l6390 STMicroelectronics, l6390 Datasheet - Page 17

• Manufacturer Part Number: l6390
• Description: High-voltage High And Low Side Driver
• Manufacturer: STMicroelectronics

L6390
9
9.1
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
7.b.
An internal charge pump
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 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
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:
BOOT
gate
selection and charging
is 30 nC and V
EXT
. This charge on a 1 µF capacitor means a voltage drop of 1V.
BOOT
(Figure
EXT
gate
value the external MOS can be seen as an equivalent
is related to the MOS total gate charge:
is 10 V, C
charge
OUT
7.b) provides the DMOS driving voltage.
EXT
is close to GND (or lower) and in the meanwhile the
C
C
) of the C
and C
BOOT
EXT
EXT
is 3 nF. With C
BOOT
>>> C
=
BOOT
BOOT
Q
------------- -
V
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
BOOT
7.a). In the L6390 a patented
= 100 nF the drop would be
DSON
ON
is 5 ms, C
(typical value:
Bootstrap driver
BOOT
Figure
17/22
has