L6386
µ
. This charge on a 1µF ca-
supply 1 C to CEXT
BOOTSTRAP DRIVER
pacitor means a voltage drop of 1V.
A bootstrap circuitry is needed to supply the high
voltage section. This function is normally accom-
plished by a high voltage fast recovery diode (fig.
4a). In the L6386 a patented integrated structure
replaces the external diode. It is realized by a
high voltage DMOS, driven synchronously with
the low side driver (LVG), with in series a diode,
as shown in fig. 4b
The internal bootstrap driver gives great advan-
tages: the external fast recovery diode can be
avoided (it usually has great leakage current).
This structure can work only if VOUT is close to
GND (or lower) and in the meanwhile the LVG is
on. The charging time (Tcharge ) of the CBOOT is
the time in which both conditions are fulfilled and
it has to be long enough to charge the capacitor.
An internal charge pump (fig. 4b) provides the
DMOS driving voltage .
The diode connected in series to the DMOS has
been added to avoid undesirable turn on of it.
The bootstrap driver introduces a voltage drop
due to the DMOS RDSON (typical value: 125
Ohm). At low frequency this drop can be ne-
glected. Anyway increasing the frequency it
must be taken in to account.
CBOOT selection and charging
:
To choose the proper CBOOT value the external
MOS can be seen as an equivalent capacitor.
This capacitor CEXT is related to the MOS total
gate charge :
The following equation is useful to compute the
drop on the bootstrap DMOS:
Qgate
Tcharge
=
→
=
V
drop
Vdrop IchargeRdson
R
dson
Qgate
Vgate
=
CEXT
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.
The ratio between the capacitors CEXT and CBOOT
is proportional to the cyclical voltage loss .
It has to be:
For example: using a power MOS with a total
gate charge of 30nC the drop on the bootstrap
CBOOT>>>CEXT
µ
DMOS is about 1V, if the Tcharge is 5 s. In fact:
e.g.: if Qgate is 30nC and Vgate is 10V, CEXT is
3nF. With CBOOT = 100nF the drop would be
300mV.
30nC
µ
5 s
=
125Ω ~ 0.8V
Vdrop
Vdrop has to be taken into account when the volt-
age 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.
If HVG has to be supplied for a long time, the
CBOOT selection has to take into account also the
leakage losses.
e.g.: HVG steady state consumption is lower than
µ
200 A, so if HVG TON is 5ms, CBOOT has to
Figure 4. Bootstrap Driver.
DBOOT
VS
VBOOT
H.V.
VBOOT
VS
H.V.
HVG
LVG
HVG
CBOOT
CBOOT
VOUT
VOUT
TO LOAD
TO LOAD
LVG
D99IN1056
a
b
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