The selection of the inductor affects power efficiency, steady state operation as well as transient behavior and loop stability. These factors make it the most important component in power regulator design. There are three important inductor specifications, inductor value, DC resistance and saturation current. Considering inductor value alone is not enough. The inductor value determines the inductor ripple current. Choose an inductor that can handle the necessary peak current without saturating. Follow Equation 3 to Equation 4 to calculate the peak current of the inductor. To calculate the current in the worst case, use the minimum input voltage, maximum output voltage and maximum load current of application. In a boost regulator, the input DC current can be calculated as Equation 3.

Equation 3.

where

• V_OUT = boost output voltage
• I_OUT = boost output current
• V_IN = boost input voltage
• η = power conversion efficiency

The inductor current peak to peak ripple can be calculated as Equation 4.

Equation 4.

where

• ΔI_L(PP) = inductor peak-to-peak ripple
• L = inductor value
• F_S = boost switching frequency
• V_OUT = boost output voltage
• V_IN = boost input voltage

Therefore, the peak current I_L(P) seen by the inductor is calculated with Equation 5.

Equation 5.

Inductor values can have ±20% tolerance with no current bias. When the inductor current approaches saturation level, its inductance can decrease 20% to 35% from the 0A value depending on how the inductor vendor defines saturation current. Using an inductor with a smaller inductance value forces discontinuous PWM when the inductor current ramps down to zero before the end of each switching cycle. This reduces the boost converter’s maximum output current, causes large input voltage ripple and reduces efficiency. Large inductance value provides much more output current and higher conversion efficiency. For these reasons, a 4.7μH to 10μH inductor value range is recommended, and 4.7μH inductor is recommended for higher than 5V input voltage by considering inductor peak current and loop stability. Table 7-2 lists the recommended inductor for the TPS61169.