ACT4072
APPLICATIONS INFORMATION
Output Voltage Setting
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:
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 recom-
mended. Since a 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 possible traces. In the
case of tantalum or electrolytic types, they can be
further away if a small parallel 0.1µF ceramic ca-
pacitor is placed right next to the IC.
V
R
FB1
�½
R
FB2
OUT
-
1
1.222V
Figure 1:
Output Voltage Setting
(1)
Output Capacitor
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:
The output capacitor also needs to have low ESR to
keep low output voltage ripple. The output ripple
voltage is:
R
RIPPLE
�½
L
OUTMAX
K
RIPPLE
R
ESR
V
IN
2
28f
SW
LC
OUT
(3)
L
�½
V
OUT
V
IN
-
V
OUT
V
IN
f
SW
I
OUTMAX
K
RIPPLE
(2)
where I
OUTMAX
is the maximum output current,
K
RIPPLE
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 capacitance.
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, typically choose a capacitance of
about 22µF.
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 due to ESR, typically choose a ca-
pacitor with less than 50mΩ ESR.
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
= 20% to 30%
corresponding to the peak-to-peak ripple current
being 20% to 30% of the maximum output current.
With this inductor value (Table 1), the peak inductor
current is I
OUT
× (1 + K
RIPPLE
/ 2). Make sure that this
peak inductor current is less that the 2.4A current
limit. Finally, select the inductor core size so that it
does not saturate at 2.4A.
Table 1:
Typical Inductor Values
V
OUT
L
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 the reverse volt-
age rating higher than the maximum input voltage.
1.5V
6.8μH
1.8V
6.8μH
2.5V
10μH
3.3V
15μH
5V
22μH
-5-
Innovative Products.
Active Solutions.
Copyright © 2006 Active-Semi, Inc.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
