8.2.2.1 Inductor Selection

The TPS61085A-Q1 is designed to work with a wide range of inductors. The main parameter for the inductor selection is the saturation current of the inductor which must be higher than the peak switch current as calculated in Detailed Design Procedure with additional margin to cover for heavy load transients. An alternative, more conservative option is to choose an inductor with a saturation current at least as high as the maximum switch current limit of 3.2 A. The other important parameter is the inductor DC resistance. Usually, the lower the DC resistance the higher the efficiency. It is important to note that the inductor DC resistance is not the only parameter determining the efficiency. Especially for a boost converter where the inductor is the energy storage element, the type and core material of the inductor influences the efficiency as well. At high switching frequencies of 1.2-MHz inductor core losses, proximity effects and skin effects become more important. Usually, an inductor with a larger form factor gives higher efficiency. The efficiency difference between different inductors can vary between 2% to 10%. For the TPS61085A-Q1, inductor values between 3 µH and 6 µH are a good choice with a switching frequency of 1.2 MHz, typically 3.3 µH. At 650 kHz, TI recommends inductors between 6 µH and 13 µH, typically 6.8 µH. Table 3 shows a few inductors. Customers must verify and validate these components for suitability with their application before using them.

Typically, TI recommends the inductor current ripple is below 20% of the average inductor current. Calculate the inductor value using Equation 4.

Equation 4.

where

• L is the inductor value
• V_IN is input voltage
• V_S is boost output voltage
• η is efficiency
• I_{out_max} is the maximum output current
• f is frequency