Table 4. Boost Mode Design Parameters

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
• V_OUT - Output Voltage

As a result the inductor must 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 3 A is recommended for most applications. See Table 2 for inductance recommendations for the different switching frequency ranges. Resistance of the inductor must be less than 300 mΩ for good efficiency.

See detailed information in Understanding Boost Power Stages in Switch Mode Power Supplies. “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 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 boost switching frequencies are shown in Table 2. To minimize audible noise of ceramic capacitors their geometric size must typically be minimized.

9.2.1.2.3 Input Capacitor Selection

A ceramic capacitor with 2 × V_{VIN 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, since slow switching speeds and long recovery times cause the efficiency and the load regulation to suffer. Diode rating for peak repetitive current must be greater than inductor peak current (up to 3 A) to ensure reliable operation. Average current rating must 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) must be used. Current rating for the FET must 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 must 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.

9.2.2.1.1 Inductor

In SEPIC mode, coupled coil saturation rating should be higher than input side inductor peak current. Current values can be estimated using Power Stage Designer™ Tool or using equations in SLYT309.

9.2.2.1.2 Diode

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

9.2.2.1.3 Capacitor C1

A ceramic capacitor with low ESR is recommended. Voltage rating must be higher than maximum input voltage.