ACT4012A
Rev0, 16-May-08
APPLICATIONS INFORMATION
Output Voltage Setting
Figure 1:
Output Voltage Setting
V
OUT
ACT4012A
FB
R
FB2
R
FB1
Input Capacitor
The input capacitor needs to be carefully selected
to maintain sufficiently low ripple at the supply input
of the converter. A low ESR capacitor is highly rec-
ommended. Since large current flows in and out of
this capacitor during switching, its ESR also affects
efficiency.
The input capacitance needs to be higher than
10µF. The best choice is the ceramic type, how-
ever, low ESR tantalum or electrolytic types may
also be used provided that the RMS ripple current
rating is higher than 50% of the output current. The
input capacitor should be placed close to the IN and
G pins of the IC, with the shortest traces possible.
In the case of tantalum or electrolytic types, they
can be further away if a small parallel 0.1µF ce-
ramic capacitor is placed right next to the IC.
Figure 1 shows the connections for setting the out-
put voltage. Select the proper ratio of the two feed-
back resistors R
FB1
and R
FB2
based on the output
voltage. Typically, use R
FB2
≈
10kΩ and determine
R
FB1
from the output voltage:
Output Capacitor
The output capacitor also needs to have low ESR to
keep low output voltage ripple. The output ripple
voltage is:
V
RIPPLE
=
I
OUTMAX
K
RIPPLE
R
ESR
⎞
⎛
V
R
FB1
=
R
FB 2
⎜
OUT
−
1
⎟
⎠
⎝
1.293V
(1)
Inductor Selection
The inductor maintains a continuous current to the
output load. This inductor current has a ripple that is
dependent on the inductance value higher induc-
tance reduces the peak-to-peak ripple current. The
trade off for high inductance value is the increase in
inductor core size and series resistance, and the
reduction in current handling capability. In general,
select an inductance value L based on ripple current
requirement:
L
=
V
OUT
×
(
V
IN
−
V
OUT
)
V
IN
f
SW
I
OUTMAX
K
RIPPLE
+
V
IN
32
×
f
SW
LC
OUT
2
(3)
(2)
where V
IN
is the input voltage, V
OUT
is the output
voltage, f
SW
is the switching frequency, I
OUTMAX
is the
maximum output current, and K
RIPPLE
is the ripple
factor. Typically, choose K
RIPPLE
= 30% to corre-
spond to the peak-to-peak ripple current being 30%
of the maximum output current.
With this inductor value, the peak inductor current is
I
OUT
× (1 + K
RIPPLE
/2). Make sure that this peak in-
ductor current is less that the 3A current limit. Fi-
nally, select the inductor core size so that it does
not saturate at 3A.
Table 1:
Typical Inductor Values
V
OUT
L
1.5V
7.5µH
1.8V
10µH
2.5V
12µH
3.3V
15µH
5V
22µH
where I
OUTMAX
is the maximum output current, K
RIP-
PLE
is the ripple factor, R
ESR
is the ESR
resistance
of
the output capacitor, f
SW
is the switching frequency,
L is the inductor value, C
OUT
is the output capaci-
tance. In the case of ceramic output capacitors,
R
ESR
is very small and does not contribute to the
ripple. Therefore, a lower capacitance value can be
used for ceramic type. In the case of tantalum or
electrolytic type, the ripple is dominated by R
ESR
multiplied by the ripple current. In that case, the
output capacitor is chosen to have sufficiently low
ESR.
For ceramic output type, typically choose a capaci-
tance of about 22µF. For tantalum or electrolytic
type, choose a capacitor with less than 50mΩ ESR.
Rectifier Diode
Use a Schottky diode as the rectifier to conduct cur-
rent when the High-Side Power Switch is off. The
Schottky diode must have current rating higher than
the maximum output current and a reverse voltage
rating higher than the maximum input voltage.
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TM
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