BOOTSTRAP DRIVER
A bootstrap circuitry is needed to supply the high
voltage section. This function is normally accom-
plished by a high voltage fast recovery diode (fig.
4a). In the L6384 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 fig. 4b
An internal charge pump (fig. 4b) provides the
DMOS driving voltage .
The diode connected in series to the DMOS has
been added to avoid undesirable turn on of it.
CBOOT 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 :
C
EXT
=
Q
gate
V
gate
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 30nC and V
gate
is 10V, C
EXT
is
3nF. With C
BOOT
= 100nF the drop would be
300mV.
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
µ
A, so if HVG T
ON
is 5ms, C
BOOT
has to
supply 1
µ
C to C
EXT
. This charge on a 1
µ
F ca-
pacitor means a voltage drop of 1V.
The internal bootstrap driver gives great advan-
tages: 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
Ohm). At low frequency this drop can be ne-
glected. Anyway increasing the frequency it
must be taken in to account.
The following equation is useful to compute the
drop on the bootstrap DMOS:
V
drop
=
I
charge
R
dson
→
V
drop
=
Q
gate
T
charge
R
dson
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.
For example: using a power MOS with a total
gate charge of 30nC the drop on the bootstrap
DMOS is about 1V, if the T
charge
is 5
µ
s. In fact:
V
drop
=
30nC
5
µ
s
⋅
125
Ω
~ 0.8V
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.
For both high and low side buffers @25˚C Tamb
0 1 2 3 4 5 C (nF)
0
50
100
150
200
250
time
(nsec)
Tr
D99IN1015
Tf
Figure 2. Typical Rise and Fall Times vs.
Load Capacitance
02468101214V
S
(V)
10
10
2
10
3
10
4
Iq
(µA)
D99IN1016
Figure 3. Quiescent Current vs. Supply
Voltage
L6384
5/10
