l6393 STMicroelectronics, l6393 Datasheet - Page 14

• Manufacturer Part Number: l6393
• Description: Half-bridge Gate Driver
• Manufacturer: STMicroelectronics

Bootstrap driver
8
8.1
14/20
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
The diode connected in series to the DMOS has been added to avoid undesirable turn on of
it.
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 a great 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:
BOOT
gate
selection and charging
is 30nC and V
V
EXT
drop
. This charge on a 1 µF capacitor means a voltage drop of 1 V.
BOOT
(Figure
gate
EXT
=
value the external MOS can be seen as an equivalent
I
is 10 V, C
ch
is related to the MOS total gate charge:
charge
OUT
arg
6.b) provides the DMOS driving voltage.
EXT
e
is close to GND (or lower) and in the meanwhile the
C
C
R
) of the C
and C
BOOT
EXT
dson
EXT
is 3 nF. With C
BOOT
>>> C
=
→
BOOT
BOOT
Q
------------- -
V
V
gate
is proportional to the cyclical voltage loss.
gate
drop
EXT
(Figure
is the time in which both conditions are
selection has to take into account also
=
BOOT
------------------ R
T
6.a). In the L6393 a patented
Q
ch
gate
arg
= 100 nF the drop would be
e
DSon
ON
dson
(typical value:
is 5 ms, C
BOOT
Figure
L6393
has