ACT4012A
Rev0, 16-May-08
Innovative PowerTM - 5 - www.active-semi.com
Copyright © 2008 Active-Semi, Inc.
Figure 1 shows the connections for setting the out-
put voltage. Select the proper ratio of the two feed-
back resistors RFB1 and RFB2 based on the output
voltage. Typically, use RFB2 10k and determine
RFB1 from the output voltage:
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:
VOUT 1.5V 1.8V 2.5V 3.3V 5V
L 7.5µH 10µH 12µH 15µH 22µH
where IOUTMAX is the maximum output current, KRIP-
PLE is the ripple factor, RESR is the ESR resistance of
the output capacitor, fSW is the switching frequency,
L is the inductor value, COUT is the output capaci-
tance. In the case of ceramic output capacitors,
RESR 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 RESR
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 50m 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.
Inductor Selection
APPLICATIONS INFORMATION
where VIN is the input voltage, VOUT is the output
voltage, fSW is the switching frequency, IOUTMAX is the
maximum output current, and KRIPPLE is the ripple
factor. Typically, choose KRIPPLE = 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
IOUT × (1 + KRIPPLE/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.
(1)
(2)
Figure 1:
Output Voltage Setting
⎟
⎠
⎞
⎜
⎝
⎛−= 1
V293.1
V
RR OUT
2FB1FB
()
RIPPLEOUTMAXSWIN
OUTINOUT
KIfV
VVV
L−×
=
(3)
ESRRIPPLEOUTMAXRIPPLE RKIV =
OUT
2
SW
IN
LCf32
V
×
+
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.
Output Capacitor
The output capacitor also needs to have low ESR to
keep low output voltage ripple. The output ripple
voltage is:
Output Voltage Setting
Table 1:
Typical Inductor Values
RFB1
RFB2
VOUT
ACT4012A
FB