L6391 Bootstrap driver
A bootstrap circuitry is needed to supply the high voltage section. This function is usually accomplished by a high voltage fast recovery diode (
Figure 10
). In the L6391 device 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 diode in series, as shown in
Figure 11 . An internal charge pump ( Figure 11 ) provides the DMOS driving voltage.
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:
Equation 1
C
EXT
=
Q
V gate
The ratio between the capacitors C
EXT
It has to be:
and C
BOOT
is proportional to the cyclical voltage loss.
Equation 2
C
BOOT
>>>C
EXT if Q gate
is 30 nC and V gate
is 10 V, C
EXT
is 3 nF. With C
BOOT
= 100 nF the drop is 300 mV.
If HVG has to be supplied for a long time, the C
BOOT the leakage and quiescent losses.
selection has also to take into account
HVG steady-state consumption is lower than 240 supply C
EXT
A, so if HVG T
ON
is 5 ms, C
BOOT
has to with 1.2 C. This charge on a 1 F capacitor means a voltage drop of 1.2 V.
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
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
DS(on)
(typical value:
120 ). At low frequency this drop can be neglected. Anyway, the rise of frequency has to take into account.
The following equation is useful to compute the drop on the bootstrap DMOS:
Equation 3
V drop
=
I ch arg e
R
DS on
V drop
=
T
Q ch arg e
R
DS on where Q gate
is the gate charge of the external power MOS, the bootstrap DMOS and T charge
R
DS(on)
is the on resistance of
is the charging time of the bootstrap capacitor.
DocID17892 Rev 3 17/23
23
Bootstrap driver L6391
For example: using a power MOS with a total gate charge of 30 nC the drop on the bootstrap DMOS is about 1 V, if the T charge
is 5 s. In fact:
Equation 4
V drop
=
30nC
5 s
120 0.7V
V drop
has to be taken into account when the voltage drop on C
BOOT
is calculated: if this drop is too high, or the circuit topology doesn’t allow a sufficient charging time, an external diode can be used.
Figure 10. Bootstrap driver with high voltage fast recovery diode
D
BOOT
V
CC
BOOT
H.V.
HVG
C
BOOT
OUT
TO LOAD
LVG
Figure 11. Bootstrap driver with internal charge pump
V
CC
BOOT
H.V.
HVG
C
BOOT
OUT
TO LOAD
LVG
18/23 DocID17892 Rev 3
