aoz1025di Alpha & Omega Semiconductor, aoz1025di Datasheet - Page 9

• Manufacturer Part Number: aoz1025di
• Description: Ezbucktm 8a Synchronous Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

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
circuits, 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
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 20%
to 40% of output current.
When selecting the inductor, make sure 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 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
ripple current rating.
ΔI
I
Lpeak
Rev. 1.3 September 2009
L
=
---------- -
f
V
=
×
O
L
I
O
×
+
⎛
⎜
⎝
ΔI
------- -
1
2
–
L
-------- -
V
V
IN
O
⎞
⎟
⎠
CIN_RMS
www.aosmd.com
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 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
capacitor.
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
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:
For lower output ripple voltage across the entire
operating temperature range, X5R or X7R dielectric type
of ceramic, or other low ESR tantalum are recommended
to be used as output capacitors.
In a buck converter, output capacitor current is
continuous. The RMS current of output capacitor is
decided by the peak to peak inductor ripple current. It
can be calculated by:
Usually, the ripple current rating of the output capacitor is
a smaller issue because of the low current stress. When
the buck inductor is selected to be very small and
inductor ripple current is high, output capacitor could be
overstressed.
I
ΔV
ΔV
ΔV
CO_RMS
O
is output capacitor value, and
CO
O
O
O
=
is the equivalent series resistance of the output
=
=
ΔI
ΔI
ΔI
=
L
L
L
×
----------
×
×
ΔI
12
ESR
⎛
⎝
-------------------------
8
L
ESR
×
f
1
CO
×
CO
C
O
+
-------------------------
8
×
f
1
×
C
O
⎞
⎠
AOZ1025D
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