10.2.2.5 Inductor Selection

As for all switch mode power supplies two main passive components are required for storing the energy during operation. This is done by an inductor and an output capacitor. The inductor must be connected between VIN and SW pin to make sure that the TPS61251 device operates. To select the right inductor current rating the programmed input current limit as well as the current ripple through the inductor is necessary. Estimation of the maximum peak inductor current can be done using Equation 7.

Equation 7.

Regarding the example from above the current ripple (ΔI_L) will be 290 mA and therefore an inductor with a rated current of about 800 mA should be used.

The TPS61251 device is designed to work with inductor values between 1 µH and 2.2 µH. TI recommends a 1.5 µH inductor for typical applications. In space constrained applications, it might be possible to consider smaller inductor values depending on the targeted inductor ripple current. Therefore, the inductor value can be reduced down to 1 µH without degrading the stability.

In regular boost converter designs the current through the inductor is defined by the switch current limit of the converters switches and therefore bigger inductors have to be chosen. The TPS61251 device allows the design engineer to reduce the current limit to the needs of the application regardless the maximum switch current limit of the converter. Programming a lower current value allows the use of smaller inductors without the danger to get into saturation.