8.2.1.2.4 Inductor Selection

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 (DCR) and saturation current. Considering inductor value alone is not enough.

The inductance value of the inductor determines its ripple current. TI recommends setting the peak-to-peak ripple current given by Equation 3 to 30% to 40% of the DC current. Inductance values shown in the Recommended Operating Conditions table are recommended for most applications. Inductor DC current can be calculated as

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 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, inductors with large inductance and low DCR values provide much more output current and higher conversion efficiency. Inductors with smaller inductance values can give better load transient response. For these reasons, a 10-μH to 22-μH inductance value range is recommended. Table 4 lists some recommended inductors for the TPS61170-Q1.

TPS61170-Q1 device 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 become unstable. Therefore, customers must verify operation in their application if the inductor is different from the recommended values.