8.2.2.3 Output Inductor Selection (L_O)

To calculate the minimum value of the output inductor, use Equation 28.

K_IND is a ratio 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 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, the following guidelines may be used.

For designs using low ESR output capacitors such as ceramics, a value as high as K_IND = 0.3 may be desirable. When using higher ESR output capacitors, K_IND = 0.2 yields better results. Because the inductor ripple current is part of the current mode PWM control system, the inductor ripple current must always be greater than 150 mA for stable PWM operation. In a wide input-voltage regulator, it is best to choose relatively large inductor-ripple current. This provides sufficienct ripple current with the input voltage at the minimum.

For this design example, K_IND = 0.3, and the inductor value is calculated to be 7.6 μH. The nearest standard value is 7.2 μH. It is important that the RMS current and saturation current ratings of the inductor not be exceeded. The RMS and peak inductor current can be found from Equation 30 and Equation 31. For this design, the RMS inductor current is 5 A, and the peak inductor current is 5.8 A. The chosen inductor is a WE 7447798720, which has a saturation current rating of 7.9 A and an RMS current rating of 6 A.

As the equation set demonstrates, lower ripple currents reduce the output voltage ripple of the regulator but require a larger value of inductance. Selecting higher ripple currents increases the output voltage ripple of the regulator but allow for a lower inductance value.

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 peak inductor current level previously calculated. In transient conditions, the inductor current can increase up to the switch-current limit of the device. For this reason, the most conservative design approach is to choose an inductor with a saturation current rating equal to or greater than the switch-current limit of the TPS54560 which is nominally 7.5 A.

Equation 28.

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Equation 29.

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Equation 30.

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Equation 31.