ACTIVE-SEMI ACT4012ASH Wide input 1.5a step down converter Datasheet

ACT4012A
Rev PrA, 21-Nov-07
Preliminary Information - All Information Subject to Change
Wide Input 1.5A Step Down Converter
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
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 operation.
• 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
TYPICAL APPLICATION CIRCUIT
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ACT4012A
Rev PrA, 21-Nov-07
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
SOP-8
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
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 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. EN pin has
a small internal pull-up current to IN when pin is not connected.
8
N/C
Not Connected.
Switch Output. Connect this pin to the switching end of the inductor.
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ACT4012A
Rev PrA, 21-Nov-07
ABSOLUTE MAXIMUM RATINGSc
PARAMETER
VALUE
UNIT
IN to G
-0.3 to 25
V
EN to G
-0.3 to VIN +0.3
V
SW to G
-1 to VIN + 1
V
BS to G
VSW - 0.3 to VSW + 8
V
-0.3 to 6
V
Internally Limited
A
Junction to Ambient Thermal Resistance (θJA)
105
°C/W
Maximum Power Dissipation
0.76
W
Operating Junction Temperature
-40 to 150
°C
Storage Temperature
-55 to 150
°C
300
°C
FB, COMP to G
Continuous SW Current
Lead Temperature (Soldering, 10 sec)
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 = 25°C, unless otherwise specified.)
PARAMETER
SYMBOL
TEST CONDITIONS
Input Voltage
VIN
VOUT = 5V, ILOAD = 0A to 1.5A
Feedback Voltage
VFB
4.75V ≤ VIN ≤ 20V
MIN
TYP
7.5
1.261
1.293
MAX
UNIT
20
V
1.325
V
High-Side Switch On Resistance
RONH
0.5
Ω
Low-Side Switch On Resistance
RONL
13.5
Ω
SW Leakage
Current Limit
COMP to Current Limit
Transconductance
VEN = 0
ILIM
GEA
Error Amplifier DC Gain
AVEA
∆ICOMP = ±10µA
fSW
Short Circuit Switching Frequency
Maximum Duty Cycle
1.8
GCOMP
Error Amplifier Transconductance
Switching Frequency
0
DMAX
310
10
µA
2.7
A
1.5
A/V
565
µA/V
4000
V/V
420
530
kHz
VFB = 0
50
kHz
VFB = 1.1V
90
%
14
%
Minimum Duty Cycle
Enable Threshold Voltage
Hysteresis = 0.1V
Enable Pull-Up Current
Pin pulled up to IN when left
unconnected
2
Supply Current in Shutdown
VEN = 0
10
IC Supply Current in Operation
VEN = 3V, VFB = 1.4V
0.85
mA
Thermal Shutdown Temperature
Hysteresis = 10°C
160
°C
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0.7
1
1.3
V
µA
20
µA
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Copyright © 2006 Active-Semi, Inc.
ACT4012A
Rev PrA, 21-Nov-07
FUNCTIONAL BLOCK DIAGRAM
FUNCTIONAL DESCRIPTION
As seen in Functional Block Diagram, the
ACT4012A 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 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 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 1.293V 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 2.6V.
The Oscillator normally switches at 420kHz. 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 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.
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ACT4012A
Rev PrA, 21-Nov-07
Input Capacitor
APPLICATIONS INFORMATION
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.
Output Voltage Setting
Figure 1:
Output Voltage Setting
VOUT
RFB1
ACT4012A
FB
RFB2
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 ≈ 10kΩ and determine
RFB1 from the output voltage:
⎞
⎛ V
R FB1 = R FB 2 ⎜ OUT − 1 ⎟
⎠
⎝ 1.293V
(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:
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 = 30% to correspond 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 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
VOUT
1.5V
1.8V
2.5V
3.3V
5V
L
7.5µH
10µH
12µH
15µH
22µH
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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 the 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:
VRIPPLE = IOUTMAX K RIPPLE RESR
+
VIN
2
32 × fSW LC OUT
(3)
where IOUTMAX is the maximum output current, KRIPis 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. 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.
PLE
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 a reverse voltage
rating higher than the maximum input voltage.
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ACT4012A
Rev PrA, 21-Nov-07
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:
STABILITY COMPENSATION
Figure 2:
Stability Compensation
C COMP =
1 .8 × 10 −5
R COMP
(F)
(10)
If RCOMP is limited to 15kΩ, then the actual cross
over frequency is 3.4 / (VOUTCOUT). Therefore:
CCOMP = 1.2 ×10−5 VOUTCOUT
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:
AVDC =
1 .3V
AVEA GCOMP
I OUT
(5)
I OUT
2 π VOUT C OUT
(6)
1
(7)
2πRCOMP CCOMP2
And the proper value for CCOMP2 is:
1
Typical Compensation for Different Output
Voltages and Output Capacitors
Follow the following steps to compensate the IC:
STEP 1. Set the cross over frequency at 1/10 of the
switching frequency via RCOMP:
R COMP =
2π VOUT C OUT f SW
10 G EA GCOMP × 1 .3V
= 2 × 10 8 V OUT C OUT
(13)
Table 2 shows some calculated results based on
the compensation method above.
(8)
2π R COMP C COMP2
COUT RESRCOUT
RCOMP
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.
And finally, the third pole is due to RCOMP and
CCOMP2 (if CCOMP2 is used):
fP 3 =
(12)
Table 2:
The first zero Z1 is due to RCOMP and CCOMP:
f Z1 =
⎛ 1.1 × 10 −6
⎞
RESRCOUT ≥ Min⎜⎜
,0.012 × VOUT ⎟⎟ (Ω)
⎝ COUT
⎠
CCOMP2 =
The second pole P2 is the output pole:
fP 2 =
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:
(4)
The dominant pole P1 is due to CCOMP:
G EA
f P1 =
2 π AVEA C COMP
(11)
(F)
VOUT
COUT
RCOMP
CCOMP CCOMP2c
2.5V
22µF Ceramic
12kΩ
1.5nF
None
3.3V
22µF Ceramic
15kΩ
1.5nF
None
5V
22µF Ceramic
15kΩ
1.5nF
None
2.5V
47µF SP CAP
15kΩ
1.5nF
None
3.3V
47µF SP CAP
15kΩ
1.8nF
None
5V
47µF SP CAP
15kΩ
2.7nF
None
2.5V
470µF/6.3V/30mΩ
15kΩ
15nF
1nF
3.3V
470µF/6.3V/30mΩ
15kΩ
22nF
1nF
5V
470µF/6.3V/30mΩ
15kΩ
27nF
None
c: CCOMP2 is needed for high ESR output capacitor.
(Ω)
but limit RCOMP to 15kΩ maximum.
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Figure 3 shows an example ACT4012A application circuit generating a 5V/1.5A output.
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ACT4012A
Rev PrA, 21-Nov-07
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 capacitance characteristics in both operating temperature and frequency compared to ceramic, polymer, and low ESR tantalum capacitors.
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ACT4012A
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TYPICAL PERFORMANCE CHARACTERISTICS
(Circuit of Figure 3, unless otherwise specified.)
Efficiency vs. Load
Efficiency vs. Load
85
80
VIN = 12V
VIN = 20V
VIN = 8V
75
70
65
VOUT = 5V
L= 22µH
COUT = 22µF/cera
60
80
VIN = 12V
75
VIN = 20V
70
VOUT = 3.3V
L= 15µH
COUT = 22µF/cera
60
55
0.1
0.3
0.5
0.7
0.9
1.1
1.3
1.5
0.1
0.5
0.7
0.9
1.1
1.3
Load (A)
Surface Temperature vs. Load
Output Voltage vs. Input Voltage
VIN = 12V
VIN = 8V
IOUT = 0.5A
5.2
60
VIN = 20V
40
ACT4012A-004
ACT4012A-003
80
1.5
5.4
5.0
4.8
IOUT = 1.5A
IOUT = 1A
4.6
4.4
4.2
20
4.0
0.1
0.3
0.5
0.7
0.9
1.1
1.3
6
1.5
Load (A)
Shutdown Supply current (µA)
1.28
1.27
1.26
40
80
14
16
18
20
25
20
15
10
5
0
120
12
16
20
Input Voltage (V)
Junction Temperature (°C)
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ACT4012A-006
1.29
0
10
Shutdown Supply current vs. Input Voltage
ACT4012A-005
1.30
-40
8
Input Voltage (V)
Feedback Voltage vs. Junction Temperature
Feedback Voltage (V)
0.3
Load (A)
100
Surface Temperature (°C)
85
65
Output Voltage (V)
Efficiency (%)
90
VIN = 8V
90
Efficiency (%)
95
ACT4012A-002
ACT4012A-001
100
95
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ACT4012A
Rev PrA, 21-Nov-07
TYPICAL PERFORMANCE CHARACTERISTICS
(Circuit of Figure 3, unless otherwise specified.)
Switching Frequency vs. Input Voltage
Switching Frequency (kHz)
ACT4012A-007
430
425
420
415
410
8
12
16
20
Input Voltage (V)
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ACT4012A
Rev PrA, 21-Nov-07
PACKAGE OUTLINE
SOP-8 PACKAGE OUTLINE AND DIMENSIONS
D
C
SYMBOL
θ
e
B
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
θ
0°
8°
0°
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
[email protected] 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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