ir3840 International Rectifier Corp., ir3840 Datasheet - Page 18

• Manufacturer Part Number: ir3840
• Description: Integrated 12a Synchronous Buck Regulator
• Manufacturer: International Rectifier Corp.

V
the voltage V
unchanged and the 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:
Where:
D is the Duty Cycle
I
current.
Io is the output current.
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 4x10uF 16V ceramic capacitors
ECJ-3YX1C106K from Panasonic. In addition to
these, although not mandatory, a 1X330uF, 25V
SMD capacitor EEV-FK1E331P may also be
used as a bulk capacitor and is recommended if
the input power supply is not located close to the
converter.
RMS
Boot
02/09/09
I
D =
RMS
Fig. 12. Bootstrap circuit to generate
o
is the RMS value of the input capacitor
=12A and D = 0.15, the I
≅
V
V
V
=
in
o
in
I
o
..........
+
∗
V
c
D
cc
across C6 remains approximately
∗
..........
−
(
1
Vc voltage
V
−
D
D
..........
)
..........
..........
..........
..
..........
(13)
RMS
....(12)
..........
= 4.28A.
..........
(11)
Inductor Selection
The inductor is selected based on output power,
operating frequency and efficiency requirements.
A low inductor value causes large ripple current,
resulting in the smaller size, faster response to a
load transient but poor efficiency and high output
noise. Generally, the selection of the inductor
value can be reduced to the desired maximum
ripple current in the inductor
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 0.607μH. Select L=0.6 μH.
The MPL104-0R6 from Delta provides a compact,
low profile inductor suitable for this application.
.
V
in
L
−
Δi ≈ 35%(I
=
V
(
V
V
V
o
F
D
Δi
Δt
in
in
o
s
=
=
=
=
operating
−
=
=
=
L
Duty
Switching
V
Inductor
Turn
Output
Maximum
∗
o
Δ
Δ
)
∗
t
i
cycle
o
;
V
on
), then the output inductor is
in
Δ
Voltage
t
∗
ripple
time
V
=
Δ
frequency
IR3840MPbF
ripple
o
input
i
D
*
∗
F
current
s
F
1
s
voltage
..........
current
( i
Δ
)
. The optimum
..........
can
..........
18
be
.
(14)