1) where k is between 0.2 to 0.4. Select k=0.3, then ×× × OUT OUT 12V-1.8V 1.8V1 L= 0.3 10A 12V 300kHz L =1.7uH Choose LOUT=2.2uH, then coilcraft inductor DO5010P-222HC is a good choice. Current Ripple is calculated as ×× × ×= IN OUT OUT RIPPLE OUT INS V -VV 1 I= L VF 12V-1.8V 1.8V1 = 2.3A 2.2uH 12V 300kHz ...(2) Output Capacitor Selection Output capacitor is basically decided by the amount of the output voltage ripple allowed during steady state(DC) load condition as well as specification for the load transient. The optimum design may require a couple of iterations to satisfy both condition. Based on DC Load Condition The amount of voltage ripple during the DC load condition is determined by equation(3). ∆ ∆ = × ∆+ ×× RIPPLE RIPPLE RIPPLE S OUT I V ESRI 8 FC ...(3) Where ESR is the output capacitors' equivalent series resistance,C OUT is the value of output capacitors. Typically when large value capacitors are selected such as Aluminum Electrolytic,POSCAP and OSCON types are used, the amount of the output voltage ripple is dominated by the first term in equation(3) and the second term can be neglected. For this example, POSCAP are chosen as output capacitors, the ESR and inductor current typically de- termines the output voltage ripple. ∆ = =Ω ∆ RIPPLE desire RIPPLE V 20mV ESR = 8.7m I 2.3A ...(4) If low ESR is required, for most applications, mul- tiple capacitors in parallel are better than a big capaci- tor. For example, for 20mV output ripple, POSCAP 2R5TPE470M9 with 9m Ω are chosen. E R IPPLE R IPPLE ES RI N V ×∆ = ∆ ...(5) Number of Capacitor is calculated as Ω× = 9m 2.3A N 20mV N =1.03 The number of capacitor has to be round up to a integer. Choose N =2. |
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