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PDF ACT4012 Data sheet ( Hoja de datos )
| Número de pieza | ACT4012 | |
| Descripción | Wide Input 1.5A Step Down Converter | |
| Fabricantes | Active-Semi | |
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Data Sheet
Rev 1, 10/2006
ACT4012
Wide Input 1.5A Step Down Converter
FEATURES
1.5A Output Current
Up to 94% Efficiency
Up to 20V Input Range
10µA Shutdown Supply Current
400kHz Switching Frequency
Adjustable Output Voltage
Cycle-by-Cycle Current Limit Protection
Thermal Shutdown Protection
Frequency Foldback at Short Circuit
Stability with Wide Range of Capacitors,
Including Low ESR Ceramic Capacitors
SOP-8 Package
APPLICATIONS
TFT LCD Monitors
Portable DVDs
Car-Powered or Battery-Powered Equipments
Set-Top Boxes
Telecom Power Supplies
DSL and Cable Modems and Routers
Termination Supplies
GENERAL DESCRIPTION
The ACT4012 is a current-mode step-down
DC-DC converter that generates up to 1.5A
output current at 400kHz switching frequency.
The device utilizes Active-Semi’s proprietary
ISOBCD20 process for operation with input
voltage up to 20V.
Consuming only 10μA in shutdown mode, the
ACT4012 is highly efficient with peak efficiency
at 94% when in operation. Protection features
include cycle-by-cycle current limit, thermal
shutdown, and frequency foldback at short
circuit.
The ACT4012 is available in SOP-8 package
and requires very few external devices for
operation.
Active-Semi, Inc.
Figure 1. Typical Application Circuit
1
www.active-semi.com
1 page  
APPLICATION INFORMATION
OUTPUT VOLTAGE SETTING
VOU T
ACT4012
RFB 1
FB
RFB 2
Figure 3. Output Voltage Setting
Figure 3 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 ≈ 10kΩ
and determine RFB1 from the output voltage:
RFB1
=
R
FB2
VOUT
1.293V
−
1
(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=
VOUT • (VIN − VOUT )
VIN fSW IOUTMAX KRIPPLE
(2)
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.
With this inductor value (Table 1), the peak
inductor current is IOUT • (1 + KRIPPLE / 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
VOUT 1.5V 1.8V 2.5V 3.3V 5V
L 7.5μH 10μH 12μH 15μH 22μH
ACT4012
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.
OUTPUT CAPACITOR
The output capacitor also needs to have low
ESR to keep low output voltage ripple. The
output ripple voltage is:
VRIPPLE = IOUTMAX K RIPPLE RESR
+
32 •
VIN
fSW 2 LCOUT
(3)
where IOUTMAX is the maximum output current,
KRIPPLE is the ripple factor, RESR is the ESR
resistance of the output capacitor, fSW is the
switching frequency, L in the inductor value, 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 type, the
ripple is dominated by RESR 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.
Active-Semi, Inc.
5 www.active-semi.com
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| Páginas | Total 10 Páginas | |
| PDF Descargar | [ Datasheet ACT4012.PDF ] | |
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