Datasheet LM2671M-3.3, LM2671M-12, LM2671N-ADJ, LM2671N-5.0, LM2671N-12 Datasheet (NSC)

September 1998

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 of3.3V, 5.0V, 12V, and an adjustable
output version.

Requiring a minimumnumberofexternal 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 (
per 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
output voltage within specified input voltages and output
load conditions, and
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.

±

10%on the oscillator frequency. Ex-

>

90%), the cop-

±

1.5%tolerance on

To simplify the LM2671 buck regulator design procedure,
there exists computer design software,

Simple

.

LM267X Made

Features

n Efficiency up to 96
n Available in SO-8 and 8-pin DIP packages
n Computer Design Software
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 500mA output load current
n 0.25Ω 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

%

LM267X Made Simple

Typical Applications

n Simple High Efficiency (>90%) Step-Down (Buck)

Regulator

n Efficient Pre-Regulator for Linear Regulators

LM2671 SIMPLE SWITCHER Power Converter High Efficiency 500mA Step-Down Voltage

Regulator with Features

Typical Application (Fixed Output Voltage Versions)

DS100042-1

SIMPLE SWITCHER®is a registered trademark of National Semiconductor Corporation.

®

Windows

is a registered trademark of Microsoft Corporation.

© 1998 National Semiconductor Corporation DS100042 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
Switch Voltage to Ground −1V
Boost Pin Voltage V
Feedback Pin Voltage −0.3V ≤ V
ESD Susceptibility

Human Body Model (Note 2) 2 kV

Power Dissipation Internally Limited

−0.1V ≤ VSH≤ 6V

+8V

SW

≤ 14V

FB

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

≤ +125˚C

J

Electrical Characteristics Specifications with standard type face are for T

face apply over full Operating Temperature Range.

=

25˚C, and those in bold type

J

LM2671-3.3

Symbol Parameter Conditions Typical Min Max Units

(Note 4) (Note 5) (Note 5)
SYSTEM PARAMETERS Test Circuit
V
V

Output Voltage V

OUT

Output Voltage V

OUT

η Efficiency V

Figure 2

=

8V to 40V, I

IN

=

6.5V to 40V, I

IN

=

12V, I

IN

(Note 3)

=

20 mA to 500 mA 3.3 3.251/3.201 3.350/3.399 V

LOAD

=

20 mA to 250 mA 3.3 3.251/3.201 3.350/3.399 V

LOAD

=

500 mA 86

LOAD

%

LM2671-5.0

Symbol Parameter Conditions Typical Min Max Units

(Note 4) (Note 5) (Note 5)
SYSTEM PARAMETERS Test Circuit
V
V

Output Voltage V

OUT

Output Voltage V

OUT

η Efficiency V

Figure 2

=

8V to 40V, I

IN

=

6.5V to 40V, I

IN

=

12V, I

IN

(Note 3)

=

20 mA to 500 mA 5.0 4.925/4.850 5.075/5.150 V

LOAD

=

20 mA to 250 mA 5.0 4.925/4.850 5.075/5.150 V

LOAD

=

500 mA 90

LOAD

%

LM2671-12

Symbol Parameter Conditions Typical Min Max Units

(Note 4) (Note 5) (Note 5)
SYSTEM PARAMETERS Test Circuit
V

Output Voltage V

OUT

η Efficiency V

=

15V to 40V, I

IN

=

24V, I

IN

Figure 2

LOAD

(Note 3)

=

20 mA to 500 mA 12 11.82/11.64 12.18/12.36 V

LOAD

=

500 mA 94

%

LM2671-ADJ

Symbol Parameter Conditions Typ Min Max Units

(Note 4) (Note 5) (Note 5)
SYSTEM PARAMETERS Test Circuit
V

V

www.national.com 2

Feedback

FB

Voltage

Feedback

FB

Voltage

Figure 3

=

V

8V to 40V, I

IN

Programmed for 5V

V

OUT

(see Circuit of

=

V

6.5V to 40V, I

IN

Programmed for 5V

V

OUT

(see Circuit of

(Note 3)

LOAD

Figure 3

LOAD

Figure 3

=

20 mA to 500 mA

)

=

20 mA to 250 mA

)

1.210 1.192/1.174 1.228/1.246 V

1.210 1.192/1.174 1.228/1.246 V

LM2671-ADJ (Continued)

Symbol Parameter Conditions Typ Min Max Units

(Note 4) (Note 5) (Note 5)
SYSTEM PARAMETERS Test Circuit
η Efficiency V

Figure 3

(Note 3)

=

12V, I

IN

=

500 mA 90

LOAD

%

All Output Voltage Versions

Electrical Characteristics

Specifications with standard type face are for T
Range. Unless otherwise specified, V
and I

LOAD

=

100 mA.

IN

Symbol Parameters Conditions Typ Min Max Units

DEVICE PARAMETERS

I

Q

I

STBY

I

CL

I

L

R
f

O

Quiescent Current V

Standby Quiescent Current ON/OFF Pin=0V 50 100/150 µA
Current Limit 0.8 0.62/0.575 1.2/1.25 A
Output Leakage Current V

Switch On-Resistance I

DS(ON)

Oscillator Frequency Measured at Switch Pin 260 225 275 kHz

D Maximum Duty Cycle 95

Minimum Duty Cycle 0

I

BIAS

Feedback Bias V
Current ADJ Version Only

V

ON/OFF Pin 1.4 0.8 2.0 V

S/D

Voltage Thesholds

I

S/D

F

SYNC

V

V
I

SS

θ

JA

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 per-

formance. When the LM2671 is used as shown in
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 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

Simple

ON/OFF Pin Current ON/OFF Pin=0V 20 737µA
Synchronization Frequency V
Synchronization Threshold

SYNC

Voltage
Soft-Start Voltage 0.63 0.53 0.73 V

SS

Soft-Start Current 4.5 1.5 6.9 µA
Thermal Resistance N Package, Junction to Ambient (Note 6) 95 ˚C/W

Figure 2

version 1.0 software.

=

25˚C, and those in bold type face apply over full Operating Temperature

J

=

12V for the 3.3V, 5V, and Adjustable versions and 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

=

40V, ON/OFF Pin=0V

IN

=

V

SWITCH

V

SWITCH

SWITCH

FEEDBACK

SYNC

0V

=

−1V, ON/OFF Pin=0V

=

500 mA 0.25 0.40/0.60 Ω

=

1.3V 85 nA

=

3.5V, 50%duty cycle 400 kHz

M Package, Junction to Ambient (Note 6) 105

and

Figure 3

test circuits, system performance will be as specified by the system parameters section of

=

24V for the 12V version,

IN

125µA

615mA

%
%

1.4 V

LM267X Made

www.national.com3

Connection Diagram

Typical Performance Characteristics

8-Lead Package

Top View

DS100042-2

For Surface Mount Package

Order Number

LM2671M-3.3, LM2671M-5.0,

LM2671M-12 or LM2671M-ADJ

See NSC Package Number M08A

For DIP Package

Order Number

LM2671N-3.3, LM2671N-5.0,

LM2671N-12 or LM2671N-ADJ

See NSC Package Number N08E

Normalized
Output Voltage

Efficiency

DS100042-3

DS100042-5

Line Regulation

DS100042-4

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Typical Performance Characteristics (Continued)

Drain-to-Source
Resistance

Operating
Quiescent Current

Standby
Quiescent Current

Switch Current Limit

DS100042-7

DS100042-6

DS100042-8

ON/OFF Threshold
Voltage

ON/OFF Pin
Current (Sourcing)

DS100042-9

DS100042-11

DS100042-10

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Typical Performance Characteristics (Continued)

Switching Frequency

Peak Switch Current

Dropout Voltage—3.3V Option

Feedback Pin
Bias Current

DS100042-12

DS100042-13

DS100042-14

Dropout Voltage—5.0V Option

DS100042-15

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DS100042-16

Block Diagram

* Patent Number 5,514,947

†

Patent Number 5,382,918

FIGURE 1.

Typical Performance Characteristics

Continuous Mode Switching Waveforms

=

V

20V, V

IN

L=100 µH, C

A: VSWPin Voltage, 10 V/div.
B: Inductor Current, 0.2 A/div
C: Output Ripple Voltage, 50 mV/div AC-Coupled

Horizontal Time Base: 1 µs/div

OUT

OUT

=

5V, I

=

100 µF, C

LOAD

=

500 mA

ESR=0.1Ω

OUT

DS100042-18

(Circuit of

A: VSWPin Voltage, 10 V/div.
B: Inductor Current, 0.5 A/div
C: Output Ripple Voltage, 20 mV/div AC-Coupled

Figure 2

)

Discontinuous Mode Switching Waveforms

=

V

IN

L=15 µH, C

20V, V

OUT

=

5V, I

OUT

=

68 µF (2x), C

LOAD

Horizontal Time Base: 1 µs/div

=

300 mA

ESR=25 mΩ

OUT

DS100042-17

DS100042-19

www.national.com7

Typical Performance Characteristics (Circuit of

Figure 2

) (Continued)

Load Transient Response for Continuous Mode

L=100 µH, C

A: Output Voltage, 100 mV/div, AC-Coupled
B: Load Current: 100 mA to 500 mA Load Pulse

=

V

20V, V

IN

=

OUT

OUT

100 µF, C

=

5V

OUT

ESR=0.1Ω

DS100042-20

Horizontal Time Base: 50 µs/div

Test Circuit and Layout Guidelines

Load Transient Response for Discontinuous Mode

L=47 µH, C

A: Output Voltage, 100 mV/div, AC-Coupled
B: Load Current: 100 mA to 400 mA Load Pulse

=

V

20V, V

IN

=

OUT

OUT

68 µF, C

=

5V,

ESR=50 mΩ

OUT

DS100042-21

Horizontal Time Base: 200 µs/div

CIN- 22 µF, 50V Tantalum, Sprague “199D Series”

- 47 µF, 25V Tantalum, Sprague “595D Series”

C

OUT

D1 - 3.3A, 50V Schottky Rectifier, IR 30WQ05F
L1 - 68 µH Sumida

- 0.01 µF, 50V Ceramic

C

B

#

RCR110D-680L

FIGURE 2. Standard Test Circuits and Layout Guides

Fixed Output Voltage Versions

www.national.com 8

DS100042-22

Test Circuit and Layout Guidelines (Continued)

CIN- 22 µF, 50V Tantalum, Sprague “199D Series”

- 47 µF, 25V Tantalum, Sprague “595D Series”

C

OUT

D1 - 3.3A, 50V Schottky Rectifier, IR 30WQ05F
L1 - 68 µH Sumida
R1 - 1.5 kΩ,1
C

- 0.01 µF, 50V Ceramic

B

For a 5V output, select R2 to be 4.75 kΩ,1

where V

REF

%

=

1.21V

#

RCR110D-680L

%

Usea1%resistor for best stability.

FIGURE 3. Standard Test Circuits and Layout Guides

Adjustable Output Voltage Versions

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
regulators. LM267X Made Simple

3.1, NT, or 95 operating systems.

Given: Given:

=

V

OUT

V

(max)=Maximum DC Input Voltage VIN(max)=12V

IN

I

(max)=Maximum Load Current I

LOAD

1. Inductor Selection (L1)
A. Select the correct inductor value selection guide from

and

Figure 5orFigure 6

4

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).

SIMPLE SWITCHER

line of switching

is available on Windows

®

Regulated Output Voltage (3.3V, 5V, or 12V) V

1. Inductor Selection (L1)

(output voltages of 3.3V, 5V, or 12V

Figure

A. Use the inductor selection guide for the 5V version shown
in

B. From the inductor value selection guide shown in
the inductance region intersected by the 12V horizontal line
and the 500 mA vertical line is 47 µH, and the inductor code is
L13.

=

5V

OUT

(max)=500 mA

LOAD

Figure 5

.

DS100042-23

Figure 5

,

www.national.com9

LM2671 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
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
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
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

2. Output Capacitor Selection (C
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.

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

.

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 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.

Figure 8

. Each manufacturer makes a

T and transient peak

•

Figure 9

.

)

OUT

T and

•

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 (C
A. Use the 5.0V section in the output capacitor table in

. Choose a capacitor value and voltage rating from the line

10

that contains the inductance value of 47 µH. The capacitance
and voltage rating values corresponding to the 47 µH inductor
are the:

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)
A. Refer to the table shown in

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.

OUT

Figure 12

)

. In this example, a

Figure

www.national.com 10

LM2671 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)

4. Input Capacitor (C

A low ESR aluminum or tantalum bypass capacitor is needed

)

IN

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

⁄2the 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

5. Boost Capacitor (C

This capacitor develops the necessary voltage to turn the

)

B

IN

switch gate on fully. All applications should use a 0.01 µF, 50V
ceramic capacitor.

6. Soft-Start Capacitor (C

This capacitor controls the rate at which the device starts up.
The formula for the soft-start capacitor C

- optional)

SS

is:

SS

4. Input Capacitor (C

The important parameters for the input capacitor are the input

)

IN

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
The next higher capacitor voltage rating is 16V.
The RMS current rating requirement for the input capacitor in a
buck regulator is approximately

1

⁄2the DC load current. In this

) would be needed.

IN

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 16V line and note which capacitor
values have RMS current ratings greater than 250 mA.
For a through hole design, a 100 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

Figure 15

example, checking

, and the Sprague 594D series

datasheet, a Sprague 594D 15 µF, 25V capacitor is adequate.

pin.

5. Boost Capacitor (C

For this application, and all applications, use a 0.01 µF, 50V

)

B

ceramic capacitor.

6. Soft-Start Capacitor (C

For this application, selecting a start-up time of 10 ms and
using the formula for C

- optional)

SS

results in a value of:

SS

where:

=

Soft-Start Current :4.5 µA typical.

I

SS

=

Soft-Start Time :Selected.

t

SS

=

V
V
V
V

Soft-Start Threshold Voltage :0.63V typical.

SSTH

=

Output Voltage :Selected.

OUT
SCHOTTKY
IN

=

=

Schottky Diode Voltage Drop :0.4V typical.

Input Voltage :Selected.

If this feature is not desired, leave this pin open.

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LM2671 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)

7. Frequency Synchronization (optional)

The LM2671 (oscillator) can be synchronized to run with an
external oscillator, using the sync pin (pin 3). By doing so, the
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,a1kΩresistor and a 100 pF
capacitor are suitable values.

INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)

7. Frequency Synchronization (optional)

For all applications, usea1kΩresistor and a 100 pF capacitor
for the RC filter.

DS100042-29

FIGURE 4. LM2671-3.3

DS100042-30

FIGURE 5. LM2671-5.0

www.national.com 12

DS100042-31

FIGURE 6. LM2671-12

DS100042-32

FIGURE 7. LM2671-ADJ

LM2671 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

Ind.

Induc-

Ref.

Desg.

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 PE-53821 PE-53821-S DO3316-683

tance

(µH)

Current

(A)

Schott Renco Pulse Engineering Coilcraft

Through Surface Through Surface Through Surface Surface

Hole Mount Hole Mount Hole Mount Mount

FIGURE 8. Inductor Manufacturers’ Part Numbers

Coilcraft Inc. Phone [phone]

FAX [phone]

Coilcraft Inc., Europe Phone [phone]

FAX [phone]

Pulse Engineering Inc. Phone [phone]

FAX [phone]

Pulse Engineering Inc., Phone [phone]
Europe FAX [phone]
Renco Electronics Inc. Phone [phone]

FAX [phone]

Schott Corp. Phone [phone]

FAX [phone]

FIGURE 9. Inductor Manufacturers’ Phone Numbers

www.national.com13

LM2671 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

Output Capacitor

Output

Voltage

(V)

3.3

5.0

12

Inductance

(µH)

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 (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

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)

FIGURE 10. Output Capacitor Table

Nichicon Corp. Phone [phone]

FAX [phone]

Panasonic Phone [phone]

FAX [phone]

AVX Corp. Phone [phone]

FAX [phone]

Sprague/Vishay Phone [phone]

FAX [phone]

Sanyo Corp. Phone [phone]

FAX [phone]

FIGURE 11. Capacitor Manufacturers’ Phone Numbers

www.national.com 14

LM2671 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

1A Diodes 3A Diodes

Surface Through Surface Through

V

R

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

Phone [phone]

Corp.

FAX [phone]

Motorola, Inc. Phone [phone]

FAX [phone]

General Instruments

Phone [phone]

Corp.

FAX [phone]

Diodes, Inc. Phone [phone]

FAX [phone]

FIGURE 13. Diode Manufacturers’ Phone Numbers

DS100042-33

FIGURE 14. RMS Current Ratings for Low ESR Electrolytic Capacitors (Typical)

www.national.com15

LM2671 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
AVX TPS

Recommended

Application Voltage

3.3 6.3
510

Sprague 594D

Recommended

Application Voltage

2.5 4

3.3 6.3
510

Voltage

Rating

+85˚C Rating

Voltage

Rating

+85˚C Rating

Recommended

Application Voltage

+85˚C Rating

10 20
12 25
15 35

Recommended

Application Voltage

+85˚C Rating

12 20
18 25
24 35
29 50

Voltage

Rating

Voltage

Rating

816

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
regulators. LM267X Made Simple is available on

3.1, NT, or 95 operating systems.

Given: Given:

=

V

OUT

V

(max)=Maximum Input Voltage VIN(max)=28V

IN

I

(max)=Maximum Load Current I

LOAD

F=Switching Frequency

1. Programming Output Voltage (Selecting R1and R2,as

Figure 3

shown in
Use the following formula to select the appropriate resistor
values.

SIMPLE SWITCHER

line of switching

Windows

Regulated Output Voltage V

(Fixed at a nominal 260 kHz).

1. Programming Output Voltage (Selecting R

)

shown in
Select R

=

20V

OUT

(max)=500 mA

LOAD

F=Switching Frequency

Figure 3

)

to be 1 kΩ,1%. Solve for R2.

1

(Fixed at a nominal 260 kHz).

and R2,as

1

REF

=

1.21V

where V
Select a value for R1between 240Ω and 1.5 kΩ. 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.)

2. Inductor Selection (L1)
A. Calculate the inductor Volt

µs), from the following formula:

(V

•

www.national.com 16

microsecond constant E•T

•

=

1kΩ(16.53 − 1)=15.53 kΩ, closest 1%value is

R

2

15.4 kΩ.
=

15.4 kΩ.

R

2

2. Inductor Selection (L1)

A. Calculate the inductor Volt

microsecond constant (E•T),

•

LM2671 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)

where V
V

D

B. Use the E
with the E
Value Selection Guide shown in

C. On the horizontal axis, select the maximum load current. C. I
D. Identify the inductance region intersected by the E

and the Maximum Load Current value. Each region is identified
by an inductance value and an inductor code (LXX).

E. Select an appropriate inductor from the four manufacturer’s
part numbers listed in
types of inductors, see the inductor selection in the fixed
output voltage design procedure.

3. Output Capacitor SeIection (C
A. Select an output capacitor from the capacitor code selection

guide in
inductor selection guide, step 1, locate the appropriate
capacitor code corresponding to the desired output voltage.

B. Select an appropriate capacitor value and voltage rating,
using the capacitor code, from the output capacitor selection
table in
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

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
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.

=

internal switch saturation voltage=0.25V and

SAT

=

diode forward voltage drop=0.5V

T value from the previous formula and match it

•

T number on the vertical axis of the Inductor

•

Figure 7

Figure 8

. For information on the different

OUT

Figure 16

Figure 17

. Using the inductance value found in the

. There are two solid tantalum (surface

.

T=21.6 (V•µs)

B. E

•

.

(max)=500 mA

T value

•

LOAD

D. From the inductor value selection guide shown in
the inductance region intersected by the 21.6 (V
horizontal line and the 500 mA vertical line is 100 µH, and the
inductor code is L20.

Figure 8

E. From the table in

, locate line L20, and select an
inductor part number from the list of manufacturers’ part
numbers.

)

3. Output Capacitor SeIection (C
A. Use the appropriate row of the capacitor code selection

guide, in

Figure 16

. For this example, use the 15–20V row.

OUT

)

The capacitor code corresponding to an inductance of 100 µH
is C20.

B. From the output capacitor selection table in
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.

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

OUT/VIN

). The

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.

Figure 7

µs)

•

Figure 17

,

,

www.national.com17

LM2671 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)

5. Input Capacitor (C

A low ESR aluminum or tantalum bypass capacitor is needed

)

IN

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

⁄2the 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

6. Boost Capacitor (C

This capacitor develops the necessary voltage to turn the

)

B

IN

switch gate on fully. All applications should 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.

5. Input Capacitor (C

The important parameters for the input capacitor are the input

)

IN

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
The RMS current rating requirement for the input capacitor in a
buck regulator is approximately

) would be needed.

IN

1

⁄2the 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.

pin.

6. Boost Capacitor (C

For this application, and all applications, use a 0.01 µF, 50V

)

B

ceramic capacitor.

Case

Style (Note 7)

Output

Voltage (V)

22 33 47 68 100 150 220

Inductance (µH)

SM and TH 1.21–2.50 ————C1C2C3
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

www.national.com 18

LM2671 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

Output Capacitor

Cap.
Ref.

Desg.

#

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.

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)

FIGURE 17. Output Capacitor Selection Table

www.national.com19

Application Information

TYPICAL SURFACE MOUNT PC BOARD LAYOUT, FIXED OUTPUT (4X SIZE)

CIN- 15 µF, 25V, Solid Tantalum Sprague, “594D series”

- 68 µF, 10V, Solid Tantalum Sprague, “594D series”

C

OUT

D1 - 1A, 40V Schottky Rectifier, Surface Mount
L1 - 47 µH, L13, Coilcraft DO3308

- 0.01 µF, 50V, Ceramic

C

B

TYPICAL SURFACE MOUNT PC BOARD LAYOUT, ADJUSTABLE OUTPUT (4X SIZE)

CIN- 15 µF, 50V, Solid Tantalum Sprague, “594D series”

- 33 µF, 25V, Solid Tantalum Sprague, “594D series”

C

OUT

D1 - 1A, 40V Schottky Rectifier, Surface Mount
L1 - 100 µH, L20, Coilcraft DO3316

- 0.01 µF, 50V, Ceramic

C

B

%

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

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.

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 induc­tor.Allowing the inductor flux to intersect sensitive feedback,
IC ground path, and C

wiring can cause problems.

OUT

DS100042-39

DS100042-40

www.national.com 20

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

www.national.com21

Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

Regulator with Features

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

LIFE SUPPORT POLICY

NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DE­VICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMI­CONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or sys­tems which, (a) are intended for surgical implant into
the body, or (b) support or sustain life, and whose fail­ure to perform when properly used in accordance

2. A critical component in 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 safety or effectiveness.

with instructions for use provided in the labeling, can
be reasonably expected to result in a significant injury
to the user.

LM2671 SIMPLE SWITCHER Power Converter High Efficiency 500mA Step-Down Voltage

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.

National Semiconductor
Corporation

Americas
Tel: [phone]
Fax: [phone]
Email: [e-mail]

www.national.com

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Response Group

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Fax: [phone]