SLVS875C January 2009 – November 2014 TPS54332
To calculate the minimum value of the output inductor, use Equation 8.
KIND is a coefficient that represents the amount of inductor ripple current relative to the maximum output current. In general, this value is at the discretion of the designer; however, the following guidelines may be used. For designs using low-ESR output capacitors such as ceramics, a value as high as KIND = 0.4 may be used. When using higher ESR output capacitors, KIND = 0.2 yields better results.
For this design example, use KIND = 0.3 and the minimum inductor value is calculated to be 2.48 μH. For this design, a l 2.5-μH inductor is chosen.
For the output filter inductor, it is important that the RMS current and saturation current ratings not be exceeded. The peak-to-peak inductor current is calculated using Equation 9.
The RMS inductor current can be found from Equation 10.
And the peak inductor current can be determined with Equation 11.
For this design, the RMS inductor current is 3.51 A and the peak inductor current is 4.15 A. The chosen inductor is a Coilcraft MSS1038-252NX_ 2.5-μH. It has a saturation current rating of 7.62 A and an RMS current rating of 6.55 A, meeting these requirements. Smaller or larger inductor values can be used depending on the amount of ripple current the designer wishes to allow so long as the other design requirements are met. Larger value inductors will have lower AC current and result in lower output voltage ripple, while smaller inductor values will increase ac current and output voltage ripple. In general, inductor values for use with the TPS54332 are in the range of 1 μH to 47 μH.