Datasheet

Bootstrap driver L6390
18/26 Doc ID 14493 Rev 7
9 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 10.a). In the L6390 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 10.b. An internal charge pump (Figure 10.b) provides the DMOS driving voltage.
9.1 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:
Equation 1
The ratio between the capacitors C
EXT
and C
BOOT
is proportional to the cyclical voltage loss.
It must be:
Equation 2
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 must be supplied for a long time, the C
BOOT
selection must also take the leakage and
quiescent losses into account.
E.g.: HVG steady-state consumption is lower than 150 μA, so if HVG T
ON
is 5 ms, C
BOOT
must supply 0.75 μC to C
EXT
. This charge on a 1 μF capacitor means a voltage drop of 0.75
V.
The internal bootstrap driver offers important advantages: the external fast recovery diode
can be avoided (it usually has a high leakage current).
This structure can work only if V
OUT
is close to GND (or lower) and, at the same time, the
LVG is on. The charging time (T
charge
) of the C
BOOT
is the time in which both conditions are
fulfilled and it must be long enough to charge the capacitor.
The bootstrap driver introduces a voltage drop due to the DMOS R
DSon
(typical value: 120
Ω). This drop can be neglected at low switching frequency, but it should be taken into
account when operating at high switching frequency.
The following equation is useful to compute the drop on the bootstrap DMOS:
Equation 3
C
EXT
Q
gate
V
gate
----------------=
V
drop
I
ch earg
R
dson
V
drop
→
Q
gate
T
ch earg
---------------------
R
dson
==