Файл: LM2596.pdf

PROCEDURE (Fixed Output Voltage Version)

EXAMPLE (Fixed Output Voltage Version)

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) = 3A

1. Inductor Selection (L1)

1. Inductor Selection (L1)

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

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

ures Figure 4, Figure 5, or Figure 6. (Output voltages of 3.3V,

in Figure 5.

5V, or 12V respectively.) For all other voltages, see the design

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

procedure for the adjustable version.

the inductance region intersected by the 12V horizontal line

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

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

tance region intersected by the Maximum Input Voltage line

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

and the Maximum Load Current line. Each region is identified

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

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

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

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

instance, both through hole and surface mount inductors are

part numbers listed in Figure 8.

available.)

2. Output Capacitor Selection (COUT)

2. Output Capacitor Selection (COUT)

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

A. See section on output capacitors in application infor-

Resistance) electrolytic capacitors between 82 µF and 820 µF

mation section.

and low ESR solid tantalum capacitors between 10 µF and

B. From the quick design component selection table shown in

470 µF provide the best results. This capacitor should be

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

located close to the IC using short capacitor leads and short

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

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

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

For additional information, see section on output capaci-

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

tors in application information section.

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

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

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

quick design component selection table shown in Figure 2.

recommended inductors and capacitors that will provide the

best overall performance.

This table contains different input voltages, output voltages,

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

The capacitor list contains both through hole electrolytic and

pacitors that will provide the best design solutions.

surface mount tantalum capacitors from four different capaci-

C. The capacitor voltage rating for electrolytic capacitors

tor manufacturers. It is recommended that both the manufac-

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

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

be used.

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

D. For computer aided design software, see Switchers Made

different manufacturers are available with the range of ESR

Simple™ version 4.3 or later.

numbers needed.

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.

PROCEDURE (Fixed Output Voltage Version)

EXAMPLE (Fixed Output Voltage Version)

3. Catch Diode Selection (D1)

3. 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. In this example, a 5A,

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

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

supply design must withstand a continuous output short, the

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

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.

4. Input Capacitor (CIN)

4. 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 pin to prevent large voltage

voltage rating and the RMS current rating. With a nominal

transients from appearing at the input. This capacitor should

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

be located close to the IC using short leads. In addition, the

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

RMS current rating of the input capacitor should be selected to

The next higher capacitor voltage rating is 25V.

be at least 1¤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

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

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

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

typical RMS current ratings for several different aluminum

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

electrolytic capacitor values.

13 can be used to select an appropriate input capacitor. From

For an aluminum electrolytic, the capacitor voltage rating

the curves, locate the 35V line and note which capacitor

should be approximately 1.5 times the maximum input volt-

values have RMS current ratings greater than 1.5A. A 680 µF/

age. Caution must be exercised if solid tantalum capacitors

35V capacitor could be used.

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

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

tantalum capacitor voltage rating should be 2 times the maxi-

(Panasonic HFQ series or Nichicon PL series or equivalent)

mum input voltage and it is recommended that they be surge

would be adequate. other types or other manufacturers ca-

current tested by the manufacturer.

pacitors can be used provided the RMS ripple current ratings

Use caution when using ceramic capacitors for input bypass-

are adequate.

ing, because it may cause severe ringing at the VIN pin.

For surface mount designs, solid tantalum capacitors can be

For additional information, see section on input capaci-

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

tors in Application Information section.

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.

LM2596 Series Buck Regulator Design Procedure (Fixed Output)

(Continued)

Conditions

Inductor

Output Capacitor

Through Hole Electrolytic

Surface Mount Tantalum

Output

Load

Max Input

Inductance

Inductor

Panasonic

Nichicon

AVX TPS

Sprague

Voltage

Current

Voltage

(µH)

( #)

HFQ Series

PL Series

Series

595D Series

(V)

(A)

(V)

(µF/V)

(µF/V)

(µF/V)

(µF/V)

3.3

3

5

22

L41

470/25

560/16

330/6.3

390/6.3

7

22

L41

560/35

560/35

330/6.3

390/6.3

10

22

L41

680/35

680/35

330/6.3

390/6.3

40

33

L40

560/35

470/35

330/6.3

390/6.3

6

22

L33

470/25

470/35

330/6.3

390/6.3

2

10

33

L32

330/35

330/35

330/6.3

390/6.3

40

47

L39

330/35

270/50

220/10

330/10

5

3

8

22

L41

470/25

560/16

220/10

330/10

10

22

L41

560/25

560/25

220/10

330/10

15

33

L40

330/35

330/35

220/10

330/10

40

47

L39

330/35

270/35

220/10

330/10

9

22

L33

470/25

560/16

220/10

330/10

2

20

68

L38

180/35

180/35

100/10

270/10

40

68

L38

180/35

180/35

100/10

270/10

12

3

15

22

L41

470/25

470/25

100/16

180/16

18

33

L40

330/25

330/25

100/16

180/16

30

68

L44

180/25

180/25

100/16

120/20

40

68

L44

180/35

180/35

100/16

120/20

15

33

L32

330/25

330/25

100/16

180/16

2

20

68

L38

180/25

180/25

100/16

120/20

40

150

L42

82/25

82/25

68/20

68/25

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

Given:

Given:

VOUT = Regulated Output Voltage

VOUT = 20V

VIN(max) = Maximum Input Voltage

VIN(max) = 28V

ILOAD(max) = Maximum Load Current

ILOAD(max) = 3A

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

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

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

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

shown in Figure 1 )

shown in Figure 1 )

Use the following formula to select the appropriate resistor

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

values.

Select a value for R1 between 240Ω and 1.5 kΩ. The lower

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

R2 = 15.4 kΩ.

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)

2. Inductor Selection (L1)

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

A. Calculate the inductor Volt · microsecond constant

· µs), from the following formula:

(E · T),

where VSAT = internal switch saturation voltage = 1.16V

and VD = diode forward voltage drop = 0.5V

B. E · T = 34.2 (V · µs)

B. Use the E · T value from the previous formula and match it

C. ILOAD(max) = 3A

with the E · T number on the vertical axis of the Inductor Value

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

Selection Guide shown in Figure 7.

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

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

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

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

L39.

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

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

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

inductor part number from the list of manufacturers part num-

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

bers.

part numbers listed in Figure 8.