MB39A214APFT-G-JNERE1 Datasheet, PDF (40/56 Pages) Fujitsu Component Limited. – 2ch DC/DC converter IC with PFM/ PWM synchronous rectification

MB39A214A
Selection of bootstrap capacitor
To drive the gate of high-side FET, the bootstrap capacitor must have enough stored charge. 0.1 µF is
assumed to be standard, however, it is necessary to adjust it when the high-side FET Q_G is big. Consider the
capacitor value calculated by the following formula as the lowest value for the bootstrap capacitor and
select a thing any more.
C_BST ≥ 10¯Q_G
C_BST : Bootstrap capacitor value [F]
Q_G : Total quantity of charge for the high-side FET gate [C]
Calculate ratings of the bootstrap capacitor by the following formula:
V_CBST > V_B
V_CBST : Withstand voltage of the bootstrap capacitor [V]
V_B
: VB voltage [V]
VB pin capacitor
4.7 µF is assumed to be a standard, and when Q_G of switching FET used is large, it is necessary to adjust it.
To suppress the ripple voltage by the switching FET gate drive, consider the capacitor value calculated by
the following formula as the lowest value for VB capacitor and select a thing any more.
C_VB ≥ 50¯Q_G
C_VB : VB pin capacitor value [F]
Q_G : Total amount of gate charge of high-side FET and low-side switching FET for 2CH [C]
Calculate ratings of the VB pin capacitor by the following formula:
V_CVB > V_B
V_CVB
V_B
: Withstand voltage of the VB pin capacitor [V]
: VB voltage [V]
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DS405-00007-1v0-E