L6599A
Application information
Equation 13
Qg
VDrop = IchargeR(DS)on + VF =
R(DS)on + VF
Tcharge
where Qg is the gate charge of the external Power MOSFET, R(DS)ON is the on-resistance of
the bootstrap DMOS (150 W, typ.) and Tcharge is the ON-time of the bootstrap driver, which
equals about half the switching period minus the deadtime TD. For example, using a
MOSFET with a total gate charge of 30 nC, the drop on the bootstrap driver is about 3 V at a
switching frequency of 200 kHz:
Equation 14
30⋅10−9
2.5⋅10−6 − 0.27⋅10−6
VDrop
=
150 + 0.6 = 2.7 V
If a significant drop on the bootstrap driver is an issue, an external ultra-fast diode can be
used, therefore saving the drop on the R(DS)ON of the internal DMOS.
Doc ID 15308 Rev 7
29/35
STMICROELECTRONICS [ ST ]
