ACT4501
Rev 2, 14-Nov-12
APPLICATIONS INFORMATION
Output Voltage Setting
Figure 1:
Output Voltage Setting
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:
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
V
OUT
×
(
V
IN
_
V
OUT
)
L
=
V
IN
f
SW
I
LOADMAX
K
RIPPLE
(2)
⎛
V
⎞
R
FB1
=
R
FB2
⎜
OUT
−
1
⎟
⎝
0.808V
⎠
(1)
where V
IN
is the input voltage, V
OUT
is the output
voltage, f
SW
is the switching frequency, I
LOADMAX
is
the maximum load 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 load current.
With a selected inductor value the peak-to-peak
inductor current is estimated as:
I
LPK
_
PK
V
OUT
×
(
V
IN
_
V
OUT
)
=
L
×
V
IN
×
f
SW
CC Current Setting
ACT4501 constant current value is set by a resistor
connected between the ISET pin and GND. The CC
output current is linearly proportional to the current
flowing out of the ISET pin. The voltage at ISET is
roughly 1V and the current gain from ISET to output
is roughly 25000 (25mA/1µA). To determine the
proper resistor for a desired current, please refer to
Figure 2 below.
Figure 2:
Curve for Programming Output CC Current
(3)
The peak inductor current is estimated as:
1
I
LPK
=
I
LOADMAX
+
I
LPK
_
PK
2
(4)
The selected inductor should not saturate at I
LPK.
The maximum output current is calculated as:
I
OUTMAX
=
I
LIM
_
ACT4501-002
Output Current vs. R
ISET
1800
1600
1
I
_
2
LPK PK
(5)
Output Current (mA)
1400
1200
1000
800
600
400
200
0
0
10
20
30
40
50
60
70
80
90
L
LIM
is the internal current limit, which is typically
2.5A, as shown in Electrical Characteristics Table.
External High Voltage Bias Diode
It is recommended that an external High Voltage
Bias diode be added when the system has a 5V
fixed input or the power supply generates a 5V
output. This helps improve the efficiency of the
regulator. The High Voltage Bias diode can be a
low cost one such as IN4148 or BAT54.
R
ISET
(kΩ)
Innovative Power
TM
-6-
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