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FEATURES
* * * * * * * * * *
2A Output Current Up to 95% Efficiency Up to 30V Input Range 6A 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 6A 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.
* SOP-8 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
BS IN SW VOUT
VIN
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
-40C to 85C -40C to 85C
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. 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 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 6A 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 (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 VIN + 0.3 -1 to VIN + 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
(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
SYMBOL
VIN VFB RONH RONL
TEST CONDITIONS
VIN = 12V
MIN
4.5 1.198
TYP
1.222 0.13 10
MAX UNIT
30 1.246 V V 10 A A A/V A/V V/V 500 kHz kHz % % 1.3 V A 20 2 A mA C
VEN = 0 ILIM GCOMP GEA AVEA fSW VFB = 0 DMAX DMIN VFB = 1.1V, PWM mode VFB = 1.4V, PFM mode Hysteresis = 0.1V 0.7 340 ICOMP = 10A 2.4
0 3.3 2 550 4000 420 60 88 0 1 2
VEN = 0 VEN = 3V, not switching Hysteresis = 10C
6 0.8 160
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FUNCTIONAL BLOCK DIAGRAM
IN EN
2A ENABLE REGULATOR & REFERENCE
ACT4072
Rev2, 27-May-08
BS
CURRENT SENSE AMPLIFIER
COMP
1.222V
ERROR AMPLIFIER
+ 0.13O HIGH-SIDE POWER SWITCH
FB
+ FOLDBACK CONTROL OSCILLATOR & RAMP
+- +PWM
COMP LOGIC
SW
THERMAL SHUTDOWN
10O LOW-SIDE 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 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 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 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 VSW + 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 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 420kHz. However, if the FB voltage is less than 0.7V, then the switching frequency decreases until it reaches a minimum of 60kHz at VFB = 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 6A low current shutdown mode and the 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 ACT4072 automatically turns off when its junction temperature exceeds 160C.
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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:
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 recommended. 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 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 possible traces. 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 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 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:
RRIPPLE = LOUTMAXK RIPPLERESR
+ VIN 2 28 fSW LCOUT
(3)
L=
VOUT x (VIN - VOUT ) VIN fSW IOUTMAXK RIPPLE
(2)
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 is 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, typically choose a capacitance of about 22F. 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 due to ESR, typically choose a capacitor with less than 50m ESR.
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 = 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 IOUT x (1 + KRIPPLE / 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 VOUT
L
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
10H
3.3V
15H
5V
22H
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Stability compensation
Figure 2: Stability Compensation
COMP
ACT4072
Rev2, 27-May-08 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:
C COMP =
CCOMP RCOMP CCOPM2
1 . 8 x 10 - 5 R COMP
(F)
(10)
ACT4072
If RCOMP is limited to 15k, then the actual cross over frequency is 3.4/(VOUTCOUT). Therefore:
CCOMP = 1.3 x10 -5VOUTCOUT
(F)
(11)
: CCOMP2 is needed only for high ESR output capacitors or PCB parasitics
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:
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:
AVDC =
1.222V AVEAGCOMP IOUT
(4)
1.1x10 -6 R ESROUT Min ,0.012VOUT C OUT
And the proper value for CCOMP2 is:
()
(12)
The dominant pole P1 is due to CCOMP:
fP 1
GEA = 2 AVEA CCOMP
CCOMP2 =
(5)
COUT RESROUT RCOMP
(13)
The second pole P2 is the output pole:
A small value CCOMP2 such as 100pF may improve stability against PCB layout parasitic effects. (6) Table 2 shows some calculated results based on the compensation method above. Table 2: (7) Typical Compensation for Different Output voltages and Output Capacitors VOUT
2.5V 3.3V
fP 2 =
IOUT 2VOUTCOUT 1 2RCOMP CCOMP
The first zero Z1 is due to RCOMP and CCOMP:
fZ 1 =
And finally, the third pole is due to RCOMP and CCOMP2 (if CCOMP2 is used):
COUT
22F Ceramic 22F Ceramic 22F Ceramic 47F SP CAP 47F SP CAP 47F SP CAP 470F/6.3V/30m 470F/6.3V/30m 470F/6.3V/30m
RCOMP CCOMP CCOMP2
8.2k 12k 15k 15k 15k 15k 15k 15k 15k 2.2nF 1.5nF 1.5nF 1.5nF 1.8nF 2.7nF 15nF 22nF 27nF 100pF 100pF 100pF 100pF 100pF 100pF 1nF 1nF 1nF
fP 3 =
1 2RCOMP CCOMP2
(8)
5V 2.5V 3.3V 5V 2.5V 3.3V 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:
RCOMP =
2VOUT COUT fSW 10GEAGCOMP 1.222V
8
= 2.3 x10 VOUT COUT
but limit RCOMP to 15k maximum.
()
(9)
: CCOMP2 is needed for board parasitic and high ESR output capacitor.
Figure 3 shows an example ACT4072 application circuit generating a 5V/2A output.
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Figure 3: ACT4072 5V/2A Output Application
ACT4072
Rev2, 27-May-08
VIN
Up to 30V IN
BS
ENABLE
IC1 ACT4072 EN
G
SW FB
C3 10nF
L1 22H/3A R1 39.2k
5V/2A
VOUT
COMP C2 1.5nF R3 15k C4 22F/10V ceramic or 47H/6.3 SP Cap
+ C1 10F/35V
C5 R2 100pF 12.1k
D1
: 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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TYPICAL PERFORMANCE CHARACTERISTICS
(Circuit of Figure 3, unless otherwise specified.)
ACT4072
Rev2, 27-May-08
Efficiency vs. Output Current
100 90 80 100 90 80 ACT4072-001
Efficiency vs. Output Current
ACT4072-002
Efficiency (%)
Efficiency (%)
70 60 50 40 30 20 10 0 0.01 0.1 VIN = 12V VIN = 25V
VIN = 6V
70 60 50 40 30 20 10 0
VIN = 30V VIN = 12V VIN = 8V
VOUT = 3.3V L = 15H CIN = 10F COUT = 22F
VOUT = 5V L = 22H CIN = 10F COUT = 22F
1
10
0.0
0.1
1
10
Output Current (A)
Output Current (A)
Switching Frequency vs. Input Voltage Shutdown Supply Current (mA)
430 18 16 14 12 10 8 6 4 2 0 ACT4072-003
Shutdown Supply Current vs. Input Voltage
ACT4072-004
Switching Frequency(kHz)
425 420 415 410 405 0.0 0.5 1.0 1.5 2.0
2.5
5
10
15
20
25
30
Temperature (C)
Input Voltage (V)
Surface Temperature vs. Output Current
80 1.27 ACT4072-005
Feedback Voltage vs. Temperature
ACT4072-006 VIN = 12V
Surface Temperature (C)
Feedback Voltage (V)
70 60 50 40 30 20 0.0 0.5
VIN = 30V
1.25
1.23
1.21
VOUT=5V VIN = 12V L=22H CIN=10F COUT=22F 1.0 1.5 2.0
1.19
1.17 -40 -20 0 20 40 60 80 100 120
Output Current (A)
Temperature (C)
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TYPICAL PERFORMANCE CHARACTERISTICS
(Circuit of Figure 3, unless otherwise specified.)
ACT4072
Rev2, 27-May-08
Load Transient Response
ACT4072-007
Load Transient Response
ACT4072-008
VOUT 200mV/div
VOUT 200mV/div
1A IOUT 0A
2A IOUT 1A VIN = 12V VIN = 12V
100s/div
100s/div
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PACKAGE OUTLINE
SOP-8 PACKAGE OUTLINE AND DIMENSIONS
ACT4072
Rev2, 27-May-08
C
D
SYMBOL
A
E1 E
DIMENSION IN MILLIMETERS MIN
1.350 0.100 1.350 0.330 0.190 4.700 3.800 5.800
DIMENSION IN INCHES MIN
0.053 0.004 0.053 0.013 0.007 0.185 0.150 0.228
L
MAX
1.750 0.250 1.550 0.510 0.250 5.100 4.000 6.300
MAX
0.069 0.010 0.061 0.020 0.010 0.201 0.157 0.248
A1 A2
? e B
B C D
A1
E E1
A
A2
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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