aoz1212 Alpha & Omega Semiconductor, aoz1212 Datasheet - Page 9

• Manufacturer Part Number: aoz1212
• Description: 3a Simple Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

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 ef ciency 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 speci cation and
ripple current rating.
The selected output capacitor must have a higher rated
voltage speci cation than the maxim um desired output
voltage including ripple. De-rating needs to be consid-
ered for long term reliability.
Output ripple voltage speci cation is another impor tant
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
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 simpli ed to:
I
Lpeak
O
V
Rev. 1.1 October 2007
V
is output capacitor value and
O
CO
O
=
=
is the Equivalent Series Resistor of output capacitor.
=
I
I
I
L
L
O
+
-------- -
ES R
-------------------------- -
8 f
2
I
L
1
CO
C
+
O
-------------------------- -
8 f
1
C
O
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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 simpli ed to:
For lower output ripple voltage across the entire operat-
ing temperature range, X5R or X7R dielectric type of
ceramic, or other low ESR tantalum capacitor or
aluminum electrolytic capacitor may also be used as out-
put capacitors.
In a buck converter, output capacitor current is continu-
ous. The RMS current of output capacitor is decided by
the peak to peak inductor ripple current. It can be calcu-
lated 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 induc-
tor ripple current is high, output capacitor could be over-
stressed.
Schottky Diode Selection
The external freewheeling diode supplies the current to
the inductor when the high side NMOS switch is off. To
reduce the losses due to the forward voltage drop and
recovery of diode, a Schottky diode is recommended.
The maximum reverse voltage rating of the chosen
Schottky diode should be greater than the maximum
input voltage, and the current rating should be greater
than the maximum load current.
Loop Compensation
The AOZ1212 employs peak current mode control for
easy use and fast transient response. Peak current mode
control eliminates the double pole effect of the output
L&C lter . It greatly simpli es the compensation loop
design.
With peak current mode control, the buck power stage
can be simpli ed to be a one-pole and one-z ero system
in frequency domain. The pole is the dominant pole and
can be calculated by:
I
f
CO _RMS
p1
V
O
=
=
----------------------------------- -
2
I
=
L
C
----------
1
O
ES R
12
I
L
R
L
CO
AOZ1212
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