SLVSE78D March 2020 – July 2021 TPS548A29
PRODUCTION DATA
To calculate the value of the output inductor (LOUT), use Equation 10. The output capacitor filters the inductor-ripple current (IIND(ripple)). Therefore, selecting a high inductor-ripple current 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. On the other hand, larger ripple current increases output ripple voltage, but improves signal-to-noise ratio and helps to stabilize operation. Generally speaking, the inductance value should set the ripple current at approximately 15% to 40% of the maximum output current for a balanced performance.
For this design, the inductor-ripple current is set to 30% of 15-A output current. With a 0.8-MHz switching frequency, 16 V as maximum VIN, and 2.5 V as the output voltage, the Equation 10 calculated inductance is 0.586 µH. A nearest standard value of 0.80 µH is chosen.
The inductor requires a low DCR to achieve good efficiency. The inductor also requires enough room above peak inductor current before saturation. The peak inductor current is estimated using Equation 12. For this design, by selecting 4.02 kΩ as the RTRIP, IOC(valley) is set to 14.9 A, thus peak inductor current under maximum VIN is calculated as 16.65 A.
The selected inductance is a XAL7070-801MEB. This has a saturation current rating of 37.8 A, RMS current rating of 20.8 A and a DCR of 2.29 mΩ max. This inductor was selected for its low DCR to get high efficiency.