l6386ad013tr STMicroelectronics, l6386ad013tr Datasheet - Page 8

• Manufacturer Part Number: l6386ad013tr
• Description: High-voltage High And Low Side Driver
• Manufacturer: STMicroelectronics

Bootstrap driver
4
4.1
8/17
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 in series a diode, as shown in
Figure 4
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 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 great advantages: 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
125 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
b. An internal charge pump
selection and charging
is 30 nC and V
is the gate charge of the external power MOS, R
EXT
. This charge on a 1 µF capacitor means a voltage drop of 1 V.
BOOT
charge
V
drop
EXT
gate
value the external MOS can be seen as an equivalent
is the charging time of the bootstrap capacitor.
=
is related to the MOS total gate charge:
is 10 V, C
charge
OUT
I
ch
EXT
arg
is close to GND (or lower) and in the meanwhile the
(Figure 4
) of the C
e
and C
R
C
C
EXT
dson
EXT
BOOT
is 3 nF. With C
BOOT
→
>>>C
=
b) provides the DMOS driving voltage. The
V
BOOT
Q
-------------- -
V
BOOT
drop
gate
gate
EXT
is proportional to the cyclical voltage loss.
(Figure 4
is the time in which both conditions are
=
selection has to take into account also
------------------ - R
T
Q
ch
gate
arg
BOOT
e
dson
a). In the L6386AD a patented
dson
= 100 nF the drop would be
is the on resistance of the
DS(on)
ON
is 5ms, C
(typical value:
L6386AD
BOOT
has