8.2.2.3 Output Inductor Selection

To calculate the value of the output inductor, use Equation 10. K_IND is a coefficient that represents the amount of inductor ripple current relative to the maximum output current. The inductor ripple current is filtered by the output capacitor. Therefore, choosing high inductor ripple currents impact 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. For this part, TI recommends the range of K_IND from 30% to 50%.

Equation 10.

where

• I_OUT = 3 A, the rated output current of the device

For this design example, use K_IND = 50% and the inductor value is calculated to be 5.28 μH. For this design, a nearest standard value was chosen: 5.6 μH. For the output filter inductor, it is important that the RMS current and saturation current ratings not be exceeded. The inductor peak-to-peak ripple current, peak current and RMS current are calculated using Equation 11, Equation 12, and Equation 13.

Equation 11.

Equation 12.

Equation 13.

For this design example, the calculated peak current is 4 A and the calculated RMS current is 3.03 A. The chosen inductor is a Vishay-Dale IHLP3232DZER5R6M11 5.6-μH. It has a saturation current rating of 7.6 A and a RMS current rating of 7.4 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 above. 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.