ACT4533C
Rev 1, 14-Nov-14
APPLICATIONS INFORMATION
CC Current Line Compensation
Output Voltage Setting
When operating at constant current mode, the
current limit increase slightly with input voltage. For
wide input voltage applications, a resistor RC may
be added to compensate line change and keep
output high CC accuracy, as shown in Figure 3.
Figure 1:
Output Voltage Setting
Figure 3:
Iutput Line Compensation
Figure 1 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. Adding a capacitor in parallel with
R
FB1 helps the system stability. Typically, use RFB2 ≈
10kꢀ and determine RFB1 from the following
equation:
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:
VOUT
⎛
⎜
⎞
⎟
RFB1 = RFB2
−1
(1)
0.808V
⎝
⎠
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:
CC Current Setting
ACT4533C 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 1.1V and the current gain from
ISET to output is roughly 21000 (21mA/1µA). To
determine the proper resistor for a desired current,
please refer to Figure 2 as below.
_
V
OUT × V VOUT
IN
(2)
L =
VINfSWILOADMAXKRIPPLE
where VIN is the input voltage, VOUT is the output
voltage, fSW is the switching frequency, ILOADMAX is
the maximum load current, and KRIPPLE is the ripple
Figure 2:
Curve for Programming Output CC Current
Output Current vs. RISET
factor. Typically, choose KRIPPLE
=
30% to
correspond to the peak-to-peak ripple current being
30% of the maximum load current.
4500
4000
3500
3000
2500
2000
1500
1000
With a selected inductor value the peak-to-peak
inductor current is estimated as:
_
VOUT × VIN VOUT
ILPK _
=
(3)
PK
L×VIN ×fSW
The peak inductor current is estimated as:
1
(4)
ILPK =ILOADMAX
+
ILPK_
PK
2
VIN = 24V, VOUT = 4V
18 22 26
500
The selected inductor should not saturate at ILPK.
The maximum output current is calculated as:
2
6
10
14
RISET (kꢀ)
Innovative PowerTM
- 6 -
www.active-semi.com
Copyright © 2014 Active-Semi, Inc.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
