SLVSDV8 July 2017 TPS54424
To calculate the value of the output inductor, use Equation 14. KIND 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 since the output capacitor must have a ripple current rating equal to or greater than the inductor ripple current. Additionally with current mode control the sensed inductor current ripple is used in the PWM modulator. Choosing small inductor ripple currents can degrade the transient response performance or introduce jitter in the high-side MOSFET on-time. The inductor ripple, KIND, is normally from 0.2 to 0.4 for the majority of applications giving a peak to peak ripple current range of 0.8 A to 1.6 A. For applications requiring operation near the minimum on-time, with on-times less than 200 ns, the target Iripple must be 1.2 A or larger for best performance. For other applications the target Iripple should be 0.8 A or larger.
For this design example, KIND = 0.3 is used and the inductor value is calculated to be 1.92 μH. The nearest standard value 1.8 µH is selected. 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 16 and Equation 17. For this design, the RMS inductor current is 4.0 A and the peak inductor current is 4.6 A. The chosen inductor is a Würth Elektronik 74438357018. It has a saturation current rating of 8.0 A (20% inductance loss) and a RMS current rating of 5.8 A (40 °C temperature rise). The DC series resistance is 18 mΩ typical.
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 in Equation 17. 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 the ratings of the inductor based on the switch current limit rather than the steady-state peak inductor current.