AOZ1031AI Alpha & Omega Semiconductor Inc, AOZ1031AI Datasheet - Page 9

• Manufacturer Part Number: AOZ1031AI
• Description: IC BUCK SYNC ADJ 3A SO8
• Manufacturer: Alpha & Omega Semiconductor Inc
• Series: EZBuck™r
• Type: Step-Down (Buck), PWM - Current Moder

Specifications of AOZ1031AI

• Featured Product: The AOZ103x Family Synchronous Buck Regulator
• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 V ~ 18 V
• Current - Output: 3A
• Frequency - Switching: 600kHz
• Voltage - Input: 4.5 V ~ 18 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• Other names: 785-1253-2

For reliable operation and best performance, the input
capacitors must have current rating higher than I
at worst operating conditions. Ceramic capacitors are
preferred for input capacitors because of their low ESR
and high current rating. Depending on the application cir-
cuits, other low ESR tantalum capacitor may also be
used. When selecting ceramic capacitors, X5R or X7R
type dielectric ceramic capacitors should be used for
their better temperature and voltage characteristics. Note
that the ripple current rating from capacitor manufactures
are based on certain amount of life time. Further
de-rating may be necessary in practical design.
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:
The peak inductor current is:
High inductance gives low inductor ripple current but
requires larger size inductor to avoid saturation. Low rip-
ple current reduces inductor core losses. It also reduces
RMS current through inductor and switches, which
results in less conduction loss. Usually, peak to peak rip-
ple current on inductor is designed to be 20%
to 30% of output current.
When selecting the inductor, make sure it is able to han-
dle the peak current without saturation even at the high-
est operating temperature.
ΔI
I
I
CIN_RMS
Lpeak
Rev. 1.6 March 2010
L
I
O
=
(m)
Figure 2. I
---------- -
f
=
V
×
O
I
L
0.5
0.4
0.3
0.2
0.1
O
0
×
+
0
⎛
⎜
⎝
ΔI
------- -
1
CIN
2
–
L
-------- -
V
vs. Voltage Conversion Ratio
V
IN
O
⎞
⎟
⎠
0.5
m
CIN_RMS
www.aosmd.com
1
The inductor takes the highest current in a buck circuit.
The conduction loss on inductor need 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 they
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 rip-
ple current rating.
The selected output capacitor must have a higher rated
voltage specification than the maximum desired output
voltage including ripple. De-rating needs to be consid-
ered for long term reliability.
Output ripple voltage specification is another important
factor for selecting the output capacitor. In a buck con-
verter circuit, output ripple voltage is determined by
inductor value, switching frequency, output capacitor
value and ESR. It can be calculated by the equation
below:
where;
C
ESR
When low ESR ceramic capacitor is used as output
capacitor, the impedance of the capacitor at the switching
frequency dominates. Output ripple is mainly caused by
capacitor value and inductor ripple current. The output
ripple voltage calculation can be simplified to:
If the impedance of ESR at switching frequency domi-
nates, the output ripple voltage is mainly decided by
capacitor ESR and inductor ripple current. The output rip-
ple voltage calculation can be further simplified to:
For lower output ripple voltage across the entire operat-
ing temperature range, X5R or X7R dielectric type of
ceramic, or other low ESR tantalum are recommended to
be used as output capacitors.
ΔV
ΔV
ΔV
O
is output capacitor value, and
CO
O
O
O
=
=
is the Equivalent Series Resistor of output capacitor.
=
ΔI
ΔI
ΔI
L
L
L
×
×
×
ESR
⎛
⎝
-------------------------
8 f C
ESR
×
1
×
CO
CO
O
+
-------------------------
8
×
f
1
×
C
O
⎞
⎠
AOZ1031AI
Page 9 of 15