LM2596T-5.0_技术文档

May 2002

LM2596

SIMPLE SWITCHER^®Power Converter 150 kHz

3A Step-Down Voltage Regulator

General Description

Features

n 3.3V, 5V, 12V, and adjustable output versions

The LM2596 series of regulators are monolithic integrated

circuits that provide all the active functions for a step-down

(buck) switching regulator, capable of driving a 3A load with

excellent line and load regulation. These devices are avail-

able in fixed output voltages of 3.3V, 5V, 12V, and an adjust-

able output version.

n Adjustable version output voltage range, 1.2V to 37V

±

4% max over line and load conditions

n Available in TO-220 and TO-263 packages

n Guaranteed 3A output load current

n Input voltage range up to 40V

Requiring a minimum number of external components, these

regulators are simple to use and include internal frequency

n Requires only 4 external components

n Excellent line and load regulation specifications

n 150 kHz fixed frequency internal oscillator

n TTL shutdown capability

n Low power standby mode, I_Qtypically 80 µA

n High efficiency

†

compensation , and a fixed-frequency oscillator.

The LM2596 series operates at a switching frequency of

150 kHz thus allowing smaller sized filter components than

what would be needed with lower frequency switching regu-

lators. Available in a standard 5-lead TO-220 package with

several different lead bend options, and a 5-lead TO-263

surface mount package.

n Uses readily available standard inductors

n Thermal shutdown and current limit protection

A standard series of inductors are available from several

different manufacturers optimized for use with the LM2596

series. This feature greatly simplifies the design of

switch-mode power supplies.

Applications

n Simple high-efficiency step-down (buck) regulator

n On-card switching regulators

n Positive to negative converter

±

Other features include a guaranteed 4% tolerance on out-

put voltage under specified input voltage and output load

†

Note: Patent Number 5,382,918.

±

conditions, and 15% on the oscillator frequency. External

shutdown is included, featuring typically 80 µA standby cur-

rent. Self protection features include a two stage frequency

reducing current limit for the output switch and an over

temperature shutdown for complete protection under fault

conditions.

Typical Application (Fixed Output Voltage

Versions)

01258301

SIMPLE SWITCHER^®and Switchers Made Simple^®are registered trademarks of National Semiconductor Corporation.

DS012583

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Connection Diagrams and Ordering Information

Bent and Staggered Leads, Through Hole

Package

Surface Mount Package

5-Lead TO-263 (S)

5-Lead TO-220 (T)

01258303

01258302

Order Number LM2596S-3.3, LM2596S-5.0,

LM2596S-12 or LM2596S-ADJ

Order Number LM2596T-3.3, LM2596T-5.0,

LM2596T-12 or LM2596T-ADJ

See NS Package Number TS5B

See NS Package Number T05D

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2

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

Human Body Model (Note 2)

Lead Temperature

2 kV

S Package

Vapor Phase (60 sec.)

Infrared (10 sec.)

+215˚C

+245˚C

+260˚C

+150˚C

Maximum Supply Voltage

ON /OFF Pin Input Voltage

Feedback Pin Voltage

Output Voltage to Ground

(Steady State)

45V

−0.3 ≤ V ≤ +25V

−0.3 ≤ V ≤+25V

T Package (Soldering, 10 sec.)

Maximum Junction Temperature

−1V

Internally limited

−65˚C to +150˚C

Operating Conditions

Temperature Range

Supply Voltage

Power Dissipation

−40˚C ≤ T_J≤ +125˚C

Storage Temperature Range

ESD Susceptibility

4.5V to 40V

LM2596-3.3

Electrical Characteristics

Specifications with standard type face are for T_J= 25˚C, and those with boldface type apply over full Operating Tempera-

ture Range

LM2596-3.3

Units

Symbol

Parameter

Conditions

Typ

Limit

(Limits)

(Note 3)

(Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1

V_OUT

Output Voltage

Efficiency

4.75V ≤ V_IN≤ 40V, 0.2A ≤ I_LOAD≤ 3A

3.3

73

V

3.168/3.135

3.432/3.465

V(min)

V(max)

%

η

V_IN= 12V, I_LOAD= 3A

LM2596-5.0

Electrical Characteristics

Specifications with standard type face are for T_J= 25˚C, and those with boldface type apply over full Operating Tempera-

ture Range

LM2596-5.0

Units

Symbol

Parameter

Conditions

Typ

Limit

(Limits)

(Note 3)

(Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1

V_OUT

Output Voltage

Efficiency

7V ≤ V_IN≤ 40V, 0.2A ≤ I_LOAD≤ 3A

5.0

80

V

4.800/4.750

5.200/5.250

V(min)

V(max)

%

η

V_IN= 12V, I_LOAD= 3A

LM2596-12

Electrical Characteristics

Specifications with standard type face are for T_J= 25˚C, and those with boldface type apply over full Operating Tempera-

ture Range

LM2596-12

Units

Symbol

Parameter

Conditions

Typ

Limit

(Limits)

(Note 3)

(Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1

V_OUT

Output Voltage

Efficiency

15V ≤ V_IN≤ 40V, 0.2A ≤ I_LOAD≤ 3A

12.0

90

V

11.52/11.40

12.48/12.60

V(min)

V(max)

%

η

V_IN= 25V, I_LOAD= 3A

3

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LM2596-ADJ

Electrical Characteristics

Specifications with standard type face are for T_J= 25˚C, and those with boldface type apply over full Operating Tempera-

ture Range

LM2596-ADJ

Units

Symbol

Parameter

Conditions

Typ

Limit

(Limits)

(Note 3)

(Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1

V_FB

Feedback Voltage 4.5V ≤ V_IN≤ 40V, 0.2A ≤ I_LOAD≤ 3A

1.230

73

V

V_OUTprogrammed for 3V. Circuit of Figure 1

1.193/1.180

1.267/1.280

V(min)

V(max)

%

η

Efficiency

V_IN= 12V, V_OUT= 3V, I_LOAD= 3A

All Output Voltage Versions

Electrical Characteristics

Specifications with standard type face are for T_J= 25˚C, and those with boldface type apply over full Operating Tempera-

ture Range. Unless otherwise specified, V_IN= 12V for the 3.3V, 5V, and Adjustable version and V_IN= 24V for the 12V ver-

sion. I_LOAD= 500 mA

LM2596-XX

Units

Symbol

Parameter

Conditions

Typ

Limit

(Limits)

(Note 3)

(Note 4)

DEVICE PARAMETERS

I_b

Feedback Bias Current

Adjustable Version Only, V_FB= 1.3V

(Note 6)

10

nA

nA (max)

kHz

50/100

f_O

Oscillator Frequency

Saturation Voltage

150

127/110

173/173

kHz(min)

kHz(max)

V

V_SAT

DC

I_OUT= 3A (Notes 7, 8)

1.16

1.4/1.5

V(max)

%

Max Duty Cycle (ON)

Min Duty Cycle (OFF)

Current Limit

(Note 8)

100

0

(Note 9)

I_CL

Peak Current (Notes 7, 8)

4.5

A

3.6/3.4

6.9/7.5

50

A(min)

A(max)

µA(max)

mA

I_L

Output Leakage Current

Quiescent Current

Output = 0V (Notes 7, 9)

Output = −1V (Note 10)

2

5

30

10

mA(max)

mA

I_Q

(Note 9)

mA(max)

µA

I_STBY

Standby Quiescent Current ON/OFF pin = 5V (OFF)

(Note 10)

80

200/250

µA(max)

˚C/W

θ_JC

θ_JA

Thermal Resistance

TO-220 or TO-263 Package, Junction to Case

TO-220 Package, Junction to Ambient (Note 11)

TO-263 Package, Junction to Ambient (Note 12)

TO-263 Package, Junction to Ambient (Note 13)

TO-263 Package, Junction to Ambient (Note 14)

2

50

30

20

˚C/W

ON/OFF CONTROL Test Circuit Figure 1

ON /OFF Pin Logic Input

1.3

V

V_IH

V_IL

Threshold Voltage

Low (Regulator ON)

High (Regulator OFF)

0.6

2.0

V(max)

V(min)

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All Output Voltage Versions

Electrical Characteristics ^(Continued)

Specifications with standard type face are for T_J= 25˚C, and those with boldface type apply over full Operating Tempera-

ture Range. Unless otherwise specified, V_IN= 12V for the 3.3V, 5V, and Adjustable version and V_IN= 24V for the 12V ver-

sion. I_LOAD= 500 mA

LM2596-XX

Units

Symbol

I_H

Parameter

Conditions

Typ

(Note 3)

5

Limit

(Limits)

(Note 4)

ON /OFF Pin Input Current V_LOGIC= 2.5V (Regulator OFF)

V_LOGIC= 0.5V (Regulator ON)

µA

15

5

µA(max)

µA

I_L

0.02

µA(max)

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 do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.

Note 2: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.

Note 3: Typical numbers are at 25˚C and represent the most likely norm.

Note 4: 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 5: External components such as the catch diode, inductor, input and output capacitors, and voltage programming resistors can affect switching regulator

system performance. When the LM2596 is used as shown in the Figure 1 test circuit, system performance will be as shown in system parameters section of Electrical

Characteristics.

Note 6: The switching frequency is reduced when the second stage current limit is activated.

Note 7: No diode, inductor or capacitor connected to output pin.

Note 8: Feedback pin removed from output and connected to 0V to force the output transistor switch ON.

Note 9: Feedback pin removed from output and connected to 12V for the 3.3V, 5V, and the ADJ. version, and 15V for the 12V version, to force the output transistor

switch OFF.

Note 10: V = 40V.

IN

Note 11: Junction to ambient thermal resistance (no external heat sink) for the TO-220 package mounted vertically, with the leads soldered to a printed circuit board

2

with (1 oz.) copper area of approximately 1 in .

2

Note 12: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single printed circuit board with 0.5 in of (1 oz.) copper area.

2

Note 13: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 2.5 in of (1 oz.) copper area.

2

Note 14: Junction to ambient thermal resistance with the TO-263 package tab soldered to a double sided printed circuit board with 3 in of (1 oz.) copper area on

2

the LM2596S side of the board, and approximately 16 in of copper on the other side of the p-c board. See Application Information in this data sheet and the thermal

™

model in Switchers Made Simple version 4.3 software.

Typical Performance Characteristics (Circuit of Figure 1)

Normalized

Output Voltage

Line Regulation

Efficiency

01258304

01258306

01258305

5

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Typical Performance Characteristics (Circuit of Figure 1) (Continued)

Switch Saturation

Voltage

Switch Current Limit

Dropout Voltage

01258307

01258310

01258313

01258308

01258309

Operating

Quiescent Current

Shutdown

Quiescent Current

Minimum Operating

Supply Voltage

01258311

01258312

ON /OFF Threshold

Voltage

ON /OFF Pin

Current (Sinking)

Switching Frequency

01258315

01258314

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Typical Performance Characteristics (Circuit of Figure 1) (Continued)

Feedback Pin

Bias Current

01258316

7

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Typical Performance Characteristics

Continuous Mode Switching Waveforms

V_IN= 20V, V_OUT= 5V, I_LOAD= 2A

Discontinuous Mode Switching Waveforms

V_IN= 20V, V_OUT= 5V, I_LOAD= 500 mA

L = 32 µH, C_OUT= 220 µF, C_OUTESR = 50 mΩ

L = 10 µH, C_OUT= 330 µF, C_OUTESR = 45 mΩ

01258317

01258318

Horizontal Time Base: 2 µs/div.

A: Output Pin Voltage, 10V/div.

B: Inductor Current 1A/div.

A: Output Pin Voltage, 10V/div.

B: Inductor Current 0.5A/div.

C: Output Ripple Voltage, 50 mV/div.

C: Output Ripple Voltage, 100 mV/div.

Load Transient Response for Continuous Mode

V_IN= 20V, V_OUT= 5V, I_LOAD= 500 mA to 2A

L = 32 µH, C_OUT= 220 µF, C_OUTESR = 50 mΩ

Load Transient Response for Discontinuous Mode

V_IN= 20V, V_OUT= 5V, I_LOAD= 500 mA to 2A

L = 10 µH, C_OUT= 330 µF, C_OUTESR = 45 mΩ

01258320

Horizontal Time Base: 200 µs/div.

01258319

A: Output Voltage, 100 mV/div. (AC)

B: 500 mA to 2A Load Pulse

Horizontal Time Base: 100 µs/div.

A: Output Voltage, 100 mV/div. (AC)

B: 500 mA to 2A Load Pulse

Test Circuit and Layout Guidelines

Fixed Output Voltage Versions

01258322

C

— 470 µF, 50V, Aluminum Electrolytic Nichicon “PL Series”

— 220 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”

IN

OUT

D1 — 5A, 40V Schottky Rectifier, 1N5825

L1 — 68 µH, L38

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Test Circuit and Layout Guidelines (Continued)

Adjustable Output Voltage Versions

01258323

where V

= 1.23V

REF

Select R to be approximately 1 kΩ, use a 1% resistor for best stability.

1

C

— 470 µF, 50V, Aluminum Electrolytic Nichicon “PL Series”

— 220 µF, 35V Aluminum Electrolytic, Nichicon “PL Series”

IN

OUT

D1 — 5A, 40V Schottky Rectifier, 1N5825

L1 — 68 µH, L38

R1 — 1 kΩ, 1%

C

— See Application Information Section

FF

FIGURE 1. Standard Test Circuits and Layout Guides

As in any switching regulator, layout is very important. 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 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 lC as possible using ground plane construction or

single point grounding.

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. (See application section for more

information.)

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,

lC groundpath and C_OUTwiring can cause problems.

9

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LM2596 Series Buck Regulator Design Procedure (Fixed Output)

PROCEDURE (Fixed Output Voltage Version)

Given:

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

_IN(max) = Maximum DC Input Voltage

_LOAD(max) = Maximum Load Current

EXAMPLE (Fixed Output Voltage Version)

Given:

V_OUT= 5V

_IN(max) = 12V

_LOAD(max) = 3A

V

I

1. Inductor Selection (L1)

A. Select the correct inductor value selection guide from Fig-

ures Figure 4, 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.

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

in Figure 5.

B. From the inductor value selection guide shown in Figure 5,

the inductance region intersected by the 12V horizontal line

and the 3A vertical line is 33 µH, and the inductor code is L40.

B. From the inductor value selection guide, identify the induc-

tance region intersected by the Maximum Input Voltage line

and the Maximum Load Current line. Each region is identified

by an inductance value and an inductor code (LXX).

C. The inductance value required is 33 µH. From the table in

Figure 8, go to the L40 line and choose an inductor part

number from any of the four manufacturers shown. (In most

instance, both through hole and surface mount inductors are

available.)

C. Select an appropriate inductor from the four manufacturer’s

part numbers listed in Figure 8.

2. Output Capacitor Selection (C_OUT

)

2. Output Capacitor Selection (C_OUT)

A. In the majority of applications, low ESR (Equivalent Series

Resistance) electrolytic capacitors between 82 µF and 820 µF

and low ESR solid tantalum capacitors between 10 µF and

470 µF provide the best results. This capacitor should be

located close to the IC using short capacitor leads and short

copper traces. Do not use capacitors larger than 820 µF .

A. See section on output capacitors in application infor-

mation section.

B. From the quick design component selection table shown in

Figure 2, locate the 5V output voltage section. In the load

current column, choose the load current line that is closest to

the current needed in your application, for this example, use

the 3A line. In the maximum input voltage column, select the

line that covers the input voltage needed in your application, in

this example, use the 15V line. Continuing on this line are

recommended inductors and capacitors that will provide the

best overall performance.

For additional information, see section on output capaci-

tors in application information section.

B. To simplify the capacitor selection procedure, refer to the

quick design component selection table shown in Figure 2.

This table contains different input voltages, output voltages,

and load currents, and lists various inductors and output ca-

pacitors that will provide the best design solutions.

The capacitor list contains both through hole electrolytic and

surface mount tantalum capacitors from four different capaci-

tor manufacturers. It is recommended that both the manufac-

turers and the manufacturer’s series that are listed in the table

be used.

C. The capacitor voltage rating for electrolytic capacitors

should be at least 1.5 times greater than the output voltage,

and often much higher voltage ratings are needed to satisfy

the low ESR requirements for low output ripple voltage.

In this example aluminum electrolytic capacitors from several

different manufacturers are available with the range of ESR

numbers needed.

D. For computer aided design software, see Switchers Made

™

Simple version 4.3 or later.

330 µF 35V Panasonic HFQ Series

330 µF 35V Nichicon PL Series

C. For a 5V output, a capacitor voltage rating at least 7.5V or

more is needed. But even a low ESR, switching grade, 220 µF

10V aluminum electrolytic capacitor would exhibit approxi-

mately 225 mΩ of ESR (see the curve in Figure 14 for the ESR

vs voltage rating). This amount of ESR would result in rela-

tively high output ripple voltage. To reduce the ripple to 1% of

the output voltage, or less, a capacitor with a higher value or

with a higher voltage rating (lower ESR) should be selected. A

16V or 25V capacitor will reduce the ripple voltage by approxi-

mately half.

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10

LM2596 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

PROCEDURE (Fixed Output Voltage Version)

3. Catch Diode Selection (D1)

EXAMPLE (Fixed Output Voltage Version)

3. Catch Diode Selection (D1)

A. The catch diode current rating must be at least 1.3 times

greater than the maximum load current. Also, 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 LM2596. The most stressful condition for

this diode is an overload or shorted output condition.

A. Refer to the table shown in Figure 11. In this example, a 5A,

20V, 1N5823 Schottky diode will provide the best perfor-

mance, and will not be overstressed even for a shorted output.

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

1.25 times the maximum input voltage.

C. This diode must be fast (short reverse recovery time) and

must be located close to the LM2596 using short leads and

short printed circuit traces. Because of their fast switching

speed and low forward voltage drop, Schottky diodes provide

the best performance and efficiency, and should be the first

choice, especially in low output voltage applications. Ultra-fast

recovery, or High-Efficiency rectifiers also provide good re-

sults. Ultra-fast recovery diodes typically have reverse recov-

ery times of 50 ns or less. Rectifiers such as the 1N5400

series are much too slow and should not be used.

4. Input Capacitor (C_IN

)

4. Input Capacitor (C_IN)

A low ESR aluminum or tantalum bypass capacitor is needed

between the input pin and ground pin 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

The important parameters for the Input capacitor are the input

voltage rating and the RMS current rating. With a nominal

input voltage of 12V, an aluminum electrolytic capacitor with a

voltage rating greater than 18V (1.5 x V_IN) would be needed.

The next higher capacitor voltage rating is 25V.

RMS current rating of the input capacitor should be selected to

1

be at least ⁄

2

the DC load current. The capacitor manufactur-

The RMS current rating requirement for the input capacitor in

ers data sheet must be checked to assure that this current

rating is not exceeded. The curve shown in Figure 13 shows

typical RMS current ratings for several different aluminum

electrolytic capacitor values.

1

a buck regulator is approximately

⁄2 the DC load current. In

this example, with a 3A load, a capacitor with a RMS current

rating of at least 1.5A is needed. The curves shown in Figure

13 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 1.5A. A 680 µF/

35V capacitor could be used.

For an aluminum electrolytic, the capacitor voltage rating

should be approximately 1.5 times the maximum input volt-

age. Caution must be exercised if solid tantalum capacitors

are used (see Application Information on input capacitor). The

tantalum capacitor voltage rating should be 2 times the maxi-

mum input voltage and it is recommended that they be surge

current tested by the manufacturer.

For a through hole design, a 680 µF/35V electrolytic capacitor

(Panasonic HFQ series or Nichicon PL series or equivalent)

would be adequate. other types or other manufacturers ca-

pacitors can be used provided the RMS ripple current ratings

are adequate.

Use caution when using ceramic capacitors for input bypass-

ing, because it may cause severe ringing at the V_INpin.

For surface mount designs, solid tantalum capacitors can be

used, but caution must be exercised with regard to the capaci-

tor surge current rating (see Application Information on input

capacitors in this data sheet). The TPS series available from

AVX, and the 593D series from Sprague are both surge cur-

rent tested.

For additional information, see section on input capaci-

tors in Application Information section.

11

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LM2596 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

Conditions

Inductor

Output Capacitor

Through Hole Electrolytic Surface Mount Tantalum

Output

Voltage

(V)

Load

Current

(A)

Max Input

Voltage

(V)

Inductance

(µH)

Inductor

Panasonic

HFQ Series

(µF/V)

Nichicon

PL Series

(µF/V)

AVX TPS

Series

(µF/V)

Sprague

595D Series

(µF/V)

#

( )

3.3

3

5

22 L41

470/25

560/35

680/35

560/35

470/25

330/35

470/25

560/25

330/35

470/25

180/35

470/25

330/25

180/25

180/35

330/25

180/25

82/25

560/16

560/35

680/35

470/35

330/35

270/50

560/16

560/25

330/35

270/35

560/16

180/35

470/25

330/25

180/25

180/35

330/25

180/25

82/25

330/6.3

220/10

100/10

100/16

68/20

390/6.3

330/10

270/10

180/16

120/20

180/16

120/20

68/25

7

10

40

6

22 L41

33 L40

22 L33

33 L32

47 L39

22 L41

33 L40

47 L39

22 L33

68 L38

22 L41

33 L40

68 L44

33 L32

68 L38

150 L42

2

3

10

40

8

5

10

15

40

9

2

3

20

40

15

18

30

40

15

20

40

12

2

FIGURE 2. LM2596 Fixed Voltage Quick Design Component Selection Table

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12

LM2596 Series Buck Regulator Design Procedure (Adjustable Output)

PROCEDURE (Adjustable Output Voltage Version)

Given:

V_OUT= Regulated Output Voltage

_IN(max) = Maximum Input Voltage

_LOAD(max) = Maximum Load Current

EXAMPLE (Adjustable Output Voltage Version)

Given:

V_OUT= 20V

_IN(max) = 28V

I_LOAD(max) = 3A

V

I

F = Switching Frequency (Fixed at a nominal 150 kHz).

1. Programming Output Voltage (Selecting R₁and R₂, as

shown in Figure 1 )

Use the following formula to select the appropriate resistor

values.

Select R₁to be 1 kΩ, 1%. Solve for R₂.

R₂= 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ.

R₂= 15.4 kΩ.

Select a value for R₁between 240Ω and 1.5 kΩ. The lower

resistor values minimize noise pickup in the sensitive feed-

back 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 • microsecond constant

(E • T),

A. Calculate the inductor Volt • microsecond constant E • T (V

• µs), from the following formula:

where V_SAT= internal switch saturation voltage = 1.16V

and V_D= diode forward voltage drop = 0.5V

B. E • T = 34.2 (V • µs)

C. I_LOAD(max) = 3A

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.

D. From the inductor value selection guide shown in Figure 7,

the inductance region intersected by the 34 (V • µs) horizontal

line and the 3A vertical line is 47 µH, and the inductor code is

L39.

C. on the horizontal axis, select the maximum load current.

D. Identify the inductance region intersected by the E • T value

and the Maximum Load Current value. Each region is identi-

fied by an inductance value and an inductor code (LXX).

E. From the table in Figure 8, locate line L39, and select an

inductor part number from the list of manufacturers part num-

bers.

E. Select an appropriate inductor from the four manufacturer’s

part numbers listed in Figure 8.

13

www.national.com

LM2596 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

3. Output Capacitor Selection (C_OUT

EXAMPLE (Adjustable Output Voltage Version)

3. Output Capacitor SeIection (C_OUT

)

A. In the majority of applications, low ESR electrolytic or solid

tantalum capacitors between 82 µF and 820 µF provide the

best results. This capacitor should be located close to the IC

using short capacitor leads and short copper traces. Do not

use capacitors larger than 820 µF. For additional informa-

tion, see section on output capacitors in application in-

formation section.

A. See section on C_OUTin Application Information section.

B. From the quick design table shown in Figure 3, locate the

output voltage column. From that column, locate the output

voltage closest to the output voltage in your application. In this

example, select the 24V line. Under the output capacitor sec-

tion, select a capacitor from the list of through hole electrolytic

or surface mount tantalum types from four different capacitor

manufacturers. It is recommended that both the manufactur-

ers and the manufacturers series that are listed in the table be

used.

B. To simplify the capacitor selection procedure, refer to the

quick design table shown in Figure 3. This table contains

different output voltages, and lists various output capacitors

that will provide the best design solutions.

In this example, through hole aluminum electrolytic capacitors

from several different manufacturers are available.

C. The capacitor voltage rating should be at least 1.5 times

greater than the output voltage, and often much higher voltage

ratings are needed to satisfy the low ESR requirements

needed for low output ripple voltage.

220 µF/35V Panasonic HFQ Series

150 µF/35V Nichicon PL Series

C. For a 20V output, a capacitor rating of at least 30V or more

is needed. In this example, either a 35V or 50V capacitor

would work. A 35V rating was chosen, although a 50V rating

could also be used if a lower output ripple voltage is needed.

Other manufacturers or other types of capacitors may also be

used, provided the capacitor specifications (especially the 100

kHz ESR) closely match the types listed in the table. Refer to

the capacitor manufacturers data sheet for this information.

4. Feedforward Capacitor (C_FF) (See Figure 1)

4. Feedforward Capacitor (C_FF)

For output voltages greater than approximately 10V, an addi-

tional capacitor is required. The compensation capacitor is

typically between 100 pF and 33 nF, and is wired in parallel

with the output voltage setting resistor, R₂. It provides addi-

tional stability for high output voltages, low input-output volt-

ages, and/or very low ESR output capacitors, such as solid

tantalum capacitors.

The table shown in Figure 3 contains feed forward capacitor

values for various output voltages. In this example, a 560 pF

capacitor is needed.

This capacitor type can be ceramic, plastic, silver mica, etc.

(Because of the unstable characteristics of ceramic capacitors

made with Z5U material, they are not recommended.)

www.national.com

14

LM2596 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

5. Catch Diode Selection (D1)

EXAMPLE (Adjustable Output Voltage Version)

5. Catch Diode Selection (D1)

A. The catch diode current rating must be at least 1.3 times

greater than the maximum load current. Also, 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 LM2596. The most stressful condition for

this diode is an overload or shorted output condition.

A. Refer to the table shown in Figure 11. Schottky diodes

provide the best performance, and in this example a 5A, 40V,

1N5825 Schottky diode would be a good choice. The 5A diode

rating is more than adequate and will not be overstressed

even for a shorted output.

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

1.25 times the maximum input voltage.

C. This diode must be fast (short reverse recovery time) and

must be located close to the LM2596 using short leads and

short printed circuit traces. Because of their fast switching

speed and low forward voltage drop, Schottky diodes provide

the best performance and efficiency, and should be the first

choice, especially in low output voltage applications. Ultra-fast

recovery, or High-Efficiency rectifiers are also a good choice,

but some types with an abrupt turn-off characteristic may

cause instability or EMl problems. Ultra-fast recovery diodes

typically have reverse recovery times of 50 ns or less. Recti-

fiers such as the 1N4001 series are much too slow and should

not be used.

6. Input Capacitor (C_IN

)

6. 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. In addition, the RMS

The important parameters for the Input capacitor are the input

voltage rating and the RMS current rating. With a nominal

input voltage of 28V, an aluminum electrolytic aluminum elec-

trolytic capacitor with a voltage rating greater than 42V (1.5 x

current rating of the input capacitor should be selected to be at

1

least

⁄2

the DC load current. The capacitor manufacturers

V_IN) would be needed. Since the the next higher capacitor

data sheet must be checked to assure that this current rating

is not exceeded. The curve shown in Figure 13 shows typical

RMS current ratings for several different aluminum electrolytic

capacitor values.

voltage rating is 50V, a 50V capacitor should be used. The

capacitor voltage rating of (1.5 x V_IN) is a conservative guide-

line, and can be modified somewhat if desired.

The RMS current rating requirement for the input capacitor of

1

This capacitor should be located close to the IC using short

leads and the voltage rating should be approximately 1.5

times the maximum input voltage.

a buck regulator is approximately

⁄2 the DC load current. In

this example, with a 3A load, a capacitor with a RMS current

rating of at least 1.5A is needed.

If solid tantalum input capacitors are used, it is recomended

that they be surge current tested by the manufacturer.

The curves shown in Figure 13 can be used to select an

appropriate input capacitor. From the curves, locate the 50V

line and note which capacitor values have RMS current ratings

greater than 1.5A. Either a 470 µF or 680 µF, 50V capacitor

could be used.

Use caution when using a high dielectric constant ceramic

capacitor for input bypassing, because it may cause severe

ringing at the V_INpin.

For a through hole design, a 680 µF/50V electrolytic capacitor

(Panasonic HFQ series or Nichicon PL series or equivalent)

would be adequate. Other types or other manufacturers ca-

pacitors can be used provided the RMS ripple current ratings

are adequate.

For additional information, see section on input capaci-

tors in application information section.

For surface mount designs, solid tantalum capacitors can be

used, but caution must be exercised with regard to the capaci-

tor surge current rting (see Application Information or input

capacitors in this data sheet). The TPS series available from

AVX, and the 593D series from Sprague are both surge cur-

rent tested.

To further simplify the buck regulator design procedure, Na-

tional Semiconductor is making available computer design

software to be used with the Simple Switcher line ot switching

regulators. Switchers Made Simple (version 4.3 or later) is

available on a 3¹⁄

" diskette for IBM compatible computers.

2

15

www.national.com

LM2596 Series Buck Regulator Design Procedure (Adjustable Output)

Output

Voltage

(V)

Through Hole Output Capacitor

Surface Mount Output Capacitor

Panasonic

Nichicon PL

Series

(µF/V)

Feedforward

AVX TPS

Sprague

595D Series

(µF/V)

Feedforward

HFQ Series

(µF/V)

Capacitor

Series

(µF/V)

330/6.3

220/10

100/16

68/20

Capacitor

2

4

820/35

560/35

470/25

330/25

220/35

100/50

820/35

470/35

470/25

330/25

220/35

150/35

100/50

33 nF

10 nF

3.3 nF

1.5 nF

1 nF

470/4

33 nF

10 nF

3.3 nF

1.5 nF

1 nF

390/6.3

330/10

180/16

120/20

33/25

6

9

1 2

1 5

2 4

2 8

680 pF

560 pF

390 pF

680 pF

220 pF

33/25

10/35

15/50

FIGURE 3. Output Capacitor and Feedforward Capacitor Selection Table

LM2596 Series Buck Regulator Design Procedure

INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)

01258326

01258324

FIGURE 6. LM2596-12

FIGURE 4. LM2596-3.3

01258325

01258327

FIGURE 5. LM2596-5.0

FIGURE 7. LM2596-ADJ

www.national.com

16

LM2596 Series Buck Regulator Design Procedure (Continued)

Inductance Current

Schott

Renco

Through

Hole

Pulse Engineering

Coilcraft

Surface

Mount

(µH)

(A)

Through

Hole

Surface

Mount

Surface

Mount

Through

Hole

Surface

Mount

L15

L21

L22

L23

L24

L25

L26

L27

L28

L29

L30

L31

L32

L33

L34

L35

L36

L37

L38

L39

L40

L41

L42

L43

L44

22 0.99

68 0.99

47 1.17

33 1.40

22 1.70

15 2.10

330 0.80

220 1.00

150 1.20

100 1.47

68 1.78

47 2.20

33 2.50

22 3.10

15 3.40

220 1.70

150 2.10

100 2.50

68 3.10

47 3.50

33 3.50

22 3.50

150 2.70

100 3.40

68 3.40

67148350 67148460 RL-1284-22-43 RL1500-22 PE-53815 PE-53815-S DO3308-223

67144070 67144450 RL-5471-5

67144080 67144460 RL-5471-6

67144090 67144470 RL-5471-7

67148370 67148480 RL-1283-22-43

67148380 67148490 RL-1283-15-43

67144100 67144480 RL-5471-1

67144110 67144490 RL-5471-2

67144120 67144500 RL-5471-3

67144130 67144510 RL-5471-4

67144140 67144520 RL-5471-5

67144150 67144530 RL-5471-6

67144160 67144540 RL-5471-7

67148390 67148500 RL-1283-22-43

67148400 67148790 RL-1283-15-43

RL1500-68 PE-53821 PE-53821-S DO3316-683

—

PE-53822 PE-53822-S DO3316-473

PE-53823 PE-53823-S DO3316-333

PE-53824 PE-53825-S DO3316-223

PE-53825 PE-53824-S DO3316-153

PE-53826 PE-53826-S DO5022P-334

PE-53827 PE-53827-S DO5022P-224

PE-53828 PE-53828-S DO5022P-154

PE-53829 PE-53829-S DO5022P-104

PE-53830 PE-53830-S DO5022P-683

PE-53831 PE-53831-S DO5022P-473

PE-53932 PE-53932-S DO5022P-333

PE-53933 PE-53933-S DO5022P-223

PE-53934 PE-53934-S DO5022P-153

67144170

67144180

67144190

67144200

67144210

—

RL-5473-1

RL-5473-4

RL-5472-1

RL-5472-2

RL-5472-3

PE-53935 PE-53935-S

PE-54036 PE-54036-S

PE-54037 PE-54037-S

PE-54038 PE-54038-S

PE-54039 PE-54039-S

PE-54040 PE-54040-S

PE-54041 PE-54041-S

PE-54042 PE-54042-S

PE-54043

—

67144220 67148290 RL-5472-4

67144230 67148300 RL-5472-5

67148410

67144240

67144250

—

RL-5473-4

RL-5473-2

RL-5473-3

PE-54044

FIGURE 8. Inductor Manufacturers Part Numbers

Coilcraft Inc.

Phone (800) 322-2645

FAX (708) 639-1469

Phone +11 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

Phone (612) 475-1173

FAX (612) 475-1786

Coilcraft Inc., Europe

Schott Corp.

FIGURE 9. Inductor Manufacturers Phone Numbers

17

www.national.com

LM2596 Series Buck Regulator Design Procedure (Continued)

Nichicon Corp.

Panasonic

Phone

FAX

(708) 843-7500

(708) 843-2798

(714) 373-7857

(714) 373-7102

(803) 448-9411

(803) 448-1943

(207) 324-4140

(207) 324-7223

Phone

FAX

AVX Corp.

Phone

FAX

Sprague/Vishay

Phone

FAX

FIGURE 10. Capacitor Manufacturers Phone Numbers

VR

3A Diodes

4A–6A Diodes

Surface Mount Through Hole

Surface Mount

Through Hole

Schottky

Ultra Fast

Schottky

Ultra Fast

Schottky

Ultra Fast

Schottky

Ultra Fast

Recovery

All of

Recovery

All of

Recovery

All of

Recovery

20V

1N5820

SR302

SR502

1N5823

SB520

All of

these

diodes

are

these

SK32

diodes

are

diodes

are

diodes

are

MBR320

1N5821

MBR330

31DQ03

1N5822

SR304

30V 30WQ03

SK33

rated to

at least

50V.

rated to

at least

50V.

rated to

at least

50V.

rated to

at least

50V.

50WQ03

50WQ04

SR503

1N5824

SB530

SR504

1N5825

SB540

40V SK34

MBRS340

MBR340

31DQ04

SR305

30WQ04

MURS320

30WF10

MUR320

MURS620

50WF10

MUR620

HER601

50V SK35

or MBRS360

More 30WQ05

MBR350

31DQ05

50WQ05

SB550

50SQ080

FIGURE 11. Diode Selection Table

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18

Block Diagram

01258321

FIGURE 12.

relatively high RMS currents flowing in a buck regulator’s

input capacitor, this capacitor should be chosen for its RMS

current rating rather than its capacitance or voltage ratings,

although the capacitance value and voltage rating are di-

rectly related to the RMS current rating.

Application Information

PIN FUNCTIONS

+V_IN—This is the positive input supply for the IC switching

regulator. A suitable input bypass capacitor must be present

at this pin to minimize voltage transients and to supply the

switching currents needed by the regulator.

The RMS current rating of a capacitor could be viewed as a

capacitor’s power rating. The RMS current flowing through

the capacitors internal ESR produces power which causes

the internal temperature of the capacitor to rise. The RMS

current rating of a capacitor is determined by the amount of

current required to raise the internal temperature approxi-

mately 10˚C above an ambient temperature of 105˚C. The

ability of the capacitor to dissipate this heat to the surround-

ing air will determine the amount of current the capacitor can

safely sustain. Capacitors that are physically large and have

a large surface area will typically have higher RMS current

ratings. For a given capacitor value, a higher voltage elec-

trolytic capacitor will be physically larger than a lower voltage

capacitor, and thus be able to dissipate more heat to the

surrounding air, and therefore will have a higher RMS cur-

rent rating.

Ground —Circuit ground.

Output —Internal switch. The voltage at this pin switches

between (+V_IN− V_SAT) and approximately −0.5V, with a duty

cycle of approximately V_OUT/V_IN. To minimize coupling to

sensitive circuitry, the PC board copper area connected to

this pin should be kept to a minimum.

Feedback —Senses the regulated output voltage to com-

plete the feedback loop.

ON /OFF —Allows the switching regulator circuit to be shut

down using logic level signals thus dropping the total input

supply current to approximately 80 µA. Pulling this pin below

a threshold voltage of approximately 1.3V turns the regulator

on, and pulling this pin above 1.3V (up to a maximum of 25V)

shuts the regulator down. If this shutdown feature is not

needed, the ON /OFF pin can be wired to the ground pin or

it can be left open, in either case the regulator will be in the

ON condition.

The consequences of operating an electrolytic capacitor

above the RMS current rating is a shortened operating life.

The higher temperature speeds up the evaporation of the

capacitor’s electrolyte, resulting in eventual failure.

Selecting an input capacitor requires consulting the manu-

facturers data sheet for maximum allowable RMS ripple

current. For a maximum ambient temperature of 40˚C, a

general guideline would be to select a capacitor with a ripple

current rating of approximately 50% of the DC load current.

For ambient temperatures up to 70˚C, a current rating of

75% of the DC load current would be a good choice for a

conservative design. The capacitor voltage rating must be at

least 1.25 times greater than the maximum input voltage,

and often a much higher voltage capacitor is needed to

satisfy the RMS current requirements.

EXTERNAL COMPONENTS

INPUT CAPACITOR

C_IN—A low ESR aluminum or tantalum bypass capacitor is

needed between the input pin and ground pin. It must be

located near the regulator using short leads. This capacitor

prevents large voltage transients from appearing at the in-

put, and provides the instantaneous current needed each

time the switch turns on.

The important parameters for the Input capacitor are the

voltage rating and the RMS current rating. Because of the

19

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RMS ripple current rating, voltage rating, and capacitance

value. For the output capacitor, the ESR value is the most

important parameter.

Application Information (Continued)

A graph shown in Figure 13 shows the relationship between

an electrolytic capacitor value, its voltage rating, and the

RMS current it is rated for. These curves were obtained from

the Nichicon “PL” series of low ESR, high reliability electro-

lytic capacitors designed for switching regulator applications.

Other capacitor manufacturers offer similar types of capaci-

tors, but always check the capacitor data sheet.

The output capacitor requires an ESR value that has an

upper and lower limit. For low output ripple voltage, a low

ESR value is needed. This value is determined by the maxi-

mum allowable output ripple voltage, typically 1% to 2% of

the output voltage. But if the selected capacitor’s ESR is

extremely low, there is a possibility of an unstable feedback

loop, resulting in an oscillation at the output. Using the

capacitors listed in the tables, or similar types, will provide

design solutions under all conditions.

“Standard” electrolytic capacitors typically have much higher

ESR numbers, lower RMS current ratings and typically have

a shorter operating lifetime.

Because of their small size and excellent performance, sur-

face mount solid tantalum capacitors are often used for input

bypassing, but several precautions must be observed. A

small percentage of solid tantalum capacitors can short if the

inrush current rating is exceeded. This can happen at turn on

when the input voltage is suddenly applied, and of course,

higher input voltages produce higher inrush currents. Sev-

eral capacitor manufacturers do a 100% surge current test-

ing on their products to minimize this potential problem. If

high turn on currents are expected, it may be necessary to

limit this current by adding either some resistance or induc-

tance before the tantalum capacitor, or select a higher volt-

age capacitor. As with aluminum electrolytic capacitors, the

RMS ripple current rating must be sized to the load current.

If very low output ripple voltage (less than 15 mV) is re-

quired, refer to the section on Output Voltage Ripple and

Transients for a post ripple filter.

An aluminum electrolytic capacitor’s ESR value is related to

the capacitance value and its voltage rating. In most cases,

higher voltage electrolytic capacitors have lower ESR values

(see Figure 14 ). Often, capacitors with much higher voltage

ratings may be needed to provide the low ESR values re-

quired for low output ripple voltage.

The output capacitor for many different switcher designs

often can be satisfied with only three or four different capaci-

tor values and several different voltage ratings. See the

quick design component selection tables in Figure 2 and 4

for typical capacitor values, voltage ratings, and manufactur-

ers capacitor types.

FEEDFORWARD CAPACITOR

(Adjustable Output Voltage Version)

Electrolytic capacitors are not recommended for tempera-

tures below −25˚C. The ESR rises dramatically at cold tem-

C_FF—A Feedforward Capacitor C_FF, shown across R2 in

Figure 1 is used when the ouput voltage is greater than 10V

or when C_OUThas a very low ESR. This capacitor adds lead

compensation to the feedback loop and increases the phase

margin for better loop stability. For C_FFselection, see the

design procedure section.

@

peratures and typically rises 3X

−25˚C and as much as

10X at −40˚C. See curve shown in Figure 15.

Solid tantalum capacitors have a much better ESR spec for

cold temperatures and are recommended for temperatures

below −25˚C.

01258329

01258328

FIGURE 14. Capacitor ESR vs Capacitor Voltage Rating

(Typical Low ESR Electrolytic Capacitor)

FIGURE 13. RMS Current Ratings for Low ESR

Electrolytic Capacitors (Typical)

CATCH DIODE

Buck regulators require a diode to provide a return path for

the inductor current when the switch turns off. This must be

a fast diode and must be located close to the LM2596 using

short leads and short printed circuit traces.

OUTPUT CAPACITOR

C_OUT—An output capacitor is required to filter the output

and provide regulator loop stability. Low impedance or low

ESR Electrolytic or solid tantalum capacitors designed for

switching regulator applications must be used. When select-

ing an output capacitor, the important capacitor parameters

are; the 100 kHz Equivalent Series Resistance (ESR), the

Because of their very fast switching speed and low forward

voltage drop, Schottky diodes provide the best performance,

especially in low output voltage applications (5V and lower).

Ultra-fast recovery, or High-Efficiency rectifiers are also a

www.national.com

20

Application Information (Continued)

good choice, but some types with an abrupt turnoff charac-

teristic may cause instability or EMI problems. Ultra-fast

recovery diodes typically have reverse recovery times of 50

ns or less. Rectifiers such as the 1N5400 series are much

too slow and should not be used.

01258331

FIGURE 16. (∆I_IND) Peak-to-Peak Inductor

Ripple Current (as a Percentage of the Load Current)

vs Load Current

By allowing the percentage of inductor ripple current to

increase for low load currents, the inductor value and size

can be kept relatively low.

01258330

FIGURE 15. Capacitor ESR Change vs Temperature

When operating in the continuous mode, the inductor current

waveform ranges from a triangular to a sawtooth type of

waveform (depending on the input voltage), with the average

value of this current waveform equal to the DC output load

current.

INDUCTOR SELECTION

All switching regulators have two basic modes of operation;

continuous and discontinuous. The difference between the

two types relates to the inductor current, whether it is flowing

continuously, or if it drops to zero for a period of time in the

normal switching cycle. Each mode has distinctively different

operating characteristics, which can affect the regulators

performance and requirements. Most switcher designs will

operate in the discontinuous mode when the load current is

low.

Inductors are available in different styles such as pot core,

toroid, E-core, bobbin core, etc., as well as different core

materials, such as ferrites and powdered iron. The least

expensive, the bobbin, rod or stick core, consists of wire

wound on a ferrite bobbin. This type of construction makes

for an inexpensive inductor, but since the magnetic flux is not

completely contained within the core, it generates more

Electro-Magnetic Interference (EMl). This magnetic flux can

induce voltages into nearby printed circuit traces, thus caus-

ing problems with both the switching regulator operation and

nearby sensitive circuitry, and can give incorrect scope read-

ings because of induced voltages in the scope probe. Also

see section on Open Core Inductors.

The LM2596 (or any of the Simple Switcher family) can be

used for both continuous or discontinuous modes of opera-

tion.

In many cases the preferred mode of operation is the con-

tinuous mode. It offers greater output power, lower peak

switch, inductor and diode currents, and can have lower

output ripple voltage. But it does require larger inductor

values to keep the inductor current flowing continuously,

especially at low output load currents and/or high input volt-

ages.

When multiple switching regulators are located on the same

PC board, open core magnetics can cause interference

between two or more of the regulator circuits, especially at

high currents. A torroid or E-core inductor (closed magnetic

structure) should be used in these situations.

To simplify the inductor selection process, an inductor selec-

tion guide (nomograph) was designed (see Figure 4 through

8). This guide assumes that the regulator is operating in the

continuous mode, and selects an inductor that will allow a

peak-to-peak inductor ripple current to be a certain percent-

age of the maximum design load current. This peak-to-peak

inductor ripple current percentage is not fixed, but is allowed

to change as different design load currents are selected.

(See Figure 16.)

The inductors listed in the selection chart include ferrite

E-core construction for Schott, ferrite bobbin core for Renco

and Coilcraft, and powdered iron toroid for Pulse Engineer-

ing.

Exceeding an inductor’s maximum current rating may cause

the inductor to overheat because of the copper wire losses,

or the core may saturate. If the inductor begins to saturate,

the inductance decreases rapidly and the inductor begins to

look mainly resistive (the DC resistance of the winding). This

can cause the switch current to rise very rapidly and force

the switch into a cycle-by-cycle current limit, thus reducing

the DC output load current. This can also result in overheat-

ing of the inductor and/or the LM2596. Different inductor

types have different saturation characteristics, and this

should be kept in mind when selecting an inductor.

The inductor manufacturer’s data sheets include current and

energy limits to avoid inductor saturation.

21

www.national.com

voltage, the ESR of the output capacitor must be low, how-

ever, caution must be exercised when using extremely low

ESR capacitors because they can affect the loop stability,

resulting in oscillation problems. If very low output ripple

voltage is needed (less than 20 mV), a post ripple filter is

recommended. (See Figure 1.) The inductance required is

typically between 1 µH and 5 µH, with low DC resistance, to

maintain good load regulation. A low ESR output filter ca-

pacitor is also required to assure good dynamic load re-

sponse and ripple reduction. The ESR of this capacitor may

be as low as desired, because it is out of the regulator

feedback loop. The photo shown in Figure 17 shows a

typical output ripple voltage, with and without a post ripple

filter.

Application Information (Continued)

DISCONTINUOUS MODE OPERATION

The selection guide chooses inductor values suitable for

continuous mode operation, but for low current applications

and/or high input voltages, a discontinuous mode design

may be a better choice. It would use an inductor that would

be physically smaller, and would need only one half to one

third the inductance value needed for a continuous mode

design. The peak switch and inductor currents will be higher

in a discontinuous design, but at these low load currents (1A

and below), the maximum switch current will still be less than

the switch current limit.

Discontinuous operation can have voltage waveforms that

are considerable different than a continuous design. The

output pin (switch) waveform can have some damped sinu-

soidal ringing present. (See Typical Performance Character-

istics photo titled Discontinuous Mode Switching Wave-

forms) This ringing is normal for discontinuous operation,

and is not caused by feedback loop instabilities. In discon-

tinuous operation, there is a period of time where neither the

switch or the diode are conducting, and the inductor current

has dropped to zero. During this time, a small amount of

energy can circulate between the inductor and the switch/

diode parasitic capacitance causing this characteristic ring-

ing. Normally this ringing is not a problem, unless the ampli-

tude becomes great enough to exceed the input voltage, and

even then, there is very little energy present to cause dam-

age.

When observing output ripple with a scope, it is essential

that a short, low inductance scope probe ground connection

be used. Most scope probe manufacturers provide a special

probe terminator which is soldered onto the regulator board,

preferable at the output capacitor. This provides a very short

scope ground thus eliminating the problems associated with

the 3 inch ground lead normally provided with the probe, and

provides a much cleaner and more accurate picture of the

ripple voltage waveform.

The voltage spikes are caused by the fast switching action of

the output switch and the diode, and the parasitic inductance

of the output filter capacitor, and its associated wiring. To

minimize these voltage spikes, the output capacitor should

be designed for switching regulator applications, and the

lead lengths must be kept very short. Wiring inductance,

stray capacitance, as well as the scope probe used to evalu-

ate these transients, all contribute to the amplitude of these

spikes.

Different inductor types and/or core materials produce differ-

ent amounts of this characteristic ringing. Ferrite core induc-

tors have very little core loss and therefore produce the most

ringing. The higher core loss of powdered iron inductors

produce less ringing. If desired, a series RC could be placed

in parallel with the inductor to dampen the ringing. The

computer aided design software Switchers Made Simple

(version 4.3) will provide all component values for continu-

ous and discontinuous modes of operation.

When a switching regulator is operating in the continuous

mode, the inductor current waveform ranges from a triangu-

lar to a sawtooth type of waveform (depending on the input

voltage). For

a given input and output voltage, the

peak-to-peak amplitude of this inductor current waveform

remains constant. As the load current increases or de-

creases, the entire sawtooth current waveform also rises

and falls. The average value (or the center) of this current

waveform is equal to the DC load current.

If the load current drops to a low enough level, the bottom of

the sawtooth current waveform will reach zero, and the

switcher will smoothly change from a continuous to a discon-

tinuous mode of operation. Most switcher designs (irregard-

less how large the inductor value is) will be forced to run

discontinuous if the output is lightly loaded. This is a per-

fectly acceptable mode of operation.

01258332

FIGURE 17. Post Ripple Filter Waveform

OUTPUT VOLTAGE RIPPLE AND TRANSIENTS

The output voltage of a switching power supply operating in

the continuous mode will contain a sawtooth ripple voltage at

the switcher frequency, and may also contain short voltage

spikes at the peaks of the sawtooth waveform.

The output ripple voltage is a function of the inductor saw-

tooth ripple current and the ESR of the output capacitor. A

typical output ripple voltage can range from approximately

0.5% to 3% of the output voltage. To obtain low ripple

www.national.com

22

1. Peak Inductor or peak switch current

Application Information (Continued)

2. Minimum load current before the circuit becomes dis-

continuous

3. Output Ripple Voltage = (∆I_IND)x(ESR of C_OUT

)

= 0.62Ax0.1Ω=62 mV p-p

4.

01258333

FIGURE 18. Peak-to-Peak Inductor

Ripple Current vs Load Current

In a switching regulator design, knowing the value of the

peak-to-peak inductor ripple current (∆I_IND) can be useful for

determining a number of other circuit parameters. Param-

eters such as, peak inductor or peak switch current, mini-

mum load current before the circuit becomes discontinuous,

output ripple voltage and output capacitor ESR can all be

calculated from the peak-to-peak ∆I_IND. When the inductor

nomographs shown in Figure 4 through 8 are used to select

an inductor value, the peak-to-peak inductor ripple current

can immediately be determined. The curve shown in Figure

18 shows the range of (∆I_IND) that can be expected for

different load currents. The curve also shows how the

peak-to-peak inductor ripple current (∆I_IND) changes as you

go from the lower border to the upper border (for a given load

current) within an inductance region. The upper border rep-

resents a higher input voltage, while the lower border repre-

sents a lower input voltage (see Inductor Selection Guides).

OPEN CORE INDUCTORS

Another possible source of increased output ripple voltage or

unstable operation is from an open core inductor. Ferrite

bobbin or stick inductors have magnetic lines of flux flowing

through the air from one end of the bobbin to the other end.

These magnetic lines of flux will induce a voltage into any

wire or PC board copper trace that comes within the induc-

tor’s magnetic field. The strength of the magnetic field, the

orientation and location of the PC copper trace to the mag-

netic field, and the distance between the copper trace and

the inductor, determine the amount of voltage generated in

the copper trace. Another way of looking at this inductive

coupling is to consider the PC board copper trace as one

turn of a transformer (secondary) with the inductor winding

as the primary. Many millivolts can be generated in a copper

trace located near an open core inductor which can cause

stability problems or high output ripple voltage problems.

These curves are only correct for continuous mode opera-

tion, and only if the inductor selection guides are used to

select the inductor value

If unstable operation is seen, and an open core inductor is

used, it’s possible that the location of the inductor with

respect to other PC traces may be the problem. To deter-

mine if this is the problem, temporarily raise the inductor

away from the board by several inches and then check

circuit operation. If the circuit now operates correctly, then

the magnetic flux from the open core inductor is causing the

problem. Substituting a closed core inductor such as a tor-

roid or E-core will correct the problem, or re-arranging the

PC layout may be necessary. Magnetic flux cutting the IC

device ground trace, feedback trace, or the positive or nega-

tive traces of the output capacitor should be minimized.

Consider the following example:

V_OUT= 5V, maximum load current of 2.5A

V_IN= 12V, nominal, varying between 10V and 16V.

The selection guide in Figure 5 shows that the vertical line

for a 2.5A load current, and the horizontal line for the 12V

input voltage intersect approximately midway between the

upper and lower borders of the 33 µH inductance region.

A 33 µH inductor will allow a peak-to-peak inductor current

(∆I_IND) to flow that will be a percentage of the maximum load

current. Referring to Figure 18, follow the 2.5A line approxi-

mately midway into the inductance region, and read the

peak-to-peak inductor ripple current (∆I_IND) on the left hand

axis (approximately 620 mA p-p).

Sometimes, locating a trace directly beneath a bobbin in-

ductor will provide good results, provided it is exactly in the

center of the inductor (because the induced voltages cancel

themselves out), but if it is off center one direction or the

other, then problems could arise. If flux problems are

present, even the direction of the inductor winding can make

a difference in some circuits.

As the input voltage increases to 16V, it approaches the

upper border of the inductance region, and the inductor

ripple current increases. Referring to the curve in Figure 18,

it can be seen that for

a load current of 2.5A, the

This discussion on open core inductors is not to frighten the

user, but to alert the user on what kind of problems to watch

out for when using them. Open core bobbin or “stick” induc-

tors are an inexpensive, simple way of making a compact

efficient inductor, and they are used by the millions in many

different applications.

peak-to-peak inductor ripple current (∆I_IND) is 620 mA with

12V in, and can range from 740 mA at the upper border (16V

in) to 500 mA at the lower border (10V in).

Once the ∆I_INDvalue is known, the following formulas can be

used to calculate additional information about the switching

regulator circuit.

23

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material and the DC resistance, it could either act as a heat

sink taking heat away from the board, or it could add heat to

the board.

Application Information (Continued)

THERMAL CONSIDERATIONS

The LM2596 is available in two packages, a 5-pin TO-220

(T) and a 5-pin surface mount TO-263 (S).

The TO-220 package needs a heat sink under most condi-

tions. The size of the heatsink depends on the input voltage,

the output voltage, the load current and the ambient tem-

perature. The curves in Figure 19 show the LM2596T junc-

tion temperature rises above ambient temperature for a 3A

load and different input and output voltages. The data for

these curves was taken with the LM2596T (TO-220 pack-

age) operating as a buck switching regulator in an ambient

temperature of 25˚C (still air). These temperature rise num-

bers are all approximate and there are many factors that can

affect these temperatures. Higher ambient temperatures re-

quire more heat sinking.

The TO-263 surface mount package tab is designed to be

soldered to the copper on a printed circuit board. The copper

and the board are the heat sink for this package and the

other heat producing components, such as the catch diode

and inductor. The PC board copper area that the package is

soldered to should be at least 0.4 in², and ideally should

have 2 or more square inches of 2 oz. (0.0028) in) copper.

Additional copper area improves the thermal characteristics,

but with copper areas greater than approximately 6 in², only

small improvements in heat dissipation are realized. If fur-

ther thermal improvements are needed, double sided, mul-

tilayer PC board with large copper areas and/or airflow are

recommended.

01258334

Circuit Data for Temperature Rise Curve

TO-220 Package (T)

Capacitors Through hole electrolytic

Inductor

Diode

Through hole, Renco

Through hole, 5A 40V, Schottky

3 square inches single sided 2 oz. copper

(0.0028")

PC board

The curves shown in Figure 20 show the LM2596S (TO-263

package) junction temperature rise above ambient tempera-

ture with a 2A load for various input and output voltages. This

data was taken with the circuit operating as a buck switching

regulator with all components mounted on a PC board to

simulate the junction temperature under actual operating

conditions. This curve can be used for a quick check for the

approximate junction temperature for various conditions, but

be aware that there are many factors that can affect the

junction temperature. When load currents higher than 2A are

used, double sided or multilayer PC boards with large cop-

per areas and/or airflow might be needed, especially for high

ambient temperatures and high output voltages.

FIGURE 19. Junction Temperature Rise, TO-220

For the best thermal performance, wide copper traces and

generous amounts of printed circuit board copper should be

used in the board layout. (One exception to this is the output

(switch) pin, which should not have large areas of copper.)

Large areas of copper provide the best transfer of heat

(lower thermal resistance) to the surrounding air, and moving

air lowers the thermal resistance even further.

Package thermal resistance and junction temperature rise

numbers are all approximate, and there are many factors

that will affect these numbers. Some of these factors include

board size, shape, thickness, position, location, and even

board temperature. Other factors are, trace width, total

printed circuit copper area, copper thickness, single- or

double-sided, multilayer board and the amount of solder on

the board. The effectiveness of the PC board to dissipate

heat also depends on the size, quantity and spacing of other

components on the board, as well as whether the surround-

ing air is still or moving. Furthermore, some of these com-

ponents such as the catch diode will add heat to the PC

board and the heat can vary as the input voltage changes.

For the inductor, depending on the physical size, type of core

01258335

Circuit Data for Temperature Rise Curve

TO-263 Package (S)

Capacitors Surface mount tantalum, molded “D” size

Inductor

Diode

Surface mount, Pulse Engineering, 68 µH

Surface mount, 5A 40V, Schottky

9 square inches single sided 2 oz. copper

(0.0028")

PC board

FIGURE 20. Junction Temperature Rise, TO-263

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24

tures a constant threshold voltage for turn on and turn off

(zener voltage plus approximately one volt). If hysteresis is

needed, the circuit in Figure 24 has a turn ON voltage which

is different than the turn OFF voltage. The amount of hyster-

esis is approximately equal to the value of the output volt-

age. If zener voltages greater than 25V are used, an addi-

tional 47 kΩ resistor is needed from the ON /OFF pin to the

ground pin to stay within the 25V maximum limit of the ON

/OFF pin.

Application Information (Continued)

INVERTING REGULATOR

The circuit in Figure 25 converts a positive input voltage to a

negative output voltage with a common ground. The circuit

operates by bootstrapping the regulator’s ground pin to the

negative output voltage, then grounding the feedback pin,

the regulator senses the inverted output voltage and regu-

lates it.

01258336

FIGURE 21. Delayed Startup

01258337

01258338

This circuit has an ON/OFF threshold of approximately 13V.

FIGURE 22. Undervoltage Lockout

for Buck Regulator

FIGURE 23. Undervoltage Lockout

for Inverting Regulator

DELAYED STARTUP

This example uses the LM2596-5.0 to generate a −5V out-

put, but other output voltages are possible by selecting other

output voltage versions, including the adjustable version.

Since this regulator topology can produce an output voltage

that is either greater than or less than the input voltage, the

maximum output current greatly depends on both the input

and output voltage. The curve shown in Figure 26 provides a

guide as to the amount of output load current possible for the

different input and output voltage conditions.

The circuit in Figure 21 uses the the ON /OFF pin to provide

a time delay between the time the input voltage is applied

and the time the output voltage comes up (only the circuitry

pertaining to the delayed start up is shown). As the input

voltage rises, the charging of capacitor C1 pulls the ON /OFF

pin high, keeping the regulator off. Once the input voltage

reaches its final value and the capacitor stops charging, and

resistor R₂pulls the ON /OFF pin low, thus allowing the

circuit to start switching. Resistor R₁is included to limit the

maximum voltage applied to the ON /OFF pin (maximum of

25V), reduces power supply noise sensitivity, and also limits

the capacitor, C1, discharge current. When high input ripple

voltage exists, avoid long delay time, because this ripple can

be coupled into the ON /OFF pin and cause problems.

The maximum voltage appearing across the regulator is the

absolute sum of the input and output voltage, and this must

be limited to a maximum of 40V. For example, when convert-

ing +20V to −12V, the regulator would see 32V between the

input pin and ground pin. The LM2596 has a maximum input

voltage spec of 40V.

This delayed startup feature is useful in situations where the

input power source is limited in the amount of current it can

deliver. It allows the input voltage to rise to a higher voltage

before the regulator starts operating. Buck regulators require

less input current at higher input voltages.

Additional diodes are required in this regulator configuration.

Diode D1 is used to isolate input voltage ripple or noise from

coupling through the C_INcapacitor to the output, under light

or no load conditions. Also, this diode isolation changes the

topology to closley resemble a buck configuration thus pro-

viding good closed loop stability. A Schottky diode is recom-

mended for low input voltages, (because of its lower voltage

drop) but for higher input voltages, a fast recovery diode

could be used.

UNDERVOLTAGE LOCKOUT

Some applications require the regulator to remain off until

the input voltage reaches a predetermined voltage. An und-

ervoltage lockout feature applied to a buck regulator is

shown in Figure 22, while Figure 23 and 24 applies the same

feature to an inverting circuit. The circuit in Figure 23 fea-

Without diode D3, when the input voltage is first applied, the

charging current of C_INcan pull the output positive by sev-

eral volts for a short period of time. Adding D3 prevents the

output from going positive by more than a diode voltage.

25

www.national.com

Application Information (Continued)

01258339

This circuit has hysteresis

Regulator starts switching at V = 13V

IN

Regulator stops switching at V = 8V

IN

FIGURE 24. Undervoltage Lockout with Hysteresis for Inverting Regulator

01258340

C

— 68 µF/25V Tant. Sprague 595D

470 µF/50V Elec. Panasonic HFQ

— 47 µF/20V Tant. Sprague 595D

220 µF/25V Elec. Panasonic HFQ

IN

OUT

FIGURE 25. Inverting −5V Regulator with Delayed Startup

be narrowed down to just a few values. Using the values

shown in Figure 25 will provide good results in the majority of

inverting designs.

This type of inverting regulator can require relatively large

amounts of input current when starting up, even with light

loads. Input currents as high as the LM2596 current limit

(approx 4.5A) are needed for at least 2 ms or more, until the

output reaches its nominal output voltage. The actual time

depends on the output voltage and the size of the output

capacitor. Input power sources that are current limited or

sources that can not deliver these currents without getting

loaded down, may not work correctly. Because of the rela-

tively high startup currents required by the inverting topology,

the delayed startup feature (C1, R₁and R₂) shown in Figure

25 is recommended. By delaying the regulator startup, the

input capacitor is allowed to charge up to a higher voltage

before the switcher begins operating. A portion of the high

input current needed for startup is now supplied by the input

capacitor (C_IN). For severe start up conditions, the input

capacitor can be made much larger than normal.

01258341

FIGURE 26. Inverting Regulator Typical Load Current

Because of differences in the operation of the inverting

regulator, the standard design procedure is not used to

select the inductor value. In the majority of designs, a 33 µH,

3.5A inductor is the best choice. Capacitor selection can also

www.national.com

26

OFF. With the inverting configuration, some level shifting is

required, because the ground pin of the regulator is no

longer at ground, but is now setting at the negative output

voltage level. Two different shutdown methods for inverting

regulators are shown in Figure 27 and 28.

Application Information (Continued)

INVERTING REGULATOR SHUTDOWN METHODS

To use the ON /OFF pin in a standard buck configuration is

@

simple, pull it below 1.3V ( 25˚C, referenced to ground) to

turn regulator ON, pull it above 1.3V to shut the regulator

01258342

FIGURE 27. Inverting Regulator Ground Referenced Shutdown

01258343

FIGURE 28. Inverting Regulator Ground Referenced Shutdown using Opto Device

27

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Application Information (Continued)

TYPICAL THROUGH HOLE PC BOARD LAYOUT, FIXED OUTPUT (1X SIZE), DOUBLE SIDED

01258344

C

— 470 µF, 50V, Aluminum Electrolytic Panasonic, “HFQ Series”

IN

— 330 µF, 35V, Aluminum Electrolytic Panasonic, “HFQ Series”

OUT

D1 — 5A, 40V Schottky Rectifier, 1N5825

L1 — 47 µH, L39, Renco, Through Hole

Thermalloy Heat Sink #7020

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28

Application Information (Continued)

TYPICAL THROUGH HOLE PC BOARD LAYOUT, ADJUSTABLE OUTPUT (1X SIZE), DOUBLE SIDED

01258345

C

— 470 µF, 50V, Aluminum Electrolytic Panasonic, “HFQ Series”

IN

— 220 µF, 35V Aluminum Electrolytic Panasonic, “HFQ Series”

OUT

D1 — 5A, 40V Schottky Rectifier, 1N5825

L1 — 47 µH, L39, Renco, Through Hole

R

C

— 1 kΩ, 1%

1

— Use formula in Design Procedure

2

— See Figure 3.

FF

Thermalloy Heat Sink #7020

FIGURE 29. PC Board Layout

29

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Physical Dimensions inches (millimeters)

unless otherwise noted

5-Lead TO-220 (T)

Order Number LM2596T-3.3, LM2596T-5.0,

LM2596T-12 or LM2596T-ADJ

NS Package Number T05D

www.national.com

30

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

5-Lead TO-263 Surface Mount Package (S)

Order Number LM2596S-3.3, LM2596S-5.0,

LM2596S-12 or LM2596S-ADJ

NS Package Number TS5B

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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 AND GENERAL

COUNSEL 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 with instructions for use provided in the

labeling, can be reasonably expected to result in a

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

safety or effectiveness.

National Semiconductor

Corporation

Americas

National Semiconductor

Europe

National Semiconductor

Asia Pacific Customer

Response Group

Tel: 65-2544466

Fax: 65-2504466

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Tel: 81-3-5639-7560

Fax: 81-3-5639-7507

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Email: ap.support@nsc.com

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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.


型号：	LM2596T-5.0
厂家：	National Semiconductor
描述：	SIMPLE SWITCHER Power Converter 150 kHz 3A Step-Down Voltage Regulator SIMPLE SWITCHER系列电源转换器150千赫3A降压型稳压器转换器稳压器
文件：	总31页 (文件大小：736K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LM2596T-5.0 [NSC]

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