SLVS839F July 2008 – October 2014 TPS54331
To calculate the minimum value of the output inductor, use Equation 8.
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 = 0.3 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 as 5.7 μH. For this design, a large value was selected: 6.8 μH.
For the output filter inductor, do not exceed the RMS current and saturation current ratings. Use Equation 9 to calculate the inductor ripple current (ILPP).
Use Equation 10 to calculate the RMS inductor current.
Use Equation 11 to calculate the peak inductor current.
For this design, the RMS inductor current is 3.01 A and the peak inductor current is 3.47 A. The selected inductor is a Sumida CDRH103-6R8, 6.8 μH. This inductor has a saturation current rating of 3.84 A and an RMS current rating of 3.6 A, which meets these requirements. Smaller or larger inductor values can be used depending on the amount of ripple current the designer wants 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 TPS54331 device are in the range of 6.8 to 47 μH.