MB39A132QN-G-ERE1 Datasheet, PDF (42/56 Pages) Fujitsu Component Limited. – ASSP For Power Management Applications (Rechargeable Battery) Synchronous Rectification DC/DC Converter IC for Charging Li-ion Battery

MB39A132
• VB capacitor
Although the typical capacitance value for a VB capacitor is 1 μF, it has to be adjusted if the switching FET
used has a large Qg. The bootstrap capacitor needs to be sufficiently charged to drive the gate of the high-
side FET. Therefore, select a capacitor that can store charge at least 100 times the total of Qg of the high-
side FET and Qg of the low-side switching FET as the VB capacitor.
Qg
CVB ≥ 100 ×
VB
CVB : VB pin capacitance [F]
Qg : Total gate charge of high-side FET and low-side switching FET [C]
VB : VB voltage [V]
The rating of VB capacitor can be found by the formula.
VCVB > VB
VCVB : Withstand voltage of VB capacitor [V]
VB :VB voltage [V]
42
DS04–27265–3E