IRDC3629 International Rectifier, IRDC3629 Datasheet - Page 12

• Manufacturer Part Number: IRDC3629
• Description: BOARD EVAL SYNC BUCK CONTROLLER
• Manufacturer: International Rectifier

Input Capacitor Selection
The ripple current generated during the on time
of upper the 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 Io=25A and D=0.15, the 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. However, for the large
pulsing input current at full load, 2x270uF 16V
OS-CON
recommended, as well as 2x10uF 16V ceramic
capacitors from Murata.
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:
RMS
V
V
11/29/2007
F
D
Δ
Δ
in
o
i
t
s
=
=
=
=
=
=
Switching
Duty
Output
Inductor
Turn
is the RMS value of the input capacitor
Maximum
V
L
in
D =
I
=
cycle
RMS
operating
−
on
(
V
V
Voltage
o
capacitors
in
=
time
V
V
=
ripple
−
in
I
o
frequency
o
input
L
V
∗
o
∗
)
∗
Δ
Δ
D
V
t
i
current
in
;
voltage
∗
∗
ripple
( −
Δ
V
1
Δ
o
t
i
*
=
D
F
D
from
s
)
∗
RMS
( i
F
--(
current
Δ
1
--(
s
=8.9A.
8
)
) 7
)
. The optimum
Sanyo
can
are
be
If
L ≈ 0.6uH
The
compact, low profile inductor suitable for this
application.
Output Capacitor Selection
The voltage ripple and transient requirements
determine the output capacitors type and values.
The criteria is normally based on the value of the
Effective Series Resistance (ESR). However the
actual capacitance value and the Equivalent
Series Inductance (ESL) are other contributing
components.
described as:
Since the output capacitor has a major role in the
overall performance of the converter and
determines the result of transient response,
selection of the capacitor is critical. The IR3629A
can perform well with all types of capacitor.
As a rule, the capacitor must have low enough
ESR to meet output ripple and load transient
requirements.
The goal for this design is to meet the voltage
ripple requirement in the smallest possible
capacitor size. Therefore a SP capacitor is
selected due to large capacitance and small size.
Two of the Panasonic SP-CAP EEFSX0D331XE
(330uF, 2V, 6mOhm) is a good choice. In this
case, the ESR dominates the output voltage
ripple, equation (9) can be used to calculate the
required ESR for the specific voltage ripple.
Δ
Δ
Δ
Δ
Δ
Δ
V
V
V
V
V
I
Δ
L
o
o
o
o
o
i ≈
(
(
(
=
C
ESR
ESL
=
=
)
Inductor
40
MPL104-0R6
Output
=
Δ
)
IR3629/IR3629A MPbF
)
V
=
=
%(
8
o
⎛
⎜
⎝
Δ
(
*
ESR
V
I
I
L
C
Δ
o
L
in
)
)
I
o
*
⎞
⎟
⎠
L
+
, then the output inductor will be:
*
voltage
ESR
*
ripple
F
Δ
ESL
These
s
V
o
(
ESL
current
)
-
ripple
+
-(9)
from
Δ
V
components
o
(
C
)
Delta
provides
can
be
a
12