l6392 STMicroelectronics, l6392 Datasheet - Page 13

• Manufacturer Part Number: l6392
• Description: High-voltage High And Low Side Driver
• Manufacturer: STMicroelectronics

L6392
8
8.1
Bootstrap driver
A bootstrap circuitry is needed to supply the high voltage section. This function is normally
accomplished by a high voltage fast recovery diode
integrated structure replaces the external diode. It is realized by a high voltage DMOS,
driven synchronously with the low side driver (LVG), with diode in series, as shown in
6.b.
An internal charge pump
C
To choose the proper C
capacitor. This capacitor C
The ratio between the capacitors C
It has to be:
e.g.: if Q
300 mV.
If HVG has to be supplied for a long time, the C
the leakage and quiescent losses.
e.g.: HVG steady state consumption is lower than 200 µA, so if HVG T
to supply 1 µC to C
The internal bootstrap driver gives agreat advantage: the external fast recovery diode can
be avoided (it usually has great leakage current).
This structure can work only if V
LVG is on. The charging time (T
fulfilled and it has to be long enough to charge the capacitor.
The bootstrap driver introduces a voltage drop due to the DMOS R
120 Ω). At low frequency 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:
where Q
bootstrap DMOS, and T
BOOT
gate
gate
selection and charging
is 30 nC and V
is the gate charge of the external power MOS, R
V
EXT
drop
. This charge on a 1µF capacitor means a voltage drop of 1 V.
BOOT
charge
(Figure
=
EXT
gate
I
value the external MOS can be seen as an equivalent
ch
is the charging time of the bootstrap capacitor.
is related to the MOS total gate charge:
is 10 V, C
charge
arg
OUT
6.b) provides the DMOS driving voltage.
EXT
e
R
is close to GND (or lower) and in the meanwhile the
C
C
) of the C
dson
and C
BOOT
EXT
EXT
→
is 3 nF. With C
BOOT
>>> C
=
BOOT
V
BOOT
Q
------------- -
V
drop
gate
is proportional to the cyclical voltage loss.
gate
EXT
(Figure
is the time in which both conditions are
selection has to take into account also
=
------------------ R
T
BOOT
Q
ch
6.a). In the L6392 a patented
dson
gate
arg
= 100 nF the drop would be
is the on resistance of the
e
DSON
dson
ON
is 5 ms, C
(typical value:
Bootstrap driver
BOOT
Figure
13/19
has