The selection of the inductor affects the steady state of the power supply operation, transient behavior, loop stability, and boost converter efficiency, the inductor is the most important component in switching power regulator design. The three most important specifications to the performance of the inductor are the inductance value, DC resistance, and saturation current.

The TPS61377 is designed to work with inductor values between 2.2 µH and 10 µH. A 2.2 µH inductor is typically available in a smaller or lower-profile package, while a 10-µH inductor produces lower inductor current ripple. If the boost output current is limited by the peak current protection of the IC, using a inductor with bigger inductance can maximize the output current capability of the converter.

Inductance values can be ±20% or even ±30% of the value at 0 A bias. When the inductor current approaches saturation level, its inductance can decrease 20% to 35% from the value at 0-A bias current, depending on how the inductor vendor defines saturation current. When selecting an inductor, make sure its rated current, especially the saturation current, is larger than boost converter peak current under all operating conditions.

Normally, it is advisable that the inductor peak-to-peak current is less than 40% of the average inductor current at maximum output current. Follow Equation 5 to Equation 7 to calculate the average, peak and ripple 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 the application. To leave enough design margin, TI recommends using the minimum switching frequency, the inductance with -30% tolerance, and a low power conversion efficiency for the calculation.

In a boost regulator, calculate the inductor DC current as in Equation 5.

Equation 5.

where

• V_OUT is the output voltage of the boost regulator.
• I_OUT is the output current of the boost regulator.
• V_IN is the input voltage of the boost regulator.
• η is the power conversion efficiency.

Calculate the inductor current peak-to-peak ripple as in Equation 6.

Equation 6.

where

• I_PP is the inductor peak-to-peak ripple.
• L is the inductor value.
• ƒ_SW is the switching frequency.
• V_OUT is the output voltage.
• V_IN is the input voltage.

Therefore, the peak current, I_Lpeak, seen by the inductor is calculated with Equation 7.

Equation 7.

It is important that the peak current does not exceed the inductor saturation current.

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. Usually, a data sheet of an inductor does not provide the core loss information. If needed, consult the inductor vendor for detailed information. An inductor with lower DCR is basically recommended for higher efficiency. However, it is usually a tradeoff between the loss and foot print. The table below lists some recommended inductors.