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PDF ACT4070B Data sheet ( Hoja de datos )

Número de pieza ACT4070B
Descripción Wide Input 3A Step Down Converter
Fabricantes Active-Semi 
Logotipo Active-Semi Logotipo



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ACT4070B
Rev 0, 23-Apr-12
Wide Input 3A Step Down Converter
FEATURES
3A Output Current
Up to 95% Efficiency
6.5V to 30V Input Range
100µA Shutdown Supply Current
4mA Standby Input Current
300kHz Switching Frequency
Output Voltage Up to 12V
Cycle-by-Cycle Current Limit Protection
Thermal Shutdown Protection
Internal Soft Start Function
Frequency Fold Back at Short Circuit
Stability with Wide Range of Capacitors
Including Low ESR Ceramic Capacitors
SOP-8/EP (Exposed Pad) Package
APPLICATIONS
TFT LCD Monitors or Televisions and HDTV
Portable DVD Players
Car-Powered or Battery-Powered Equipment
Set-Top Boxes
Telecom Power Supplies
DSL and Cable Modems and Routers
GENERAL DESCRIPTION
ACT4070B is a wide input voltage step-down
DC/DC converter that provides up to 3A output cur-
rent at 300kHz switching frequency. ACT4070B is a
replacement part for ACT4070 with advanced fea-
tures such as lower standby current and higher light
load efficiency. ACT4070B can be dropped into
ACT4070 socket with only feedback resistance
value changed.
ACT4070B’s protection features include Cycle-by-
Cycle current limit, thermal shutdown, and fre-
quency foldback at over current and short circuit.
The devices are available in a SOP-8EP package
and require very few external devices for operation.
NOTE:
ACT4070B is the replacement part for
ACT4070.
TYPICAL APPLICATION CIRCUIT
100
80
60
40
20
0
1
Efficiency vs. Load Current
VIN = 12V
VIN = 24V
VOUT = 5V
10
100
1000
10000
Load Current (mA)
Innovative PowerTM
- 1 - www.active-semi.com
Copyright © 2012 Active-Semi, Inc.

1 page




ACT4070B pdf
ACT4070B
Rev 0, 23-Apr-12
APPLICATIONS INFORMATION
Output Voltage Setting
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 10kand determine
RFB1 from the output voltage:
R FB 1
=
R FB2
⎜⎛ V OUT
0 . 808
V
- 1 ⎟⎞
Figure 1:
Output Voltage Setting
(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 -VOUT
V f I KIN SW OUTMAX 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 = between 20% and
30% to correspond to the peak-to-peak ripple current
being a percentage 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 5A current
limit. Finally, select the inductor core size so that it
does not saturate at 5A.
Table 1:
Typical Inductor Values
VOUT
L
1.5V
6.8μH
1.8V
6.8μH
2.5V
8.5μH
3.3V
15μH
5V
15μH
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 recom-
mended. Since large current flows in and out of this
capacitor during switching, its ESR also affects effi-
ciency.
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 ca-
pacitor 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 RIPPLE
+
28
×
VIN
fSW 2LCOUT
(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. There-
fore, a lower capacitance value can be used for ce-
ramic 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 50mESR.
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 the reverse volt-
age rating higher than the maximum input voltage.
Innovative PowerTM
- 5 - www.active-semi.com
Copyright © 2012 Active-Semi, Inc.

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