Datasheet

Bootstrap driver L6386E
8/18 Doc ID 13989 Rev 2
4 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 (Figure 4 a). In the L6386E a patented
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 b. An internal charge pump (Figure 4 b) provides the DMOS driving voltage. The
diode connected in series to the DMOS has been added to avoid undesirable turn on of it.
4.1 C
BOOT
selection and charging
To choose the proper C
BOOT
value the external MOS can be seen as an equivalent
capacitor. This capacitor C
EXT
is related to the MOS total gate charge:
The ratio between the capacitors C
EXT
and C
BOOT
is proportional to the cyclical voltage loss.
It has to be:
C
BOOT
>>>C
EXT
e.g.: if Q
gate
is 30 nC and V
gate
is 10 V, C
EXT
is 3 nF. With C
BOOT
= 100 nF the drop would be
300 mV.
If HVG has to be supplied for a long time, the C
BOOT
selection has to take into account also
the leakage losses.
e.g.: HVG steady state consumption is lower than 200 μA, so if HVG T
ON
is 5 ms, C
BOOT
has
to supply 1 μC to C
EXT
. This charge on a 1 μF capacitor means a voltage drop of 1 V.
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
OUT
is close to GND (or lower) and in the meanwhile the
LVG is on. The charging time (T
charge
) of the C
BOOT
is the time in which both conditions are
fulfilled and it has to be long enough to charge the capacitor.
The bootstrap driver introduces a voltage drop due to the DMOS R
DSon
(typical value: 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
gate
is the gate charge of the external power MOS, R
DSon
is the on resistance of the
bootstrap DMOS, and T
charge
is the charging time of the bootstrap capacitor.
C
EXT
Q
gate
V
gate
-------------- -=
V
drop
I
ch earg
R
dson
V
drop
→
Q
gate
T
ch earg
-------------------
R
dson
==