FREQ datasheet(40/56 Pages) FUJITSU

40 / 56 page

MB39A214A

Limitation : development tool vendor use only

FUJITSU SEMICONDUCTOR CONFIDENTIAL

DS405-00007-1v0-E

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 QG 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.

CBST ≥ 10 QG

CBST : Bootstrap capacitor value [F]

: Total quantity of charge for the high-side FET gate [C]

Calculate ratings of the bootstrap capacitor by the following formula:

VCBST > VB

VCBST

: Withstand voltage of the bootstrap capacitor [V]

: VB voltage [V]

VB pin capacitor

4.7 µF is assumed to be a standard, and when QG 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.

CVB ≥ 50 QG

CVB

: VB pin capacitor value [F]

: 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:

VCVB > VB

VCVB

: Withstand voltage of the VB pin capacitor [V]

: VB voltage [V]

FREQ Datasheet(PDF) 40 Page - fujitsu semiconductor