전자부품 데이터시트 검색엔진 

HM1480 데이터시트(HTML) 7 Page  Shenzhen Huazhimei Semiconductor Co., Ltd 

HM1480 데이터시트(HTML) 7 Page  Shenzhen Huazhimei Semiconductor Co., Ltd 
7 / 10 page 7 Setting Output Voltage The output voltage is set with a resistor divider from the output node to the FB pin. It is recommended to use divider resistors with 1% tolerance or better. To improve efficiency at very light loads consider using larger value resistors. If the values are too high the regulator is more susceptible to noise and voltage errors from the FB input current are noticeable. For most applications, a resistor in the 10k? to 1M? range is suggested for R4. R3 is then given by: R3 = R4 ? [(VOUT / VREF) ? 1] where VREF is 1.21V. Inductor Selection For most applications, the value of the inductor will fall in the range of 4.7?H to 47?H. Its value is chosen based on the desired ripple current. Large value inductors lower ripple current and small value inductors result in higher ripple currents. Higher VIN or VOUT also increases the ripple current as shown in equation. A reasonable starting point for setting ripple current is ?IL = 800mA (40% of 2A). The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation. Thus, a 2.8A rated inductor should be enough for most applications (2A + 800mA). For better efficiency, choose a low DCresistance inductor. Different core materials and shapes will change the size/current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or perm alloy materials are small and don?t radiate much energy, but generally cost more than powdered iron core inductors with similar electrical characteristics. The choice of which style inductor to use often depends more on the price vs. size requirements and any radiated field/EMI requirements than on what the HM1480 requires to operate. Output and Input Capacitor Selection In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worstcase condition is commonly used for design because even significant deviations do not offer much relief. Note that the capacitor manufacturer?s ripple current ratings are often based on 2000 hours of life. This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Always consult the manufacturer if there is any question. 
