L6388 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6388' PDF : 11 Pages View PDF
L6388
has to be long enough to charge the capacitor.
The bootstrap driver introduces a voltage drop due to the DMOS RDSON (typical value: 125 Ohm). At low fre-
quency this drop can be neglected. Anyway increasing the frequency it must be taken in to account.
The following equation is useful to compute the drop on the bootstrap DMOS:
Vdrop
=
Ich argeRdson → Vdrop
=
---Q-----g---a---t--e---
Tch arg e
Rd
so
n
where Qgate is the gate charge of the external power MOS, Rdson is the on resistance of the bootstrap DMOS,
and Tcharge is the charging time of the bootstrap capacitor.
For example: using a power MOS with a total gate charge of 30nC the drop on the bootstrap DMOS is about
1V, if the Tcharge is 5µs. In fact:
Vdrop = 3---5-0---µ-n---sC--- ⋅ 125Ω ∼ 0.8V
Vdrop has to be taken into account when the voltage drop on CBOOT is calculated: if this drop is too high, or the
circuit topology doesn't allow a sufficient charging time, an external diode can be used.
Figure 8. Bootstrap Driver.
DBOOT
VS
VBOOT
H.V.
HVG
VOUT
CBOOT
TO LOAD
LVG
a
VS
VBOOT
H.V.
HVG
VOUT
CBOOT
TO LOAD
LVG
b
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