9.1.2 Inductor Selection

Selection of the inductor affects 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, according to half of the peak-to-peak ripple current given by Equation 1, pause the inductor DC current given by:

Equation 3.

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 maximum output current of the boost convert, causes large input voltage ripple, and reduces efficiency. Large inductance value provides much more output current and higher conversion efficiency. For these reasons, a 10-μH to 22-μH inductor value range is recommended. A 22-μH inductor optimized the efficiency for most application while maintaining low inductor peak to peak ripple. Table 2 lists the recommended inductor for the TPS61165. When recommending inductor value, the factory has considered –40% and 20% tolerance from its nominal value.

TPS61165 has built-in slope compensation to avoid subharmonic oscillation associated with current mode control. If the inductor value is lower than 10 μH, the slope compensation may not be adequate, and the loop can be unstable. Therefore, customers need to verify the inductor in their application if it is different from the recommended values.