Файл: LM2596.pdf

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

3. Output Capacitor Selection (COUT)

3. Output Capacitor SeIection (COUT)

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

A. See section on COUT in Application Information section.

tantalum capacitors between 82 µF and 820 µF provide the

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

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

output voltage column. From that column, locate the output

using short capacitor leads and short copper traces. Do not

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

use capacitors larger than 820 µF. For additional informa-

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

tion, see section on output capacitors in application in-

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

formation section.

or surface mount tantalum types from four different capacitor

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

manufacturers. It is recommended that both the manufactur-

quick design table shown in Figure 3. This table contains

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

different output voltages, and lists various output capacitors

used.

that will provide the best design solutions.

In this example, through hole aluminum electrolytic capacitors

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

from several different manufacturers are available.

greater than the output voltage, and often much higher voltage

220 µF/35V Panasonic HFQ Series

ratings are needed to satisfy the low ESR requirements

150 µF/35V Nichicon PL Series

needed for low output ripple voltage.

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 (CFF) (See Figure 1)

4. Feedforward Capacitor (CFF)

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

The table shown in Figure 3 contains feed forward capacitor

tional capacitor is required. The compensation capacitor is

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

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

capacitor is needed.

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.

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

5. Catch Diode Selection (D1)

5. Catch Diode Selection (D1)

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

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

greater than the maximum load current. Also, if the power

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

supply design must withstand a continuous output short, the

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

diode should have a current rating equal to the maximum

rating is more than adequate and will not be overstressed

current limit of the LM2596. The most stressful condition for

even for a shorted output.

this diode is an overload or shorted output condition.

6. Input Capacitor (CIN)

6. Input Capacitor (CIN)

A low ESR aluminum or tantalum bypass capacitor is needed

The important parameters for the Input capacitor are the input

between the input pin and ground to prevent large voltage

voltage rating and the RMS current rating. With a nominal

transients from appearing at the input. In addition, the RMS

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

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

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

least 1¤2 the DC load current. The capacitor manufacturers

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

IN

data sheet must be checked to assure that this current rating

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

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

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

RMS current ratings for several different aluminum electrolytic

line, and can be modified somewhat if desired.

capacitor values.

The RMS current rating requirement for the input capacitor of

This capacitor should be located close to the IC using short

a buck regulator is approximately 1¤2 the DC load current. In

leads and the voltage rating should be approximately 1.5

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

times the maximum input voltage.

rating of at least 1.5A is needed.

If solid tantalum input capacitors are used, it is recomended

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

that they be surge current tested by the manufacturer.

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

Use caution when using a high dielectric constant ceramic

line and note which capacitor values have RMS current ratings

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

capacitor for input bypassing, because it may cause severe

could be used.

ringing at the VIN pin.

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

For additional information, see section on input capaci-

tors in application information section.

(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 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¤2" diskette for IBM compatible computers.

Output

Through Hole Output Capacitor

Surface Mount Output Capacitor

Voltage

Panasonic

Nichicon PL

Feedforward

AVX TPS

Sprague

Feedforward

(V)

HFQ Series

Series

Capacitor

Series

595D Series

Capacitor

(µF/V)

(µF/V)

(µF/V)

(µF/V)

2

820/35

820/35

33 nF

330/6.3

470/4

33 nF

4

560/35

470/35

10 nF

330/6.3

390/6.3

10 nF

6

470/25

470/25

3.3 nF

220/10

330/10

3.3 nF

9

330/25

330/25

1.5 nF

100/16

180/16

1.5 nF

1 2

330/25

330/25

1 nF

100/16

180/16

1 nF

1 5

220/35

220/35

680 pF

68/20

120/20

680 pF

2 4

220/35

150/35

560 pF

33/25

33/25

220 pF

2 8

100/50

100/50

390 pF

10/35

15/50

220 pF

FIGURE 3. Output Capacitor and Feedforward Capacitor Selection Table

01258324

01258326

FIGURE 4. LM2596-3.3

FIGURE 6. LM2596-12

Inductance

Current

Schott

Renco

Pulse Engineering

Coilcraft

(µH)

(A)

Through

Surface

Through

Surface

Through

Surface

Surface

Hole

Mount

Hole

Mount

Hole

Mount

Mount

L15

22

0.99

67148350

67148460

RL-1284-22-43

RL1500-22

PE-53815

PE-53815-S

DO3308-223

L21

68

0.99

67144070

67144450

RL-5471-5

RL1500-68

PE-53821

PE-53821-S

DO3316-683

L22

47

1.17

67144080

67144460

RL-5471-6

Ð

PE-53822

PE-53822-S

DO3316-473

L23

33

1.40

67144090

67144470

RL-5471-7

Ð

PE-53823

PE-53823-S

DO3316-333

L24

22

1.70

67148370

67148480

RL-1283-22-43

Ð

PE-53824

PE-53825-S

DO3316-223

L25

15

2.10

67148380

67148490

RL-1283-15-43

Ð

PE-53825

PE-53824-S

DO3316-153

L26

330

0.80

67144100

67144480

RL-5471-1

Ð

PE-53826

PE-53826-S

DO5022P-334

L27

220

1.00

67144110

67144490

RL-5471-2

Ð

PE-53827

PE-53827-S

DO5022P-224

L28

150

1.20

67144120

67144500

RL-5471-3

Ð

PE-53828

PE-53828-S

DO5022P-154

L29

100

1.47

67144130

67144510

RL-5471-4

Ð

PE-53829

PE-53829-S

DO5022P-104

L30

68

1.78

67144140

67144520

RL-5471-5

Ð

PE-53830

PE-53830-S

DO5022P-683

L31

47

2.20

67144150

67144530

RL-5471-6

Ð

PE-53831

PE-53831-S

DO5022P-473

L32

33

2.50

67144160

67144540

RL-5471-7

Ð

PE-53932

PE-53932-S

DO5022P-333

L33

22

3.10

67148390

67148500

RL-1283-22-43

Ð

PE-53933

PE-53933-S

DO5022P-223

L34

15

3.40

67148400

67148790

RL-1283-15-43

Ð

PE-53934

PE-53934-S

DO5022P-153

L35

220

1.70

67144170

Ð

RL-5473-1

Ð

PE-53935 PE-53935-S

Ð

L36

150

2.10

67144180

Ð

RL-5473-4

Ð

PE-54036 PE-54036-S

Ð

L37

100

2.50

67144190

Ð

RL-5472-1

Ð

PE-54037 PE-54037-S

Ð

L38

68

3.10

67144200

Ð

RL-5472-2

Ð

PE-54038 PE-54038-S

Ð

L39

47

3.50

67144210

Ð

RL-5472-3

Ð

PE-54039 PE-54039-S

Ð

L40

33

3.50

67144220

67148290

RL-5472-4

Ð

PE-54040 PE-54040-S

Ð

L41

22

3.50

67144230

67148300

RL-5472-5

Ð

PE-54041 PE-54041-S

Ð

L42

150

2.70

67148410

Ð

RL-5473-4

Ð

PE-54042 PE-54042-S

Ð

L43

100

3.40

67144240

Ð

RL-5473-2

Ð

PE-54043

Ð

L44

68

3.40

67144250

Ð

RL-5473-3

Ð

PE-54044

Ð

Coilcraft Inc.

Phone

(800)

322-2645

FAX

(708)

639-1469

Coilcraft Inc., Europe

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

Schott Corp.

Phone

(612)

475-1173

FAX

(612)

475-1786

Nichicon Corp.

Phone

(708)

843-7500

FAX

(708)

843-2798

Panasonic

Phone

(714)

373-7857

FAX

(714)

373-7102

AVX Corp.

Phone

(803)

448-9411

FAX

(803)

448-1943

Sprague/Vishay

Phone

(207)

324-4140

FAX

(207)

324-7223

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

Recovery

Recovery

Recovery

20V

All of

1N5820

All of

All of

SR502

All of

these

these

these

these

SK32

SR302

1N5823

diodes

diodes

diodes

diodes

MBR320

SB520

are

are

are

are

30V

30WQ03

1N5821

rated to

rated to

rated to

rated to

SK33

MBR330

50WQ03

SR503

at least

at least

at least

at least

31DQ03

1N5824

50V.

50V.

50V.

50V.

1N5822

SB530

40V

SK34

SR304

50WQ04

SR504

MBRS340

MBR340

1N5825

30WQ04

MURS320

31DQ04

MUR320

MURS620

SB540

MUR620

50V

SK35

30WF10

SR305

50WF10

HER601

or

MBRS360

MBR350

50WQ05

SB550

More

30WQ05

31DQ05

50SQ080