ACT4050
Active- Semi
Rev 2, 01-Jul-11
APPLICATIONS INFORMATION
LLIM is the internal current limit, as shown in
Electrical Characteristics Table.
Output Voltage Setting
Figure 1:
Input Capacitor
Output Voltage Setting
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.
VOUT
ACT4050
FB
RFB1
RFB2
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 the
output voltage. Select the proper ratio of the two
feedback resistors RFB1 and RFB2 based on the
output voltage. Typically, use RFB2 ≈ 10kꢀ and
determine RFB1 from the following equation:
VOUT
⎛
⎜
⎞
⎟
RFB 1 = RFB 2
− 1
(1)
0.817 V
⎝
⎠
Note: To achieve best performance with 12V input application,
we recommend to use output voltage greater than 1.4V.
Inductor Selection
Output Capacitor
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:
The output capacitor also needs to have low ESR to
keep low output voltage ripple. The output ripple
voltage is:
VIN
VRIPPLE = IOUTMAXKRIPPLERESR
+
(6)
28 × fSW 2 LCOUT
where IOUTMAX is the maximum output current, KRIPPLE
is the ripple factor, RESR is the ESR of the output
capacitor, fSW is the switching frequency, L is the
inductor value, and COUT is the output capacitance. In
the case of ceramic output capacitors, RESR 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 RESR multiplied
by the ripple current. In that case, the output
capacitor is chosen to have sufficiently low ESR.
VOUT
×
(
VIN −VOUT
)
L =
(2)
VINfSW IOUTMAXKRIPPLE
where VIN is the input voltage, VOUT is the output
voltage, fSW is the switching frequency, IOUTMAX is the
maximum output current, and KRIPPLE is the ripple
factor. Typically, choose KRIPPLE
=
30% to
correspond to the peak-to-peak ripple current being
30% of the maximum output current.
For ceramic output capacitor, typically choose a
capacitance of about 22µF. For tantalum or
electrolytic capacitors, choose a capacitor with less
than 50mꢀ ESR.
With a selected inductor value the peak-to-peak
inductor current is estimated as:
VOUT × VIN -VOUT
ILPK
=
-PK
(3)
L ×VIN × fSW
Rectifier Diode
The peak inductor current is estimated as:
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
(4)
ILPK = ILOADMAX
+
ILPK -PK
2
The selected inductor should not saturate at ILPK.
The maximum output current is calculated as:
1
IOUTMAX
= ILIM
-
ILPK
- PK
(5)
2
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