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Chapter 17 Gate Drive Unit (GDUV4)
MC9S12ZVM Family Reference Manual Rev. 1.3
Freescale Semiconductor
661
17.5.2
Calculation of Bootstrap Capacitor
The size of the bootstrap capacitor C
BS
depends on the total gate charge Q
G
needed to turn on the power
FET used in the application. If the bootstrap capacitor is too small there can be a large voltage drop due to
charge sharing between bootstrap capacitor C
BS
and the total gate capacitance of the power FET C
G
. The
resulting voltage on the gate of the power FET can be calculated as follow:
Eqn. 17-1
For example if C
BS
= 20 C
G
then the resulting gate voltage is V
G
= 0.95 V
BS
.
VG
QBS
CBS CG
+
--------------------------
VBS
1
CG
CBS
-----------
+
--------------------
=
=