ACT4065
High Input 2A Step Down Converter
FEATURES
2A Output Current
Up to 95% Efficiency
Up to 28V Input Range
8µA Shutdown Supply Current
200kHz 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
Set-Top Boxes
Telecom Power Supplies
DSL and Cable Modems and Routers
Termination Supplies
GENERAL DESCRIPTION
The ACT4065 is a current-mode step-down
DC-DC converter that generates up to 2A output
current at 200kHz switching frequency. The
device utilizes Active-Semi’s proprietary
ISOBCD30 process for operation with input
voltage up to 28V.
Consuming only 8μA in shutdown mode, the
ACT4065 is highly efficient with peak efficiency
at 95% when in operation. Protection features
include cycle-by-cycle current limit, thermal
shutdown, and frequency foldback at short
circuit.
The ACT4065 is available in SOP-8 package
and requires very few external devices for
operation.
Active-Semi, Inc. - 1 - www.active-semi.com
Data Sheet
Rev 2, 6/2006
Figure 1. Typical Application Circuit
ACT4065
IN
BS
EN FB
G COMP
SW
+
8.5V to 28V
ENABLE
5V/1.7A
5V/2A (<23V INPUT)
5V/1.7A (>23V INPUT)
ACT4065
ORDERING INFORMATION
PART NUMBER TEMPERATURE RANGE PACKAGE PINS PACKING
ACT4065SH -40°C to 85°C SOP-8 8 TUBE
ACT4065SH-T -40°C to 85°C SOP-8 8 TAPE & REEL
PIN CONFIGURATION
PIN DESCRIPTION
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.
5 FB Feedback Input. The voltage at this pin is regulated to 1.293V. Connect to the resistor
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. This pin has a
small internal pull up current to a high level voltage when pin is not connected. Do not
allow EN pin to exceed 6V.
8 N/C Not Connected.
Active-Semi, Inc. - 2 - www.active-semi.com
1
BS
SOP-8
2
IN
3
SW
4
G
8
7
6
5
N/C
EN
COMP
FB
ACT4065SH
ACT4065
ABSOLUTE MAXIMUM RATINGS
(Note: Exceeding these limits may damage the device. Exposure to absolute maximum rating conditions for long periods may affect device
reliability.)
PARAMETER VALUE UNIT
IN Supply Voltage -0.3 to 30 V
SW Voltage -1 to V
IN
+ 1 V
BS Voltage V
SW
– 0.3 to V
SW
+ 8 V
EN, FB, COMP Voltage -0.3 to 6 V
Continuous SW Current Internally limited A
Maximum Power Dissipation 0.76 W
Junction to Ambient Thermal Resistance (
θ
JA
)105 °C/W
Operating Junction Temperature -40 to 150 °C
Storage Temperature -55 to 150 °C
Lead Temperature (Soldering, 10 sec) 300 °C
ELECTRICAL CHARACTERISTICS
(V
IN
= 12V, T
J
= 25°C unless otherwise specified.)
PARAMETER SYMBOL TEST CONDITIONS MIN TYP MAX UNIT
Input Voltage V
IN
V
OUT
= 5V, I
LOAD
= 1A 6 28 V
Feedback Voltage V
FB
V
COMP
= 1.5V 1.267 1.293 1.319 V
High-Side Switch On Resistance R
ONH
0.2 Ω
Low-Side Switch On Resistance R
ONL
8 Ω
SW Leakage V
EN
= 0 0 10 µA
Current Limit I
LIM
3 3.5 A
COMP to Current Limit
Transconductance G
COMP
1.8 A/V
Error Amplifier Transconductance G
EA
ΔI
COMP
= ±10µA 550 µA/V
Error Amplifier DC Gain A
VEA
4000 V/V
Switching Frequency f
SW
160 200 240 kHz
Short Circuit Switching Frequency V
FB
= 0 50 kHz
Maximum Duty Cycle D
MAX
V
FB
= 1.1V 93 %
Minimum Duty Cycle V
FB
= 1.4V 0 %
Enable Threshold Voltage Hysteresis = 0.1V 0.7 1 1.3 V
Enable Pull Up Current Pin pulled up to 4.5V typically when
left unconnected 1 µA
Supply Current in Shutdown V
EN
= 0 8 20 µA
IC Supply Current in Operation V
EN
= 3V, V
FB
= 1.4V 0.7 mA
Thermal Shutdown Temperature Hysteresis = 10°C 160 °C
Active-Semi, Inc. - 3 - www.active-semi.com
ACT4065
Figure 2. Functional Block Diagram
FUNCTIONAL DESCRIPTION
As seen in Figure 2, Functional Block
Diagram, the ACT4065 is a current mode pulse
width modulation (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 connected 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
resulting 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 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 bootstrap pin 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
reference. 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 200kHz.
However, 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 ACT4065 has an enable input EN for
turning the IC on or off. When EN is less than
0.7V, the IC is in 8μA low current shutdown
mode. 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 low voltage input with a maximum
voltage of 6V; it should never be directly
connected to IN.
THERMAL SHUTDOWN
The ACT4065 automatically turns off when its
junction temperature exceeds 160°C.
Active-Semi, Inc. - 4 - www.active-semi.com
IN
EN
COMP
FB
BS
SW
REGULATOR
&
REFERENCE
OSCILLATOR
&
RAMP
+
–
+
–
+ – + –
PWM
COMPARATOR
LOGIC
THERMAL
SHUTDOWN
G
2μA
ENABLE
ERROR
AMPLIFIER
1.293V
FOLDBACK
CONTROL
0.2Ω
HIGH-SIDE
POWER
SWITCH
8Ω LOW-SIDE
POWER SWITCH
CURRENT SENSE
AMPLIFIER
ACT4065
APPLICATION INFORMATION
OUTPUT VOLTAGE SETTING
Figure 3. Output Voltage Setting
Figure 3 shows the connections for setting
the output voltage. Select the proper ratio of the
two feedback resistors RFB1 and RFB2 based on
the output voltage. Typically, use RFB2 ≈ 10kΩ
and determine RFB1 from the output voltage:
−=
1
V293.1
V
RR
OUT
2FB1FB
(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 inductance 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:
RIPPLEOUTMAXSWIN
OUTINOUT
KIfV
)VV(V
L
−•
=
(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
correspond to the peak-to-peak ripple current
being 30% 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 3A current limit. Finally, select the inductor
core size so that it does not saturate at 3A.
Table 1. Typical Inductor Values
V
OUT
1.5V 1.8V 2.5V 3.3V 5V
L
10μH 10μH 15μH 22μH 33μH
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 recommended. 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;
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 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:
ESRRIPPLEOUTMAXRIPPLE
RKIV
=
OUT
2
SW
IN
LCf28
V
•
+
(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. 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
capacitance 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 current when the High-Side Power
Switch is off. The Schottky diode must have
current rating higher than the maximum output
current and the reverse voltage rating higher
than the maximum input voltage.
Active-Semi, Inc. - 5 - www.active-semi.com
ACT4065
STABILITY COMPENSATION
*CCOMP2 is needed only for high ESR output capacitor
Figure 4. Stability Compensation
The feedback system of the IC is stabilized
by the components at COMP pin, as shown in
Figure 4. The DC loop gain of the system is
determined by the following equation:
COMPVEA
OUT
VDC
GA
I
V293.1
A
=
(4)
The dominant pole P1 is due to CCOMP:
COMPVEA
EA
1P
CAπ2
G
f
=
(5)
The second pole P2 is the output pole:
OUTOUT
OUT
2P
CVπ2
I
f
=
(6)
The first zero Z1 is due to RCOMP and CCOMP:
COMPCOMP
1Z
CRπ2
1
f
=
(7)
And finally, the third pole is due to RCOMP and
CCOMP2 (if CCOMP2 is used):
2COMPCOMP
3P
CRπ2
1
f
=
(8)
Follow the following steps to compensate the
IC:
STEP 1. Set the cross over frequency at 1/5 of
the switching frequency via RCOMP:
V293.1GG10
fCVπ2
R
COMPEA
SWOUTOUT
COMP
•
=
)(CV108.9
OUTOUT
7
Ω
×=
(9)
but limit RCOMP to 15kΩ maximum.
STEP 2. Set the zero fZ1 at 1/4 of the cross over
frequency. If RCOMP is less than 15kΩ, the
equation for CCOMP is:
)F(
R
106.1
C
COMP
5
COMP
−
×
=
(10)
If RCOMP is limited to 15kΩ, then the actual cross
over frequency is 6.1 / (VOUTCOUT). Therefore:
)F(CV1096.6C
OUTOUT
6
COMP
−
×=
(11)
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:
ESRCOUT
R
)Ω(V012.0,
C
101.1
Min
OUT
OUT
6
•
×
≥
−
(12)
And the proper value for CCOMP2 is:
COMP
ESRCOUTOUT
2COMP
R
RC
C
=
(13)
Though CCOMP2 is unnecessary when the
output capacitor 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
Output Voltages and Output Capacitors
V
OUT
C
OUT
R
COMP
C
COMP
C
COMP2
2.5V 22μF Ceramic 5.6kΩ 2.7nF None
3.3V 22μF Ceramic 7.2kΩ 2.2nF None
5V 22μF Ceramic 10kΩ 1.5nF None
2.5V 47μF SP Cap 11kΩ 1.5nF None
3.3V 47μF SP Cap 15kΩ 1nF None
5V 47μF SP Cap 15kΩ 1.5nF None
2.5V 470μF/6.3V/30mΩ 15kΩ 8.2nF 1nF
3.3V 470μF/6.3V/30mΩ 15kΩ 10nF 1nF
5V 470μF/10V/30mΩ 15kΩ 15nF None
Figure 5 shows a sample ACT4065
application circuit generating 2.5V/2A output.
Active-Semi, Inc. - 6 - www.active-semi.com
ACT4065
COMP
C
C O M P
R
C O M P
C
C O M P 2
*
ACT4065
Figure 5. ACT4065 2.5V/2A Output Application
Active-Semi, Inc. - 7 - www.active-semi.com
IC1
ACT4065
IN
BS
EN FB
G COMP
SW
6V to 25V
ENABLE
2.5V/2A
C1
10μF/35V R3
5.6k
R2
13k
C5
(OPTIONAL)
C2
2.7nF
L1 15 μH/3A
C3
10nF
D1
+
R1 12K
C4
22μF/10V
Ceramic
ACT4065
TYPICAL PERFORMANCE CHARACTERISTICS
(Circuit of Figure 5, unless otherwise specified .)
Active-Semi, Inc. - 8 - www.active-semi.com
ACT4065
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 4.780 5.000 0.188 0.197
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
θ 0° 8° 0° 8°
Active-Semi, Inc. - 9 - www.active-semi.com
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 data sheet, 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 www.active-semi.com. For other inquiries, please send to:
1270 Oakmead Parkway, Sunnyvale, California 94085, USA