Active-Semi
FEATURES
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2A Output Current
Up to 95% Efficiency
Up to 30V Input Range
6µA Shutdown Supply Current
420kHz Switching Frequency
Adjustable Output Voltage
Cycle-by-Cycle Current Limit Protection
Thermal Shutdown Protection
Frequency Fold-Back at Short Circuit
Stability with Wide Range of Capacitors,
Including Low ESR Ceramic Capacitors
ACT4072
Rev2, 27-May-08
Wide Input 2A Step Down Converter
GENERAL DESCRIPTION
The ACT4072 is a current-mode step-down DC/DC
converter that generates up to 2A output current at
420kHz switching frequency. The device utilizes
Active-Semi’s proprietary ISOBCD30 process for
operation with input voltage up to 30V.
Consuming only 6µA in shutdown mode, the
ACT4072 is highly efficient with peak efficiency at
95% when in operation. Protection features include
cycle-by-cycle current limit, thermal shutdown, and
frequency fold-back at short circuit.
The ACT4072 is available in SOP-8 package and
requires very few external devices for operation.
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SOP-8 Package
APPLICATIONS
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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
TYPICAL APPLICATION CIRCUIT
BS
IN
SW
V
OUT
V
IN
Up to 30V
ACT4072
ENABLE
EN
G
FB
COMP
+
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ORDERING INFORMATION
PART NUMBER
ACT4072SH
ACT4072SH-T
ACT4072
Rev2, 27-May-08
TEMPERATURE RANGE
-40°C to 85°C
-40°C to 85°C
PACKAGE
SOP-8
SOP-8
PINS
8
8
PACKING
TUBE
TAPE & REEL
PIN CONFIGURATION
BS
IN
SW
G
1
2
8
7
N/C
EN
COMP
FB
ACT4072SH
3
4
6
5
SOP-8
PIN DESCRIPTIONS
PIN
NUMBER
1
2
3
4
5
6
7
8
PIN NAME
BS
IN
SW
G
FB
COMP
EN
N/C
PIN DESCRIPTION
Bootstrap. This pin acts as the positive rail for the high-side switch’s gate driver. Con-
nect a 10nF between this pin and SW.
Input Supply. Bypass this pin to G with a low ESR capacitor. See
Input Capacitor
in
Ap-
plication Information
section.
Switch Output. Connect this pin to the switching end of the inductor.
Ground.
Feedback Input. The voltage at this pin is regulated to 1.222V. Connect to the resistor
divider between the output and ground to set output voltage.
Compensation Pin. See
Compensation Techniques
in
Application Information
section.
Enable Input. Drive higher than 1.3V or leave unconnected to enable the IC. Drive lower
than 0.7V to disable the IC. When disabled, the IC is in 6µA low current shutdown mode
and the output is discharged through the Low-Side Power Switch. This pin has a small
internal pull up current to a high level voltage when pin is not connected.
Not Connected.
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ABSOLUTE MAXIMUM RATINGS
PARAMETER
IN to G
EN to G
SW to G
BS to SW
FB, COMP to G
Continuous SW Current
Junction to Ambient Thermal Resistance (θ
JA
)
Maximum Power Dissipation
Operating Junction Temperature
Storage Temperature
Lead Temperature (Soldering, 10 sec)
Ambient Operating Temperature
ACT4072
Rev2, 27-May-08
VALUE
-0.3 to +34
-0.3 to V
IN
+ 0.3
-1 to V
IN
+ 1
-0.3 to +8
-0.3 to +6
Internally limited
105
0.76
-40 to 150
-55 to 150
300
-40 to 85
UNIT
V
V
V
V
V
A
°C/W
W
°C
°C
°C
°C
: Do not exceed these limits to prevent damage to the device. Exposure to absolute maximum rating conditions for long periods may
affect device reliability.
ELECTRICAL CHARACTERISTICS
(V
IN
= 12V, T
A
= 25°C, unless otherwise specified.)
PARAMETER
Input Voltage
Feedback Voltage
High-Side Switch On Resistance
Low-Side Switch On Resistance
SW Leakage
Current Limit
COMP to Current Limit Transconductance
Error Amplifier Transconductance
Error Amplifier DC Gain
Switching Frequency
Short Circuit Switching Frequency
Maximum Duty Cycle
Minimum Duty Cycle
Enable Threshold Voltage
Enable Pull Up Current
Supply Current in Shutdown
IC Supply Current in Operation
Thermal Shutdown Temperature
SYMBOL
V
IN
V
FB
R
ONH
R
ONL
TEST CONDITIONS
V
IN
= 12V
MIN
4.5
1.198
TYP
1.222
0.13
10
MAX UNIT
30
1.246
V
V
Ω
Ω
10
µA
A
A/V
µA/V
V/V
500
kHz
kHz
%
%
1.3
V
µA
20
2
µA
mA
°C
V
EN
= 0
I
LIM
G
COMP
G
EA
A
VEA
f
SW
V
FB
= 0
D
MAX
D
MIN
V
FB
= 1.1V, PWM mode
V
FB
= 1.4V, PFM mode
Hysteresis = 0.1V
0.7
340
∆I
COMP
= ±10µA
2.4
0
3.3
2
550
4000
420
60
88
0
1
2
V
EN
= 0
V
EN
= 3V, not switching
Hysteresis = 10°C
6
0.8
160
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FUNCTIONAL BLOCK DIAGRAM
IN
EN
2µA
ENABLE
REGULATOR
&
REFERENCE
ACT4072
Rev2, 27-May-08
BS
CURRENT SENSE
AMPLIFIER
COMP
1.222V
ERROR
AMPLIFIER
+
-
0.13O
Ω
HIGH-SIDE
POWER
SWITCH
FB
+
-
FOLDBACK
CONTROL
OSCILLATOR
&
RAMP
+ - + -
PWM
COMP
LOGIC
SW
THERMAL
SHUTDOWN
10O LOW-SIDE
Ω
POWER SWITCH
G
FUNCTIONAL DESCRIPTION
As seen in the
Functional Block Diagram,
the
ACT4072 is a current mode pulse width modulation
(PWM) converter. The converter operates as fol-
lows:
A switching cycle starts when the rising edge of the
Oscillator clock output causes the High-Side Power
Switch to turn on and the Low-Side Power Switch to
turn off. With the SW side of the inductor now con-
nected to IN, the inductor current ramps up to store
energy in its magnetic field. The inductor current
level is measured by the Current Sense Amplifier
and added to the Oscillator ramp signal. If the result-
ing summation is higher than the COMP voltage, the
output of the PWM Comparator goes high. When
this happens or when Oscillator clock output goes
low, the High-Side Power Switch turns off and the
Low-Side Power Switch turns on. At this point, the
SW side of the inductor swings to a diode voltage
below ground, causing the inductor current to de-
crease and magnetic energy to be transferred to the
output. This state continues until the cycle starts
again.
The High-Side Power Switch is driven by logic using
the BS bootstrap pin as the positive rail. This pin is
charged to V
SW
+ 6V when the Low-Side Power
Switch turns on.
The COMP voltage is the integration of the error
between the FB input and the internal 1.222V refer-
ence. If FB is lower than the reference voltage,
COMP tends to go higher to increase current to the
output. Current limit happens when COMP reaches
its maximum clamp value of 2.55V.
The Oscillator normally switches at 420kHz. How-
ever, if the FB voltage is less than 0.7V, then the
switching frequency decreases until it reaches a
minimum of 60kHz at V
FB
= 0.5V.
Shutdown Control
The ACT4072 has an enable input EN for turning
the IC on or off. When EN is less than 0.7V, the IC
is in 6µA low current shutdown mode and the out-
put is discharged through the Low-Side Power
Switch. When EN is higher than 1.3V, the IC is in
normal operation mode. EN is internally pulled up
with a 2µA current source and can be left uncon-
nected for always-on operation.
Thermal Shutdown
The ACT4072 automatically turns off when its junc-
tion temperature exceeds 160°C.
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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 R
FB1
and R
FB2
based on the output
voltage. Typically, use R
FB2
≈
10kΩ and determine
R
FB1
from the output voltage:
ACT4072
Rev2, 27-May-08
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 a 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 shortest possible traces. 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.
⎛
V
⎞
R
FB1
=
R
FB2
⎜
OUT
-
1
⎟
⎝
1 .222 V
⎠
Figure 1:
Output Voltage Setting
V
OUT
(1)
ACT4072
FB
R
FB1
R
FB2
Output Capacitor
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:
The output capacitor also needs to have low ESR to
keep low output voltage ripple. The output ripple
voltage is:
R
RIPPLE
=
L
OUTMAX
K
RIPPLE
R
ESR
+
V
IN
2
28 f
SW
LC
OUT
(3)
L
=
V
OUT
×
(
V
IN
-
V
OUT
)
V
IN
f
SW
I
OUTMAX
K
RIPPLE
(2)
where I
OUTMAX
is the maximum output current,
K
RIPPLE
is the ripple factor, R
ESR
is the ESR
resistance of the output capacitor, f
SW
is the
switching frequency, L is the inductor value, C
OUT
is
the output capacitance.
In the case of ceramic output capacitors, R
ESR
is
very small and does not contribute to the ripple.
Therefore, a lower capacitance value can be used
for ceramic type, typically choose a capacitance of
about 22µF.
In the case of tantalum or electrolytic type, the ripple
is dominated by R
ESR
multiplied by the ripple current.
In that case, the output capacitor is chosen to have
sufficiently low due to ESR, typically choose a ca-
pacitor with less than 50mΩ ESR.
where V
IN
is the input voltage, V
OUT
is the output
voltage, f
SW
is the switching frequency, I
OUTMAX
is the
maximum output current, and K
RIPPLE
is the ripple
factor. Typically, choose K
RIPPLE
= 20% to 30%
corresponding to the peak-to-peak ripple current
being 20% to 30% of the maximum output current.
With this inductor value (Table 1), the peak inductor
current is I
OUT
× (1 + K
RIPPLE
/ 2). Make sure that this
peak inductor current is less that the 2.4A current
limit. Finally, select the inductor core size so that it
does not saturate at 2.4A.
Table 1:
Typical Inductor Values
V
OUT
L
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.
1.5V
6.8µH
1.8V
6.8µH
2.5V
10µH
3.3V
15µH
5V
22µH
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