STMicroelectronics 19V - 90W Adapter with PFC for Laptop computers using the L6563H and L6699 EVL6699-90WADP EVL6699-90WADP Ficha De Dados
Códigos do produto
EVL6699-90WADP
L6699
Bootstrap section
Doc ID 022835 Rev 2
33/38
This concern applies to converters designed with a high resonance frequency (indicatively,
> 150 kHz), so that they run at high frequency also at full load. Otherwise, the converter runs
at high frequency at light load, where the current flowing in the MOSFETs of the half bridge
leg is low, so that, generally, a slight R
> 150 kHz), so that they run at high frequency also at full load. Otherwise, the converter runs
at high frequency at light load, where the current flowing in the MOSFETs of the half bridge
leg is low, so that, generally, a slight R
DS(on)
rise is not an issue. However, it is wise to check
this point anyway and the following equation is useful to estimate the drop on the bootstrap
driver:
driver:
Equation 14
where Qg is the gate charge of the external Power MOSFET, R
DS(on)
is the ON-resistance of
the bootstrap DMOS (150
Ω
, typ.) and T
charge
is the ON-time of the bootstrap driver, which
equals about half the switching period minus the deadtime T
D
. For example, using a
MOSFET with a total gate charge of 30 nC, the drop on the bootstrap driver is about 3 V at a
switching frequency of 200 kHz and with a deadtime of 300 ns:
switching frequency of 200 kHz and with a deadtime of 300 ns:
Equation 15
If a significant drop on the bootstrap driver is an issue, an external ultra-fast diode can be
used, therefore saving the drop on the R
used, therefore saving the drop on the R
DS(on)
of the internal DMOS. In this case it is also
recommended to add a small resistor (in the ten
Ω) in series to the diode to prevent the
bootstrap capacitor from being charged to a voltage exceeding the absolute maximum rating
(18 V), in case of significant negative spikes on the half bridge midpoint when the high-side
MOSFET turns off.
(18 V), in case of significant negative spikes on the half bridge midpoint when the high-side
MOSFET turns off.
Figure 23.
Bootstrap supply: standard
circuit
circuit
Figure 24.
Bootstrap supply: internal
bootstrap synchronous diode
bootstrap synchronous diode
!-V
/54
6"//4
6CC
$
"//4
#
"//4
!-V
,
6CC
,6'
#
"//4
F
on
)
DS
(
e
arg
ch
g
F
on
)
DS
(
e
arg
ch
Drop
V
+
R
T
Q
=
V
+
R
I
=
V
V
6
.
2
=
6
.
0
+
150
10
·
3
.
0
10
·
5
.
2
10
·
30
=
V
6
6
9
Drop
-
-
-
-