User Manual

11/04/08
12
PD-60357
IR3802AMPbF
Input Capacitor Selection
The input filter capacitor should be selected
based on how much ripple the supply can
tolerate on the DC input line. The ripple current
generated during the on time of 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
RMS
is the RMS value of the input capacitor
current.
Io is the output current.
For Io=6A and D=0.15, the I
RMS
=2.14A.
Ceramic capacitors are recommended due to
their peak current capabilities. They also feature
low ESR and ESL at higher frequency which
results in better efficiency.
Use 3x10uF, 16V ceramic capacitors from
Panasonic.
Inductor Selection
The inductor is selected based on output power,
operating frequency and efficiency requirements.
A low inductor value causes a 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 . The
optimum point is usually found between 20% and
50% ripple of the output current.
For the buck converter, the inductor value for the
desired operating ripple current can be
determined using the following:
Where:
)7 --(D1DII
oRMS
)( −∗∗=
in
o
V
V
D =
s
oin
F
1
Dt
t
i
LVV ∗=∗=−
Δ
Δ
Δ
;
)( i
Δ
()
)8 --(
FiV
V
VVL
sin
o
oin
* Δ ∗
∗−=
cycle DutyD
time on Turnt
frequency SwitchingF
current ripple Inductori
Voltage OutputV
voltage input MaximumV
s
o
in
=
=
=
=
=
=
Δ
Δ
If , then the output inductor will be:
L = 2.2uH
ACT provides a range of inductors in different
values, low profile suitable for large currents.
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. These components can be
described as:
Since the output capacitor has a major role in the
overall performance of the converter and
determine the result of transient response,
selection of the capacitor is critical. The IR3802A
can perform well with all types of capacitors.
As a rule the capacitor must have low enough
ESR to meet output ripple and load transient
requirements, yet have high enough ESR to
satisfy stability requirements.
The goal for this design is to meet the voltage
ripple requirement in the smallest possible
capacitor size. Therefore, a ceramic capacitor is
selected due to its low ESR and small size. Six of
the Panasonic ECJ2FB0J226M (22uF, 6.3V, X5R
and EIA 0805 case size) are a good choice.
In the case of tantalum or low ESR electrolytic
capacitors, the ESR dominates the output
voltage ripple, equation (9) can be used to
calculate the required ESR for the specific
voltage ripple.
)%(40≈
o
Ii
Δ
current ripple InductorI
ripple voltage OutputV
FC8
I
V
ESL
L
V
V
-(9)- ESRIV
VVVV
L
o
so
L
Co
in
ESLo
LESRo
CoESLoESRoo
=
=
=
⎟
⎠
⎞
⎜
⎝
⎛
=
=
+
+
=
Δ
Δ
Δ
Δ
Δ
ΔΔ
Δ
Δ
Δ
Δ
**
*
*
)(
)(
)(
)()()(