l6398n STMicroelectronics, l6398n Datasheet - Page 11

• Manufacturer Part Number: l6398n
• Description: High Voltage High And Low Side Driver
• Manufacturer: STMicroelectronics

L6398
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
Figure
C
To choose the proper C
capacitor. This capacitor C
Equation 1
The ratio between the capacitors C
It has to be:
Equation 2
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 190 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:
Equation 3
BOOT
7. An internal charge pump
gate
selection and charging
is 30 nC and V
V
EXT
drop
. This charge on a 1 μF capacitor means a voltage drop of 1V.
BOOT
=
EXT
gate
I
value the external MOS can be seen as an equivalent
ch
Doc ID 18199 Rev 3
is related to the MOS total gate charge:
is 10 V, C
charge
arg
OUT
EXT
(Figure
e
is close to GND (or lower) and in the meanwhile the
R
) of the C
C
C
dson
and C
BOOT
EXT
EXT
7) provides the DMOS driving voltage.
→
is 3 nF. With C
BOOT
>>> C
=
BOOT
V
BOOT
Q
------------- -
V
drop
gate
is proportional to the cyclical voltage loss.
gate
EXT
is the time in which both conditions are
(Figure
selection has to take into account also
=
------------------ R
T
BOOT
Q
6). In the L6398 a patented
ch
gate
arg
= 100 nF the drop would be
e
DSon
dson
ON
(typical value:
is 5 ms, C
Bootstrap driver
BOOT
11/16
has