8.2.1.2.5 Selecting the Inductor

The 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.

Inductor values can have ±20% tolerance with no current bias. When the inductor current approaches saturation level, its inductance can falls to some percentage of its 0-A value depending on how the inductor vendor defines saturation current.

Using an inductor with a smaller inductance value forces discontinuous PWM where the inductor current ramps down to zero before the end of each switching cycle. This reduces the maximum output current of the boost converter, causes large input voltage ripple and reduces efficiency. In general, large inductance value provides much more output and higher conversion efficiency. Small inductance value can give better the load transient response. For these reasons, TI recommends a 4.7-μH to 22-μH inductor value range. Table 4 lists the recommended inductor for the TPS61500.

Meanwhile, the TPS61500 can program the switching frequency. Normally, small inductance value is suitable for high frequency and vice versa. The device has built-in slope compensation to avoid sub-harmonic oscillation associated with current mode control. If the inductor value is lower than 4.7 μH, the slope compensation may not be adequate, and the loop can be unstable. Therefore, customers should verify the inductor in their application if it is different from the recommended values.