ACT4012
APPLICATION INFORMATION
OUTPUT VOLTAGE SETTING
Figure 3. Output Voltage Setting
Figure 3 shows the connections for setting
the output voltage. Select the proper ratio of the
two feedback resistors RFB1 and RFB2 based on
the output voltage. Typically, use RFB2 ≈ 10kΩ
and determine RFB1 from the output voltage:
−
=
1
V
293
.
1
V
R
R
OUT
2
FB
1
FB
(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 inductance 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:
RIPPLE
OUTMAX
SW
IN
OUT
IN
OUT
K
I
f
V
)
V
V
(
V
L
−
•
=
(2)
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
correspond to the peak-to-peak ripple current
being 30% of the maximum output current.
With this inductor value (Table 1), the peak
inductor current is IOUT • (1 + KRIPPLE / 2). Make
sure that this peak inductor current is less that
the 3A current limit. Finally, select the inductor
core size so that it does not saturate at 3A.
Table 1. Typical Inductor Values
VOUT
1.5V
1.8V
2.5V
3.3V
5V
L
6.8μH 6.8μH 10μH 15μH 22μH
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 recommended. 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;
however, 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
shortest traces possible. In the case of tantalum
or electrolytic types, they can be further away if a
small parallel 0.1µF ceramic 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:
ESR
RIPPLE
OUTMAX
RIPPLE
R
K
I
V
=
OUT
2
SW
IN
LC
f
28
V
•
+
(3)
where IOUTMAX is the maximum output current,
KRIPPLE is the ripple factor, RESR is the ESR
resistance of the output capacitor, fSW is the
switching frequency, L in the inductor value, COUT
is the output capacitance. 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
capacitance 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 current when the High-Side Power
Switch is off. The Schottky diode must have
current rating higher than the maximum output
current and the reverse voltage rating higher
than the maximum input voltage.
Active-Semi, Inc.
- 5 -
www.active-semi.com
ACT4012
FB
VOUT
RFB 1
RFB 2
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