AOZ1232-01
Output Capacitor
Thermal Management and Layout
Consideration
The output capacitor is selected based on the DC output
voltage rating, output ripple voltage specification and
ripple current rating.
In the AOZ1232-01 buck regulator circuit, high pulsing
current flows through two circuit loops. The first loop
starts from the input capacitors, to the VIN pin, to the LX
pins, to the filter inductor, to the output capacitor and
load, and then returns to the input capacitor through
ground. Current flows in the first loop when the high side
switch is on. The second loop starts from the inductor, to
the output capacitors and load, to the low side switch.
Current flows in the second loop when the low side
switch is on.
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:
In PCB layout, minimizing the two loops area reduces the
noise of this circuit and improves efficiency. A ground
plane is strongly recommended to connect the input
capacitor, output capacitor and PGND pin of the
AOZ1232-01.
1
-------------------------
V = I ESR
+
O
L
CO
8 f C
O
where,
In the AOZ1232-01 buck regulator circuit, the major
power dissipating components are the AOZ1232-01 and
output inductor. The total power dissipation of the con-
verter circuit can be measured by input power minus out-
put power.
CO is output capacitor value and
ESRCO is the Equivalent Series Resistor of output capacitor.
When a 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:
P
= V I – V I
IN IN O O
total_loss
The power dissipation of inductor can be approximately
calculated by output current and DCR of inductor and
output current.
1
-------------------------
V = I
O
L
8 f C
O
2
P
= I R
1.1
inductor
inductor_loss
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:
The actual junction temperature can be calculated with
power dissipation in the AOZ1232-01 and thermal
impedance from junction to ambient.
V = I ESR
CO
T
= P
–P
inductor_loss JA
O
L
junction
total_loss
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.
The maximum junction temperature of AOZ1232-01 is
150ºC, which limits the maximum load current capability.
The thermal performance of the AOZ1232-01 is strongly
affected by the PCB layout. Extra care should be taken
by users during design process to ensure that the IC will
operate under the recommended environmental
conditions.
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:
I
L
----------
I
=
CO_RMS
12
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, the output capacitor could
be overstressed.
Rev. 1.3 October 2012
www.aosmd.com
Page 12 of 18
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