AOZ1284
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 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.
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 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:
1
V I ESR
O
L
CO
8 f C
O
where;
CO is output capacitor value and
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:
ESRCO is the Equivalent Series Resistor of output
capacitor.
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
simplified to:
V
V
O
O
I
1
L
f L
V
IN
The peak inductor current is:
1
V I
O
L
IL
2
8 f C
O
ILPEAK IO
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:
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.
VO IL ESRCO
When selecting the inductor, make sure it is able to
handle the peak current without saturation even at the
highest operating temperature.
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.
The inductor takes the highest current in a buck
circuit. The conduction loss on inductor needs to be
checked for thermal and efficiency requirements.
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:
Surface mount inductors in different shape and styles
are available from Wurth, Sumida, Coilcraft, 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.
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, output
capacitor could be overstressed.
Output Capacitor
The output capacitor is selected based on the DC
output voltage rating, output ripple voltage
specification and ripple current rating.
www.aosmd.com
Page 9 of 14
Rev. 0.5 March 2012
AOS [ ALPHA & OMEGA SEMICONDUCTORS ]
