AOZ1232-01
Application Information
The basic AOZ1232-01 application circuit is shown in
pages 2 and 3. Component selection is explained below.
For reliable operation and best performance, the input
capacitors must have current rating higher than I
CIN-RMS
at 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 manufactures is
based on certain amount of life time. Further de-rating
may be necessary for practical design requirement.
Input Capacitor
The input capacitor must be connected to the IN pins and
PGND pin of the AOZ1232-01 to maintain steady input
voltage and filter out the pulsing input current. A small
decoupling capacitor, usually 1F, should be connected
to the VCC pin and AGND pin for stable operation of the
AOZ1232-01. The voltage rating of input capacitor must
be greater than maximum input voltage plus ripple
voltage.
The input ripple voltage can be approximated by
equation below:
Inductor
The inductor is used to supply constant current to output
when it is driven by a switching voltage. For given input
and output voltage, inductance and switching frequency
together decide the inductor ripple current, which is:
I
V
V
O
O
O
-----------------
--------
--------
1 –
V
f C
V
IN
IN
IN
V
V
O
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 input capacitor current can be
calculated by:
O
----------
--------
I
=
1 –
L
V
f L
IN
The peak inductor current is:
I
L
V
V
O
O
--------
I
= I +
Lpeak
O
--------
--------
I
= I
1 –
2
CIN_RMS
O
V
V
IN
IN
High inductance gives 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. Usually, peak to
peak ripple current on inductor is designed to be 30% to
50% of output current.
if let m equal the conversion ratio:
V
O
--------
= m
V
IN
The relation between the input capacitor RMS current
and voltage conversion ratio is calculated and shown in
Figure 3. It can be seen that when V is half of V , C it
When selecting the inductor, make sure it is able to
handle the peak current without saturation even at the
highest operating temperature.
O
IN
IN
is under the worst current stress. The worst current
stress on C is 0.5 x I .
IN
0.5
O
The inductor takes the highest current in a buck circuit.
The conduction loss on the inductor needs to be checked
for thermal and efficiency requirements.
0.4
0.3
0.2
0.1
0
Surface mount inductors in different shapes and styles
are available from Coilcraft, Elytone and Murata.
Shielded inductors are small and radiate less EMI noise,
but they do cost more than unshielded inductors. The
choice depends on EMI requirement, price and size.
ICIN_RMS(m)
IO
0
0.5
m
1
Figure 3. ICIN vs. Voltage Conversion Ratio
Rev. 1.3 October 2012
www.aosmd.com
Page 11 of 18
AOS [ ALPHA & OMEGA SEMICONDUCTORS ]
