ACT4065
APPLICATION INFORMATION
OUTPUT VOLTAGE SETTING
V
OU T
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.
ACT4065
FB
R
FB 1
R
2
FB
Figure 3. Output Voltage Setting
Figure 3 shows the connections for setting
the output voltage. Select the proper ratio of the
two feedback resistors R
FB1
and R
FB2
based on
the output voltage. Typically, use R
FB2
≈ 10kΩ
and determine R
FB1
from the output voltage:
R
FB1
V
=
R
FB2
OUT
−
1
1.293 V
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
IN
28
•
f
SW 2
LC
OUT
(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:
L
=
V
OUT
•
( V
IN
−
V
OUT
)
V
IN
f
SW
I
OUTMAX
K
RIPPLE
(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
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 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.
Table 1. Typical Inductor Values
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 in 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.
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
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.
V
OUT
L
1.5V
10μH
1.8V
10μH
2.5V
3.3V
5V
15μH 22μH 33μH
Active-Semi, Inc.
-5-
www.active-semi.com
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
