aoz1281di Alpha & Omega Semiconductor, aoz1281di Datasheet - Page 8

• Manufacturer Part Number: aoz1281di
• Description: Ezbucktm 1.8 A Simple Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

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
For reliable operation and best performance, the input
capacitors must have a current rating higher than
I
capacitors are preferred for use as input capacitors
because of their low ESR and high ripple current rating.
Depending on the application circuits, other low ESR
tantalum capacitors or aluminum electrolytic capacitors
may 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 is based on a fixed life time. Further
de-rating may be necessary for practical design
requirements.
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 provides a low inductor ripple current
but requires larger size inductor to avoid saturation.
Low ripple current reduces inductor core losses and also
reduces RMS current through inductor and switches.
This results in less conduction loss.
CIN_RMS
ΔI
I
I
IN
CIN_RMS
Lpeak
Rev. 1.0 June 2011
, C
L
I
O
=
IN
(m)
Figure 2. I
---------- -
f
is under the worst current stress. The worst
=
V
at the worst operating conditions. Ceramic
×
O
I
L
0.5
0.4
0.3
0.2
0.1
O
0
×
+
0
⎛
⎜
⎝
ΔI
------- -
1
CIN
2
–
L
IN
-------- -
V
vs. Voltage Conversion Ratio
V
is 0.5 x I
IN
O
⎞
⎟
⎠
O
.
0.5
m
O
is half of
www.aosmd.com
1
When selecting the inductor, confirm it is able to handle
the peak current without saturation 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.
The selected output capacitor must have a higher rated
voltage specification than the maximum desired output
voltage including ripple. De-rating needs to be
considered for long term reliability.
Output ripple voltage specification is another important
factor for selecting the output capacitor. In a buck
converter 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
capacitor.
When a low ESR ceramic capacitor is used as the 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
dominates, the output ripple voltage is mainly decided by
capacitor ESR and inductor ripple current. The output
ripple voltage calculation can be further simplified to:
ΔV
ΔV
ΔV
O
is output capacitor value, and
O
CO
O
O
=
=
is the equivalent series resistance of the output
=
ΔI
ΔI
ΔI
L
L
L
×
×
×
⎛
⎝
ESR
⎛
⎝
-------------------------
8
ESR
×
f
1
CO
×
CO
C
O
+
⎞
⎠
-------------------------
8
×
f
1
×
C
O
⎞
⎠
AOZ1281
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