LM2672
SIMPLE SWITCHER
®
Power Converter High Efficiency
1A Step-Down Voltage Regulator with Features
General Description
The LM2672 series of regulators are monolithic integrated
circuits built with a LMDMOS process. These regulators pro-
vide all the active functions for a step-down (buck) switching
regulator, capable of driving a 1A 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 LM2672 series operates at a switching frequency of
260 kHz, thus allowing smaller sized filter components than
what would be needed with lower frequency switching regu-
lators. Because of its very high efficiency (
>
90%), the cop-
per traces on the printed circuit board are the only heat sink-
ing needed.
A family of standard inductors for use with the LM2672 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. Ex-
ternal 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 LM2672 buck regulator design procedure,
there exists computer design software,
LM267X Made
Simple
version 6.0.
Features
n
Efficiency up to 96%
n
Available in SO-8 and 8-pin DIP packages
n
Computer Design Software
LM267X Made Simple
version 6.0
n
Simple and easy to design with
n
Requires only 5 external components
n
Uses readily available standard inductors
n
3.3V, 5.0V, 12V, and adjustable output versions
n
Adjustable version output voltage range: 1.21V to 37V
n
±
1.5% max output voltage tolerance over line and load
conditions
n
Guaranteed 1A output load current
n
0.25
Ω
DMOS Output Switch
n
Wide input voltage range: 8V to 40V
n
260 kHz fixed frequency internal oscillator
n
TTL shutdown capability, low power standby mode
n
Soft-start and frequency synchronization
n
Thermal shutdown and current limit protection
Typical Applications
n
Simple High Efficiency (
>
90%) Step-Down (Buck)
Regulator
n
Efficient Pre-Regulator for Linear Regulators
Typical Application
(Fixed Output Voltage Versions)
SIMPLE SWITCHER
®
is a registered trademark of National Semiconductor Corporation.
Windows
®
is a registered trademark of Microsoft Corporation.
DS012934-1
August 2000
LM2672
SIMPLE
SWITCHER
Power
Converter
High
Efficiency
1A
Step-Down
V
oltage
Regulator
with
Features
© 2000 National Semiconductor Corporation
DS012934
www.national.com
Absolute Maximum Ratings
(Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage
45V
ON/OFF Pin Voltage
−0.1V
≤
V
SH
≤
6V
Switch Voltage to Ground
−1V
Boost Pin Voltage
V
SW
+ 8V
Feedback Pin Voltage
−0.3V
≤
V
FB
≤
14V
ESD Susceptibility
Human Body Model (Note 2)
2 kV
Power Dissipation
Internally Limited
Storage Temperature Range
−65˚C to +150˚C
Lead Temperature
M Package
Vapor Phase (60s)
+215˚C
Infrared (15s)
+220˚C
N Package (Soldering, 10s)
+260˚C
Maximum Junction Temperature
+150˚C
Operating Ratings
Supply Voltage
6.5V to 40V
Temperature Range
−40˚C
≤
T
J
≤
+125˚C
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those in bold type
face apply over full Operating Temperature Range.
LM2672-3.3
Symbol
Parameter
Conditions
Typical
Min
Max
Units
(Note 4)
(Note 5)
(Note 5)
SYSTEM PARAMETERS Test Circuit
Figure 2
(Note 3)
V
OUT
Output Voltage
V
IN
= 8V to 40V, I
LOAD
= 20 mA to 1A
3.3
3.251/3.201
3.350/3.399
V
V
OUT
Output Voltage
V
IN
= 6.5V to 40V, I
LOAD
= 20 mA to 500 mA
3.3
3.251/3.201
3.350/3.399
V
η
Efficiency
V
IN
= 12V, I
LOAD
= 1A
86
%
LM2672-5.0
Symbol
Parameter
Conditions
Typical
Min
Max
Units
(Note 4)
(Note 5)
(Note 5)
SYSTEM PARAMETERS Test Circuit
Figure 2
(Note 3)
V
OUT
Output Voltage
V
IN
= 8V to 40V, I
LOAD
= 20 mA to 1A
5.0
4.925/4.850
5.075/5.150
V
V
OUT
Output Voltage
V
IN
= 6.5V to 40V, I
LOAD
= 20 mA to 500 mA
5.0
4.925/4.850
5.075/5.150
V
η
Efficiency
V
IN
= 12V, I
LOAD
= 1A
90
%
LM2672-12
Symbol
Parameter
Conditions
Typical
Min
Max
Units
(Note 4)
(Note 5)
(Note 5)
SYSTEM PARAMETERS Test Circuit
Figure 2
(Note 3)
V
OUT
Output Voltage
V
IN
= 15V to 40V, I
LOAD
= 20 mA to 1A
12
11.82/11.64
12.18/12.36
V
η
Efficiency
V
IN
= 24V, I
LOAD
= 1A
94
%
LM2672-ADJ
Symbol
Parameter
Conditions
Typ
Min
Max
Units
(Note 4)
(Note 5)
(Note 5)
SYSTEM PARAMETERS Test Circuit
Figure 3
(Note 3)
V
FB
Feedback
Voltage
V
IN
= 8V to 40V, I
LOAD
= 20 mA to 1A
1.210
1.192/1.174
1.228/1.246
V
V
OUT
Programmed for 5V
(see Circuit of
Figure 3
)
V
FB
Feedback
Voltage
V
IN
= 6.5V to 40V, I
LOAD
= 20 mA to 500 mA
1.210
1.192/1.174
1.228/1.246
V
V
OUT
Programmed for 5V
(see Circuit of
Figure 3
)
LM2672
www.national.com
2
LM2672-ADJ
(Continued)
Symbol
Parameter
Conditions
Typ
Min
Max
Units
(Note 4)
(Note 5)
(Note 5)
SYSTEM PARAMETERS Test Circuit
Figure 3
(Note 3)
η
Efficiency
V
IN
= 12V, I
LOAD
= 1A
90
%
All Output Voltage Versions
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those in bold type face apply over full Operating Temperature
Range. Unless otherwise specified, V
IN
= 12V for the 3.3V, 5V, and Adjustable versions and V
IN
= 24V for the 12V version,
and I
LOAD
= 100 mA.
Symbol
Parameters
Conditions
Typ
Min
Max
Units
DEVICE PARAMETERS
I
Q
Quiescent Current
V
FEEDBACK
= 8V
2.5
3.6
mA
For 3.3V, 5.0V, and ADJ Versions
V
FEEDBACK
= 15V
2.5
mA
For 12V Versions
I
STBY
Standby Quiescent Current
ON/OFF Pin = 0V
50
100/150
µA
I
CL
Current Limit
1.55
1.25/1.2
2.1/2.2
A
I
L
Output Leakage Current
V
IN
= 40V, ON/OFF Pin = 0V
1
25
µA
V
SWITCH
= 0V
V
SWITCH
= −1V, ON/OFF Pin = 0V
6
15
mA
R
DS(ON)
Switch On-Resistance
I
SWITCH
= 1A
0.25
0.30/0.50
Ω
f
O
Oscillator Frequency
Measured at Switch Pin
260
225
275
kHz
D
Maximum Duty Cycle
95
%
Minimum Duty Cycle
0
%
I
BIAS
Feedback Bias
V
FEEDBACK
= 1.3V
85
nA
Current
ADJ Version Only
V
S/D
ON/OFF Pin
1.4
0.8
2.0
V
Voltage Thesholds
I
S/D
ON/OFF Pin Current
ON/OFF Pin = 0V
20
7
37
µA
F
SYNC
Synchronization Frequency
V
SYNC
= 3.5V, 50% duty cycle
400
kHz
V
SYNC
Synchronization Threshold
Voltage
1.4
V
V
SS
Soft-Start Voltage
0.63
0.53
0.73
V
I
SS
Soft-Start Current
4.5
1.5
6.9
µA
θ
JA
Thermal Resistance
N Package, Junction to Ambient (Note 6)
95
˚C/W
M Package, Junction to Ambient (Note 6)
105
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is in-
tended 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 k
Ω
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 per-
formance. When the LM2672 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% pro-
duction tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to cal-
culate 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.
LM2672
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3
Connection Diagram
Typical Performance Characteristics
8-Lead Package
Top View
DS012934-2
For Surface Mount Package
Order Number
LM2672M-3.3, LM2672M-5.0,
LM2672M-12 or LM2672M-ADJ
See NSC Package Number M08A
For DIP Package
Order Number
LM2672N-3.3, LM2672N-5.0,
LM2672N-12 or LM2672N-ADJ
See NSC Package Number N08E
Normalized
Output Voltage
DS012934-3
Line Regulation
DS012934-4
Efficiency
DS012934-5
Drain-to-Source
Resistance
DS012934-6
Switch Current Limit
DS012934-7
Operating
Quiescent Current
DS012934-8
LM2672
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4
Typical Performance Characteristics
(Continued)
Standby
Quiescent Current
DS012934-9
ON/OFF Threshold
Voltage
DS012934-10
ON/OFF Pin
Current (Sourcing)
DS012934-11
Switching Frequency
DS012934-12
Feedback Pin
Bias Current
DS012934-13
Peak Switch Current
DS012934-14
Dropout Voltage — 3.3V Option
DS012934-15
Dropout Voltage — 5.0V Option
DS012934-16
LM2672
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5
Block Diagram
Typical Performance Characteristics
(Circuit of
Figure 2
)
DS012934-17
* Patent Number 5,514,947
†
Patent Number 5,382,918
FIGURE 1.
Continuous Mode Switching Waveforms
V
IN
= 20V, V
OUT
= 5V, I
LOAD
= 1A
L = 47 µH, C
OUT
= 68 µF, C
OUT
ESR = 50 m
Ω
DS012934-18
A: V
SW
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
Discontinuous Mode Switching Waveforms
V
IN
= 20V, V
OUT
= 5V, I
LOAD
= 300 mA
L = 15 µH, C
OUT
= 68 µF (2x), C
OUT
ESR = 25 m
Ω
DS012934-19
A: V
SW
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
LM2672
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6
Typical Performance Characteristics
(Circuit of
Figure 2
) (Continued)
Test Circuit and Layout Guidelines
Load Transient Response for Continuous Mode
V
IN
= 20V, V
OUT
= 5V, I
LOAD
= 1A
L = 47 µH, C
OUT
= 68 µF, C
OUT
ESR = 50 m
Ω
DS012934-20
A: Output Voltage, 100 mV/div, AC-Coupled
B: Load Current: 200 mA to 1A Load Pulse
Horizontal Time Base: 50 µs/div
Load Transient Response for Discontinuous Mode
V
IN
= 20V, V
OUT
= 5V,
L = 47 µH, C
OUT
= 68 µF, C
OUT
ESR = 50 m
Ω
DS012934-21
A: Output Voltage, 100 mV/div, AC-Coupled
B: Load Current: 100 mA to 300 mA Load Pulse
Horizontal Time Base: 200 µs/div
DS012934-22
C
IN
- 22 µF, 50V Tantalum, Sprague “199D Series”
C
OUT
- 47 µF, 25V Tantalum, Sprague “595D Series”
D1 - 3.3A, 50V Schottky Rectifier, IR 30WQ05F
L1 - 68 µH Sumida #RCR110D-680L
C
B
- 0.01 µF, 50V Ceramic
FIGURE 2. Standard Test Circuits and Layout Guides
Fixed Output Voltage Versions
LM2672
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7
Test Circuit and Layout Guidelines
(Continued)
LM2672 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:
V
OUT
= Regulated Output Voltage (3.3V, 5V, or 12V)
V
OUT
= 5V
V
IN
(max) = Maximum DC Input Voltage
V
IN
(max) = 12V
I
LOAD
(max) = Maximum Load Current
I
LOAD
(max) = 1A
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.
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, 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).
B. From the inductor value selection guide shown in
Figure 5
,
the inductance region intersected by the 12V horizontal line
and the 1A vertical line is 33 µH, and the inductor code is L23.
DS012934-23
C
IN
- 22 µF, 50V Tantalum, Sprague “199D Series”
C
OUT
- 47 µF, 25V Tantalum, Sprague “595D Series”
D1 - 3.3A, 50V Schottky Rectifier, IR 30WQ05F
L1 - 68 µH Sumida #RCR110D-680L
R1 - 1.5 k
Ω
, 1%
C
B
- 0.01 µF, 50V Ceramic
For a 5V output, select R2 to be 4.75 k
Ω
, 1%
where V
REF
= 1.21V
Use a 1% resistor for best stability.
FIGURE 3. Standard Test Circuits and Layout Guides
Adjustable Output Voltage Versions
LM2672
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8
LM2672 Series Buck Regulator Design Procedure (Fixed Output)
(Continued)
PROCEDURE (Fixed Output Voltage Version)
EXAMPLE (Fixed Output Voltage Version)
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:
C. The inductance value required is 33 µH. From the table in
Figure 8
, go to the L23 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.)
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 (C
OUT
)
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.
2. Output Capacitor Selection (C
OUT
)
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 33 µH. The capacitance
and voltage rating values corresponding to the 33 µH inductor
are the:
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.
220 µF/35V
Sanyo MV-GX Series.
220 µF/35V
Nichicon PL Series.
220 µF/35V
Panasonic HFQ Series.
3. 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 the output
voltage divided by the input voltage). The largest value of the
catch diode average 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 LM2672. The most stressful
condition for this diode is a shorted output condition.
3. Catch Diode Selection (D1)
A. Refer to the table shown in
Figure 12
. In this example, a
1A, 20V Schottky diode will provide the best performance. If
the circuit must withstand a continuous shorted output, a
higher current Schottky diode is recommended.
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 LM2672 using short leads and short printed circuit traces.
LM2672
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9
LM2672 Series Buck Regulator Design Procedure (Fixed Output)
(Continued)
PROCEDURE (Fixed Output Voltage Version)
EXAMPLE (Fixed Output Voltage Version)
4. Input Capacitor (C
IN
)
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
1
⁄
2
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 V
IN
pin.
4. Input Capacitor (C
IN
)
The important parameters for the input capacitor are the input
voltage rating and the RMS current rating. With a maximum
input voltage of 12V, an aluminum electrolytic capacitor with a
voltage rating greater than 15V (1.25 x V
IN
) would be needed.
The next higher capacitor voltage rating is 16V.
The RMS current rating requirement for the input capacitor in a
buck regulator is approximately
1
⁄
2
the DC load current. In this
example, with a 1A load, a capacitor with a RMS current rating
of at least 500 mA is needed. The curves shown in
Figure 14
can be used to select an appropriate input capacitor. From the
curves, locate the 16V line and note which capacitor values
have RMS current ratings greater than 500 mA.
For a through hole design, a 330 µF/16V 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, 25V capacitor is adequate.
5. Boost Capacitor (C
B
)
This capacitor develops the necessary voltage to turn the
switch gate on fully. All applications should use a 0.01 µF, 50V
ceramic capacitor.
5. Boost Capacitor (C
B
)
For this application, and all applications, use a 0.01 µF, 50V
ceramic capacitor.
6. Soft-Start Capacitor (C
SS
- optional)
This capacitor controls the rate at which the device starts up.
The formula for the soft-start capacitor C
SS
is:
6. Soft-Start Capacitor (C
SS
- optional)
For this application, selecting a start-up time of 10 ms and
using the formula for C
SS
results in a value of:
where:
I
SS
= Soft-Start Current
:4.5 µA typical.
t
SS
= Soft-Start Time
:Selected.
V
SSTH
= Soft-Start Threshold Voltage
:0.63V typical.
V
OUT
= Output Voltage
:Selected.
V
SCHOTTKY
= Schottky Diode Voltage Drop
:0.4V typical.
V
IN
= Input Voltage
:Selected.
If this feature is not desired, leave this pin open.
LM2672
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10
LM2672 Series Buck Regulator Design Procedure (Fixed Output)
(Continued)
PROCEDURE (Fixed Output Voltage Version)
EXAMPLE (Fixed Output Voltage Version)
7. Frequency Synchronization (optional)
The LM2672 (oscillator) can be synchronized to run with an
external oscillator, using the sync pin (pin 3). By doing so, the
LM2672 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 k
Ω
resistor and a 100 pF
capacitor are suitable values.
7. Frequency Synchronization (optional)
For all applications, use a 1 k
Ω
resistor and a 100 pF capacitor
for the RC filter.
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)
DS012934-29
FIGURE 4. LM2672-3.3
DS012934-30
FIGURE 5. LM2672-5.0
DS012934-31
FIGURE 6. LM2672-12
DS012934-32
FIGURE 7. LM2672-ADJ
LM2672
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11
LM2672 Series Buck Regulator Design Procedure (Fixed Output)
(Continued)
Ind.
Ref.
Desg.
Induc-
tance
(µH)
Current
(A)
Schott
Renco
Pulse Engineering
Coilcraft
Through
Surface
Through
Surface
Through
Surface
Surface
Hole
Mount
Hole
Mount
Hole
Mount
Mount
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
PE-53821
PE-53821-S
DO3316-683
L22
47
1.17
67144080
67144460
RL-5471-6
—
PE-53822
PE-53822-S
DO3316-473
L23
33
1.40
67144090
67144470
RL-5471-7
—
PE-53823
PE-53823-S
DO3316-333
L24
22
1.70
67148370
67148480
RL-1283-22-43
—
PE-53824
PE-53824-S
DO3316-223
L27
220
1.00
67144110
67144490
RL-5471-2
—
PE-53827
PE-53827-S
DO5022P-224
L28
150
1.20
67144120
67144500
RL-5471-3
—
PE-53828
PE-53828-S
DO5022P-154
L29
100
1.47
67144130
67144510
RL-5471-4
—
PE-53829
PE-53829-S
DO5022P-104
L30
68
1.78
67144140
67144520
RL-5471-5
—
PE-53830
PE-53830-S
DO5022P-683
FIGURE 8. Inductor Manufacturers’ Part Numbers
Coilcraft Inc.
Phone
(800) 322-2645
FAX
(708) 639-1469
Coilcraft Inc., Europe
Phone
+44 1236 730 595
FAX
+44 1236 730 627
Pulse Engineering Inc.
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
Schott Corp.
Phone
(612) 475-1173
FAX
(612) 475-1786
FIGURE 9. Inductor Manufacturers’ Phone Numbers
LM2672
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12
LM2672 Series Buck Regulator Design Procedure (Fixed Output)
(Continued)
Output
Voltage
(V)
Inductance
(µH)
Output Capacitor
Surface Mount
Through Hole
Sprague
AVX TPS
Sanyo OS-CON
Sanyo MV-GX
Nichicon
Panasonic
594D Series
Series
SA Series
Series
PL Series
HFQ Series
(µF/V)
(µF/V)
(µF/V)
(µF/V)
(µF/V)
(µF/V)
3.3
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
5.0
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
12
22
120/20
(2x) 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.
Phone
(847) 843-7500
FAX
(847) 843-2798
Panasonic
Phone
(714) 373-7857
FAX
(714) 373-7102
AVX Corp.
Phone
(803) 448-9411
FAX
(803) 448-1943
Sprague/Vishay
Phone
(207) 324-4140
FAX
(207) 324-7223
Sanyo Corp.
Phone
(619) 661-6322
FAX
(619) 661-1055
FIGURE 11. Capacitor Manufacturers’ Phone Numbers
LM2672
www.national.com
13
LM2672 Series Buck Regulator Design Procedure (Fixed Output)
(Continued)
V
R
1A Diodes
3A Diodes
Surface
Through
Surface
Through
Mount
Hole
Mount
Hole
20V
SK12
1N5817
SK32
1N5820
B120
SR102
SR302
30V
SK13
1N5818
SK33
1N5821
B130
11DQ03
30WQ03F
31DQ03
MBRS130
SR103
40V
SK14
1N5819
SK34
1N5822
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.
Phone
(310) 322-3331
FAX
(310) 322-3332
Motorola, Inc.
Phone
(800) 521-6274
FAX
(602) 244-6609
General Instruments
Corp.
Phone
(516) 847-3000
FAX
(516) 847-3236
Diodes, Inc.
Phone
(805) 446-4800
FAX
(805) 446-4850
FIGURE 13. Diode Manufacturers’ Phone Numbers
DS012934-33
FIGURE 14. RMS Current Ratings for Low ESR Electrolytic Capacitors (Typical)
LM2672
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14
LM2672 Series Buck Regulator Design Procedure (Fixed Output)
(Continued)
LM2672 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 version 6.0 is available on
Windows
3.1, NT, or 95 operating systems.
Given:
Given:
V
OUT
= Regulated Output Voltage
V
OUT
= 20V
V
IN
(max) = Maximum Input Voltage
V
IN
(max) = 28V
I
LOAD
(max) = Maximum Load Current
I
LOAD
(max) = 1A
F = Switching Frequency
(Fixed at a nominal 260 kHz).
F = Switching Frequency
(Fixed at a nominal 260 kHz).
1. Programming Output Voltage (Selecting R
1
and R
2
, as
shown in
Figure 3
)
Use the following formula to select the appropriate resistor
values.
1. Programming Output Voltage (Selecting R
1
and R
2
, as
shown in
Figure 3
)
Select R
1
to be 1 k
Ω
, 1%. Solve for R
2
.
where V
REF
= 1.21V
Select a value for R
1
between 240
Ω
and 1.5 k
Ω
. The lower
resistor values minimize noise pickup in the sensitive feedback
pin. (For the lowest temperature coefficient and the best
stability with time, use 1% metal film resistors.)
R
2
= 1 k
Ω
(16.53 − 1) = 15.53 k
Ω
, closest 1% value is
15.4 k
Ω
.
R
2
= 15.4 k
Ω
.
AVX TPS
Recommended
Application Voltage
Voltage
Rating
+85˚C Rating
3.3
6.3
5
10
10
20
12
25
15
35
Sprague 594D
Recommended
Application Voltage
Voltage
Rating
+85˚C Rating
2.5
4
3.3
6.3
5
10
8
16
12
20
18
25
24
35
29
50
FIGURE 15. Recommended Application Voltage for AVX TPS and
Sprague 594D Tantalum Chip Capacitors Derated for 85˚C.
LM2672
www.national.com
15
LM2672 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
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 V
SAT
=internal switch saturation voltage=0.25V and
V
D
= diode forward voltage drop = 0.5V
B. Use the E
•
T value from the previous formula and match it
with the E
•
T number on the vertical axis of the Inductor
Value Selection Guide shown in
Figure 7
.
B. E
•
T = 21.6 (V
•
µs)
C. On the horizontal axis, select the maximum load current.
C. I
LOAD
(max) = 1A
D. Identify the inductance region intersected by the E
•
T value
and the Maximum Load Current value. Each region is identified
by an inductance value and an inductor code (LXX).
D. From the inductor value selection guide shown in
Figure 7
,
the inductance region intersected by the 21.6 (V
•
µs)
horizontal line and the 1A vertical line is 68 µH, and the
inductor code is L30.
E. Select an appropriate inductor from the four manufacturer’s
part numbers listed in
Figure 8
. For information on the different
types of inductors, see the inductor selection in the fixed
output voltage design procedure.
E. From the table in
Figure 8
, locate line L30, and select an
inductor part number from the list of manufacturers’ part
numbers.
3. Output Capacitor SeIection (C
OUT
)
A. Select an output capacitor from the capacitor code selection
guide in
Figure 16
. Using the inductance value found in the
inductor selection guide, step 1, locate the appropriate
capacitor code corresponding to the desired output voltage.
3. Output Capacitor SeIection (C
OUT
)
A. Use the appropriate row of the capacitor code selection
guide, in
Figure 16
. For this example, use the 15–20V row.
The capacitor code corresponding to an inductance of 68 µH is
C20.
B. Select an appropriate capacitor value and voltage rating,
using the capacitor code, from the output capacitor selection
table in
Figure 17
. There are two solid tantalum (surface
mount) capacitor manufacturers and four electrolytic (through
hole) capacitor manufacturers to choose from. 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
.
B. From the output capacitor selection table in
Figure 17
,
choose a capacitor value (and voltage rating) that intersects
the capacitor code(s) selected in section A, C20.
The capacitance and voltage rating values corresponding to
the capacitor code C20 are the:
Surface Mount:
33 µF/25V
Sprague 594D Series.
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.
LM2672
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16
LM2672 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
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 V
OUT
/V
IN
). 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 LM2672. The most stressful
condition for this diode is a shorted output condition.
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 2.2A) Schottky diode is recommended.
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 LM2672 using short leads and short printed circuit traces.
5. Input Capacitor (C
IN
)
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
1
⁄
2
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 V
IN
pin.
5. Input Capacitor (C
IN
)
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 x V
IN
) would be needed.
The RMS current rating requirement for the input capacitor in a
buck regulator is approximately
1
⁄
2
the DC load current. In this
example, with a 1A load, a capacitor with a RMS current rating
of at least 500 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 500 mA.
For a through hole design, a 330 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 (C
B
)
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. Boost Capacitor (C
B
)
For this application, and all applications, use a 0.01 µF, 50V
ceramic capacitor.
If the soft-start and frequency synchronization features are
desired, look at steps 6 and 7 in the fixed output design
procedure.
LM2672
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17
LM2672 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
Case
Style (Note 7)
Output
Voltage (V)
Inductance (µH)
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
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
LM2672
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18
LM2672 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
Output Capacitor
Cap.
Ref.
Desg.
#
Surface Mount
Through Hole
Sprague
AVX TPS
Sanyo OS-CON
Sanyo MV-GX
Nichicon
Panasonic
594D Series
Series
SA Series
Series
PL Series
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
(2x) 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
(2x) 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
LM2672
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19
Application Information
TYPICAL SURFACE MOUNT PC BOARD LAYOUT, FIXED OUTPUT (4X SIZE)
TYPICAL SURFACE MOUNT PC BOARD LAYOUT, ADJUSTABLE OUTPUT (4X SIZE)
Layout is very important in switching regulator designs. Rap-
idly 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.
If open core inductors are used, special care must be
taken as to the location and positioning of this type of induc-
tor. Allowing the inductor flux to intersect sensitive feedback,
IC ground path, and C
OUT
wiring can cause problems.
When using the adjustable version, special care must be
taken as to the location of the feedback resistors and the as-
sociated wiring. Physically locate both resistors near the IC,
and route the wiring away from the inductor, especially an
open core type of inductor.
DS012934-39
C
IN
- 15 µF, 50V, Solid Tantalum Sprague, “594D series”
C
OUT
- 68 µF, 16V, Solid Tantalum Sprague, “594D series”
D1 - 1A, 40V Schottky Rectifier, Surface Mount
L1 - 33 µH, L23, Coilcraft DO3316
C
B
- 0.01 µF, 50V, Ceramic
DS012934-40
C
IN
- 15 µF, 50V, Solid Tantalum Sprague, “594D series”
C
OUT
- 33 µF, 25V, Solid Tantalum Sprague, “594D series”
D1 - 1A, 40V Schottky Rectifier, Surface Mount
L1 - 68 µH, L30, Coilcraft DO3316
C
B
- 0.01 µF, 50V, Ceramic
R1 - 1k, 1%
R2 - Use formula in Design Procedure
FIGURE 18. PC Board Layout
LM2672
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20
Physical Dimensions
inches (millimeters) unless otherwise noted
8-Lead (0.150" Wide) Molded Small Outline Package, JEDEC
Order Number LM2672M-3.3, LM2672M-5.0,
LM2672M-12 or LM2672M-ADJ
NS Package Number M08A
LM2672
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21
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
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accordance with instructions for use provided in the
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significant injury to the user.
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
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www.national.com
8-Lead (0.300" Wide) Molded Dual-In-Line Package
Order Number LM2672N-3.3, LM2672N-5.0,
LM2672N-12 or LM2672N-ADJ
NS Package Number N08E
LM2672
SIMPLE
SWITCHER
Power
Converter
High
Efficiency
1A
Step-Down
V
oltage
Regulator
with
Features
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.