MB39A132QN-G-ERE1 Datasheet, PDF (41/56 Pages) Fujitsu Component Limited. – ASSP For Power Management Applications (Rechargeable Battery) Synchronous Rectification DC/DC Converter IC for Charging Li-ion Battery

MB39A132
• Bootstrap diode selection
Select a Schottky barrier diode (SBD) that has a small forward voltage drop.
The current to drive the gate of High-side FET flows to the SBD of the bootstrap circuit. The average current
can be found by the following formula. Select a bootstrap diode that keep the average current below the
current rating.
ID ≥ Qg × fOSC
ID : Forward current [A]
Qg : FET total gate electric charge of high-side [C]
fOSC : Oscillation frequency [Hz]
The rating of the bootstrap diode can be found by the following formula.
VR_BOOT > VIN
VR_BOOT : Bootstrap diode DC reverse voltage [V]
VIN : Switching power supply voltage [V]
• Bootstrap capacitor selection
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 10 times Qg of the high-side FET as the bootstrap capacitor.
Qg
CBOOT ≥ 10 ×
VB
CBOOT: Bootstrap capacitance [F]
Qg : Withstand voltage FET gate charge [C]
VB : VB voltage [V]
The rating of bootstrap capacitor can be found by the following formula.
VCBOOT > VIN
VCBOOT : Rating of bootstrap capacitor [V]
VIN : Switching power supply voltage [V]
DS04–27265–3E
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