ZHCSOA1A July 2021 – July 2021 TPS548B27
PRODUCTION DATA
To calculate the value of the output inductor (LOUT), use Equation 9. 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 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 20-A output current. With a 600-kHz switching frequency, 16 V as maximum VIN, and 1.0 V as the output voltage, the calculated inductance is 0.260 µH.
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 11. For this design, by selecting 5.24kΩ as the RTRIP, IOC(valley) is set to 23 A, thus peak inductor current under maximum VIN is calculated as 5.208A.
The selected inductance is a Coilcraft XAL7070-301MEB. This has a saturation current rating of 55.6 A, RMS current rating of 26.1 A, and a DCR of 1.17-mΩ max. This inductor was selected for its low DCR to get high efficiency.