AOZ1020 Просмотр технического описания (PDF) - Alpha and Omega Semiconductor
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AOZ1020 Datasheet PDF : 15 Pages
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AOZ1020
For reliable operation and best performance, the input
capacitors must have current rating higher than ICIN_RMS
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 manu-
factures 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:
ΔIL
=
f--V--×--O--L--
×
⎛
⎜
1
⎝
–
V-V----IO-N--⎠⎟⎞
The peak inductor current is:
ILpeak = IO + Δ---2--I-L--
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 30% 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.
Table 2 lists some inductors for typical output voltage
design.
Output Capacitor
The output capacitor is selected based on the DC output
voltage rating, output ripple voltage specification and
ripple current rating.
Table 2.
Vout
5.0V
3.3V
1.8V
1.2V
0.8V
L1
Unshielded, 4.7uH
LQH55DN4R7M03
Shielded, 4.7uH
LQH66SN4R7M03
Shield, 5.8uH
ET553-5R8
Un-shielded, 4.7uH
DO3316P-472MLD
Unshielded, 1.5uH
LQH55DN1R5M03
Shield, 1.5uH
LQH66SN1R5M03
Shield, 2.2uH
ET553-2R2
Un-shielded, 1.5uH
DO3316P-152MLD
Un-shielded, 1.5uH
DO1813P-152HC
Manufacture
MURATA
MURATA
ELYTONE
Coilcraft
MURATA
MURATA
ELYTONE
Coilcraft
Coilcraft
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:
ΔVO
=
ΔIL
×
⎛
⎝
E
SRC
O
+
-8----×-----f--1-×-----C-----O--⎠⎞
where,
CO is output capacitor value, and
ESRCO is the equivalent series resistance of the output
capacitor.
When low ESR ceramic capacitor is used as output
capacitor, the impedance of the capacitor at the switch-
ing frequency dominates. Output ripple is mainly caused
by capacitor value and inductor ripple current. The output
ripple voltage calculation can be simplified to:
ΔVO
=
ΔIL
×
⎛
⎝
-8----×-----f--1-×-----C-----O--⎠⎞
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:
ΔVO = ΔIL × ESRCO
Rev. 1.5 December 2010
www.aosmd.com
Page 9 of 15
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