aoz1083ci Alpha & Omega Semiconductor, aoz1083ci Datasheet - Page 7

• Manufacturer Part Number: aoz1083ci
• Description: 1.2 A Buck Led Driver
• Manufacturer: Alpha & Omega Semiconductor

Application Information
The basic AOZ1083 application circuit is shown in
Figure 1. Component selection is explained below.
Input Capacitor
The input capacitor must be connected to the VIN pin
and PGND pin of the AOZ1083 to maintain steady input
voltage and filter out the pulsing input current. The
voltage rating of the input capacitor must be greater than
the maximum input voltage plus ripple voltage.
The input ripple voltage can be approximated by
equation below::
Since the input current is discontinuous in a buck
converter, the current stress on the input capacitor is
another concern when selecting the capacitor. For a buck
circuit, the RMS value of the input capacitor current can
be calculated by:
if we let m equal the conversion ratio:
The relationship between the input capacitor RMS
current and voltage conversion ratio is calculated and
shown in Figure 2. It can be seen that when V
V
current stress on C
ΔV
I
-------- -
V
V
CIN_RMS
I
IN
CIN_RMS
Rev. 1.0 July 2011
IN
O
, C
IN
I
O
=
IN
=
(m)
Figure 2. I
m
is under the worst current stress. The worst
---------------- -
f
×
=
0.5
0.4
0.3
0.2
0.1
I
O
C
0
I
0
IN
O
×
CIN
×
IN
⎛
⎜
⎝
1
vs. Voltage Conversion Ratio
-------- - 1
V
is at 0.5 x I
V
–
IN
O
-------- -
V
V
⎛
⎜
⎝
IN
O
–
⎞
⎟
⎠
-------- -
V
×
V
IN
O
O
-------- -
V
V
0.5
m
.
⎞
⎟
⎠
IN
O
O
is half of
www.aosmd.com
1
For reliable operation and best performance, the input
capacitors must have current rating higher than I
at the worst operating conditions. Ceramic capacitors are
preferred for input capacitors because of their low ESR
and high ripple current rating. Depending on the
application circuits, other low ESR tantalum capacitor or
aluminum electrolytic capacitor may also be used. When
selecting ceramic capacitors, X5R or X7R type dielectric
ceramic capacitors are preferred for their better
temperature and voltage characteristics. Note that the
ripple current rating from capacitor manufacturers are
based on a certain life time. Further de-rating may need
to be considered for long term reliability.
Inductor
The inductor is used to supply constant current to output
when it is driven by a switching voltage. For a given input
and output voltage, inductance and switching frequency
together decide the inductor ripple current, which is:
The peak inductor current is:
High inductance provides low inductor ripple current but
requires a larger size inductor to avoid saturation. Low
ripple current reduces inductor core losses. It also
reduces RMS current through inductor and switches,
which results in less conduction loss.
When selecting the inductor, confirm it is able to handle
the peak current without saturation even at the highest
operating temperature.
The inductor takes the highest current in a buck circuit.
The conduction loss on inductor needs to be checked for
thermal and efficiency requirements.
Surface mount inductors in different shape and styles are
available from Coilcraft, Elytone and Murata. Shielded
inductors are small and radiate less EMI noise but cost
more than unshielded inductors. The choice depends on
EMI requirement, price and size.
Output Capacitor
The output capacitor is selected based on the DC output
voltage rating, output ripple voltage specification and
ripple current rating.
ΔI
I
Lpeak
L
=
---------- -
f
=
V
×
O
I
L
O
×
+
⎛
⎜
⎝
ΔI
------- -
1
2
–
L
-------- -
V
V
IN
O
⎞
⎟
⎠
AOZ1083
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