aoz1092d Alpha & Omega Semiconductor, aoz1092d Datasheet - Page 9

• Manufacturer Part Number: aoz1092d
• Description: Ezbucktm 3a Simple 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 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 are
based on certain amount of life time. Further de-rating
may be necessary for practical design requirement.
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.
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 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 they
cost more than unshielded inductors. The choice
depends on EMI requirement, price and size.
Table 2 lists some inductors for typical output voltage
design.
ΔI
I
Lpeak
Rev. 1.3 February 2009
L
=
---------- -
f
=
V
×
O
I
L
O
×
+
⎛
⎜
⎝
ΔI
------- -
1
2
–
L
-------- -
V
V
IN
O
⎞
⎟
⎠
CIN_RMS
www.aosmd.com
Table 2.
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 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:
ΔV
ΔV
O
V
is output capacitor value, and
5.0V
3.3V
1.8V
CO
O
O
OUT
=
is the equivalent series resistance of the output
=
ΔI
ΔI
L
L
Shielded, 6.8µH
MSS1278-682MLD
Shielded, 6.8µH
MSS1260-682MLD
Un-shielded, 4.7µH
DO3316P-472MLD
Shielded, 4.7µH
DO1260-472NXD
Shielded, 3.3µH
ET553-3R3
Shielded, 2.2µH
ET553-2R2
Un-shielded, 3.3µH
DO3316P-222MLD
Shielded, 2.2µH
MSS1260-222NXD
×
×
⎛
⎝
⎛
⎝
ESR
-------------------------
8
×
f
1
×
L1
CO
C
O
+
⎞
⎠
-------------------------
8
×
f
1
×
C
O
⎞
⎠
Manufacturer
AOZ1092D
ELYTONE
ELYTONE
Coilcraft
Coilcraft
Coilcraft
Page 9 of 16