LM2671MX-ADJ - National Semiconductor

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 pro-

vide all the active functions for a step-down (buck) switching

regulator, capable of driving a 500mA load current with ex-

cellent 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 fre-

quency 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 regu-

lators. 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 out-

put 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 cur-

rent. 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 Sim-

ple (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

WEBENCH® is a registered trademark of National Semiconductor Corporation.

Windows® is a registered trademark of Microsoft Corporation.

PRODUCTION DATA information is current as of

publication date. Products conform to specifications per

the terms of the Texas Instruments standard warranty.

Production processing does not necessarily include

testing of all parameters.

Connection Diagrams

16-Lead LLP Surface Mount Package

Top View

10004241

LLP Package

See NSC Package Drawing Number LDA16A

8-Lead Package

Top View

10004202

SO-8/DIP Package

See NSC Package Drawing Number MO8A/N08E

TABLE 1. Package Marking and Ordering Information

Output Voltage Order Information Package Marking Supplied as:

16 Lead LLP

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

SO-8

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

DIP

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 Rails

LM2671

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for

availability and specifications.

Supply Voltage 45V

ON/OFF Pin Voltage −0.1V ≤ VSH ≤ 6V

Switch Voltage to Ground −1V

Boost Pin Voltage VSW + 8V

Feedback Pin Voltage −0.3V ≤ VFB ≤ 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

LLP Package (See AN-1187)

Maximum Junction Temperature +150°C

Operating Ratings

Supply Voltage 6.5V to 40V

Temperature Range −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 Min Max Units

(Note 4) (Note 5) (Note 5)

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 V

VOUT Output Voltage VIN = 6.5V to 40V, ILOAD = 20 mA to 250 mA 3.3 3.251/3.201 3.350/3.399 V

ηEfficiency VIN = 12V, ILOAD = 500 mA 86 %

LM2671-5.0

Symbol Parameter Conditions Typical Min Max Units

(Note 4) (Note 5) (Note 5)

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 V

VOUT Output Voltage VIN = 6.5V to 40V, ILOAD = 20 mA to 250 mA 5.0 4.925/4.850 5.075/5.150 V

ηEfficiency VIN = 12V, ILOAD = 500 mA 90 %

LM2671-12

Symbol Parameter Conditions Typical Min Max Units

(Note 4) (Note 5) (Note 5)

SYSTEM PARAMETERS Test Circuit Figure 2 (Note 3)

VOUT Output Voltage VIN = 15V to 40V, ILOAD = 20 mA to 500 mA 12 11.82/11.64 12.18/12.36 V

ηEfficiency VIN = 24V, ILOAD = 500 mA 94 %

LM2671

LM2671-ADJ

Symbol Parameter Conditions Typ Min Max Units

(Note 4) (Note 5) (Note 5)

SYSTEM PARAMETERS Test Circuit Figure 3 (Note 3)

VFB Feedback Voltage VIN = 8V to 40V, ILOAD = 20 mA to 500 mA 1.210 1.192/1.174 1.228/1.246 V

VOUT Programmed for 5V

(see Circuit of Figure 3)

VFB Feedback Voltage VIN = 6.5V to 40V, ILOAD = 20 mA to 250 mA 1.210 1.192/1.174 1.228/1.246 V

VOUT Programmed for 5V

(see Circuit of Figure 3)

ηEfficiency 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

DEVICE PARAMETERS

IQQuiescent Current VFEEDBACK = 8V 2.5 3.6 mA

For 3.3V, 5.0V, and ADJ Versions

VFEEDBACK = 15V 2.5 mA

For 12V Versions

ISTBY Standby Quiescent Current ON/OFF Pin = 0V 50 100/150 μA

ICL Current Limit 0.8 0.62/0.575 1.2/1.25 A

ILOutput Leakage Current VIN = 40V, ON/OFF Pin = 0V 1 25 μA

VSWITCH = 0V

VSWITCH = −1V, ON/OFF Pin = 0V 6 15 mA

RDS(ON) Switch On-Resistance ISWITCH = 500 mA 0.25 0.40/0.60 Ω

fOOscillator Frequency Measured at Switch Pin 260 225 275 kHz

D Maximum Duty Cycle 95 %

Minimum Duty Cycle 0 %

IBIAS Feedback Bias VFEEDBACK = 1.3V 85 nA

Current ADJ Version Only

VS/D ON/OFF Pin 1.4 0.8 2.0 V

Voltage Thesholds

IS/D ON/OFF Pin Current ON/OFF Pin = 0V 20 7 37 μA

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 V

ISS Soft-Start Current 4.5 1.5 6.9 μA

θJA 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

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.

LM2671

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.

LM2671

Typical Performance Characteristics

Normalized

Output Voltage

10004203

Line Regulation

10004204

Efficiency

10004205

Drain-to-Source

Resistance

10004206

Switch Current Limit

10004207

Operating

Quiescent Current

10004208

LM2671

Standby

Quiescent Current

10004209

ON/OFF Threshold

Voltage

10004210

ON/OFF Pin

Current (Sourcing)

10004211

Switching Frequency

10004212

Feedback Pin

Bias Current

10004213

Peak Switch Current

10004214

LM2671

Dropout Voltage—3.3V Option

10004215

Dropout Voltage—5.0V Option

10004216

Block Diagram

10004217

* Patent Number 5,514,947

† Patent Number 5,382,918

FIGURE 1.

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

10004218

A: VSW Pin 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

Discontinuous Mode Switching Waveforms

VIN = 20V, VOUT = 5V, ILOAD = 300 mA

L = 15 μH, COUT = 68 μF (2×), COUTESR = 25 mΩ

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

Load Transient Response for Continuous Mode

VIN = 20V, VOUT = 5V

L = 100 μH, COUT = 100 μF, COUTESR = 0.1Ω

10004220

A: Output Voltage, 100 mV/div, AC-Coupled

B: Load Current: 100 mA to 500 mA Load Pulse

Horizontal Time Base: 50 μs/div

Load Transient Response for Discontinuous Mode

VIN = 20V, VOUT = 5V,

L = 47 μH, COUT = 68 μF, COUTESR = 50 mΩ

10004221

A: Output Voltage, 100 mV/div, AC-Coupled

B: Load Current: 100 mA to 400 mA Load Pulse

Horizontal Time Base: 200 μs/div

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

LM2671

Application Hints

The LM2671 provides all of the active functions required for a step-down (buck) switching regulator. The internal power switch is

a DMOS power MOSFET to provide power supply designs with high current capability, up to 0.5A, and highly efficient operation.

The LM2671 is part of the SIMPLE SWITCHER®® family of power converters. A complete design uses a minimum number of

external components, which have been pre-determined from a variety of manufacturers. Using either this data sheet or TI's

WEBENCH® design tool, a complete switching power supply can be designed quickly. Also, refer to the LM2670 data sheet for

additional applications information.

SWITCH OUTPUT

This is the output of a power MOSFET switch connected directly to the input voltage. The switch provides energy to an inductor,

an output capacitor and the load circuitry under control of an internal pulse-width-modulator (PWM). The PWM controller is internally

clocked by a fixed 260kHz oscillator. In a standard step-down application the duty cycle (Time ON/Time OFF) of the power switch

is proportional to the ratio of the power supply output voltage to the input voltage. The voltage on the VSW pin cycles between Vin

(switch ON) and below ground by the voltage drop of the external Schottky diode (switch OFF).

INPUT

The input voltage for the power supply is connected to the VIN pin. In addition to providing energy to the load the input voltage also

provides bias for the internal circuitry of the LM2671. For guaranteed performance the input voltage must be in the range of 6.5V

to 40V. For best performance of the power supply the VIN pin should always be bypassed with an input capacitor located close to

this pin and GND.

C BOOST

A capacitor must be connected from the CB pin to the VSW pin. This capacitor boosts the gate drive to the internal MOSFET above

Vin to fully turn it ON. This minimizes conduction losses in the power switch to maintain high efficiency. The recommended value

for C Boost is 0.01μF.

GROUND

This is the ground reference connection for all components in the power supply. In fast-switching, high-current applications such

as those implemented with the LM2671, it is recommended that a broad ground plane be used to minimize signal coupling through-

out the circuit

SYNC

This input allows control of the switching clock frequency. If left open-circuited the regulator will be switched at the internal oscillator

frequency, typically 260 kHz. An external clock can be used to force the switching frequency and thereby control the output ripple

frequency of the regulator. This capability provides for consistent filtering of the output ripple from system to system as well as

precise frequency spectrum positioning of the ripple frequency which is often desired in communications and radio applications.

This external frequency must be greater than the LM2671 internal oscillator frequency, which could be as high as 275 kHz, to

prevent an erroneous reset of the internal ramp oscillator and PWM control of the power switch. The ramp oscillator is reset on the

positive going edge of the sync input signal. It is recommended that the external TTL or CMOS compatible clock (between 0V and

a level greater than 3V) be ac coupled to the SYNC pin through a 100pF capacitor and a 1KΩ resistor to ground.

When the SYNC function is used, current limit frequency foldback is not active. Therefore, the device may not be fully protected

against extreme output short circuit conditions.

FEEDBACK

This is the input to a two-stage high gain amplifier, which drives the PWM controller. Connect the FB pin directly to the output for

proper regulation. For the fixed output devices (3.3V, 5V and 12V outputs), a direct wire connection to the output is all that is required

as internal gain setting resistors are provided inside the LM2671. For the adjustable output version two external resistors are

required to set the dc output voltage. For stable operation of the power supply it is important to prevent coupling of any inductor

flux to the feedback input.

ON/OFF

This input provides an electrical ON/OFF control of the power supply. Connecting this pin to ground or to any voltage less than

0.8V will completely turn OFF the regulator. The current drain from the input supply when OFF is only 50μA. The ON/OFF input

has an internal pull-up current source of approximately 20μA and a protection clamp zener diode of 7V to ground. When electrically

driving the ON/OFF pin the high voltage level for the ON condition should not exceed the 6V absolute maximum limit. When ON/

OFF control is not required this pin should be left open.

DAP (LLP PACKAGE)

The Die Attach Pad (DAP) can and should be connected to the PCB Ground plane/island. For CAD and assembly guidelines refer

to Application Note SNAO401 at http://www.ti.com/lit/an/snoa401q/snoa401q.pdf.

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.

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

500 mA vertical line is 47 μH, and the inductor code is L13.

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 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.)

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.

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:

LM2671

PROCEDURE (Fixed Output Voltage Version) 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)

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.

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 LM2671 using

short leads and short printed circuit traces.

4. 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. 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 12V, an aluminum electrolytic capacitor with a voltage

rating greater than 15V (1.25 × VIN) 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 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 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 example, checking Figure

15, and the Sprague 594D series datasheet, a Sprague 594D 15

μF, 25V capacitor is adequate.

LM2671

PROCEDURE (Fixed Output Voltage Version) EXAMPLE (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.

5. Boost Capacitor (CB)

For this application, and all applications, 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:

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)

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,

a 1 kΩ resistor and a 100 pF capacitor are suitable values.

7. Frequency Synchronization (optional)

For all applications, use a 1 kΩ resistor and a 100 pF capacitor for

the RC filter.

LM2671

INDUCTOR VALUE SELECTION GUIDES

(For Continuous Mode Operation)

10004229

FIGURE 4. LM2671-3.3

10004230

FIGURE 5. LM2671-5.0

10004231

FIGURE 6. LM2671-12

LM2671

10004232

FIGURE 7. LM2671-ADJ

Ind.

Ref.

Desg.

Inducta

nce

(μH)

Current

(A)

Schott Renco Pulse Engineering Coilcraft

Through Surface Through Surface Through Surface Surface

Hole Mount Hole Mount Hole Mount 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 PE-53821 PE-53821-S DO3316-683

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

LM2671

FIGURE 9. Inductor Manufacturers' Phone Numbers

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

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. Phone (847) 843-7500

FAX (847) 843-2798

Panasonic Phone (714) 373-7857

FAX (714) 373-7102

AVX Corp. Phone (845) 448-9411

FAX (845) 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

1A Diodes 3A Diodes

Surface Through Surface Through

Mount Hole Mount Hole

20V SK12 1N5817 SK32 1N5820

B120 SR102 SR302

LM2671

1A Diodes 3A Diodes

Surface Through Surface Through

Mount Hole Mount Hole

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

10004233

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

LM2671

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.

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

in Figure 3)

Use the following formula to select the appropriate resistor values.

1. Programming Output Voltage (Selecting R1 and R2, as shown

in Figure 3)

Select R1 to be 1 kΩ, 1%. Solve for R2.

where VREF = 1.21V

Select a value for R1 between 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.)

R2 = 1 kΩ (16.53 − 1) = 15.53 kΩ, closest 1% value is 15.4 kΩ.

R2 = 15.4 kΩ.

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),

LM2671

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

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

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. ILOAD(max) = 500 mA

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 500 mA vertical line is 100 μH, and the inductor code is

L20.

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 L20, and select an inductor

part number from the list of manufacturers' part numbers.

3. Output Capacitor SeIection (COUT)

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 (COUT)

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 100 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.

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.

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.

B. The reverse voltage rating of the diode should be at least 1.25

times the maximum input voltage.

LM2671

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

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)