A boost converter normally requires two main passive components for storing energy during power conversion: an inductor and an output capacitor. The inductor affects the steady state efficiency (including the ripple and efficiency), transient behavior, and loop stability, which makes the inductor the most critical component in application.

When selecting the inductor and the inductance, the other important parameters are:

• Maximum current rating (consider RMS and peak current)
• Series resistance
• Operating temperature

The TPS61382A-Q1 has built-in slope compensation to avoid subharmonic oscillation associated with current mode control. If the inductor value is too low and makes the inductor peak-to-peak ripple higher than 6A, the slew rate of the slope compensation cannot be adequate, and the loop can be unstable. Therefore, it is recommended to make the peak-to-peak current ripple is from 1A to 3A when selecting the inductor.

The inductance can be calculated by:

As a result, TI recommends 0.47μH for 2.2MHz switching frequency.

The current flowing through the inductor is the inductor ripple current plus the average input current. During power up, load faults, or transient load conditions, the inductor current can increase above the peak inductor current calculated.

Inductor values can have ±20%, or even ±30%, tolerance with no current bias. When the inductor current approaches the saturation level, the inductance can decrease 20% to 35% from the value at 0A bias current, depending on how the inductor vendor defines saturation. When selecting an inductor, make sure the rated current, especially the saturation current, is larger than the peak current during the operation.

The inductor peak current varies as a function of the load, switching frequency, and input and output voltages. The peak current can be calculated by:

Select the inductor with a saturation current rating higher than the maximum inductor current.

where

• I_peak is the peak current of the inductor
• I_OUT is the output current
• D is the duty cycle
• eff is the efficiency
• V_BUB is the input voltage
• L is the inductance
• ƒ_SW is the switching frequency

The heat rating current (RMS) is can be calculated with:

where

• I_LRMS is the RMS current of the inductor
• I_BUB is the input current of the inductor
• ΔI_L is the ripple current of the inductor

It is important that the peak current does not exceed the inductor saturation current and the RMS current is not over the temperature-related rating current of the inductors.

For a given physical inductor size, increasing inductance usually results in an inductor with lower saturation current. The total losses of the coil consists of the DC resistance (DCR) loss and the following frequency-dependent loss:

• The losses in the core material (magnetic hysteresis loss, especially at high switching frequencies)
• Additional losses in the conductor from the skin effect (current displacement at high frequencies)
• Magnetic field losses of the neighboring windings (proximity effect)

For a certain inductor, the larger current ripple (smaller inductor) generates the higher DC and also the frequency-dependent loss. An inductor with lower DCR is basically recommended for higher efficiency. However, it is usually a tradeoff between the loss and footprint. Table 8-3 lists some recommended inductors. In this application example, the Coilcraft™ inductor XGL6060-471 is selected for the small size, high saturation current, and small DCR.