EA4533BYH Datasheet, PDF (7/17 Pages) Active-Semi, Inc – Wide-Input Sensorless CC/CV Step-Down DC/DC Converter

APPLICATIONS INFORMATION
Output Voltage Setting
Figure 1:
Output Voltage Setting
ACT4533A/B
Rev 0, 14-Aug-13
CC Current Line Compensation
When operating at constant current mode, the
current limit increase slightly with input voltage. For
wide input voltage applications, a resistor R_C may
be added to compensate line change and keep
output high CC accuracy, as shown in Figure 3.
Figure 3:
Iutput Line Compensation
Figure 1 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. Adding a capacitor in parallel with
R_FB1 helps the system stability. Typically, use R_FB2 ≈
10kΩ and determine R_FB1 from the following
equation:
R _FB1
=
R_{FB 2}
⎜⎛
⎝
V_OUT
0.808V
− 1 ⎟⎞
⎠
(1)
CC Current Setting
ACT4533A/B 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 below.
Figure 2:
Curve for Programming Output CC Current
Output Current vs. R_ISET
4500
4000
3500
3000
2500
2000
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
(2)
V f I K _{IN SW LOADMAX RIPPLE}
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:
( ) _{ILPK _PK}
=
V_OUT
×
V_IN
V_
OUT
L ×V_IN × f_SW
(3)
The peak inductor current is estimated as:
1500
1000
VIN = 24V, VOUT = 4V
500
2
6
10
14
18
22
26
R_ISET (kΩ)
1
I_LPK
=
I_LOADMAX
+
2
I
LPK
__PK
(4)
The selected inductor should not saturate at I_LPK.
The maximum output current is calculated as:
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