9.2.1.4.1 Inductor

There are two main considerations when choosing an inductor; the inductor should not saturate, and the inductor current ripple should be small enough to achieve the desired output voltage ripple. Different saturation current rating specifications are followed by different manufacturers so attention must be given to details. Saturation current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of application should be requested from the manufacturer. Shielded inductors radiate less noise and should be preferred.

The saturation current should be greater than the sum of the maximum load current and the worst case average to peak inductor current.

Figure 44 shows the worst case conditions.

Figure 44. Calculating Inductor Maximum Current

• I_RIPPLE: peak inductor current
• I_OUTMAX: maximum load current
• V_IN: minimum input voltage in application
• L : min inductor value including worst case tolerances
• f : minimum switching frequency
• V_OUT: output voltage
• D: Duty Cycle for CCM Operation
• V_OUT : Output Voltage

As a result the inductor should be selected according to the I_SAT. A more conservative and recommended approach is to choose an inductor that has a saturation current rating greater than the maximum current limit of 3.1 A. A 4.7-µH to 6.8-µH inductor with a saturation current rating of at least 3.1 A is recommended for most applications. The inductor’s resistance should be less than 300 mΩ for good efficiency.

9.2.1.4.2 Output Capacitor

A ceramic capacitor with 50-V voltage rating is recommended for the output capacitor. The DC-bias effect can reduce the effective capacitance by up to 80% especially with small package size capacitors, which needs to be considered in capacitance value and package selection. Typically one 10-µF or two 4.7-µF capacitors is sufficient. Effectively the capacitance should be at least 2 µF at boost maximum output voltage.

9.2.1.4.3 LDO Capacitor

A ceramic capacitor with at least 10 V voltage rating is recommended for the output capacitor of the LDO. The DC-bias effect can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Typically 10 µF capacitor is sufficient.

9.2.1.4.4 VDD Capacitor

A ceramic capacitor with at least 10-V voltage rating is recommended for the VDD input capacitor. If input voltage is higher, then the rating should be selected accordingly. The DC-bias effect can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Typically, a 1-μF capacitor is sufficient. X5R/X7R are recommended types.

9.2.1.4.5 Boost Input Capacitor

A ceramic capacitor with at least 10-V voltage rating is recommended for the boost input capacitor. If input voltage is higher, then the rating should be selected accordingly. The DC-bias effect can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Typically, a 10-μF capacitor per boost is sufficient. X5R/X7R are recommended types.

9.2.1.4.6 Diode

A Schottky diode should be used for the output diode. Peak repetitive current should be greater than inductor peak current (3.1 A) to ensure reliable operation. Average current rating should be greater than the maximum output current. Schottky diodes with a low forward drop and fast switching speeds are ideal for increasing efficiency in portable applications. Choose a reverse breakdown voltage of the Schottky diode significantly larger (~40 V) than the output voltage. Do not use ordinary rectifier diodes, since slow switching speeds and long recovery times cause the efficiency and the load regulation to suffer.