aoz1242 Alpha & Omega Semiconductor, aoz1242 Datasheet - Page 9

• Manufacturer Part Number: aoz1242
• Description: Ezbucktm 3a Simple Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

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.
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 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 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:
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:
ΔV
ΔV
ΔV
O
Rev. 1.4 November 2010
is output capacitor value and
CO
O
O
O
is the Equivalent Series Resistor of output capacitor.
=
=
=
ΔI
ΔI
ΔI
L
L
L
×
×
×
ESR
⎛
⎝
-------------------------
8
ESR
×
f C
1
×
CO
CO
O
+
-------------------------
8
×
f C
1
×
O
⎞
⎠
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For lower output ripple voltage across the entire
operating temperature range, X5R or X7R dielectric
type of ceramic, or other low ESR tantalum capacitor
or aluminum electrolytic capacitor may also 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.
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, Schottky diode is recommended to
use. 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 AOZ1242 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 filter. It greatly simplifies the compensation loop
design.
With peak current mode control, the buck power stage
can be simplified to be a one-pole and one-zero system
in frequency domain. The pole is dominant pole and can
be calculated by:
The zero is a ESR zero due to output capacitor and its
ESR. It is can be calculated by:
where;
C
R
ESR
I
f
f
CO_RMS
O
L
p1
Z1
is load resistor value, and
is the output filter capacitor,
CO
=
=
is the equivalent series resistance of output capacitor.
---------------------------------- -
2π
------------------------------------------------
2π
×
×
=
C
C
----------
1
ΔI
O
O
12
L
×
1
×
R
ESR
L
CO
AOZ1242
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