L6385 データシートの表示（PDF） - STMicroelectronics
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L6385 Datasheet PDF : 9 Pages
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L6385
leakage losses.
e.g.: HVG steady state consumption is lower than
200µA, so if HVG TON is 5ms, CBOOT has to
supply 1µC to CEXT. This charge on a 1µF ca-
pacitor means a voltage drop of 1V.
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.
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.
The following equation is useful to compute the
Figure 4. Bootstrap Driver.
drop on the bootstrap DMOS:
Vdrop = IchargeRdson → Vdrop = TQchgaartgeeRdson
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
=
30nC
5µs
⋅
125Ω
~
0.8V
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.
DBOOT
VS
HVG
LVG
VBOOT
H.V.
VOUT
VS
CBOOT
TO LOAD
HVG
LVG
VBOOT
H.V.
VOUT
CBOOT
TO LOAD
a
b
D99IN1056
Figure 5. Turn On Time vs. Temperature
250
@ Vcc = 15V
200
150
Typ.
100
50
0
-45 -25 0
25 50 75 100 125
Tj (°C)
Figure 6. Turn Off Time vs. Temperature
250
@ Vcc = 15V
200
150
Typ.
100
50
0
-45 -25 0
25 50 75 100 125
Tj (°C)
5/9
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