L6390_12 STMICROELECTRONICS [STMicroelectronics], L6390_12 Datasheet - Page 18

• Manufacturer Part Number: L6390_12
• Description: High-voltage high/low-side driver
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

Bootstrap driver
9
9.1
18/26
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
Figure
C
To choose the proper C
capacitor. This capacitor C
Equation 1
The ratio between the capacitors C
It must be:
Equation 2
E.g.: if Q
300 mV.
If HVG must be supplied for a long time, the C
quiescent losses into account.
E.g.: HVG steady-state consumption is lower than 150 μA, so if HVG T
must supply 0.75 μC to C
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
LVG is on. The charging time (T
fulfilled and it must be long enough to charge the capacitor.
The bootstrap driver introduces a voltage drop due to the DMOS R
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
BOOT
10.b. An internal charge pump
gate
selection and charging
is 30 nC and V
V
drop
BOOT
EXT
EXT
=
gate
. This charge on a 1 μF capacitor means a voltage drop of 0.75
value the external MOS can be seen as an equivalent
I
ch
Doc ID 14493 Rev 7
is related to the MOS total gate charge:
is 10 V, C
charge
OUT
arg
EXT
e
C
is close to GND (or lower) and, at the same time, the
R
) of the C
C
(Figure
EXT
and C
dson
BOOT
EXT
=
→
is 3 nF. With C
BOOT
>>> C
BOOT
10.b) provides the DMOS driving voltage.
Q
--------------- -
V
V
BOOT
drop
gate
gate
is proportional to the cyclical voltage loss.
selection must also take the leakage and
EXT
is the time in which both conditions are
(Figure
=
--------------------- R
T
Q
ch
BOOT
gate
arg
10.a). In the L6390 a patented
e
= 100 nF the drop would be
dson
DSon
ON
(typical value: 120
is 5 ms, C
BOOT
L6390