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NX9548CMTR Datasheet(PDF) 11 Page - Microsemi Corporation |
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NX9548CMTR Datasheet(HTML) 11 Page - Microsemi Corporation |
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11 / 18 page  NX9548 11 Rev.1.6 03/06/09 APPLICATION INFORMATION Symbol Used In Application Information: VIN - Input voltage VOUT - Output voltage IOUT - Output current DVRIPPLE - Output voltage ripple FS - Working frequency DIRIPPLE - Inductor current ripple Design Example The following is typical application for NX9548, the schematic is figure 1. VIN = 8 to 22V VOUT=1.5V FS=220kHz IOUT=9A DVRIPPLE <=60mV DVDROOP<=60mV @ 3A step On_Time and Frequency Calculation The constant on time control technique used in NX9548 delivers high efficiency, excellent transient dy- namic response, make it a good candidate for step down notebook applications. An internal one shot timer turns on the high side driver with an on time which is proportional to the input supply VIN as well inversely proportional to the output voltage VOUT. During this time, the output inductor charges the output cap increasing the output voltage by the amount equal to the output ripple. Once the timer turns off, the Hdrv turns off and cause the output voltage to decrease until reaching the internal FB voltage of 0.75V on the PFM comparator. At this point the comparator trips causing the cycle to repeat itself. A minimum off time of 400nS is internally set. The equation setting the On Time is as follows: 12 TON OUT IN 4.45 10 RV TON V 0.5V − ××× = − ...(1) OUT S IN V F V TON = × ...(2) In this application example, the RTON is chosen to be 1Mohm, when VIN=22V, the TON is 310nS and F S is around 220kHz. Output Inductor Selection The value of inductor is decided by inductor ripple current and working frequency. Larger inductor value nor- mally means smaller ripple current. However if the in- ductance is chosen too large, it brings slow response and lower efficiency. The ripple current is a design free- dom which can be decided by design engineer accord- ing to various application requirements. The inductor value can be calculated by using the following equations: ( ) IN OUT ON OUT RIPPLE RIPPLE OUTPUT V -VT L= I I =kI × × ...(3) where k is percentage of output current. In this example, inductor from COILCRAFT DO5010H-332 with L=3.3uH is chosen. Current Ripple is recalculated as below: IN OUT ON RIPPLE OUT (V -V )T I= L (22V-1.5V) 310nS = 3.3uH =1.925A × × ...(4) 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 conditions. Based on DC Load Condition The amount of voltage ripple during the DC load condition is determined by equation(5). ∆ ∆ = × ∆+ ×× RIPPLE RIPPLE RIPPLE S OUT I V ESRI 8 FC ...(5) Where ESR is the output capacitors' equivalent series resistance,C OUT is the value of output capacitors. Typically POSCAP is recommended to use in NX9548's applications. The amount of the output voltage ripple is dominated by the first term in equation(5) and the second term can be neglected. For this example, one POSCAP 2R5TPE330MC |
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