9.2.1.2.1.1 Inductor Selection

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.

Equation 3 below shows the worst case conditions.

Equation 3.

where

• I_RIPPLE: Average to peak inductor current
• I_OUTMAX: Maximum load current
• V_IN: Maximum input voltage in application
• L: Min inductor value including worst case tolerances
• f: Minimum switching frequency
• D: Duty cycle for CCM Operation
• V_OUT: Output voltage

Example using above equations:

• V_IN = 12 V
• V_OUT = 38 V
• I_OUT = 400 mA
• L = 15 μH − 20% = 12 μH
• f = 1.25 MHz
• I_SAT = 1.6 A

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 2.5 A. A 15-μH inductor with a saturation current rating of 2.5 A is recommended for most applications. The inductor’s resistance should be less than 300 mΩ for good efficiency. For high efficiency choose an inductor with high frequency core material such as ferrite to reduce core losses. To minimize radiated noise, use shielded core inductor. Inductor should be placed as close to the SW pin and the IC as possible. Special care should be used when designing the PCB layout to minimize radiated noise and to get good performance from the boost converter. For more information on the PCB layout recommendations, please see Layout Guidelines.

9.2.1.2.1.2 Output Capacitor

A ceramic capacitor with 50-V voltage rating or higher is recommended for the output capacitor. The DC-bias effect can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. For light loads a 4.7-μF capacitor is sufficient. Effectively the capacitance should be 4 μF for < 150-mA loads. For maximum output voltage/current 10-μF capacitor (or two 4.7-μF capacitors) is recommended to minimize the output ripple.

9.2.1.2.1.3 LDO Capacitor

A 1-μF ceramic capacitor with 10-V voltage rating is recommended for the LDO capacitor.

9.2.1.2.1.4 Output Diode

A Schottky diode should be used for the output diode. Peak repetitive current should be greater than inductor peak current (2.5 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 (approximately 60 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.