JAJSI75A April 2018 – November 2019 TPS57112C-Q1
PRODUCTION DATA.
The inductor selected works for the entire TPS57112C-Q1 input-voltage range. To calculate the value of the output inductor, use Equation 21. The k(IND) coefficient represents the amount of ripple current in the inductor relative to the maximum output current. The output capacitor filters the inductor ripple current. Therefore, choosing high inductor ripple currents impacts the selection of the output capacitor, because the output capacitor must have a ripple-current rating equal to or greater than the inductor ripple current. In general, the inductor ripple value is at the discretion of the designer; however, k(IND) is normally from 0.1 to 0.3 for the majority of applications.
For this design example, use k(IND) = 0.3; the calculated value of the inductor is 2.2 µH. For this design, the choice is a nearest standard value of 1.5 μH. For the output-filter inductor, it is important not to exceed the rms current and saturation current ratings. Use Equation 23 and Equation 24 to find the rms and peak inductor currents.
For this design, the rms inductor current is 2 A and the peak inductor current is 2.42 A. The chosen inductor is a Coilcraft XLA4020-152ME_. It has a saturation current rating of 9.6 A and an rms current rating of 7.5 A.
The current flowing through the inductor is the inductor ripple current plus the output current. During power up, faults, or transient load conditions, the inductor current can increase above the calculated peak inductor current level calculated previously. In transient conditions, the inductor current can increase up to the switch-current limit of the device. For this reason, the most conservative approach is to specify an inductor with a saturation current rating equal to or greater than the switch-current limit rather than the peak inductor current.