IRDC3831 International Rectifier, IRDC3831 Datasheet - Page 17

• Manufacturer Part Number: IRDC3831
• Description: BOARD EVAL SYNC BUCK CONVERTER
• Manufacturer: International Rectifier

Bootstrap Capacitor Selection
To drive the Control FET, it is necessary to
supply a gate voltage at least 4V greater than the
voltage at the SW pin, which is connected the
source of the Control FET . This is achieved by
using a bootstrap configuration, which comprises
the internal bootstrap diode and an external
bootstrap capacitor (C6). The operation of the
circuit is as follows: When the lower MOSFET is
turned on, the capacitor node connected to SW is
pulled down to ground. The capacitor charges
towards V
which has a forward voltage drop V
V
approximately given as
When the upper MOSFET turns on in the next
cycle, the capacitor node connected to SW rises
to the bus voltage V
is appropriately chosen, the voltage V
remains
voltage at the Boot pin becomes
A bootstrap capacitor of value 0.1uF is suitable
for most applications.
Input Capacitor Selection
The ripple current generated during the on time of
the upper MOSFET should be provided by the
input capacitor. The RMS value of this ripple is
expressed by:
V
11/05/10
c
Boot
Fig. 11. Bootstrap circuit to generate
across
V
I
≅
RMS
c
V
≅
in
cc
approximately
=
V
+
I
through the internal bootstrap diode,
cc
o
V
the
∗
cc
−
Vc voltage
V
D
−
D
∗
V
bootstrap
(
..........
in
D
1
. However, if the value of C6
−
..........
D
)
..........
..........
unchanged
..........
..........
capacitor
..........
..........
D
....(12)
. The voltage
c
..........
across C6
and
..........
C6
(10)
the
(11)
is
Where:
D is the Duty Cycle
I
current.
I
For I
Ceramic capacitors are recommended due to
their peak current capabilities. They also feature
low ESR and ESL at higher frequency which
enables better efficiency. For this application, it is
advisable to have 3x10uF 16V ceramic capacitors
ECJ-3YX1C106K from Panasonic. In addition to
these, although not mandatory, a 330uF 25V
SMD capacitor EEV-FK1E331P from Panasonic
may be used as a bulk capacitor, and is
recommended if the input power supply is not
located close to the converter.
Inductor Selection
The inductor is selected based on output power,
operating frequency and efficiency requirements.
A low inductor value results in a smaller size and
faster response to a load transient but poor
efficiency and high output noise due to large
ripple current. Generally, the selection of the
inductor value can be reduced to the desired
maximum ripple current in the inductor
optimum point is usually found between 20% and
50% ripple of the output current.
For the buck converter, the inductor value for the
desired
determined using the following relation:
Where:
If
calculated to be 0.63μH. Select L=0.6 μH.
The MPL104-0R6 from Delta provides a compact,
low profile inductor suitable for this application.
RMS
o
is the output current.
V
in
L
Δi ≈ 35%(I
o
is the RMS value of the input capacitor
=8 A and D = 0.0625, the I
−
=
D =
V
(
V
V
V
F
D
Δi
Δt
o
in
o
in
s
operating
=
V
=
V
=
=
=
=
=
−
in
Duty
o
L
Switching
Inductor
Turn
Output
V
Maximum
..........
∗
o
Δ
Δ
)
o
∗
i
t
cycle
), then the output inductor is
on
;
V
..........
in
Δ
Voltage
ripple
time
t
∗
IR3831MPbF
ripple
V
frequency
=
Δ
input
o
i
D
..........
*
∗
F
current
s
F
1
voltage
s
current
..
..........
RMS
(13)
= 1.94 A
..........
can
( i
Δ
...
)
17
. The
(14)
be