AOZ1016 Datasheet, PDF (9/15 Pages) Alpha & Omega Semiconductors – EZBuck™ 2A Simple Buck Regulator

AOZ1016
0.5
0.4
I_{CIN_RMS}(m) ^0.3
I_O
0.2
0.1
0
0
0.5
1
m
Figure 2. I_CIN vs. Voltage Conversion Ratio
For reliable operation and best performance, the input
capacitors must have current rating higher than I_{CIN_RMS}
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 is
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:
∆I_L
=
--^V-----^O----
×

1
–
-^V-----^O----
f × L  _{V IN}
The peak inductor current is:
I_Lpeak
=
I
O
+
∆_-----I_--L-
2
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 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. 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.
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
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:
∆V _O = ∆I _L × _ES R_CO + -₈----_×-----_f---¹-_×-----_C-----_O--_
where,
C_O is output capacitor value, and
ESR_CO is the equivalent series resistance of the output
capacitor.
When a low ESR ceramic capacitor is used as an 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:
∆V _O
=
∆I_L
×
-------------¹-------------
8 × f × 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:
∆V _O = ∆I_L × ESR_CO
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.
Rev. 1.1 September 2007
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