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Copyright © 2008 Active-Semi, Inc.
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
ACT4012A
Rev0, 16-May-08
Wide Input 1.5A Step Down Converter
ACT4012A
BS
IN
EN
COMP
FB
SW
G
ENABLE
12V
+
5V/1.5A
VIN
ACT4012A
Rev0, 16-May-08
Innovative PowerTM - 2 - www.active-semi.com
Copyright © 2008 Active-Semi, Inc.
ORDERING INFORMATION
PART NUMBER TEMPERATURE RANGE PACKAGE PINS PACKING
ACT4012ASH -40°C to 85°C SOP-8 8 TUBE
ACT4012ASH-T -40°C to 85°C SOP-8 8 TAPE & REEL
PIN CONFIGURATION
PIN DESCRIPTIONS
PIN NUMBER PIN NAME PIN DESCRIPTION
1 BS
Bootstrap. This pin acts as the positive rail for the high-side switch’s gate driver.
Connect a 10nF between this pin and SW.
2 IN
Input Supply. Bypass this pin to G with a low ESR capacitor. See Input Capacitor in
Application Information section.
3 SW Switch Output. Connect this pin to the switching end of the inductor.
4 G
Ground and Heat sink. Connect to a large, uncovered PCB copper area for best
heat dissipation.
5 FB
Feedback Input. The voltage at this pin is regulated to 1.293V. Connect to the resis-
tor divider between output and ground to set output voltage.
6 COMP Compensation Pin. See Compensation Technique in Application Information section.
7 EN
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. EN pin has
a small internal pull-up current to IN when pin is not connected.
8 N/C Not Connected.
SOP-8
18
7
5
6
2
3
4
BS
IN
SW
G
N/C
EN
COMP
FB
ACT4012ASH
ACT4012A
Rev0, 16-May-08
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Copyright © 2008 Active-Semi, Inc.
ABSOLUTE MAXIMUM RATINGSc
PARAMETER VALUE UNIT
IN to G -0.3 to 25 V
SW to G -1 to VIN + 1 V
BS to G VSW - 0.3 to VSW + 8 V
FB, COMP to G -0.3 to 6 V
Continuous SW Current Internally Limited A
Junction to Ambient Thermal Resistance (JA) 105 °C/W
Maximum Power Dissipation 0.76 W
Operating Junction Temperature -40 to 150 °C
Storage Temperature -55 to 150 °C
Lead Temperature (Soldering, 10 sec) 300 °C
EN to G -0.3 to VIN +0.3 V
ELECTRICAL CHARACTERISTICS
(VIN = 12V, TA = 25°C, unless otherwise specified.)
PARAMETER SYMBOL TEST CONDITIONS MIN TYP MAX UNIT
Input Voltage VIN V
OUT = 5V, ILOAD = 0A to 1.5A 7.5 20 V
Feedback Voltage VFB 4.75V VIN 20V 1.261 1.293 1.325 V
High-Side Switch On Resistance RONH 0.5
Low-Side Switch On Resistance RONL 13.5
SW Leakage VEN = 0 0 10 µA
Current Limit ILIM 1.8 2.7 A
COMP to Current Limit
Transconductance GCOMP 1.5 A/V
Error Amplifier Transconductance GEA ICOMP = ±10µA 565 µA/V
Error Amplifier DC Gain AVEA 4000 V/V
Switching Frequency fSW 310 420 530 kHz
Short Circuit Switching Frequency VFB = 0 50 kHz
Maximum Duty Cycle DMAX V
FB = 1.1V 90 %
Minimum Duty Cycle 14 %
Enable Threshold Voltage Hysteresis = 0.1V 0.7 1 1.3 V
Enable Pull-Up Current Pin pulled up to IN when left
unconnected 2 µA
Supply Current in Shutdown VEN = 0 10 20 µA
IC Supply Current in Operation VEN = 3V, VFB = 1.4V 0.85 mA
Thermal Shutdown Temperature Hysteresis = 10°C 160 °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.
ACT4012A
Rev0, 16-May-08
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Copyright © 2008 Active-Semi, Inc.
FUNCTIONAL BLOCK DIAGRAM
FUNCTIONAL DESCRIPTION
As seen in Functional Block Diagram, the
ACT4012A is a current mode pulse width modula-
tion (PWM) converter. The converter operates as
follows:
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 magnetic field. The inductor current
level is measured by the Current Sense Amplifier
and added to the Oscillator ramp signal. If the re-
sulting 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 volt-
age below ground, causing the inductor current to
decrease 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 as the positive rail. This pin is charged to VSW +
6V when the Low-Side Power Switch turns on.
The COMP voltage is the integration of the error
between FB input and the internal 1.293V 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 2.6V.
The Oscillator normally switches at 420kHz. How-
ever, if FB voltage is less than 0.7V, then the
switching frequency decreases until it reaches a
minimum of 50kHz at VFB = 0.5V.
Shutdown Control
The ACT4012A has an enable input EN for turning
the IC on or off. When EN is less than 0.7V, the IC
is 10µA 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. Note that EN is a high voltage
input that can with stand voltages up to VIN.
Thermal Shutdown
The ACT4012A automatically turns off when its
junction temperature exceeds 160°C.
IN
EN
COMP
FB
BS
SW
LOGIC
THERMAL
SHUTDOWN
REGULATOR
&
REFERENCE
+
-
OSCILLATOR
&
RAMP
FOLDBACK
CONTROL
1.293V
ERROR
AMPLIFIER
ENABLE
0.5
HIGH-SIDE
POWER
SWITCH
G
+-+-
-
+
PWM
COMP
CURRENT SENSE
AMPLIFIER
13 LOW-SIDE
POWER SWITCH
2µA
ACT4012A
Rev0, 16-May-08
Innovative PowerTM - 5 - www.active-semi.com
Copyright © 2008 Active-Semi, Inc.
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 10k and determine
RFB1 from the output voltage:
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:
VOUT 1.5V 1.8V 2.5V 3.3V 5V
L 7.5µH 10µH 12µH 15µH 22µH
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 50m 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.
Inductor Selection
APPLICATIONS INFORMATION
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.
(1)
(2)
Figure 1:
Output Voltage Setting
⎟
⎠
⎞
⎜
⎝
⎛−= 1
V293.1
V
RR OUT
2FB1FB
()
RIPPLEOUTMAXSWIN
OUTINOUT
KIfV
VVV
L−×
=
(3)
ESRRIPPLEOUTMAXRIPPLE RKIV =
OUT
2
SW
IN
LCf32
V
×
+
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:
Output Voltage Setting
Table 1:
Typical Inductor Values
RFB1
RFB2
VOUT
ACT4012A
FB
ACT4012A
Rev0, 16-May-08
Innovative PowerTM - 6 - www.active-semi.com
Copyright © 2008 Active-Semi, Inc.
STABILITY COMPENSATION STEP 2. Set the zero fZ1 at 1/4 of the cross over
frequency. If RCOMP is less than 15k, the equation
for CCOMP is:
If RCOMP is limited to 15k, then the actual cross
over frequency is 3.4 / (VOUTCOUT). Therefore:
STEP 3. If the output capacitor’s ESR is high
enough to cause a zero at lower than 4 times the
cross over frequency, an additional compensation
capacitor CCOMP2 is required. The condition for using
CCOMP2 is:
And the proper value for CCOMP2 is:
Though CCOMP2 is unnecessary when the output ca-
pacitor has sufficiently low ESR, a small value
CCOMP2 such as 100pF may improve stability against
PCB layout parasitic effects.
Table 2 shows some calculated results based on
the compensation method above.
Table 2:
Typical Compensation for Different O u tput
Voltages and Output Capacitor s
VOUT C
OUT R
COMP C
COMP C
COMP2c
2.5V 22µF Ceramic 12k 1.5nF None
3.3V 22µF Ceramic 15k 1.5nF None
5V 22µF Ceramic 15k 1.5nF None
2.5V 47µF SP CAP 15k 1.5nF None
3.3V 47µF SP CAP 15k 1.8nF None
5V 47µF SP CAP 15k 2.7nF None
2.5V 470µF/6.3V/30m 15k 15nF 1nF
3.3V 470µF/6.3V/30m 15k 22nF 1nF
5V 470µF/6.3V/30m 15k 27nF None
c: CCOMP2 is needed for high ESR output capacitor.
Figure 3 shows an example ACT4012A application circuit gener-
ating a 5V/1.5A output.
COMP
ESRCOUTOUT
2COMP R
RC
C=(13)
Figure 2:
Stability Compensation
(10)
(F)
COMP
5
COMP R
108.1
C
−
×
=
(11)
(F)
OUTOUT
5
COMP CV102.1C −
×=
c: CCOMP2 is needed only for high ESR output capacitor
The feedback system of the IC is stabilized by the
components at the COMP pin, as shown in Figure 2.
The DC loop gain of the system is determined by
the following equation:
The dominant pole P1 is due to CCOMP:
And finally, the third pole is due to RCOMP and
CCOMP2 (if CCOMP2 is used):
Follow the following steps to compensate the IC:
STEP 1. Set the cross over frequency at 1/10 of the
switching frequency via RCOMP:
but limit RCOMP to 15k maximum.
The first zero Z1 is due to RCOMP and CCOMP:
The second pole P2 is the output pole:
COMP2COMP
3P CRπ2
1
f=(8)
(Ω) (9)
V3.1GG10
fCVπ2
R
COMPEA
SWOUTOUT
COMP ×
=
OUTOUT
8CV102 ×=
(4)
COMPVEA
OUT
VDC GA
I
V3.1
A=
(5)
COMPVEA
EA
1P CAπ2
G
f=
(6)
OUTOUT
OUT
2P CVπ2
I
f=
(12)
(Ω)
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛×
×
≥
−
OUT
OUT
6
ESRCOUT V012.0,
C
101.1
MinR
(7)
COMP2COMP
1Z CRπ2
1
f=
CCOMP2
CCOMP
RCOMP
ACT4012A
COMP
1
ACT4012A
Rev0, 16-May-08
Innovative PowerTM - 7 - www.active-semi.com
Copyright © 2008 Active-Semi, Inc.
Figure 3:
ACT4012A 5V/1.5A Output Applicationc
c: D1 is a 30V, 2A Schottky diode with low forward voltage, an IR 20BQ030 or SK23 equivalent. C4 can be either a ceramic capacitor
(Panasonic ECJ-3YB1C226M) or SP-CAP (Specialty Polymer) Aluminum Electrolytic Capacitor such as Panasonic EEFCD0J470XR.
The SP-Cap is based on aluminum electrolytic capacitor technology, but uses a solid polymer electrolyte and has very stable capaci-
tance characteristics in both operating temperature and frequency compared to ceramic, polymer, and low ESR tantalum capacitors.
ACT4012A
BS
IN
EN
COMP
FB
SW
G
ENABLE
7.5V to 20V
+
5V/1.5A
VIN VOUT
R1 37k
D1
C4
22µF/10V
Ceramic
R2
13k
C5
(OPTIONAL)
R3
15k
C1
10µF/
25V
C2
1.5nF
L1 22µH/2A
C3
10nF
ACT4012A
Rev0, 16-May-08
Innovative PowerTM - 8 - www.active-semi.com
Copyright © 2008 Active-Semi, Inc.
TYPICAL PERFORMANCE CHARACTERISTICS
Load (A)
0.1 0.5 1.1 1.5
ACT4012A-002
70
80
60
85
95
0.1
Load (A)
0.3 0.5 0.7 0.9
ACT4012A-001
100
95
65
55
Efficiency vs. Load
(Circuit of Figure 3, unless otherwise specified.)
Input Voltage (V)
20
Shutdown Supply current (µA)
ACT4012A-006
0 12 16 20
Shutdown Supply current vs. Input Voltage
25
15
10
5
Load (A)
80
Surface Temperature (°C)
ACT4012A-003
0.1 0.3 0.7 0.9
Surface Temperature vs. Load
1.3
100
60
40
20
Junction Temperature (°C)
1.29
ACT4012A-005
-40 0 40 80
Feedback Voltage vs. Junction Temperature
120
1.30
1.28
1.27
1.26
90
85
80
70
60
1.1 1.3 1.5
Efficiency (%)
Efficiency (%)
Feedback Voltage (V)
90
75
65
0.3 0.7 0.9 1.3
VOUT = 3.3V
L= 15µH
COUT = 22µF/cera
0.5 1.1 1.5
VIN = 12V VIN = 20V
VIN = 8V
VOUT = 5V
L= 22µH
COUT = 22µF/cera
VIN = 8V
VIN = 12V
VIN = 20V
Efficiency vs. Load
75
5.0
Output Voltage (V)
ACT4012A-004
6 10 14
Output Voltage vs. Input Voltage
20
5.4
4.6
4.2
4.0
5.2
4.8
4.4
12 8
Input Voltage (V)
18 16
IOUT = 0.5A
IOUT = 1A IOUT = 1.5A
VIN = 12V
VIN = 20V
VIN = 8V
ACT4012A
Rev0, 16-May-08
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Copyright © 2008 Active-Semi, Inc.
8
Input Voltage (V)
12 16 20
ACT4012A-007
425
430
415
410
Switching Frequency (kHz)
Switching Frequency vs. Input Voltage
420
TYPICAL PERFORMANCE CHARACTERISTICS
(Circuit of Figure 3, unless otherwise specified.)
ACT4012A
Rev0, 16-May-08
Innovative PowerTM - 10 - www.active-semi.com
Copyright © 2008 Active-Semi, Inc.
Active-Semi, Inc. reserves the right to modify the circuitry or specifications without notice. Users should evaluate each
product to make sure that it is suitable for their applications. Active-Semi products are not intended or authorized for use
as critical components in life-support devices or systems. Active-Semi, Inc. does not assume any liability arising out of
the use of any product or circuit described in this datasheet, nor does it convey any patent license.
Active-Semi and its logo are trademarks of Active-Semi, Inc. For more information on this and other products, contact
sales@active-semi.com or visit http://www.active-semi.com. For other inquiries, please send to:
1270 Oakmead Parkway, Suite 310, Sunnyvale, California 94085-4044, USA
PACKAGE OUTLINE
SOP-8 PACKAGE OUTLINE AND DIMENSIONS
SYMBOL DIMENSION IN
MILLIMETERS DIMENSION IN
INCHES
MIN MAX MIN MAX
A 1.350 1.750 0.053 0.069
A1 0.100 0.250 0.004 0.010
A2 1.350 1.550 0.053 0.061
B 0.330 0.510 0.013 0.020
C 0.190 0.250 0.007 0.010
D 4.700 5.100 0.185 0.201
E 3.800 4.000 0.150 0.157
E1 5.800 6.300 0.228 0.248
e 1.270 TYP 0.050 TYP
L 0.400 1.270 0.016 0.050
0° 8° 0° 8°
CD
B
e
