9.2.1.2.1 Inductor Selection

There are two main considerations when choosing an inductor; the inductor must not saturate, and the inductor current ripple must 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 are preferred. The saturation current must be greater than the sum of the maximum load current, and the worst case average-to-peak inductor current. Equation 4 shows the worst case conditions

Equation 4.

where

• 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

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. A saturation current rating of at least 2.5 A is recommended for most applications. See Table 2 for recommended inductance value for the different switching frequency ranges. The inductor’s resistance should be less than
300 mΩ for good efficiency.

See detailed information in Understanding Boost Power Stages in Switch Mode Power Supplies. Power Stage Designer™ Tool can be used for the boost calculation: http://www.ti.com/tool/powerstage-designer.

9.2.1.2.2 Output Capacitor Selection

A ceramic capacitor with 2 × V_{MAX BOOST} or more voltage rating 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. Capacitance recommendations for different switching frequencies are shown in Table 2. To minimize audible noise of ceramic capacitors their physical size should typically be minimized.

9.2.1.2.3 Input Capacitor Selection

A ceramic capacitor with 2 × V_{IN MAX} or more voltage rating is recommended for the input capacitor. The DC-bias effect can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Capacitance recommendations for different boost switching frequencies are shown in Table 2.

9.2.1.2.4 LDO Output 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 a 1-µF capacitor is sufficient.

9.2.1.2.5 Diode

A Schottky diode should be used for the boost output diode. Do not use ordinary rectifier diodes because slow switching speeds and long recovery times degrade the efficiency and the load regulation. Diode rating for peak repetitive current should be greater than inductor peak current (up to 3 A) to ensure reliable operation in boost mode. 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. Choose a reverse breakdown voltage of the Schottky diode significantly larger than the output voltage.

9.2.2.2.1 Inductor

In SEPIC mode, currents flowing through the coupled inductors or the two separate inductors L1 and L2 are the input current and output current, respectively. Values can be calculated using Power Stage Designer™ Tool or using equations in Designing DC/DC Converters Based on SEPIC Topology.

9.2.2.2.2 Diode

In SEPIC mode diode peak current is equal to the sum of input and output currents. Diode rating for peak repetitive current should be greater than SW pin current limit (up to 3 A for transients) to ensure reliable operation in boost mode. Average current rating should be greater than the maximum output current. Diode voltage rating must be higher than sum of input and output voltages.

9.2.2.2.3 Capacitor C1

Ti recommends a ceramic capacitor with low ESR. Diode voltage rating must be higher than maximum input voltage.