Application Information
Ceramic capacitors are now available in values over 10µF.
Since the ceramic capacitor has low ESR and ESL, a single
ceramic capacitor can be adequate for both low frequency
and high frequency noises. The disadvantage of ceramic
capacitors are their high cost. Solid tantalum capacitors can
have low ESR and small size. However, the reliability of
the tantalum capacitor is always a concern in the applica-
tion where the capacitor may experience surge current.
Output Capacitor
In a buck converter, the requirements on the output capaci-
tor are not as critical as those on the input capacitor. The
current to the output capacitor comes from the inductor
and thus is triangular. In most applications, this makes the
RMS ripple current not an issue in selecting output capaci-
tors.
The output ripple voltage is the sum of a triangular wave
caused by ripple current flowing through ESR, and a
square wave due to ESL. Capacitive reactance is assumed
to be small compared to ESR and ESL. The peak to peak
ripple current of the inductor is:
Figure 8: Input voltage ripple in a Buck converter.
To calculate the RMS current, multiply the load current
with the constant given by Figure 9 at each duty cycle. It is
a common practice to select the input capacitor with an
RMS current rating more than half the maximum load cur-
rent. If multiple capacitors are paralleled, the RMS current
for each capacitor should be the total current divided by
the number of capacitors.
VO(VIN – VO)
IP – P
=
(VIN)(L)(fS)
V
RIPPLE(ESR), the output ripple due to the ESR, is equal to the
product of IP – P and ESR. The voltage developed across the
ESL is proportional to the di/ dt of the output capacitor. It
is realized that the di/ dt of the output capacitor is the same
as the di/ dt of the inductor current. Therefore, when the
switch turns on, the di/ dt is equal to (VIN – VO)/ L, and it
becomes VO/ L when the switch turns off. The total ripple
voltage induced by ESL can then be derived from
0.6
0.5
0.4
0.3
0.2
0.1
0
V
L
VIN – V
L
VIN
L
V
RIPPLE(ESL) = ESL( IN ) + ESL(
O ) = ESL(
)
The total output ripple is the sum of the VRIPPLE(ESR) and
VRIPPLE(ESR)
.
0.8
1
0.2
0.4
0.6
0
Duty Cycle
Figure 9: Input capacitor RMS current can be calculated by multiplying
Y value with maximum load current at any duty cycle.
Selecting the capacitor type is determined by each design’s
constraint and emphasis. The aluminum electrolytic capaci-
tors are widely available at lowest cost. Their ESR and ESL
(equivalent series inductor) are relatively high. Multiple
capacitors are usually paralleled to achieve lower ESR. In
addition, electrolytic capacitors usually need to be paral-
leled with a ceramic capacitor for filtering high frequency
noises. The OS-CON are solid aluminum electrolytic capac-
itors, and therefore has a much lower ESR. Recently, the
price of the OS-CON capacitors has dropped significantly
so that it is now feasibly to use them for some low cost
designs. Electrolytic capacitors are physically large, and not
used in applications where the size, and especially height is
the major concern.
Figure 10: The output voltage ripple using two 10µF ceramic capacitors
in parallel.
9
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
