LM2595T-5.0/NOPB_技术文档

May 1999

LM2595

SIMPLE SWITCHER^®Power Converter 150 kHz

1A Step-Down Voltage Regulator

General Description

Features

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

The LM2595 series of regulators are monolithic integrated

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

(buck) switching regulator, capable of driving a 1A 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 (surface mount)

packages

n Guaranteed 1A output load current

n Input voltage range up to 40V

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 85 µA

n High efficiency

Requiring a minimum number of external components, these

regulators are simple to use and include internal frequency

†

compensation , and a fixed-frequency oscillator.

The LM2595 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. Typically, for output voltages less

than 12V, and ambient temperatures less than 50˚C, no heat

sink is required.

n Uses readily available standard inductors

n Thermal shutdown and current limit protection

Applications

A standard series of inductors are available from several dif-

ferent manufacturers optimized for use with the LM2595 se-

ries. This feature greatly simplifies the design of

switch-mode power supplies.

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

n Efficient pre-regulator for linear regulators

n On-card switching regulators

±

Other features include a guaranteed 4% tolerance on out-

n Positive to negative converter

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 85 µA stand-by cur-

rent. Self protection features include a two stage frequency

reducing current limit for the output switch and an over tem-

perature shutdown for complete protection under fault condi-

tions.

Typical Application (Fixed Output Voltage Versions)

DS012565-1

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

DS012565

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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)

DS012565-2

DS012565-3

Order Number LM2595T-3.3, LM2595T-5.0,

LM2595T-12 or LM2595T-ADJ

Order Number LM2595S-3.3, LM2595S-5.0,

LM2595S-12 or LM2595S-ADJ

See NS Package Number T05D

See NS Package Number TS5B

16-Lead Ceramic Dual-in-Line Package (J)

DS012565-57

Order Number LM2595J-3.3-QML (5962-9687901QEA),

LM2595J-5.0-QML (5962-9650301QEA),

LM2595J-12-QML (5962-9650201QEA),

or LM2595J-ADJ-QML (5962-9650401QEA)

See NS Package Number J16A

For specifications and information about Military-Aerospace products, please see the Mil-Aero web page at

http://www.national.com/appinfo/milaero/index.html.

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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

T Package (Soldering, 10 sec.)

Maximum Junction Temperature

Operating Conditions

Temperature Range

Supply Voltage

−1V

Internally limited

−65˚C to +150˚C

Power Dissipation

−40˚C ≤ T_J≤ +125˚C

Storage Temperature Range

ESD Susceptibility

4.5V to 40V

LM2595-3.3

Electrical Characteristics

=

Specifications with standard type face are for T_J25˚C, and those with boldface type apply over full Operating Tempera-

ture Range.

Symbol

Parameter

Conditions

LM2595-3.3

Limit

Units

(Limits)

Typ

(Note 3)

(Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1

V_OUT

Output Voltage

4.75V ≤ V_IN≤ 40V, 0.1A ≤ I_LOAD≤ 1A

3.3

78

V

3.168/3.135

3.432/3.465

V(min)

V(max)

%

= =

V_IN12V, I_LOAD1A

η

Efficiency

LM2595-5.0

Electrical Characteristics

=

Specifications with standard type face are for T_J25˚C, and those with boldface type apply over full Operating Tempera-

ture Range.

Symbol

Parameter

Conditions

LM2595-5.0

Limit

Units

(Limits)

Typ

(Note 3)

(Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1

V_OUT

Output Voltage

7V ≤ V_IN≤ 40V, 0.1A ≤ I_LOAD≤ 1A

5.0

82

V

4.800/4.750

5.200/5.250

V(min)

V(max)

%

= =

V_IN12V, I_LOAD1A

η

Efficiency

LM2595-12

Electrical Characteristics

=

Specifications with standard type face are for T_J25˚C, and those with boldface type apply over full Operating Tempera-

ture Range.

Symbol

Parameter

Conditions

LM2595-12

Units

(Limits)

Typ

Limit

(Note 3)

(Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1

V_OUT

Output Voltage

15V ≤ V_IN≤ 40V, 0.1A ≤ I_LOAD≤ 1A

12.0

90

V

11.52/11.40

12.48/12.60

V(min)

V(max)

%

= =

V_IN25V, I_LOAD1A

η

Efficiency

3

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LM2595-ADJ

Electrical Characteristics

=

Specifications with standard type face are for T_J25˚C, and those with boldface type apply over full Operating Tempera-

ture Range.

Symbol

Parameter

Conditions

LM2595-ADJ

Units

(Limits)

Typ

Limit

(Note 3)

1.230

(Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1

V_FB

Feedback Voltage

4.5V ≤ V_IN≤ 40V, 0.1A ≤ I_LOAD≤ 1A

V

V_OUTprogrammed for 3V. Circuit of Figure 1

1.193/1.180

1.267/1.280

V(min)

V(max)

%

=

η

Efficiency

V_IN12V, V_OUT3V, I_LOAD1A

78

All Output Voltage Versions

Electrical Characteristics

=

Specifications with standard type face are for T_J25˚C, and those with boldface type apply over full Operating Tempera-

=

ture Range. Unless otherwise specified, V_IN12V for the 3.3V, 5V, and Adjustable version and V_IN24V for the 12V ver-

=

sion. I_LOAD200 mA.

Symbol

Parameter

Conditions

LM2595-XX

Units

(Limits)

Typ

Limit

(Note 3)

10

(Note 4)

DEVICE PARAMETERS

=

I_b

Feedback Bias Current

Adjustable Version Only,V_FB1.3V

nA

nA (max)

kHz

50/100

f_O

Oscillator Frequency

Saturation Voltage

(Note 6)

150

127/110

173/173

kHz(min)

kHz(max)

V

=

V_SAT

DC

I_OUT1A (Notes 7, 8)

1

1.2/1.3

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)

1.5

A

1.2/1.15

2.4/2.6

50

A(min)

A(max)

µA(max)

mA

=

I_L

Output Leakage Current

Quiescent Current

Output 0V

(Notes 7, 9) and (Note 10)

=

Output −1V

2

5

15

10

mA(max)

mA

I_Q

(Note 9)

mA(max)

µA

=

I_STBY

Standby Quiescent

Current

ON/OFF pin 5V (OFF)

(Note 10)

85

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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4

All Output Voltage Versions

Electrical Characteristics ^(Continued)

=

Specifications with standard type face are for T_J25˚C, and those with boldface type apply over full Operating Tempera-

=

ture Range. Unless otherwise specified, V_IN12V for the 3.3V, 5V, and Adjustable version and V_IN24V for the 12V ver-

=

sion. I_LOAD200 mA.

Symbol

Parameter

Conditions

LM2595-XX

Units

(Limits)

Typ

Limit

(Note 3)

5

(Note 4)

ON/OFF CONTROL Test Circuit Figure 1

=

I_H

ON/OFF Pin

Input Current

V_LOGIC2.5V (Regulator OFF)

µA

15

5

µA(max)

µA

=

I_L

V_LOGIC0.5V (Regulator ON)

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 in-

tended 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% produc-

tion 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 sys-

tem performance. When the LM2595 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. The amount of reduction is determined by the severity of current over-

load.

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.

=

40V.

Note 10:

V

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 LM2595S 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

DS012565-13

DS012565-12

DS012565-11

5

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

Switch Saturation

Voltage

Switch Current Limit

Dropout Voltage

DS012565-15

DS012565-16

DS012565-14

Operating

Quiescent Current

Shutdown

Quiescent Current

Minimum Operating

Supply Voltage

DS012565-4

DS012565-5

DS012565-6

ON /OFF Threshold

Voltage

ON /OFF Pin

Current (Sinking)

Switching Frequency

DS012565-9

DS012565-7

DS012565-8

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6

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

Feedback Pin

Bias Current

DS012565-10

Continuous Mode Switching Waveforms

Discontinuous Mode Switching Waveforms

=

V_IN20V, V_OUT5V, I_LOAD1A

=

V_IN20V, V_OUT5V, I_LOAD600 mA

=

= =

68 µH, C_OUT120 µF, C_OUTESR 100 mΩ

L

=

= =

22 µH, C_OUT220 µF, C_OUTESR 50 mΩ

L

DS012565-17

DS012565-18

A: Output Pin Voltage, 10V/div.

B: Inductor Current 0.5A/div.

C: Output Ripple Voltage, 50 mV/div.

A: Output Pin Voltage, 10V/div.

B: Inductor Current 0.5A/div.

C: Output Ripple Voltage, 50 mV/div.

Horizontal Time Base: 2 µs/div.

Load Transient Response for Continuous Mode

Load Transient Response for Discontinuous Mode

=

V_IN20V, V_OUT5V, I_LOAD250 mA to 750 mA

=

V_IN20V, V_OUT5V, I_LOAD250 mA to 750 mA

=

= =

68 µH, C_OUT120 µF, C_OUTESR 100 mΩ

L

=

= =

22 µH, C_OUT220 µF, C_OUTESR 50 mΩ

L

DS012565-19

DS012565-20

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

B: 250 mA to 750 mA Load Pulse

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

B: 250 mA to 750 mA Load Pulse

Horizontal Time Base: 100 µs/div.

Horizontal Time Base: 200 µs/div.

7

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Test Circuit and Layout Guidelines

Fixed Output Voltage Versions

DS012565-22

C

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

IN

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

OUT

D1 — 3A, 40V Schottky Rectifier, 1N5822

L1 — 100 µH, L29

Adjustable Output Voltage Versions

DS012565-23

C

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

IN

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

OUT

D1 — 3A, 40V Schottky Rectifier, 1N5822

L1 — 100 µH, L29

R

C

— 1 kΩ, 1%

1

— 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, ex-

ternal components should be located as close to the

switcher lC as possible using ground plane construction or

single point grounding.

If open core inductors are used, special care must be

taken as to the location and positioning of this type of induc-

tor. Allowing the inductor flux to intersect sensitive feedback,

lC groundpath and C_OUTwiring can cause problems.

When using the adjustable version, special care must be

taken as to the location of the feedback resistors and the as-

sociated 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.)

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8

LM2595 Series Buck Regulator Design Procedure (Fixed Output)

PROCEDURE (Fixed Output Voltage Version)

EXAMPLE (Fixed Output Voltage Version)

Given:

=

V_OUT5V

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

=

V_IN(max) 12V

V_IN(max) Maximum DC Input Voltage

=

I_LOAD(max) Maximum Load Current

I_LOAD(max) 1A

1. Inductor Selection (L1)

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

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

12V respectively.) For all other voltages, see the design pro-

cedure 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 1A vertical line is 68 µH, and the inductor code is

L30.

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 68 µH. From the table in

Figure 8, go to the L30 line and choose an inductor part num-

ber from any of the four manufacturers shown. (In most in-

stance, both through hole and surface mount inductors are

available.)

C. Select an appropriate inductor from the four manufactur-

er’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 47 µF and 330

µF and low ESR solid tantalum capacitors between 56 µF

and 270 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 330 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 1A 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 4.2 or later.

220 µF 25V Panasonic HFQ Series

220 µF 25V Nichicon PL Series

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

more is needed. But, in this example, even a low ESR,

switching grade, 220 µF 10V aluminum electrolytic capacitor

would exhibit approximately 225 mΩ of ESR (see the curve

in Figure 14 for the ESR vs voltage rating). This amount of

ESR would result in relatively high output ripple voltage. To

reduce the ripple to 1% of the output voltage, or less, a ca-

pacitor with a higher voltage rating (lower ESR) should be se-

lected. A 16V or 25V capacitor will reduce the ripple voltage

by approximately half.

Procedure continued on next page.

Example continued on next page.

9

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LM2595 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 LM2595. 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

3A, 20V, 1N5820 Schottky diode will provide the best perfor-

mance, and will not be overstressed even for a shorted out-

put.

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 LM2595 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 results. Ultra-fast recovery diodes typically have re-

verse recovery 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 volt-

age transients from appearing at the input. This capacitor

should be located close to the IC using short leads. In addi-

The important parameters for the Input capacitor are the in-

put 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.

tion, the RMS current rating of the input capacitor should be

1

selected to be at least

⁄2 the DC load current. The capacitor

The RMS current rating requirement for the input capacitor in

manufacturers data sheet must be checked to assure that

this current rating is not exceeded. The curve shown in Fig-

ure 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 1A load, a capacitor with a RMS current

rating of at least 500 mA is needed. The curves shown in Fig-

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

From the curves, locate the 25V line and note which capaci-

tor values have RMS current ratings greater than 500 mA. Ei-

ther a 180 µF or 220 µF, 25V 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

maximum input voltage and it is recommended that they be

surge current tested by the manufacturer.

For a through hole design, a 220 µF/25V electrolytic capaci-

tor (Panasonic HFQ series or Nichicon PL series or equiva-

lent) would be adequate. other types or other manufacturers

capacitors can be used provided the RMS ripple current rat-

ings 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 ca-

pacitor 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

current tested.

For additional information, see section on input capaci-

tors in Application Information section.

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10

LM2595 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

HFQ Series

(µF/V)

Nichicon

AVX TPS

Series

(µF/V)

220/10

100/16

220/10

100/16

220/10

100/16

68/20

Sprague

595D Series

(µF/V)

Voltage Current

Voltage

(V)

5

(µH)

(#)

PL Series

(µF/V)

330/16

270/25

220/35

220/16

150/25

82/35

(V)

(A)

22

33

L24

L23

L31

L30

L13

L21

L20

L28

L31

L30

L29

L21

L19

L31

L30

L36

L35

L21

L19

L26

330/16

270/25

220/25

180/35

220/25

150/35

330/16

220/25

180/35

180/16

120/25

100/25

220/25

180/35

82/25

330/10

270/10

220/10

180/10

220/10

150/16

100/20

270/10

220/10

150/16

120/16

150/16

100/20

68/25

7

1

10

40

6

47

3.3

68

47

0.5

1

10

40

8

68

100

33

330/16

220/25

180/35

120/35

180/16

1200/25

100/25

220/25

120/25

82/25

10

15

40

9

47

68

5

100

68

0.5

1

20

40

15

18

30

40

15

20

40

150

47

68/20

120/20

100/20

68/25

68

68/20

150

220

68

68/20

12

82/25

68/20

180/25

82/25

180/25

82/25

68/20

120/20

100/20

68/25

0.5

150

330

68/20

56/25

68/20

FIGURE 2. LM2595 Fixed Voltage Quick Design Component Selection Table

LM2595 Series Buck Regulator Design Procedure (Adjustable Output)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

Given:

=

V_OUT20V

V_OUTRegulated Output Voltage

=

V_IN(max) 28V

V_IN(max) Maximum Input Voltage

=

I_LOAD(max) 1A

I_LOAD(max) Maximum Load Current

=

Switching Frequency (Fixed at a nominal 150 kHz).

F

Switching Frequency (Fixed at a nominal 150 kHz).

F

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.

=

R₂1k (16.26 − 1) 15.26k, closest 1% value is 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.)

=

R₂15.4 kΩ.

Procedure continued on next page.

Example continued on next page.

11

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LM2595 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

2. Inductor Selection (L1)

EXAMPLE (Adjustable Output Voltage Version)

2. Inductor Selection (L1)

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

•

A. Calculate the inductor Volt microsecond constant

(V • µs), from the following formula:

(E • T),

=

where V_SATinternal switch saturation voltage 1V

=

and V_Ddiode forward voltage drop 0.5V

=

•

34.8 (V µs)

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

it with the E • T number on the vertical axis of the Inductor

Value Selection Guide shown in Figure 7.

B. E

T

•

=

C. I_LOAD(max) 1A

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

the inductance region intersected by the 35 (V • µs) horizon-

tal line and the 1A vertical line is 100 µH, and the inductor

code is L29.

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

identified by an inductance value and an inductor code

(LXX).

E. From the table in Figure 8, locate line L29, and select an

inductor part number from the list of manufacturers part num-

bers.

E. Select an appropriate inductor from the four manufactur-

er’s part numbers listed in Figure 8.

3. Output Capacitor Selection (C_OUT

)

3. Output Capacitor SeIection (C_OUT)

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

tantalum capacitors between 47 µF and 330 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 330 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

section, select a capacitor from the list of through hole elec-

trolytic or surface mount tantalum types from four different

capacitor manufacturers. It is recommended that both the

manufacturers 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 dif-

ferent output voltages, and lists various output capacitors

that will provide the best design solutions.

In this example, through hole aluminum electrolytic capaci-

tors 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 volt-

age ratings are needed to satisfy the low ESR requirements

needed for low output ripple voltage.

82 µF, 35V Panasonic HFQ Series

82 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 capaci-

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

ing 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. Re-

fer to the capacitor manufacturers data sheet for this informa-

tion.

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

4. Feedforward Capacitor (C_FF)

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

ditional capacitor is required. The compensation capacitor is

typically between 50 pF and 10 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 1 nF

capacitor is needed.

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

(Because of the unstable characteristics of ceramic capaci-

tors made with Z5U material, they are not recommended.)

www.national.com

12

LM2595 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

Procedure continued on next page.

EXAMPLE (Adjustable Output Voltage Version)

Example continued on next page.

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 LM2595. 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 pro-

vide the best performance, and in this example a 3A, 40V,

1N5822 Schottky diode would be a good choice. The 3A di-

ode rating is more than adequate and will not be over-

stressed 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 LM2595 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 charac-

teristic may cause instability or EMl problems. Ultra-fast re-

covery diodes typically have reverse recovery times of 50 ns

or less. Rectifiers 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

current rating of the input capacitor should be selected to be

The important parameters for the Input capacitor are the in-

put 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

at least ¹

⁄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 electro-

lytic 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 1A load, a capacitor with a RMS current

rating of at least 500 mA 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 ap-

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

line and note which capacitor values have RMS current rat-

ings greater than 500 mA. Either a 100 µF or 120 µF, 50V ca-

pacitor could be used.

se caution when using a high dielectric constant ceramic ca-

pacitor for input bypassing, because it may cause severe

ringing at the V_INpin.

For a through hole design, a 120 µF/50V electrolytic capaci-

tor (Panasonic HFQ series or Nichicon PL series or equiva-

lent) would be adequate. Other types or other manufacturers

capacitors can be used provided the RMS ripple current rat-

ings 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 ca-

pacitor surge current rting (see Application Information or in-

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

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

current 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 switch-

ing regulators. Switchers Made Simple (version 4.2 or later)

is available on a 3¹⁄

" diskette for IBM compatible computers.

2

13

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LM2595 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

Output

Voltage

(V)

Through Hole Electrolytic Output Capacitor

Surface Mount Tantalum Output Capacitor

Panasonic

HFQ Series

(µF/V)

Nichicon PL

Series

(µF/V)

Feedforward

Capacitor

AVX TPS

Series

(µF/V)

330/6.3

220/10

100/16

68/20

Sprague

595D Series

(µF/V)

Feedforward

Capacitor

1.2

4

330/50

220/25

180/25

120/25

82/35

330/50

220/25

180/25

120/25

82/35

0

330/6.3

220/10

0

4.7 nF

3.3 nF

1.5 nF

1 nF

4.7 nF

3.3 nF

1.5 nF

220 pF

6

220/10

9

180/16

1 2

1 5

2 4

2 8

120/20

68/20

100/20

33/25

33/35

82/50

1 nF

10/35

33/35

FIGURE 3. Output Capacitor and Feedforward Capacitor Selection Table

LM2595 Series Buck Regulator Design Procedure

INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)

DS012565-24

DS012565-26

FIGURE 4. LM2595-3.3

FIGURE 6. LM2595-12

DS012565-27

DS012565-25

FIGURE 7. LM2595-ADJ

FIGURE 5. LM2595-5.0

www.national.com

14

LM2595 Series Buck Regulator Design Procedure (Continued)

Inductance

(µH)

Current

(A)

Renco

Through Hole

Pulse Engineering

Coilcraft

Surface

Mount

Through

Surface

Mount

Surface Mount

Hole

L4

68

47

0.32

0.37

0.44

0.32

0.39

0.48

0.58

0.70

0.83

0.99

1.24

0.42

0.55

0.66

0.82

0.99

1.17

1.40

1.70

0.80

1.00

1.20

1.47

1.78

2.15

RL-1284-68-43

RL-1284-47-43

RL-1284-33-43

RL-5470-3

RL1500-68

RL1500-47

RL1500-33

RL1500-220

RL1500-150

RL1500-100

RL1500-68

RL1500-47

RL1500-33

RL1500-22

RL1500-15

RL1500-330

RL1500-220

RL1500-150

RL1500-100

RL1500-68

—

PE-53804

PE-53805

PE-53806

PE-53809

PE-53810

PE-53811

PE-53812

PE-53813

PE-53814

PE-53815

PE-53816

PE-53817

PE-53818

PE-53819

PE-53820

PE-53821

PE-53822

PE-53823

PE-53824

PE-53826

PE-53827

PE-53828

PE-53829

PE-53830

PE-53935

PE-53804-S

PE-53805-S

PE-53806-S

PE-53809-S

PE-53810-S

PE-53811-S

PE-53812-S

PE-53813-S

PE-53814-S

PE-53815-S

PE-53816-S

PE-53817-S

PE-53818-S

PE-53819-S

PE-53820-S

PE-53821-S

PE-53822-S

PE-53823-S

PE-53824-S

PE-53826-S

PE-53827-S

PE-53828-S

PE-53829-S

PE-53830-S

PE-53935-S

DO1608-68

DO1608-473

DO1608-333

DO3308-224

DO3308-154

DO3308-104

DO3308-683

DO3308-473

DO3308-333

DO3308-223

DO3308-153

DO3316-334

DO3316-224

DO3316-154

DO3316-104

DO3316-683

DO3316-473

DO3316-333

DO3316-223

DO5022P-334

DO5022P-224

DO5022P-154

DO5022P-104

DO5022P-683

—

L5

L6

33

L9

220

150

100

68

L10

L11

L12

L13

L14

L15

L16

L17

L18

L19

L20

L21

L22

L23

L24

L26

L27

L28

L29

L30

L35

RL-5470-4

RL-5470-5

RL-5470-6

47

RL-5470-7

33

RL-1284-33-43

RL-1284-22-43

RL-1284-15-43

RL-5471-1

22

15

330

220

150

100

68

RL-5471-2

RL-5471-3

RL-5471-4

RL-5471-5

47

RL-5471-6

33

RL-5471-7

—

22

RL-1283-22-43

RL-5471-1

—

330

220

150

100

68

—

RL-5471-2

—

RL-5471-3

—

RL-5471-4

—

RL-5471-5

—

47

RL-5473-1

—

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

Phone (619) 674-8100

FAX (619) 674-8262

Phone +353 93 24 107

FAX +353 93 24 459

Phone (800) 645-5828

FAX (516) 586-5562

Coilcraft Inc., Europe

Pulse Engineering Inc.

Pulse Engineering Inc.,

Europe

Renco Electronics Inc.

FIGURE 9. Inductor Manufacturers Phone Numbers

15

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LM2595 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

1A Diodes

3A Diodes

Surface Mount

Through Hole

Surface Mount

Through Hole

Schottky

Ultra Fast

Schottky

Ultra Fast

Schottky

Ultra Fast

Schottky

Ultra Fast

Recovery

SK12

All of

these

diodes

are

rated to

at least

50V.

1N5817

SR102

All of

these

diodes

are

rated to

at least

50V.

All of

these

diodes

are

rated to

at least

50V.

1N5820

SR302

All of

these

diodes

are

rated to

at least

50V.

20V

30V

40V

SK32

MBR320

1N5821

MBR330

31DQ03

1N5822

SR304

SK13

1N5818

SR103

MBRS130

SK33

11DQ03

SK14

MBRS140

10BQ040

10MQ040

MBRS160

10BQ050

10MQ060

1N5819

SR104

SK34

MBRS340

30WQ04

SK35

MBR340

31DQ04

SR305

MURS120

10BF10

11DQ04

SR105

MUR120

MURS320

30WF10

MUR320

30WF10

50V

or

MBR150

11DQ05

MBR360

30WQ05

MBR350

31DQ05

More

FIGURE 11. Diode Selection Table

www.national.com

16

Block Diagram

DS012565-21

FIGURE 12.

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

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 electro-

lytic capacitor will be physically larger than a lower voltage

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

rounding air, and therefore will have a higher RMS current

rating.

+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.

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 85 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 ca-

pacitor’s electrolyte, resulting in eventual failure.

Selecting an input capacitor requires consulting the manu-

facturers data sheet for maximum allowable RMS ripple cur-

rent. For a maximum ambient temperature of 40˚C, a gen-

eral 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 sat-

isfy 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 lo-

cated 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.

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-

The important parameters for the Input capacitor are the

voltage rating and the RMS current rating. Because of the

relatively high RMS currents flowing in a buck regulator’s in-

put capacitor, this capacitor should be chosen for its RMS

17

www.national.com

allowable output ripple voltage, typically 1% to 2% of the out-

put voltage. But if the selected capacitor’s ESR is extremely

low, there is a possibility of an unstable feedback loop, re-

sulting in an oscillation at the output. Using the capacitors

listed in the tables, or similar types, will provide design solu-

tions under all conditions.

Application Information (Continued)

lytic capacitors designed for switching regulator applications.

Other capacitor manufacturers offer similar types of capaci-

tors, but always check the capacitor data sheet.

“Standard” electrolytic capacitors typically have much higher

ESR numbers, lower RMS current ratings and typically have

a shorter operating lifetime.

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.

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.

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 of-

ten can be satisfied with only three or four different capacitor

values and several different voltage ratings. See the quick

design component selection tables in Figure 2 and Figure 3

for typical capacitor values, voltage ratings, and manufactur-

ers capacitor types.

Electrolytic capacitors are not recommended for tempera-

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

FEEDFORWARD CAPACITOR

@

peratures and typically rises 3X

−25˚C and as much as

(Adjustable Output Voltage Version)

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

C

_FF-A Feedforward Capacitor C_FF, shown across R2 in Fig-

Solid tantalum capacitors have a much better ESR spec for

cold temperatures and are recommended for temperatures

below −25˚C.

ure 1 is used when the output 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 de-

sign procedure section.

DS012565-29

FIGURE 14. Capacitor ESR vs Capacitor Voltage Rating

(Typical Low ESR Electrolytic Capacitor)

CATCH DIODE

DS012565-28

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 LM2595 using

short leads and short printed circuit traces.

FIGURE 13. RMS Current Ratings for Low ESR

Electrolytic Capacitors (Typical)

OUTPUT CAPACITOR

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

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

teristic may cause instability or EMI problems. Ultra-fast re-

covery 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.

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

RMS ripple current rating, voltage rating, and capacitance

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

important parameter.

The output capacitor requires an ESR value that has an up-

per and lower limit. For low output ripple voltage, a low ESR

value is needed. This value is determined by the maximum

www.national.com

18

Application Information (Continued)

DS012565-31

FIGURE 16. (∆I_IND) Peak-to-Peak

Inductor Ripple Current (as a Percentage

of the Load Current) vs Load Current

DS012565-30

FIGURE 15. Capacitor ESR Change vs Temperature

INDUCTOR SELECTION

By allowing the percentage of inductor ripple current to in-

crease for low load currents, the inductor value and size can

be kept relatively low.

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.

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.

Inductors are available in different styles such as pot core,

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

terials, such as ferrites and powdered iron. The least expen-

sive, the bobbin, rod or stick core, consists of wire wound on

a ferrite bobbin. This type of construction makes for an inex-

pensive inductor, but since the magnetic flux is not com-

pletely 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 LM2595 (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 out-

put 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 voltages.

To simplify the inductor selection process, an inductor selec-

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

Figure 7). This guide assumes that the regulator is operating

in the continuous mode, and selects an inductor that will al-

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

percentage 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.)

When multiple switching regulators are located on the same

PC board, open core magnetics can cause interference be-

tween two or more of the regulator circuits, especially at high

currents. A toroid or E-core inductor (closed magnetic struc-

ture) should be used in these situations.

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 LM2595. 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.

19

www.national.com

ing in oscillation problems. If very low output ripple voltage is

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

mended. (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 capacitor is also

required to assure good dynamic load response and ripple

reduction. The ESR of this capacitor may be as low as de-

sired, because it is out of the regulator feedback loop. The

photo shown in Figure 17 shows a typical output ripple volt-

age, 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 de-

sign. The peak switch and inductor currents will be higher in

a

discontinuous design, but at these low load currents

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.

(400 mA 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 out-

put pin (switch) waveform can have some damped sinusoi-

dal ringing present. (See Typical Performance Characteris-

tics photo titled Discontinuous Mode Switching Waveforms)

This ringing is normal for discontinuous operation, and is not

caused by feedback loop instabilities. In discontinuous op-

eration, 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 ringing. Nor-

mally this ringing is not a problem, unless the amplitude be-

comes great enough to exceed the input voltage, and even

then, there is very little energy present to cause damage.

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 pro-

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

parallel with the inductor to dampen the ringing. The com-

puter aided design software Switchers Made Simple (ver-

sion 4.3) will provide all component values for continuous

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 re-

mains constant. As the load current increases or decreases,

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 dis-

continuous if the output is lightly loaded. This is a perfectly

acceptable mode of operation.

DS012565-32

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.

DS012565-33

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 volt-

age, the ESR of the output capacitor must be low, however,

caution must be exercised when using extremely low ESR

capacitors because they can affect the loop stability, result-

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-

www.national.com

20

OPEN CORE INDUCTORS

Application Information (Continued)

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.

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 Figure 7 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).

These curves are only correct for continuous mode opera-

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

lect 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 re-

spect to other PC traces may be the problem. To determine

if this is the problem, temporarily raise the inductor away

from the board by several inches and then check circuit op-

eration. If the circuit now operates correctly, then the mag-

netic flux from the open core inductor is causing the problem.

Substituting a closed core inductor such as a torroid 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 negative

traces of the output capacitor should be minimized.

Consider the following example:

=

V_OUT5V, maximum load current of 800 mA

=

V_IN12V, nominal, varying between 10V and 14V.

The selection guide in Figure 5 shows that the vertical line

for a 0.8A load current, and the horizontal line for the 12V in-

put voltage intersect approximately midway between the up-

per and lower borders of the 68 µH inductance region. A 68

µH inductor will allow a peak-to-peak inductor current (∆I_IND

)

to flow that will be a percentage of the maximum load cur-

rent. Referring to Figure 18, follow the 0.8A 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 300 mA p-p).

Sometimes, locating a trace directly beneath a bobbin induc-

tor will provide good results, provided it is exactly in the cen-

ter 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 14V, it approaches the up-

per 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 0.8A, the peak-to-peak in-

ductor ripple current (∆I_IND) is 300 mA with 12V in, and can

range from 340 mA at the upper border (14V in) to 225 mA at

the lower border (10V in).

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 ef-

ficient inductor, and they are used by the millions in many dif-

ferent applications.

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

used to calculate additional information about the switching

regulator circuit.

1. Peak Inductor or peak switch current

THERMAL CONSIDERATIONS

The LM2595 is available in two packages, a 5-pin TO-220

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

The TO-220 package can be used without a heat sink for

ambient temperatures up to approximately 50˚C (depending

on the output voltage and load current). The curves in Figure

19 show the LM2595T junction temperature rises above am-

bient temperature for different input and output voltages. The

data tor these curves was taken with the LM2595T (TO-220

package) operating as a switching regutator 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 some heat sinking, either to the PC board or a small

external heat sink.

2. Minimum load current before the circuit becomes dis-

continuous

=

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

)

=

0.30Ax0.16Ω 48 mV p-p

4. ESR of C_OUT

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,

21

www.national.com

Application Information (Continued)

but with copper areas greater than approximately 3 in², only

small improvements in heat dissipation are realized. If fur-

ther thermal improvements are needed, double sided or mul-

tilayer PC-board with large copper areas are recommended.

The curves shown in Figure 20 show the LM2595S (TO-263

package) junction temperature rise above ambient tempera-

ture with a 1A 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 junc-

tion temperature.

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.

DS012565-34

Circuit Data for Temperature Rise Curve

TO-220 Package (T)

Capacitors

Through hole electrolytic

Inductor

Diode

Through hole, Schott, 68 µH

Through hole, 3A 40V, Schottky

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 compo-

nents 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 mate-

rial 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.

PC board

3 square inches single sided 2 oz. copper

(0.0028")

FIGURE 19. Junction Temperature Rise, TO-220

DS012565-35

Circuit Data for Temperature Rise Curve

TO-263 Package (S)

Capacitors

Surface mount tantalum, molded “D” size

Surface mount, Schott, 68 µH

Inductor

Diode

Surface mount, 3A 40V, Schottky

PC board

3 square inches single sided 2 oz. copper

(0.0028")

FIGURE 20. Junction Temperature Rise, TO-263

www.national.com

22

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

which is different than the turn OFF voltage. The amount of

hysteresis is approximately equal to the value of the output

voltage. If zener voltages greater than 25V are used, an ad-

ditional 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.

DS012565-36

FIGURE 21. Delayed Startup

DS012565-37

DS012565-46

FIGURE 22. Undervoltage Lockout

for Buck Regulator

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

FIGURE 23. Undervoltage Lockout

for Inverting Regulator

DELAYED STARTUP

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 volt-

age 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 cir-

cuit 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.

This example uses the LM2595-5.0 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 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 LM2595 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 Figure 24 applies

the same feature to an inverting circuit. The circuit in Figure

23 features a constant threshold voltage for turn on and turn

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

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.

23

www.national.com

Application Information (Continued)

DS012565-39

This circuit has hysteresis

=

Regulator starts switching at V

13V

8V

IN

Regulator stops switching at V

IN

FIGURE 24. Undervoltage Lockout with Hysteresis for Inverting Regulator

DS012565-40

C

:

—

220 µF/25V Tant. Sprague 595D

120 µF/50V Elec. Panasonic HFQ

22 µF/20V Tant. Sprague 595D

120 µF/25V Elec. Panasonic HFQ

IN

C

OUT

:

—

FIGURE 25. Inverting −5V Regulator with Delayed Startup

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 LM2595 current limit (ap-

prox 1.5A) are needed for at least 2 ms or more, until the out-

put reaches its nominal output voltage. The actual time de-

pends 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 in-

put current needed for startup is now supplied by the input

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

pacitor can be made much larger than normal.

DS012565-41

FIGURE 26. Inverting Regulator Typical Load Current

INVERTING REGULATOR SHUTDOWN METHODS

Because of differences in the operation of the inverting regu-

lator, the standard design procedure is not used to select the

inductor value. In the majority of designs, a 68 µH, 1.5A in-

ductor is the best choice. Capacitor selection can also be

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

in Figure 25 will provide good results in the majority of invert-

ing designs.

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

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 Figure 28.

www.national.com

24

Application Information (Continued)

DS012565-42

FIGURE 27. Inverting Regulator Ground Referenced Shutdown

DS012565-43

FIGURE 28. Inverting Regulator Ground Referenced Shutdown using Opto Device

25

www.national.com

Application Information (Continued)

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

DS012565-44

C

- 150 µF, 50V, Aluminium Electrolytic Nichicon, “PL series”

IN

- 120 µF, 25V Aluminium Electrolytic Nichicon, “PL series”

OUT

D1 - 3A, 40V Schottky Rectifier, 1N5822

L1 - 68 µH, L30, Schott, Through hole

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

DS012565-45

C

- 150 µF, 50V, Aluminium Electrolytic Nichicon, “PL series”

IN

- 120 µF, 25V Aluminium Electrolytic Nichicon, “PL series”

OUT

D1 - 3A, 40V Schottky Rectifier, 1N5822

L1 - 68 µH, L30, Schott, Through hole

R1 - 1 kΩ, 1%

R2 - Use formula in Design Procedure

C

FF

- See Figure 3

FIGURE 29. PC Board Layout

www.national.com

26

Physical Dimensions inches (millimeters) unless otherwise noted

5-Lead TO-220 (T)

Order Number LM2595T-3.3, LM2595T-5.0,

LM2595T-12 or LM2595T-ADJ

NS Package Number T05D

27

www.national.com

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

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

Order Number LM2595S-3.3, LM2595S-5.0,

LM2595S-12 or LM2595S-ADJ

NS Package Number TS5B

www.national.com

28

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

16-Lead Ceramic Dual-in-Line Package (J)

Order Number LM2595J-3.3-QML (5962-9687901QEA),

LM2595J-5.0-QML (5962-9650301QEA),

LM2595J-12-QML (5962-9650201QEA),

or LM2595J-ADJ-QML (5962-9650401QEA)

NS Package Number J16A

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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

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Tel: 1-800-272-9959

Fax: 1-800-737-7018

Email: 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：	LM2595T-5.0/NOPB
厂家：	National Semiconductor
描述：	IC 2.6 A SWITCHING REGULATOR, 173 kHz SWITCHING FREQ-MAX, PZFM5, BENT STAGGERED, TO-220, 5 PIN, Switching Regulator or Controller 局域网开关
文件：	总29页 (文件大小：934K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LM2595T-5.0/NOPB [NSC]

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