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 10 shows the worst-case conditions:

Equation 10.

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
• ƒ - 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. A saturation current rating at least 3 A is recommended for most applications. See Table 2 for inductance recommendation for the different switch 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 (SLVA061). Power Stage Designer™ Tools can be used for the boost calculation: http://www.ti.com/tool/powerstage-designer.

9.2.1.2.2 Output Capacitor Selection

A ceramic and electrolytic capacitors should have sufficient voltage rating. The DC-bias effect in ceramic capacitors can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Capacitance recommendation for different switching frequency range is shown in Table 2. To minimize audible of noise ceramic capacitors their geometric size is usually minimized.

9.2.1.2.3 Input Capacitor Selection

A ceramic and electrolytic capacitors should have sufficient voltage rating. The DC-bias effect in ceramic capacitors can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Capacitance recommendation for different switching frequency range is shown in Table 2. To minimize audible of noise ceramic capacitors their geometric size is usually minimized.

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 in ceramic capacitors 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. Ordinary rectifier diodes should not be used, 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. 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.

9.2.1.2.6 Power Line Transistor

A pFET transistor with necessary voltage rating (V_DS at least 5 V higher than max input voltage) should be used. Current rating for the FET should be the same as input peak current or greater. Transfer characteristic is very important for pFET. V_GS for open transistor must be less than V_IN. A 20-kΩ resistor between pFET gate and source is sufficient.

9.2.1.2.7 Input Current Sense Resistor

A high-power 50-mΩ resistor should be used for sensing the boost input current. Power rating can be calculated from the input current and sense resistor resistance value. Increasing R_ISENSE decreases VIN OCP current proportionally.

9.2.3.2.1 Diode

A Schottky diode with a low forward drop and fast switching speed should be used for the SEPIC output diode. Do not use ordinary rectifier diodes, because slow switching speeds and long recovery times degrade the efficiency and load regulation. The diode must be able to handle peak repetitive current greater than the integrated FET peak current (SW pin limit), thus 3 A or higher must be used to ensure reliable operation. Average current rating should be greater than the maximum output current. Choose a diode with reverse breakdown larger than the sum of input voltage and output voltage.

9.2.3.2.2 Inductor

Coupled or uncoupled inductors can be used in SEPIC mode. Coupled inductor typically provides better efficiency. Power Stage Designer™ Tools can be used for the SEPIC inductance calculation: http://www.ti.com/tool/powerstage-designer.