ACT4060A
APPLICATIONS INFORMATION
Output Voltage Setting
Figure 1:
Output Voltage Setting
V
OUT
ACT4060A
FB
R
FB2
R
FB1
Rev 1, 25-Jun-09
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 the 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.
Figure 1 shows the connections for setting
output voltage. Select the proper ratio of the
feedback resistors R
FB1
and R
FB2
based on
output voltage. Typically, use R
FB2
≈
10kΩ
determine R
FB1
from the following equation:
the
two
the
and
(1)
⎛
V
⎞
R
FB1
=
R
FB 2
⎜
OUT
−
1
⎟
⎝
1.293V
⎠
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
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:
+
V
IN
28
×
f
SW
LC
OUT
2
(3)
L
=
V
OUT
×
(
V
IN
−
V
OUT
)
V
IN
f
SW
I
OUTMAX
K
RIPPLE
(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
correspond 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
inductor current is less that the 3A current limit.
Finally, select the inductor core size so that it does
not saturate at 3A. Typical inductor values for
various output voltages are shown in Table 1.
Table 1:
Typical Inductor Values
V
OUT
L
where I
OUTMAX
is the maximum output current,
K
RIPPLE
is the ripple factor, R
ESR
is the ESR of the
output capacitor, f
SW
is the switching frequency, L is
the inductor value, and 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. In
the case of tantalum or electrolytic capacitors, 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 capacitor, typically choose a
capacitance of about 22µF. For tantalum or
electrolytic capacitors, 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 a reverse
voltage 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 Power
TM
www.active-semi.com
Copyright © 2009 Active-Semi, Inc.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
