SLUSDV2A May 2020 – May 2021 BQ25798
The device has 1.5 MHz switching frequency to allow the use of small inductor (1µH) and capacitor values. It also provide the 750kHz switching frequency to achieve higher efficiency for the applications which have enough design space to accommodate the larger inductor (2.2 µH) and capacitors. Please note that the 1.5 MHz switching frequency only works with the 1µH inductor and the 750 kHz switching frequency only works with the 2.2µH inductor.
Because the converter might be either operated in the buck mode or the boost mode, so the inductor current is equal to either the charging current or the input current. The inductor saturation current should be higher than the larger value of the input current (IIN) or the charging current (ICHG) plus half the ripple current (IRIPPLE):
The inductor ripple current (IRIPPLE) depends on the input voltage (VBUS), the output voltage (VSYS), the switching frequency (FSW) and the inductance (L). The inductor current ripples for buck mode and boost mode are calculated with equations (4) and (5), respectively:
The inductor current ripple in the buck mode is usually larger than that in the boost mode, since the voltage-second applied on the inductor is larger. The maximum inductor current ripple in the buck mode happens in the vicinity of D = VSYS / VBUS = 0.5. The SYS voltage is approximately 8V for the 2s battery configuration, so the worst case for the inductor ripples is with the 15V or 20V input voltage.