NSC LM2671N-12

LM2671
SIMPLE SWITCHER® Power Converter High Efficiency
500mA Step-Down Voltage Regulator with Features
General Description
The LM2671 series of regulators are monolithic integrated
circuits built with a LMDMOS process. These regulators provide all the active functions for a step-down (buck) switching
regulator, capable of driving a 500mA load current with excellent line and load regulation. These devices are available
in fixed output voltages of 3.3V, 5.0V, 12V, and an adjustable
output version.
Requiring a minimum number of external components, these
regulators are simple to use and include patented internal
frequency compensation (Patent Nos. 5,382,918 and
5,514,947), fixed frequency oscillator, external shutdown,
soft-start, and frequency synchronization.
The LM2671 series operates at a switching frequency of
260 kHz, thus allowing smaller sized filter components than
what would be needed with lower frequency switching regulators. Because of its very high efficiency (>90%), the copper
traces on the printed circuit board are the only heat sinking
needed.
A family of standard inductors for use with the LM2671 are
available from several different manufacturers. This feature
greatly simplifies the design of switch-mode power supplies
using these advanced ICs. Also included in the datasheet are
selector guides for diodes and capacitors designed to work in
switch-mode power supplies.
Other features include a guaranteed ±1.5% tolerance on output voltage within specified input voltages and output load
conditions, and ±10% on the oscillator frequency. External
shutdown is included, featuring typically 50 μA stand-by current. The output switch includes current limiting, as well as
thermal shutdown for full protection under fault conditions.
To simplify the LM2671 buck regulator design procedure,
there exists computer design software, LM267X Made Simple (version 6.0).
Features
■ Efficiency up to 96%
■ Available in SO-8, 8-pin DIP and LLP packages
■ Computer Design Software LM267X Made Simple
(version 6.0)
Simple and easy to design with
Requires only 5 external components
Uses readily available standard inductors
3.3V, 5.0V, 12V, and adjustable output versions
Adjustable version output voltage range: 1.21V to 37V
±1.5% max output voltage tolerance over line and load
conditions
■ Guaranteed 500mA output load current
■ 0.25Ω DMOS Output Switch
■ Wide input voltage range: 8V to 40V
■ 260 kHz fixed frequency internal oscillator
■ TTL shutdown capability, low power standby mode
■ Soft-start and frequency synchronization
■ Thermal shutdown and current limit protection
■
■
■
■
■
■
Applications
■ Simple High Efficiency (>90%) Step-Down (Buck)
Regulator
■ Efficient Pre-Regulator for Linear Regulators
Typical Application
(Fixed Output Voltage Versions)
10004201
SIMPLE SWITCHER® is a registered trademark of National Semiconductor Corporation
Windows® is a registered trademark of Microsoft Corporation.
© 2007 National Semiconductor Corporation
100042
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LM2671 SIMPLE SWITCHER Power Converter High Efficiency 500mA Step-Down Voltage
Regulator with Features
April 2007
LM2671
Connection Diagrams
16-Lead LLP Surface Mount Package
Top View
8-Lead Package
Top View
10004202
SO-8/DIP Package
See NSC Package Drawing Number MO8A/N08E
10004241
LLP Package
See NSC Package Drawing Number LDA16A
TABLE 1. Package Marking and Ordering Information
Output Voltage
Order Information
Package Marking
Supplied as:
12
LM2671LD-12
S0005B
1000 Units on Tape and Reel
12
LM2671LDX-12
S0005B
4500 Units on Tape and Reel
3.3
LM2671LD-3.3
S0006B
1000 Units on Tape and Reel
3.3
LM2671LDX-3.3
S0006B
4500 Units on Tape and Reel
5.0
LM2671LD-5.0
S0007B
1000 Units on Tape and Reel
5.0
LM2671LDX-5.0
S0007B
4500 Units on Tape and Reel
ADJ
LM2671LD-ADJ
S0008B
1000 Units on Tape and Reel
ADJ
LM2671LDX-ADJ
S0008B
4500 Units on Tape and Reel
12
LM2671M-12
2671M-12
Shipped in Anti-Static Rails
12
LM2671MX-12
2671M-12
2500 Units on Tape and Reel
3.3
LM2671M-3.3
2671M-3.3
Shipped in Anti-Static Rails
3.3
LM2671MX-3.3
2671M-3.3
2500 Units on Tape and Reel
5.0
LM2671M-5.0
2671M-5.0
Shipped in Anti-Static Rails
5.0
LM2671MX-5.0
2671M-5.0
2500 Units on Tape and Reel
ADJ
LM2671M-ADJ
2671M-ADJ
Shipped in Anti-Static Rails
ADJ
LM2671MX-ADJ
2671M-ADJ
2500 Units on Tape and Reel
12
LM2671N-12
LM2671N-12
Shipped in Anti-Static Rails
3.3
LM2671N-3.3
LM2671N-3.3
Shipped in Anti-Static Rails
5.0
LM2671N-5.0
LM2671N-5.0
Shipped in Anti-Static Rails
ADJ
LM2671N-ADJ
LM2671N-ADJ
Shipped in Anti-Static Rail
16 Lead LLP
SO-8
DIP
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2
If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors
for availability and specifications.
Storage Temperature Range
−65°C to +150°C
Supply Voltage
45V
Lead Temperature
ON/OFF Pin Voltage
−0.1V ≤ VSH ≤ 6V
M Package
Switch Voltage to Ground
−1V
Vapor Phase (60s)
+215°C
Boost Pin Voltage
VSW + 8V
Infrared
(15s)
+220°C
Feedback Pin Voltage
−0.3V ≤ VFB ≤ 14V
N Package (Soldering, 10s)
+260°C
ESD Susceptibility
LLP Package (See AN-1187)
Human Body Model (Note 2)
2 kV
Maximum Junction Temperature
+150°C
Power Dissipation
Internally Limited
Operating Ratings
Supply Voltage
Temperature Range
6.5V to 40V
−40°C ≤ TJ ≤ +125°C
Electrical Characteristics
LM2671-3.3 Specifications with standard type face are for TJ = 25°C, and those in bold type face apply over full
Operating Temperature Range.
Symbol
Parameter
Conditions
Typical
(Note 4)
Min
(Note 5)
Max
(Note 5)
Units
V
SYSTEM PARAMETERS Test Circuit Figure 2 (Note 3)
VOUT
Output Voltage
VIN = 8V to 40V, ILOAD = 20 mA to 500 mA
3.3
3.251/3.201
3.350/3.399
VOUT
Output Voltage
VIN = 6.5V to 40V, ILOAD = 20 mA to 250 mA
3.3
3.251/3.201
3.350/3.399
η
Efficiency
VIN = 12V, ILOAD = 500 mA
86
V
%
LM2671-5.0
Symbol
Parameter
Conditions
Typical
(Note 4)
Min
(Note 5)
Max
(Note 5)
Units
V
SYSTEM PARAMETERS Test Circuit Figure 2 (Note 3)
VOUT
Output Voltage
VIN = 8V to 40V, ILOAD = 20 mA to 500 mA
5.0
4.925/4.850
5.075/5.150
VOUT
Output Voltage
VIN = 6.5V to 40V, ILOAD = 20 mA to 250 mA
5.0
4.925/4.850
5.075/5.150
η
Efficiency
VIN = 12V, ILOAD = 500 mA
90
V
%
LM2671-12
Symbol
Parameter
Conditions
Typical
(Note 4)
Min
(Note 5)
Max
(Note 5)
11.82/11.64
12.18/12.36
Units
SYSTEM PARAMETERS Test Circuit Figure 2 (Note 3)
VOUT
Output Voltage
VIN = 15V to 40V, ILOAD = 20 mA to 500 mA
12
η
Efficiency
VIN = 24V, ILOAD = 500 mA
94
V
%
LM2671-ADJ
Symbol
Parameter
Conditions
Typ
(Note 4)
Min
(Note 5)
Max
(Note 5)
Units
1.210
1.192/1.174
1.228/1.246
V
SYSTEM PARAMETERS Test Circuit Figure 3 (Note 3)
VFB
Feedback Voltage VIN = 8V to 40V, ILOAD = 20 mA to 500 mA
VOUT Programmed for 5V
(see Circuit of Figure 3)
3
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LM2671
Absolute Maximum Ratings (Note 1)
LM2671
Symbol
VFB
Parameter
Conditions
Typ
(Note 4)
Min
(Note 5)
Max
(Note 5)
Units
1.210
1.192/1.174
1.228/1.246
V
Feedback Voltage VIN = 6.5V to 40V, ILOAD = 20 mA to 250 mA
VOUT Programmed for 5V
(see Circuit of Figure 3)
η
Efficiency
VIN = 12V, ILOAD = 500 mA
90
%
All Output Voltage Versions
Specifications with standard type face are for TJ = 25°C, and those in bold type face apply over full Operating Temperature
Range. Unless otherwise specified, VIN = 12V for the 3.3V, 5V, and Adjustable versions and VIN = 24V for the 12V version, and
ILOAD = 100 mA.
Symbol
Parameters
Conditions
Typ
Min
Max
Units
3.6
mA
DEVICE PARAMETERS
IQ
Quiescent Current
VFEEDBACK = 8V
2.5
For 3.3V, 5.0V, and ADJ Versions
VFEEDBACK = 15V
2.5
mA
For 12V Versions
ISTBY
Standby Quiescent Current
ICL
Current Limit
IL
Output Leakage Current
ON/OFF Pin = 0V
100/150
μA
1.2/1.25
A
1
25
μA
6
15
mA
50
0.8
VIN = 40V, ON/OFF Pin = 0V
0.62/0.575
VSWITCH = 0V
VSWITCH = −1V, ON/OFF Pin = 0V
0.40/0.60
Ω
275
kHz
RDS(ON)
Switch On-Resistance
ISWITCH = 500 mA
0.25
fO
Oscillator Frequency
Measured at Switch Pin
260
D
Maximum Duty Cycle
95
%
Minimum Duty Cycle
0
%
85
nA
IBIAS
VS/D
Feedback Bias
VFEEDBACK = 1.3V
Current
ADJ Version Only
ON/OFF Pin
225
1.4
0.8
2.0
7
37
V
Voltage Thesholds
μA
IS/D
ON/OFF Pin Current
ON/OFF Pin = 0V
20
FSYNC
Synchronization Frequency
VSYNC = 3.5V, 50% duty cycle
400
kHz
VSYNC
Synchronization Threshold
Voltage
1.4
V
VSS
Soft-Start Voltage
0.63
0.53
0.73
ISS
Soft-Start Current
4.5
1.5
6.9
θJA
Thermal Resistance
N Package, Junction to Ambient (Note 6)
95
M Package, Junction to Ambient (Note 6)
105
V
μA
°C/W
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but device parameter specifications may not be guaranteed under these conditions. For guaranteed specifications and test conditions,
see the Electrical Characteristics.
Note 2: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin.
Note 3: External components such as the catch diode, inductor, input and output capacitors, and voltage programming resistors can affect switching regulator
performance. When the LM2671 is used as shown in Figure 2 and Figure 3 test circuits, system performance will be as specified by the system parameters section
of the Electrical Characteristics.
Note 4: Typical numbers are at 25°C and represent the most likely norm.
Note 5: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%
production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used
to calculate Average Outgoing Quality Level (AOQL).
Note 6: Junction to ambient thermal resistance with approximately 1 square inch of printed circuit board copper surrounding the leads. Additional copper area
will lower thermal resistance further. See Application Information section in the application note accompanying this datasheet and the thermal model in LM267X
Made Simple version 6.0 software. The value θJ−A for the LLP (LD) package is specifically dependent on PCB trace area, trace material, and the number of layers
and thermal vias. For improved thermal resistance and power dissipation for the LLP package, refer to Application Note AN-1187.
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LM2671
Typical Performance Characteristics
Normalized
Output Voltage
Line Regulation
10004204
10004203
Efficiency
Drain-to-Source
Resistance
10004205
10004206
Switch Current Limit
Operating
Quiescent Current
10004207
10004208
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LM2671
Standby
Quiescent Current
ON/OFF Threshold
Voltage
10004209
10004210
ON/OFF Pin
Current (Sourcing)
Switching Frequency
10004212
10004211
Peak Switch Current
Feedback Pin
Bias Current
10004213
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10004214
6
LM2671
Dropout Voltage—3.3V Option
Dropout Voltage—5.0V Option
10004215
10004216
Block Diagram
10004217
* Patent Number 5,514,947
† Patent Number 5,382,918
FIGURE 1.
7
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LM2671
Typical Performance Characteristics
(Circuit of Figure 2)
Continuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 500 mA
L = 100 μH, COUT = 100 μF, COUTESR = 0.1Ω
Discontinuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 300 mA
L = 15 μH, COUT = 68 μF (2×), COUTESR = 25 mΩ
10004218
A: VSW Pin Voltage, 10 V/div.
B: Inductor Current, 0.2 A/div
C: Output Ripple Voltage, 50 mV/div AC-Coupled
10004219
A: VSW Pin Voltage, 10 V/div.
B: Inductor Current, 0.5 A/div
C: Output Ripple Voltage, 20 mV/div AC-Coupled
Horizontal Time Base: 1 μs/div
Horizontal Time Base: 1 μs/div
Load Transient Response for Continuous Mode
VIN = 20V, VOUT = 5V
L = 100 μH, COUT = 100 μF, COUTESR = 0.1Ω
Load Transient Response for Discontinuous Mode
VIN = 20V, VOUT = 5V,
L = 47 μH, COUT = 68 μF, COUTESR = 50 mΩ
10004220
A: Output Voltage, 100 mV/div, AC-Coupled
B: Load Current: 100 mA to 500 mA Load Pulse
10004221
A: Output Voltage, 100 mV/div, AC-Coupled
B: Load Current: 100 mA to 400 mA Load Pulse
Horizontal Time Base: 50 μs/div
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Horizontal Time Base: 200 μs/div
8
LM2671
Test Circuit and Layout Guidelines
10004222
CIN - 22 μF, 50V Tantalum, Sprague “199D Series”
COUT - 47 μF, 25V Tantalum, Sprague “595D Series”
D1 - 3.3A, 50V Schottky Rectifier, IR 30WQ05F
L1 - 68 μH Sumida #RCR110D-680L
CB - 0.01 μF, 50V Ceramic
FIGURE 2. Standard Test Circuits and Layout Guides
Fixed Output Voltage Versions
10004223
CIN - 22 μF, 50V Tantalum, Sprague “199D Series”
COUT - 47 μF, 25V Tantalum, Sprague “595D Series”
D1 - 3.3A, 50V Schottky Rectifier, IR 30WQ05F
L1 - 68 μH Sumida #RCR110D-680L
R1 - 1.5 kΩ, 1%
CB - 0.01 μF, 50V Ceramic
For a 5V output, select R2 to be 4.75 kΩ, 1%
where VREF = 1.21V
Use a 1% resistor for best stability.
FIGURE 3. Standard Test Circuits and Layout Guides
Adjustable Output Voltage Versions
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LM2671
LM2671 Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version)
EXAMPLE (Fixed Output Voltage Version)
To simplify the buck regulator design procedure, National
Semiconductor is making available computer design software to be
used with the SIMPLE SWITCHER line of switching regulators.
LM267X Made Simple (version 6.0) is available on Windows® 3.1,
NT, or 95 operating systems.
Given:
Given:
VOUT = Regulated Output Voltage (3.3V, 5V, or 12V)
VOUT = 5V
VIN(max) = Maximum DC Input Voltage
VIN(max) = 12V
ILOAD(max) = Maximum Load Current
ILOAD(max) = 500 mA
1. Inductor Selection (L1)
A. Select the correct inductor value selection guide from Figure 4
and Figure 5 or Figure 6 (output voltages of 3.3V, 5V, or 12V
respectively). For all other voltages, see the design procedure for
the adjustable version.
B. From the inductor value selection guide, identify the inductance
region intersected by the Maximum Input Voltage line and the
Maximum Load Current line. Each region is identified by an
inductance value and an inductor code (LXX).
C. Select an appropriate inductor from the four manufacturer's part
numbers listed in Figure 8. Each manufacturer makes a different
style of inductor to allow flexibility in meeting various design
requirements. Listed below are some of the differentiating
characteristics of each manufacturer's inductors:
Schott: ferrite EP core inductors; these have very low leakage
magnetic fields to reduce electro-magnetic interference (EMI) and
are the lowest power loss inductors
Renco: ferrite stick core inductors; benefits are typically lowest cost
inductors and can withstand E•T and transient peak currents above
rated value. Be aware that these inductors have an external
magnetic field which may generate more EMI than other types of
inductors.
Pulse: powered iron toroid core inductors; these can also be low
cost and can withstand larger than normal E•T and transient peak
currents. Toroid inductors have low EMI.
Coilcraft: ferrite drum core inductors; these are the smallest
physical size inductors, available only as SMT components. Be
aware that these inductors also generate EMI—but less than stick
inductors.
Complete specifications for these inductors are available from the
respective manufacturers. A table listing the manufacturers' phone
numbers is located in Figure 9.
2. Output Capacitor Selection (COUT)
A. Select an output capacitor from the output capacitor table in
Figure 10. Using the output voltage and the inductance value found
in the inductor selection guide, step 1, locate the appropriate
capacitor value and voltage rating.
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1. Inductor Selection (L1)
A. Use the inductor selection guide for the 5V version shown in
Figure 5.
B. From the inductor value selection guide shown in Figure 5, the
inductance region intersected by the 12V horizontal line and the
500 mA vertical line is 47 μH, and the inductor code is L13.
C. The inductance value required is 47 μH. From the table in Figure
8, go to the L13 line and choose an inductor part number from any
of the four manufacturers shown. (In most instances, both through
hole and surface mount inductors are available.)
2. Output Capacitor Selection (COUT)
A. Use the 5.0V section in the output capacitor table in Figure 10.
Choose a capacitor value and voltage rating from the line that
contains the inductance value of 47 μH. The capacitance and
voltage rating values corresponding to the 47 μH inductor are the:
10
EXAMPLE (Fixed Output Voltage Version)
The capacitor list contains through-hole electrolytic capacitors from
four different capacitor manufacturers and surface mount tantalum
capacitors from two different capacitor manufacturers. It is
recommended that both the manufacturers and the manufacturer's
series that are listed in the table be used. A table listing the
manufacturers' phone numbers is located in Figure 11.
Surface Mount:
68 μF/10V Sprague 594D Series.
100 μF/10V AVX TPS Series.
Through Hole:
68 μF/10V Sanyo OS-CON SA Series.
150 μF/35V Sanyo MV-GX Series.
150 μF/35V Nichicon PL Series.
150 μF/35V Panasonic HFQ Series.
3. Catch Diode Selection (D1)
3. Catch Diode Selection (D1)
A. In normal operation, the average current of the catch diode is A. Refer to the table shown in Figure 12. In this example, a 1A,
the load current times the catch diode duty cycle, 1-D (D is the
20V Schottky diode will provide the best performance. If the circuit
switch duty cycle, which is approximately the output voltage divided must withstand a continuous shorted output, a higher current
by the input voltage). The largest value of the catch diode average Schottky diode is recommended.
current occurs at the maximum load current and maximum input
voltage (minimum D). For normal operation, the catch diode current
rating must be at least 1.3 times greater than its maximum average
current. However, if the power supply design must withstand a
continuous output short, the diode should have a current rating
equal to the maximum current limit of the LM2671. The most
stressful condition for this diode is a shorted output condition.
B. The reverse voltage rating of the diode should be at least 1.25
times the maximum input voltage.
C. Because of their fast switching speed and low forward voltage
drop, Schottky diodes provide the best performance and efficiency.
This Schottky diode must be located close to the LM2671 using
short leads and short printed circuit traces.
4. Input Capacitor (CIN)
4. Input Capacitor (CIN)
A low ESR aluminum or tantalum bypass capacitor is needed
The important parameters for the input capacitor are the input
between the input pin and ground to prevent large voltage
voltage rating and the RMS current rating. With a maximum input
transients from appearing at the input. This capacitor should be
voltage of 12V, an aluminum electrolytic capacitor with a voltage
located close to the IC using short leads. In addition, the RMS
rating greater than 15V (1.25 × VIN) would be needed. The next
current rating of the input capacitor should be selected to be at least higher capacitor voltage rating is 16V.
½ the DC load current. The capacitor manufacturer data sheet must The RMS current rating requirement for the input capacitor in a
be checked to assure that this current rating is not exceeded. The buck regulator is approximately ½ the DC load current. In this
curves shown in Figure 14 show typical RMS current ratings for
example, with a 500 mA load, a capacitor with a RMS current rating
several different aluminum electrolytic capacitor values. A parallel of at least 250 mA is needed. The curves shown in Figure 14 can
connection of two or more capacitors may be required to increase be used to select an appropriate input capacitor. From the curves,
the total minimum RMS current rating to suit the application
locate the 16V line and note which capacitor values have RMS
requirements.
current ratings greater than 250 mA.
For an aluminum electrolytic capacitor, the voltage rating should be For a through hole design, a 100 μF/16V electrolytic capacitor
at least 1.25 times the maximum input voltage. Caution must be (Panasonic HFQ series, Nichicon PL, Sanyo MV-GX series or
exercised if solid tantalum capacitors are used. The tantalum
equivalent) would be adequate. Other types or other
capacitor voltage rating should be twice the maximum input
manufacturers' capacitors can be used provided the RMS ripple
voltage. The tables in Figure 15 show the recommended
current ratings are adequate. Additionally, for a complete surface
application voltage for AVX TPS and Sprague 594D tantalum
mount design, electrolytic capacitors such as the Sanyo CV-C or
capacitors. It is also recommended that they be surge current
CV-BS and the Nichicon WF or UR and the NIC Components NACZ
tested by the manufacturer. The TPS series available from AVX, series could be considered.
and the 593D and 594D series from Sprague are all surge current For surface mount designs, solid tantalum capacitors can be used,
tested. Another approach to minimize the surge current stresses but caution must be exercised with regard to the capacitor surge
on the input capacitor is to add a small inductor in series with the current rating and voltage rating. In this example, checking Figure
input supply line.
15, and the Sprague 594D series datasheet, a Sprague 594D 15
Use caution when using ceramic capacitors for input bypassing, μF, 25V capacitor is adequate.
because it may cause severe ringing at the VIN pin.
11
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LM2671
PROCEDURE (Fixed Output Voltage Version)
LM2671
PROCEDURE (Fixed Output Voltage Version)
5. Boost Capacitor (CB)
This capacitor develops the necessary voltage to turn the switch
gate on fully. All applications should use a 0.01 μF, 50V ceramic
capacitor.
6. Soft-Start Capacitor (CSS - optional)
This capacitor controls the rate at which the device starts up. The
formula for the soft-start capacitor CSS is:
EXAMPLE (Fixed Output Voltage Version)
5. Boost Capacitor (CB)
For this application, and all applications, use a 0.01 μF, 50V
ceramic capacitor.
6. Soft-Start Capacitor (CSS - optional)
For this application, selecting a start-up time of 10 ms and using
the formula for CSS results in a value of:
where:
ISS = Soft-Start Current :4.5 μA typical.
tSS = Soft-Start Time :Selected.
VSSTH = Soft-Start Threshold Voltage :0.63V typical.
VOUT = Output Voltage :Selected.
VSCHOTTKY = Schottky Diode Voltage Drop :0.4V typical.
VIN = Input Voltage :Selected.
If this feature is not desired, leave this pin open. With certain
softstart capacitor values and operating conditions, the LM2671
can exhibit an overshoot on the output voltage during turn on.
Especially when starting up into no load or low load, the softstart
function may not be effective in preventing a larger voltage
overshoot on the output. With larger loads or lower input voltages
during startup this effect is minimized. In particular, avoid using
softstart capacitors between 0.033µF and 1µF.
7. Frequency Synchronization (optional)
7. Frequency Synchronization (optional)
The LM2671 (oscillator) can be synchronized to run with an
For all applications, use a 1 kΩ resistor and a 100 pF capacitor for
external oscillator, using the sync pin (pin 3). By doing so, the
the RC filter.
LM2671 can be operated at higher frequencies than the standard
frequency of 260 kHz. This allows for a reduction in the size of the
inductor and output capacitor.
As shown in the drawing below, a signal applied to a RC filter at the
sync pin causes the device to synchronize to the frequency of that
signal. For a signal with a peak-to-peak amplitude of 3V or greater,
a 1 kΩ resistor and a 100 pF capacitor are suitable values.
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LM2671
INDUCTOR VALUE SELECTION GUIDES
(For Continuous Mode Operation)
10004231
FIGURE 6. LM2671-12
10004229
FIGURE 4. LM2671-3.3
10004232
FIGURE 7. LM2671-ADJ
10004230
FIGURE 5. LM2671-5.0
Ind. Inducta
nce
Ref.
Desg.
(μH)
Current
(A)
Schott
Through
Hole
Surface
Mount
Renco
Through
Hole
Pulse Engineering
Surface
Mount
Through
Hole
Surface
Mount
Coilcraft
Surface
Mount
L2
150
0.21
67143920 67144290 RL-5470-4
RL1500-150 PE-53802 PE-53802-S DO1608-154
L3
100
0.26
67143930 67144300 RL-5470-5
RL1500-100 PE-53803 PE-53803-S DO1608-104
L4
68
0.32
67143940 67144310
RL-1284-68-43
RL1500-68
PE-53804 PE-53804-S DO1608-683
L5
47
0.37
67148310 67148420
RL-1284-47-43
RL1500-47
PE-53805 PE-53805-S DO1608-473
L6
33
0.44
67148320 67148430
RL-1284-33-43
RL1500-33
PE-53806 PE-53806-S DO1608-333
L7
22
0.52
67148330 67148440
RL-1284-22-43
RL1500-22
PE-53807 PE-53807-S DO1608-223
L9
220
0.32
67143960 67144330
RL-5470-3
RL1500-220 PE-53809 PE-53809-S DO3308-224
L10
150
0.39
67143970 67144340
RL-5470-4
RL1500-150 PE-53810 PE-53810-S DO3308-154
L11
100
0.48
67143980 67144350
RL-5470-5
RL1500-100 PE-53811 PE-53811-S DO3308-104
L12
68
0.58
67143990 67144360
RL-5470-6
RL1500-68
PE-53812 PE-53812-S DO3308-683
L13
47
0.70
67144000 67144380
RL-5470-7
RL1500-47
PE-53813 PE-53813-S DO3308-473
L14
33
0.83
67148340 67148450
RL-1284-33-43
RL1500-33
PE-53814 PE-53814-S DO3308-333
L15
22
0.99
67148350 67148460
RL-1284-22-43
RL1500-22
PE-53815 PE-53815-S DO3308-223
L18
220
0.55
67144040 67144420
RL-5471-2
RL1500-220 PE-53818 PE-53818-S DO3316-224
L19
150
0.66
67144050 67144430
RL-5471-3
RL1500-150 PE-53819 PE-53819-S DO3316-154
L20
100
0.82
67144060 67144440
RL-5471-4
RL1500-100 PE-53820 PE-53820-S DO3316-104
L21
68
0.99
67144070 67144450
RL-5471-5
RL1500-68
13
PE-53821 PE-53821-S DO3316-683
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LM2671
FIGURE 8. Inductor Manufacturers' Part Numbers
Coilcraft Inc.
Coilcraft Inc., Europe
Pulse Engineering Inc.
Phone
(800) 322-2645
FAX
(708) 639-1469
Phone
+44 1236 730 595
FAX
+44 1236 730 627
Phone
(619) 674-8100
FAX
(619) 674-8262
Pulse Engineering Inc.,
Phone
+353 93 24 107
Europe
FAX
+353 93 24 459
Renco Electronics Inc.
Phone
(800) 645-5828
FAX
(516) 586-5562
Phone
(612) 475-1173
FAX
(612) 475-1786
Schott Corp.
FIGURE 9. Inductor Manufacturers' Phone Numbers
Output Capacitor
Output
Voltage
(V)
3.3
5.0
12
Inductance
(μH)
Surface Mount
Sprague
594D Series
Through Hole
AVX TPS
Series
Sanyo OS-CON
SA Series
Sanyo MV-GX
Series
Panasonic
HFQ Series
(μF/V)
(μF/V)
(μF/V)
(μF/V)
(μF/V)
(μF/V)
22
120/6.3
100/10
100/10
330/35
330/35
330/35
33
120/6.3
100/10
68/10
220/35
220/35
220/35
47
68/10
100/10
68/10
150/35
150/35
150/35
68
120/6.3
100/10
100/10
120/35
120/35
120/35
100
120/6.3
100/10
100/10
120/35
120/35
120/35
150
120/6.3
100/10
100/10
120/35
120/35
120/35
22
100/16
100/10
100/10
330/35
330/35
330/35
33
68/10
10010
68/10
220/35
220/35
220/35
47
68/10
100/10
68/10
150/35
150/35
150/35
68
100/16
100/10
100/10
120/35
120/35
120/35
100
100/16
100/10
100/10
120/35
120/35
120/35
150
100/16
100/10
100/10
120/35
120/35
120/35
22
120/20
(2×) 68/20
68/20
330/35
330/35
330/35
33
68/25
68/20
68/20
220/35
220/35
220/35
47
47/20
68/20
47/20
150/35
150/35
150/35
68
47/20
68/20
47/20
120/35
120/35
120/35
100
47/20
68/20
47/20
120/35
120/35
120/35
150
47/20
68/20
47/20
120/35
120/35
120/35
220
47/20
68/20
47/20
120/35
120/35
120/35
FIGURE 10. Output Capacitor Table
Nichicon Corp.
Panasonic
AVX Corp.
www.national.com
Nichicon
PL Series
Phone
(847) 843-7500
FAX
(847) 843-2798
Phone
(714) 373-7857
FAX
(714) 373-7102
Phone
(845) 448-9411
14
Sanyo Corp.
(845) 448-1943
Phone
(207) 324-4140
FAX
(207) 324-7223
Phone
(619) 661-6322
FAX
(619) 661-1055
LM2671
Sprague/Vishay
FAX
FIGURE 11. Capacitor Manufacturers' Phone Numbers
1A Diodes
3A Diodes
VR
Surface
Mount
Through
Hole
Surface
Mount
Through
Hole
20V
SK12
1N5817
SK32
1N5820
B120
SR102
SK13
1N5818
SK33
1N5821
30WQ03F
31DQ03
SK34
1N5822
30V
40V
SR302
B130
11DQ03
MBRS130
SR103
SK14
1N5819
B140
11DQ04
30BQ040
MBR340
MBRS140
SR104
30WQ04F
31DQ04
10BQ040
MBRS340
SR304
10MQ040
MBRD340
15MQ040
50V
SK15
MBR150
SK35
MBR350
B150
11DQ05
30WQ05F
31DQ05
10BQ050
SR105
SR305
FIGURE 12. Schottky Diode Selection Table
International Rectifier
Corp.
Motorola, Inc.
General Instruments
Corp.
Diodes, Inc.
Phone
(310) 322-3331
FAX
(310) 322-3332
Phone
(800) 521-6274
FAX
(602) 244-6609
Phone
(516) 847-3000
FAX
(516) 847-3236
Phone
(805) 446-4800
FAX
(805) 446-4850
FIGURE 13. Diode Manufacturers' Phone Numbers
15
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LM2671
10004233
FIGURE 14. RMS Current Ratings for Low ESR Electrolytic Capacitors (Typical)
AVX TPS
Recommended
Application Voltage
Recommended
Application Voltage
Voltage
Rating
Voltage
Rating
+85°C Rating
+85°C Rating
5
10
3.3
6.3
8
16
5
10
12
20
10
20
18
25
12
25
24
35
15
35
29
50
Sprague 594D
Recommended
Application Voltage
Voltage
Rating
+85°C Rating
2.5
3.3
4
6.3
FIGURE 15. Recommended Application Voltage for AVX TPS and
Sprague 594D Tantalum Chip Capacitors Derated for 85°C.
LM2671 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
To simplify the buck regulator design procedure, National
Semiconductor is making available computer design software to be
used with the SIMPLE SWITCHER line of switching regulators.
LM267X Made Simple is available on (version 6.0) Windows 3.1,
NT, or 95 operating systems.
Given:
Given:
VOUT = Regulated Output Voltage
VOUT = 20V
VIN(max) = Maximum Input Voltage
VIN(max) = 28V
ILOAD(max) = Maximum Load Current
ILOAD(max) = 500 mA
F = Switching Frequency (Fixed at a nominal 260 kHz).
F = Switching Frequency (Fixed at a nominal 260 kHz).
1. Programming Output Voltage (Selecting R1 and R2, as shown 1. Programming Output Voltage (Selecting R1 and R2, as shown
in Figure 3)
in Figure 3)
Use the following formula to select the appropriate resistor values. Select R1 to be 1 kΩ, 1%. Solve for R2.
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16
EXAMPLE (Adjustable Output Voltage Version)
where VREF = 1.21V
Select a value for R1 between 240Ω and 1.5 kΩ. The lower resistor R2 = 1 kΩ (16.53 − 1) = 15.53 kΩ, closest 1% value is 15.4 kΩ.
values minimize noise pickup in the sensitive feedback pin. (For the R2 = 15.4 kΩ.
lowest temperature coefficient and the best stability with time, use
1% metal film resistors.)
2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant E • T
(V • μs), from the following formula:
2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant (E • T),
where VSAT=internal switch saturation voltage=0.25V and
VD = diode forward voltage drop = 0.5V
B. Use the E • T value from the previous formula and match it with B. E • T = 21.6 (V • μs)
the E • T number on the vertical axis of the Inductor Value Selection
Guide shown in Figure 7.
C. On the horizontal axis, select the maximum load current.
C. ILOAD(max) = 500 mA
D. Identify the inductance region intersected by the E • T value and D. From the inductor value selection guide shown in Figure 7, the
the Maximum Load Current value. Each region is identified by an inductance region intersected by the 21.6 (V • μs) horizontal line
inductance value and an inductor code (LXX).
and the 500 mA vertical line is 100 μH, and the inductor code is
L20.
E. Select an appropriate inductor from the four manufacturer's part E. From the table in Figure 8, locate line L20, and select an inductor
numbers listed in Figure 8. For information on the different types of part number from the list of manufacturers' part numbers.
inductors, see the inductor selection in the fixed output voltage
design procedure.
3. Output Capacitor SeIection (COUT)
3. Output Capacitor SeIection (COUT)
A. Select an output capacitor from the capacitor code selection
A. Use the appropriate row of the capacitor code selection guide,
guide in Figure 16. Using the inductance value found in the inductor in Figure 16. For this example, use the 15–20V row. The capacitor
selection guide, step 1, locate the appropriate capacitor code
code corresponding to an inductance of 100 μH is C20.
corresponding to the desired output voltage.
B. Select an appropriate capacitor value and voltage rating, using B. From the output capacitor selection table in Figure 17, choose
the capacitor code, from the output capacitor selection table in
a capacitor value (and voltage rating) that intersects the capacitor
Figure 17. There are two solid tantalum (surface mount) capacitor code(s) selected in section A, C20.
manufacturers and four electrolytic (through hole) capacitor
The capacitance and voltage rating values corresponding to the
manufacturers to choose from. It is recommended that both the
capacitor code C20 are the:
manufacturers and the manufacturer's series that are listed in the Surface Mount:
table be used. A table listing the manufacturers' phone numbers is 33 μF/25V Sprague 594D Series.
located in Figure 11.
33 μF/25V AVX TPS Series.
Through Hole:
33 μF/25V Sanyo OS-CON SC Series.
120 μF/35V Sanyo MV-GX Series.
120 μF/35V Nichicon PL Series.
120 μF/35V Panasonic HFQ Series.
Other manufacturers or other types of capacitors may also be used,
provided the capacitor specifications (especially the 100 kHz ESR)
closely match the characteristics of the capacitors listed in the
output capacitor table. Refer to the capacitor manufacturers' data
sheet for this information.
17
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LM2671
PROCEDURE (Adjustable Output Voltage Version)
LM2671
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
4. Catch Diode Selection (D1)
A. In normal operation, the average current of the catch diode is
the load current times the catch diode duty cycle, 1-D (D is the
switch duty cycle, which is approximately VOUT/VIN). The largest
value of the catch diode average current occurs at the maximum
input voltage (minimum D). For normal operation, the catch diode
current rating must be at least 1.3 times greater than its maximum
average current. However, if the power supply design must
withstand a continuous output short, the diode should have a
current rating greater than the maximum current limit of the
LM2671. The most stressful condition for this diode is a shorted
output condition.
B. The reverse voltage rating of the diode should be at least 1.25
times the maximum input voltage.
C. Because of their fast switching speed and low forward voltage
drop, Schottky diodes provide the best performance and efficiency.
The Schottky diode must be located close to the LM2671 using
short leads and short printed circuit traces.
5. Input Capacitor (CIN)
A low ESR aluminum or tantalum bypass capacitor is needed
between the input pin and ground to prevent large voltage
transients from appearing at the input. This capacitor should be
located close to the IC using short leads. In addition, the RMS
current rating of the input capacitor should be selected to be at least
½ the DC load current. The capacitor manufacturer data sheet must
be checked to assure that this current rating is not exceeded. The
curves shown in Figure 14 show typical RMS current ratings for
several different aluminum electrolytic capacitor values. A parallel
connection of two or more capacitors may be required to increase
the total minimum RMS current rating to suit the application
requirements.
For an aluminum electrolytic capacitor, the voltage rating should be
at least 1.25 times the maximum input voltage. Caution must be
exercised if solid tantalum capacitors are used. The tantalum
capacitor voltage rating should be twice the maximum input
voltage. The tables in Figure 15 show the recommended
application voltage for AVX TPS and Sprague 594D tantalum
capacitors. It is also recommended that they be surge current
tested by the manufacturer. The TPS series available from AVX,
and the 593D and 594D series from Sprague are all surge current
tested. Another approach to minimize the surge current stresses
on the input capacitor is to add a small inductor in series with the
input supply line.
Use caution when using ceramic capacitors for input bypassing,
because it may cause severe ringing at the VIN pin.
4. Catch Diode Selection (D1)
A. Refer to the table shown in Figure 12. Schottky diodes provide
the best performance, and in this example a 1A, 40V Schottky diode
would be a good choice. If the circuit must withstand a continuous
shorted output, a higher current (at least 1.2A) Schottky diode is
recommended.
5. Input Capacitor (CIN)
The important parameters for the input capacitor are the input
voltage rating and the RMS current rating. With a maximum input
voltage of 28V, an aluminum electrolytic capacitor with a voltage
rating of at least 35V (1.25 × VIN) would be needed.
The RMS current rating requirement for the input capacitor in a
buck regulator is approximately ½ the DC load current. In this
example, with a 500 mA load, a capacitor with a RMS current rating
of at least 250 mA is needed. The curves shown in Figure 14 can
be used to select an appropriate input capacitor. From the curves,
locate the 35V line and note which capacitor values have RMS
current ratings greater than 250 mA.
For a through hole design, a 68 μF/35V electrolytic capacitor
(Panasonic HFQ series, Nichicon PL, Sanyo MV-GX series or
equivalent) would be adequate. Other types or other
manufacturers' capacitors can be used provided the RMS ripple
current ratings are adequate. Additionally, for a complete surface
mount design, electrolytic capacitors such as the Sanyo CV-C or
CV-BS and the Nichicon WF or UR and the NIC Components NACZ
series could be considered.
For surface mount designs, solid tantalum capacitors can be used,
but caution must be exercised with regard to the capacitor surge
current rating and voltage rating. In this example, checking Figure
15, and the Sprague 594D series datasheet, a Sprague 594D 15
μF, 50V capacitor is adequate.
6. Boost Capacitor (CB)
6. Boost Capacitor (CB)
This capacitor develops the necessary voltage to turn the switch For this application, and all applications, use a 0.01 μF, 50V
gate on fully. All applications should use a 0.01 μF, 50V ceramic ceramic capacitor.
capacitor.
If the soft-start and frequency synchronization features are desired,
look at steps 6 and 7 in the fixed output design procedure.
Inductance (μH)
Case
Style (Note 7)
Output
Voltage (V)
22
33
47
68
100
150
220
SM and TH
1.21–2.50
—
—
—
—
C1
C2
C3
SM and TH
2.50–3.75
—
—
—
C1
C2
C3
C3
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18
Output
Voltage (V)
22
33
47
68
100
150
220
SM and TH
3.75–5.0
—
—
C4
C5
C6
C6
C6
SM and TH
5.0–6.25
—
C4
C7
C6
C6
C6
C6
SM and TH
6.25–7.5
C8
C4
C7
C6
C6
C6
C6
SM and TH
7.5–10.0
C9
C10
C11
C12
C13
C13
C13
SM and TH
10.0–12.5
C14
C11
C12
C12
C13
C13
C13
SM and TH
12.5–15.0
C15
C16
C17
C17
C17
C17
C17
SM and TH
15.0–20.0
C18
C19
C20
C20
C20
C20
C20
SM and TH
20.0–30.0
C21
C22
C22
C22
C22
C22
C22
TH
30.0–37.0
C23
C24
C24
C25
C25
C25
C25
Note 7: SM - Surface Mount, TH - Through Hole
FIGURE 16. Capacitor Code Selection Guide
Output Capacitor
Surface Mount
Through Hole
Cap.
Ref.
Desg.
#
Sprague
594D Series
AVX TPS
Series
Sanyo OS-CON
SA Series
Sanyo MV-GX
Series
Nichicon
PL Series
Panasonic
HFQ Series
(μF/V)
(μF/V)
(μF/V)
(μF/V)
(μF/V)
(μF/V)
C1
120/6.3
100/10
100/10
220/35
220/35
220/35
C2
120/6.3
100/10
100/10
150/35
150/35
150/35
C3
120/6.3
100/10
100/35
120/35
120/35
120/35
C4
68/10
100/10
68/10
220/35
220/35
220/35
C5
100/16
100/10
100/10
150/35
150/35
150/35
C6
100/16
100/10
100/10
120/35
120/35
120/35
C7
68/10
100/10
68/10
150/35
150/35
150/35
C8
100/16
100/10
100/10
330/35
330/35
330/35
C9
100/16
100/16
100/16
330/35
330/35
330/35
C10
100/16
100/16
68/16
220/35
220/35
220/35
C11
100/16
100/16
68/16
150/35
150/35
150/35
C12
100/16
100/16
68/16
120/35
120/35
120/35
C13
100/16
100/16
100/16
120/35
120/35
120/35
C14
100/16
100/16
100/16
220/35
220/35
220/35
C15
47/20
68/20
47/20
220/35
220/35
220/35
C16
47/20
68/20
47/20
150/35
150/35
150/35
C17
47/20
68/20
47/20
120/35
120/35
120/35
C18
68/25
(2×) 33/25
47/25 (Note 8)
220/35
220/35
220/35
C19
33/25
33/25
33/25 (Note 8)
150/35
150/35
150/35
C20
33/25
33/25
33/25 (Note 8)
120/35
120/35
120/35
C21
33/35
(2×) 22/25
(Note 9)
150/35
150/35
150/35
C22
33/35
22/35
(Note 9)
120/35
120/35
120/35
C23
(Note 9)
(Note 9)
(Note 9)
220/50
100/50
120/50
C24
(Note 9)
(Note 9)
(Note 9)
150/50
100/50
120/50
C25
(Note 9)
(Note 9)
(Note 9)
150/50
82/50
82/50
Note 8: The SC series of Os-Con capacitors (others are SA series)
Note 9: The voltage ratings of the surface mount tantalum chip and Os-Con capacitors are too low to work at these voltages.
FIGURE 17. Output Capacitor Selection Table
19
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LM2671
Inductance (μH)
Case
Style (Note 7)
LM2671
Application Information
TYPICAL SURFACE MOUNT PC BOARD LAYOUT, FIXED
OUTPUT (4X SIZE)
10004239
CIN - 15 μF, 25V, Solid Tantalum Sprague, “594D series”
COUT - 68 μF, 10V, Solid Tantalum Sprague, “594D series”
D1 - 1A, 40V Schottky Rectifier, Surface Mount
L1 - 47 μH, L13, Coilcraft DO3308
CB - 0.01 μF, 50V, Ceramic
TYPICAL SURFACE MOUNT PC BOARD LAYOUT,
ADJUSTABLE OUTPUT (4X SIZE)
10004240
CIN - 15 μF, 50V, Solid Tantalum Sprague, “594D series”
COUT - 33 μF, 25V, Solid Tantalum Sprague, “594D series”
D1 - 1A, 40V Schottky Rectifier, Surface Mount
L1 - 100 μH, L20, Coilcraft DO3316
CB - 0.01 μF, 50V, Ceramic
R1 - 1k, 1%
R2 - Use formula in Design Procedure
FIGURE 18. PC Board Layout
Layout is very important in switching regulator designs.
Rapidly switching currents associated with wiring inductance
can generate voltage transients which can cause problems.
For minimal inductance and ground loops, the wires indicated
by heavy lines (in Figure 2 and Figure 3) should be wide
printed circuit traces and should be kept as short as possible. For best results, external components should be located as close to the switcher IC as possible using ground plane
construction or single point grounding.
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If open core inductors are used, special care must be taken
as to the location and positioning of this type of inductor. Allowing the inductor flux to intersect sensitive feedback, IC
ground path, and COUT wiring can cause problems.
When using the adjustable version, special care must be taken as to the location of the feedback resistors and the associated wiring. Physically locate both resistors near the IC, and
route the wiring away from the inductor, especially an open
core type of inductor.
20
21
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LM2671
The Die Attach Pad (DAP) can and should be connected to
PCB Ground plane/island. For CAD and assembly guidelines
refer
to
Application
Note
AN-1187
at
http://
power.national.com.
LLP PACKAGE DEVICES
The LM2671 is offered in the 16 lead LLP surface mount
package to allow for increased power dissipation compared
to the SO-8 and DIP.
LM2671
Physical Dimensions inches (millimeters) unless otherwise noted
8-Lead (0.150″ Wide) Molded Small Outline Package, JEDEC
Order Number LM2671M-3.3, LM2671M-5.0,
LM2671M-12 or LM2671M-ADJ
NS Package Number M08A
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22
LM2671
8-Lead (0.300″ Wide) Molded Dual-In-Line Package
Order Number LM2671N-3.3, LM2671N-5.0,
LM2671N-12 or LM2671N-ADJ
NS Package Number N08E
16-Lead LLP Surface Mount Package
NS Package Number LDA16A
23
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LM2671 SIMPLE SWITCHER Power Converter High Efficiency 500mA Step-Down Voltage
Regulator with Features
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