Rev PrA, 13-Aug-07
Advanced Product Information – All Information Subject to Change
ACT4075
Wide Input 2.5A Step Down Converter
FEATURES
2.5A Output Current
Up to 95% Efficiency
4.5V to 32V Input Range
6µA Shutdown Supply Current
410kHz Switching Frequency
Adjustable Output Voltage
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
The ACT4075 is a current-mode step-down DC/DC
converter that generates up to 2.5A output current
at 410kHz switching frequency. The device utilizes
Active-Semi’s proprietary ISOBCD30 process for
operation with input voltage up to 32V.
Consuming only 6μA in shutdown mode, the
ACT4075 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 device also
includes an internal soft start function to prevent
overshoot.
The ACT4075 is available in SOP-8/EP exposed
pad package and requires very few external de-
vices for operation.
GENERAL DESCRIPTION
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
TYPICAL APPLICATION CIRCUIT
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Copyright © 2007 Active-Semi, Inc.
Rev PrA, 13-Aug-07
ACT4075
PACKING
TUBE
TAPE & REEL
ORDERING INFORMATION
PART NUMBER
ACT4075YH
ACT4075YH-T
TEMPERATURE RANGE
-40°C to 85°C
-40°C to 85°C
PACKAGE
SOP-8/EP
SOP-8/EP
PINS
8
8
PIN CONFIGURATION
BS
IN
SW
G
1
2
8
7
N/C
EN
COMP
FB
ACT4075
3
4
EP
SOP-8/EP
6
5
PIN DESCRIPTIONS
PIN NUMBER
1
2
3
4
5
6
PIN NAME
BS
IN
SW
G
FB
COMP
PIN DESCRIPTION
Bootstrap. This pin acts as the positive rail for the high-side switch’s gate driver.
Connect a 10nF between this pin and SW.
Input Supply. Bypass this pin to G with a low ESR capacitor. See
Input Capacitor
in
Application 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 output and ground to set output voltage.
Compensation Pin. See
Compensation Technique
in
Application Information
sec-
tion.
Enable Input. When higher than 1.3V, this pin turns the IC on. When lower than
0.7V, this pin turns the IC off. Output voltage is discharged when the IC is off. This
pin has a small internal pull up current to a high level voltage when pin is not con-
nected.
Not Connected.
Exposed Pad shown as dashed box. The exposed thermal pad should be con-
nected to board ground plane and pin 4. The ground plane should include a large
exposed copper pad under the package for thermal dissipation (see package out-
line). The leads and exposed pad should be flush with the board, without offset
from the board surface.
7
8
EN
N/C
EP
EP
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Rev PrA, 13-Aug-07
ACT4075
UNIT
V
V
V
V
V
A
°C/W
W
°C
°C
°C
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)
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
46
1.8
-40 to 150
-55 to 150
300
:
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 Transcon-
ductance
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
OUT
= 2.5V, I
LOAD
= 0A to 2.5A
MIN
4.5
TYP
MAX UNIT
32
V
V
mΩ
Ω
10
µA
A
A/V
µA/V
V/V
490
kHz
kHz
%
%
1.3
V
µA
20
2
µA
mA
°C
1.198 1.222 1.246
100
10
V
EN
= 0, V
IN
= 12V, V
SW
= 0V
0
3.5
ΔI
LOAD
/ΔI
COMP
ΔI
COMP
= ±10µA
340
V
FB
= 0V
V
FB
= 1.1V, PWM mode
V
FB
= 1.4V, PFM mode
Hysteresis = 0.1V
Pin pulled up to V
IN
when left uncon-
nected
V
EN
= 0
V
EN
= 3V, not switching
Hysteresis = 10°C
0.7
5
3
550
4000
410
40
90
0
1
2
6
0.85
160
I
LIM
G
COMP
G
EA
A
VEA
f
SW
D
MAX
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Copyright © 2007 Active-Semi, Inc.
Rev PrA, 13-Aug-07
ACT4075
FUNCTIONAL BLOCK DIAGRAM
FUNCTIONAL DESCRIPTION
As seen in the
Functional Block Diagram,
the
ACT4075 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 the 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
output. This state continues until the cycle starts
again.
The High-Side Power Switch is driven by logic using
BS bootstrap pin as the positive rail. This pin is
charged to V
SW
+ 6V when the Low-Side Power
Switch turns on.
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The COMP voltage is the integration of the error
between 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 2.65V.
The Oscillator normally switches at 410kHz. How-
ever, if FB voltage is less than 0.7V, then the
switching frequency decreases until it reaches a
typical value of 40kHz at V
FB
= 0V.
Shutdown Control
The ACT4075 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 output 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 unconnected for
always-on operation.
Thermal Shutdown
The ACT4075 automatically turns off when its junc-
tion temperature exceeds 160°C and then restarts
once the temperature falls to 150°C.
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Copyright © 2007 Active-Semi, Inc.
Rev PrA, 13-Aug-07
ACT4075
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:
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.
V
R
FB1
½
R
FB2
OUT
-
1
1.222V
Figure 1:
Output Voltage Setting
(1)
V
OUT
ACT4075
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:
V
RIPPLE
½
I
OUTMAX
K
RIPPLE
R
RIPPLE
V
IN
28
f
SW
LC
OUT
2
(3)
L
½
V
OUT
V
IN
-
V
OUT
V
IN
f
SW
I
OUTMAX
K
RIPPLE
(2)
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
= 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 I
OUT
× (1 + K
RIPPLE
/ 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
V
OUT
L
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. 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 R
ESR
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 the reverse volt-
age rating higher than the maximum input voltage.
1.5V
6.8μH
1.8V
6.8μH
2.5V
6.8μH
3.3V
8.5μH
5V
15μH
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