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ADD5203ACPZRL 데이터시트(HTML) 17 Page  Analog Devices 

ADD5203ACPZRL 데이터시트(HTML) 17 Page  Analog Devices 
17 / 24 page ADD5203 Rev. 0  Page 17 of 24 EXTERNAL COMPONENT SELECTION GUIDE Inductor Selection The inductor is an integral part of the stepup converter. It stores energy during the switchon time and transfers that energy to the output through the output diode during the switchoff time. An inductor in the range of 4.7 μH to 22 μH is recommended. In general, lower inductance values result in higher saturation current and lower series resistance for a given physical size. However, lower inductance results in higher peak current, which can lead to reduced efficiency and greater input and/or output ripple and noise. Peaktopeak inductor ripple current at close to 30% of the maximum dc input current typically yields an optimal compromise. The input (VIN) and output (VOUT) voltages determine the switch duty cycle (D), which in turn can be used to determine the inductor ripple current. OUT IN OUT V V V D − = Use the duty cycle and switching frequency (fSW) to determine the on time. SW ON f D t = The inductor ripple current (ΔIL) in a steady state is L t V I IN L ON × = Δ Solve for the inductance value (L). L IN I t V L Δ × = ON Make sure that the peak inductor current (that is, the maximum input current plus half of the inductor ripple current) is less than the rated saturation current of the inductor. In addition, ensure that the maximum rated rms current of the inductor is greater than the maximum dc input current to the regulator. For duty cycles greater than 50% that occur with input voltages greater than half the output voltage, slope compensation is required to maintain stability of the currentmode regulator. The inherent openloop stability causes subharmonic instability when the duty ratio is greater than 50%. To avoid subharmonic instability, the slope of the inductor current should be less than half of the compensation slope. Inductor manufacturers include Coilcraft, Inc., Sumida Corporation, and Toko. Input and Output Capacitors Selection The ADD5203 requires input and output bypass capacitors to supply transient currents while maintaining a constant input and output voltage. Use a low effective series resistance (ESR) 10 μF or greater capacitor for the input capacitor to prevent noise at the ADD5203 input. Place the input between the VIN and GND, as close as possible to the ADD5203. Ceramic capacitors are preferred because of their low ESR characteristics. Alternatively, use a high value, medium ESR capacitor in parallel with a 0.1 μF low ESR capacitor as close as possible to the ADD5203. The output capacitor maintains the output voltage and supplies current to the load while the ADD5203 switch is on. The value and characteristics of the output capacitor greatly affect the output voltage ripple and stability of the regulator. Use a low ESR output capacitor; ceramic dielectric capacitors are preferred. For very low ESR capacitors, such as ceramic capacitors, the ripple current due to the capacitance is calculated as follows. Because the capacitor discharges during the on time (tON), the charge removed from the capacitor (QC) is the load current multiplied by the on time. Therefore, the output voltage ripple (ΔVOUT) is OUT ON L OUT C OUT C t I C Q V × = = Δ where: COUT is the output capacitance. IL is the average inductor current. Using the duty cycle and switching frequency (fSW), users can determine the on time with the following equation: SW ON f D t = The input (VIN) and output (VOUT) voltages determine the switch duty cycle (D) with the following equation: OUT IN OUT V V V D − = Choose the output capacitor based on the following equation: ( ) OUT OUT SW IN OUT L OUT V V f V V I C Δ × × − × ≥ Capacitor manufacturers include Murata Manufacturing Co., Ltd., AVX, Sanyo, and Taiyo Yuden Co., Ltd. Diode Selection The output diode conducts the inductor current to the output capacitor and loads while the switch is off. For high efficiency, minimize the forward voltage drop of the diode. Schottky diodes are recommended. However, for high voltage, high temperature applications, where the Schottky diode reverse leakage current becomes significant and can degrade efficiency, use an ultrafast junction diode. The output diode for a boost regulator must be chosen depending on the output voltage and the output current. The diode must be rated for a reverse voltage equal to or greater than the output voltage used. The average current rating must be greater than the maximum load current expected, and the peak current rating must be greater than the peak inductor current. Using Schottky diodes with lower forward voltage drop decreases power dissipation and increases efficiency. The diode must be rated to handle the average output load current. Many diode 
