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ACT4070
Rev 0, 12/2006
Wide Input 3A Step Down Converter FEATURES

3A Output Current Up to 95% Efficiency 4.5V to 30V Input Range 6A Shutdown Supply Current 400kHz 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
GENERAL DESCRIPTION
The ACT4070 is a current-mode step-down DC/DC converter that generates up to 3A output current at 400kHz switching frequency. The device utilizes Active-Semi's proprietary ISOBCD30 process for operation with input voltage up to 30V. Consuming only 6A in shutdown mode, the ACT4070 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 ACT4070 is available in SOP-8/EP exposed pad package and requires very few external devices for operation.
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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ACT4070
ORDERING INFORMATION
PART NUMBER
ACT4070YH ACT4070YH-T
TEMPERATURE RANGE
-40C to 85C -40C to 85C
PACKAGE
SOP-8/EP SOP-8/EP
PINS
8 8
PACKING
TUBE TAPE & REEL
PIN CONFIGURATION
SOP-8/EP
PIN DESCRIPTION
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 section. 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. Not Connected. Exposed Pad shown as dashed box. The exposed thermal pad should be connected 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 outline). The leads and exposed pad should be flush with the board, without offset from the board surface.
7
EN
8
N/C
EP
EP
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ACT4070
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 (JA) Maximum Power Dissipation Operating Junction Temperature Storage Temperature Lead Temperature (Soldering, 10 sec)
VALUE
-0.3 to +34 -0.3 to VIN + 0.3 -1 to VIN + 1 -0.3 to +8 -0.3 to 6 Internally limited 46 1.8 -40 to 150 -55 to 150 300
UNIT
V V V V V A C/W W 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
(VIN = 12V, TA= 25C, 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
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SYMBOL
VIN VFB RONH RONL
TEST CONDITIONS
VOUT = 2.5V, ILOAD = 0A to 3A
MIN
4.5
TYP
MAX UNIT
30 V V m 10 A A A/V A/V V/V 460 kHz kHz % % 1.3 V A 20 2 A mA C
1.198 1.222 1.246 100 10 VEN = 0, VIN = 12V, VSW = 0V 0 3.5 ILOAD/ICOMP ICOMP = 10A 5 3 550 4000 340 VFB = 0V 400 40 90 0 0.7 1 2 6 0.85 160
ILIM GCOMP GEA AVEA fSW
DMAX
VFB = 1.1V, PWM mode VFB = 1.4V, PFM mode Hysteresis = 0.1V Pin pulled up to VIN when left unconnected VEN = 0 VEN = 3V, not switching Hysteresis = 10C
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ACT4070
FUNCTIONAL BLOCK DIAGRAM
FUNCTIONAL DESCRIPTION
As seen in the Functional Block Diagram, the ACT4070 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.
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The COMP voltage is the integration of the error between FB input and the internal 1.222V 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 2.65V. The Oscillator normally switches at 400kHz. However, if FB voltage is less than 0.7V, then the switching frequency decreases until it reaches a typical value of 40kHz at VFB = 0V.
Shutdown Control
The ACT4070 has an enable input EN for turning the IC on or off. When EN is less than 0.7V, the IC is in 6A 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 2A current source and can be left unconnected for always-on operation.
Thermal Shutdown
The ACT4070 automatically turns off when its junction temperature exceeds 160C and then restarts once the temperature falls to 150C.
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ACT4070
APPLICATIONS INFORMATION
Output Voltage Setting
Figure 1 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 10k and determine RFB1 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 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 10F. 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 0.1F ceramic capacitor is placed right next to the IC.
V RFB1 RFB2 OUT - 1 1.222V
Figure 1: Output Voltage Setting
(1)
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 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: The output capacitor also needs to have low ESR to keep low output voltage ripple. The output ripple voltage is:
VRIPPLE IOUTMAX K RIPPLE RRIPPLE VIN 28 fSW LCOUT
2
(3)
L
VOUT VIN - VOUT VIN fSW IOUTMAX K 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
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 22F. For tantalum or electrolytic type, choose a capacitor with less than 50m 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.
1.5V
6.8H
1.8V
6.8H
2.5V
6.8H
3.3V
8.5H
5V
15H
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ACT4070
Stability compensation
Figure 2: 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:
CCOMP
1.6 x10 5 RCOMP
(F)
(10)
If RCOMP is limited to 15k, then the actual cross over frequency is 4.8/(VOUTCOUT). Therefore:
CCOMP 8.8 x10 6 VOUT COUT
: CCOMP2 is needed only for high ESR output capacitor
(F)
(11)
The feedback system of the IC is stabilized by the components at COMP pin, as shown in Figure 2. The DC loop gain of the system is determined by the following equation:
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:
RESROUT 1.1x10 6 Min ,0.012VOUT C OUT
And the proper value for CCOMP2 is: () (12)
AVDC
1.222V AVEAGCOMP IOUT
(4)
The dominant pole P1 is due to CCOMP:
fP 1
G EA 2 A VEA C COMP
(5)
CCOMP
COUT R ESROUT RCOMP
(13)
The second pole P2 is the output pole:
fP 2
IOUT 2 VOUT C OUT
(6)
Though CCOMP2 is unnecessary when the output capacitor has sufficiently low ESR, a small value CCOMP2 such as 220pF may improve stability against PCB layout parasitic effects. Table 2 shows some calculated results based on the compensation method above.
The first zero Z1 is due to RCOMP and CCOMP:
fZ1
1 2 RCOMP CCOMP
(7)
Table 2: Typical Compensation for Different Output Voltages and Output Capacitors VOUT COUT
22F Ceramic 22F Ceramic 22F Ceramic 100F SP CAP 100F SP CAP 100F SP CAP
And finally, the third pole is due to RCOMP and CCOMP2 (if CCOMP2 is used):
fP 3
1 2 RCOMP CCOMP2
RCOMP
4k 5.6k 12k 15k 15k 15k
CCOMP CCOMP2
3.3nF 3.3nF 1.5nF 1.5nF 2.2nF 4.7nF 220pF 220pF 220pF 220pF 220pF 220pF
(8)
1.8V 2.5V
Follow the following steps to compensate the IC: STEP 1. Set the cross over frequency at 1/10 of the switching frequency via RCOMP:
5V 1.8V 2.5V
RCOMP
2 VOUT COUT fSW 10 GEAGCOMP 1 .222 V
() (9)
5V
1 .25 x10 8 VOUT COUT
: CCOMP2 is needed for board parasitic and high ESR output capacitor.
but limit RCOMP to 15k maximum.
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Figure 3 shows a sample ACT4070 application circuit generating a 2.5V/3A output.
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ACT4070
Figure 3: ACT4070 2.5V/3A Output Application
4.5V to 30V VIN ENABLE IN
BS
C3 10nF SW
L1 10H/4A R1 14k
2.5V/3A VOUT
IC1 ACT4070 EN
G C2 (CCOMP) 3.3nF R3 (RCOMP) 5.6k
FB D1 40V 3A
COMP C4 (COUT) 22F/10V ceramic or 47H/6.3 SP Cap
+ C1 10F/35V
C5 (CCOMP2) 220pF
R2 13k
: D1 is a 40V, 3A Schottky diode with low forward voltage, an IR 30BQ040 or SK34 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 capacitance characteristics in both operating temperature and frequency compared to ceramic, polymer, and low ESR tantalum capacitors.
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ACT4070
TYPICAL PERFORMANCE CHARACTERISTICS
(Circuit of Figure 3, unless otherwise specified.)
Efficiency vs. Output Current
100 90 80 VIN = 8V 100 90 80 ACT4070-0001
Efficiency vs. Output Current
ACT4070-0002
VIN = 8V
Efficiency (%)
60 50 40 30 20 10 0 0.01 0.1 1 VIN = 20V VIN = 30V
Efficiency (%)
70 VIN = 12V
70 60 50 40 30 20 10 VIN = 12V VIN = 20V
VOUT = 2.5V L = 10H CIN = 22F COUT = 22F
VIN = 30V
VOUT = 5V L = 15H CIN = 22F COUT = 22F
10
0 0.01
0.1
1
10
Output Current (A)
Output Current (A)
Feedback Voltage vs. Temperature
1.27 500
Switching Frequency vs. Input Voltage
ACT4070-0004
1.25
Switching Frequency (kHz)
ACT4070-0003
Feedback Voltage (V)
450
1.23
400
1.21 1.19 1.17 -40 -20 0 20 40 60 80 100
350
300 8 10 12 14 16 18 20 22 24 26 28 30
Temperature (C)
Input Voltage (V)
Shutdown Supply Current vs. Input Voltage
18
Surface Temperature vs. Output Current
140 ACT4070-006 VOUT = 5V L = 15H CIN = 22F COUT = 22F
ACT4070-0005
Shutdown Supply Current (A)
Surface Temperature (C)
16 14 12 10 8 6 4 2 0 5 10 15 20 25
120 100 80 60 40
VIN = 30V
VIN = 20V
VIN = 12V 20 0
30
0
0.5
1
1.5
2
2.5
3
Input Voltage (V)
Output Current (A)
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ACT4070
TYPICAL PERFORMANCE CHARACTERISTICS
(Circuit of Figure 3, unless otherwise specified.)
Load Transient Response
ACT4070-0007
Load Transient Response
ACT4070-0008
VOUT 200mV/div
VOUT 200mV/div
1A IOUT 0A VIN = 12V
1A IOUT 0A
VIN = 12V
100s/div
100s/div
Load Transient Response
ACT4070-0009
VOUT 200mV/div
3A IOUT 2A
VIN = 12V
100s/div
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ACT4070
PACKAGE OUTLINE
SOP-8/EP PACKAGE OUTLINE AND DIMENSIONS
SYMBOL A
e D
DIMENSION IN MILLIMETERS MIN MAX 1.750 0.150 1.550 0.510 0.250 5.100 3.402 4.000 6.200 2.513
DIMENSION IN INCHES MIN 0.053 0.002 0.053 0.013 0.007 0.185 0.126 0.150 0.228 0.091 MAX 0.069 0.006 0.061 0.020 0.010 0.200 0.134 0.157 0.244 0.099
D1
1.350 0.050 1.350 0.330 0.170 4.700 3.202 3.800 5.800 2.313
A1
b
A2 b c D D1 E E1 E2 e L
1.270 TYP 0.400 0 1.270 8
0.050 TYP 0.016 0 0.050 8
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
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