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ADD5205 Datasheet(PDF) 11 Page - Analog Devices |
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ADD5205 Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page Data Sheet ADD5205 Rev. 0 | Page 11 of 16 EXTERNAL COMPONENT SELECTION GUIDE Inductor Selection The inductor is an integral part of the step-up converter. It stores energy during the switch-on time and transfers that energy to the output through the output diode during the switch-off time. An inductor in the 3.3 μH to 6.8 μH range is recommended. In general, lower inductance values result in higher saturation current and lower series resistance for a given physical size. 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 (tON). SW ON f D t = The inductor ripple current (ΔIL) in a steady state is L t V I ON IN L × = Δ Solve for the inductance value (L). L ON IN I t V L Δ × = Ensure 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. L LAVG LPK I I I Δ 2 1 + = , IN OUT OUT LAVG V η V I I × × = OUT SW IN OUT IN L V f V V V L I × − = Δ 1 where: ILPK is the peak inductor current. ILAVG is the input average current. Table 6 shows a list of recommend inductors. Table 6. Recommended Inductors Coilcraft Part No. L (μH) ISAT (A) Size (mm) XFL4020-332ML 3.3 2.7 4 × 4 × 2 LPS4012-472ML 4.7 1.6 3.9 × 3.9 × 1.1 LPS4018-472ML 4.7 1.8 3.9 × 3.9 × 1.7 LPS4018-682ML 6.8 1.2 3.9 × 3.9 × 1.7 Input and Output Capacitors Selection The ADD5205 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) 4.7 μF or greater capacitor for the input capacitor to prevent noise at the ADD5205 input. Place the input between VIN and GND, as close as possible to the ADD5205. The output capacitor maintains the output voltage and supplies current to the load while the ADD5205 switch is on. The value and characteristics of the output capacitor greatly affect the output voltage ripple and stability of the regulator. Use a ceramic X5R or X7R dielectric capacitor, and for the output capacitor, a 4.7 μF or greater capacitor is preferred. Place a 100 nF or greater capacitor as close as possible to the OVP pin of ADD5205. 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. 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 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. |
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