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PDF ACT4012A Data sheet ( Hoja de datos )
| Número de pieza | ACT4012A | |
| Descripción | Wide Input 1.5A Step Down Converter | |
| Fabricantes | Active-Semi | |
| Logotipo |  |
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ACT4012A
Rev0, 16-May-08
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
• 420kHz 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
• S-Top Boxes
• Telecom Power Supplies
• DSL and Cable Modems and Routers
• Termination Supplies
GENERAL DESCRIPTION
The ACT4012A is a current-mode step-down
DC/DC converter that generates up to 1.5A output
420kHz switching frequency. The device utilizes
Active-Semi’s proprietary ISOBCD20 process for
operation with input voltages up to 20V.
Consuming only 10µA in shutdown mode, the
ACT4012A 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 ACT4012A is available in a SOP-8 package
and requires very few external devices for opera-
tion.
TYPICAL APPLICATION CIRCUIT
12V
VIN
ENABLE
BS
IN SW
ACT4012A
EN
G
FB
COMP
+
5V/1.5A
Innovative PowerTM
- 1 - www.active-semi.com
Copyright © 2008 Active-Semi, Inc.
1 page  
ACT4012A
Rev0, 16-May-08
APPLICATIONS INFORMATION
Output Voltage Setting
Figure 1:
Output Voltage Setting
VOUT
ACT4012A
FB
RFB1
RFB2
Figure 1 shows the connections for setting the out-
put voltage. Select the proper ratio of the two feed-
back resistors RFB1 and RFB2 based on the output
voltage. Typically, use RFB2 ≈ 10kΩ and determine
RFB1 from the output voltage:
RFB1
=
R
FB
2
⎜⎛
⎝
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 induc-
tance 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 fSW
× VIN − VOUT
I KOUTMAX RIPPLE
(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 corre-
spond to the peak-to-peak ripple current being 30%
of the maximum output current.
With this inductor value, the peak inductor current is
IOUT × (1 + KRIPPLE/2). Make sure that this peak in-
ductor current is less that the 3A current limit. Fi-
nally, select the inductor core size so that it does
not saturate at 3A.
Table 1:
Typical Inductor Values
VOUT
L
1.5V
7.5µH
1.8V
10µH
2.5V
12µH
3.3V
15µH
5V
22µH
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 rec-
ommended. 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, how-
ever, 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 ce-
ramic 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:
V = I K RRIPPLE
OUTMAX RIPPLE ESR
+
32
VIN
× fSW 2 LC OUT
(3)
where IOUTMAX is the maximum output current, KRIP-
PLE is the ripple factor, RESR is the ESR resistance of
the output capacitor, fSW is the switching frequency,
L is the inductor value, COUT is the output capaci-
tance. 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 capaci-
tance 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 cur-
rent 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.
Innovative PowerTM
- 5 - www.active-semi.com
Copyright © 2008 Active-Semi, Inc.
5 Page | ||
| Páginas | Total 10 Páginas | |
| PDF Descargar | [ Datasheet ACT4012A.PDF ] | |
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