LM2598T-ADJ_技术文档

January 2001

LM2598

SIMPLE SWITCHER^®Power Converter 150 kHz

1A Step-Down Voltage Regulator, with Features

ternal shutdown is included, featuring typically 85 µA

standby current. Self protection features include a two stage

current limit for the output switch and an over temperature

shutdown for complete protection under fault conditions.

General Description

The LM2598 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.

Features

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

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

This series of switching regulators is similar to the LM2595

series, with additional supervisory and performance features

added.

±

4% max over line and load conditions

n Guaranteed 1A output current

n Available in 7-pin TO-220 and TO-263 (surface mount)

package

Requiring a minimum number of external components, these

regulators are simple to use and include internal frequency

n Input voltage range up to 40V

n Excellent line and load regulation specifications

n 150 kHz fixed frequency internal oscillator

n Shutdown /Soft-start

n Out of regulation error flag

n Error output delay

n Low power standby mode, I_Qtypically 85 µA

n High Efficiency

n Uses readily available standard inductors

n Thermal shutdown and current limit protection

†

compensation , improved line and load specifications,

fixed-frequency oscillator, Shutdown /Soft-start, error flag

delay and error flag output.

The LM2598 series operates at a switching frequency of 150

kHz thus allowing smaller sized filter components than what

would be needed with lower frequency switching regulators.

Available in a standard 7-lead TO-220 package with several

different lead bend options, and a 7-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.

A standard series of inductors (both through hole and sur-

face mount types) are available from several different manu-

facturers optimized for use with the LM2598 series. This

feature greatly simplifies the design of switch-mode power

supplies.

Applications

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 all conditions of input voltage and output

±

load conditions, and 15% on the oscillator frequency. Ex-

Typical Application (Fixed Output Voltage Versions)

DS012593-1

†

Patent Number 5,382,918.

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

DS012593

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Connection Diagrams and Order Information

Bent and Staggered Leads, Through Hole Package

7-Lead TO-220 (T)

Surface Mount Package

7-Lead TO-263 (S)

DS012593-22

DS012593-50

Order Number LM2598S-3.3, LM2598S-5.0,

LM2598S-12 or LM2598S-ADJ

Order Number LM2598T-3.3, LM2598T-5.0,

LM2598T-12 or LM2598T-ADJ

See NS Package Number TS7B

See NS Package Number TA07B

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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.

ESD Susceptibility

Human Body Model (Note 3)

Lead Temperature

2 kV

S Package

Vapor Phase (60 sec.)

Infrared (10 sec.)

+215˚C

+245˚C

+260˚C

+150˚C

Maximum Supply Voltage (V_IN

)

45V

6V

SD/SS Pin Input Voltage (Note 2)

Delay Pin Voltage (Note 2)

Flag Pin Voltage

T Package (Soldering, 10 sec.)

Maximum Junction Temperature

1.5V

−0.3 ≤ V ≤ +45V

−0.3 ≤ V ≤ +25V

Feedback Pin Voltage

Output Voltage to Ground

(Steady State)

Operating Conditions

Temperature Range

Supply Voltage

−1V

Internally limited

−65˚C to +150˚C

−25˚C ≤ T_J≤ +125˚C

Power Dissipation

4.5V to 40V

Storage Temperature Range

LM2598-3.3

Electrical Characteristics

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

ture Range.

Symbol

Parameter

Conditions

LM2598-3.3

Limit

Units

(Limits)

Typ

(Note 4)

(Note 5)

SYSTEM PARAMETERS (Note 6) 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)

%

η

Efficiency

V_IN= 12V, I_LOAD= 1A

LM2598-5.0

Electrical Characteristics

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

ture Range.

Symbol

Parameter

Conditions

LM2598-5.0

Limit

Units

(Limits)

Typ

(Note 4)

(Note 5)

SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1

V_OUT

Output Voltage

7V ≤ V_IN≤ 40V, 0.1A ≤ I_LOAD≤ 1A

5

V

4.800/4.750

5.200/5.250

V(min)

V(max)

%

η

Efficiency

V_IN= 12V, I_LOAD= 1A

82

LM2598-12

Electrical Characteristics

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

ture Range.

Symbol

Parameter

Conditions

LM2598-12

Units

(Limits)

Typ

Limit

(Note 4)

(Note 5)

SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1

V_OUT

Output Voltage

15V ≤ V_IN≤ 40V, 0.1A ≤ I_LOAD≤ 1A

12

90

V

11.52/11.40

12.48/12.60

V(min)

V(max)

%

η

Efficiency

V_IN= 25V, I_LOAD= 1A

3

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LM2598-ADJ

Electrical Characteristics

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

ture Range.

Symbol

Parameter

Conditions

LM2598-ADJ

Units

(Limits)

Typ

Limit

(Note 4)

1.230

(Note 5)

SYSTEM PARAMETERS (Note 6) 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 12.

1.193/1.180

1.267/1.280

V(min)

V(max)

%

η

Efficiency

V_IN= 12V, V_OUT= 3V, I_LOAD= 1A

78

All Output Voltage Versions

Electrical Characteristics

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

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

sion. I_LOAD= 200 mA

Symbol

Parameter

Conditions

LM2598-XX

Units

(Limits)

Typ

Limit

(Note 4)

10

(Note 5)

DEVICE PARAMETERS

I_b

Feedback Bias Current

Adjustable Version Only, V_FB= 1.3V

(Note 7)

nA

nA(max)

kHz

50/100

f_O

Oscillator Frequency

Saturation Voltage

150

127/110

173/173

kHz(min)

kHz(max)

V

V_SAT

DC

I_OUT= 1A (Note 8) (Note 9)

1

1.2/1.3

V(max)

%

Max Duty Cycle (ON)

Min Duty Cycle (OFF)

Current Limit

(Note 9)

100

0

(Note 10)

I_CL

Peak Current, (Note 8) (Note 9)

1.5

A

1.2/1.15

2.4/2.6

50

A(min)

A(max)

µA(max)

mA

I_L

Output Leakage Current

Output = 0V (Note 9) (Note 10) (Note 11)

Output = −1V

2

5

15

10

mA(max)

mA

I_Q

Operating Quiescent

Current

SD /SS Pin Open, (Note 10)

SD /SS pin = 0V, (Note 11)

mA(max)

µA

I_STBY

Standby Quiescent

Current

85

200/250

µA(max)

˚C/W

θ_JC

θ_JA

Thermal Resistance

TO220 or TO263 Package, Junction to Case

TO220 Package, Junction to Ambient (Note 12)

TO263 Package, Junction to Ambient (Note 13)

TO263 Package, Junction to Ambient (Note 14)

TO263 Package, Junction to Ambient (Note 15)

2

50

30

20

˚C/W

SHUTDOWN/SOFT-START CONTROL Test Circuit of Figure 1

V_SD

Shutdown Threshold

Voltage

1.3

V

Low, (Shutdown Mode)

0.6

2

V(max)

V(min)

V

High, (Soft-start Mode)

V_SS

Soft-start Voltage

V_OUT= 20% of Nominal Output Voltage

V_OUT= 100% of Nominal Output Voltage

2

3

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4

All Output Voltage Versions

Electrical Characteristics ^(Continued)

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

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

sion. I_LOAD= 200 mA

Symbol

Parameter

Conditions

LM2598-XX

Units

(Limits)

Typ

Limit

(Note 4)

(Note 5)

SHUTDOWN/SOFT-START CONTROL Test Circuit of Figure 1

I_SD

Shutdown Current

V_SHUTDOWN= 0.5V

5

µA

10

5

µA(max)

µA

I_SS

Soft-start Current

V_Soft-start= 2.5V

1.6

µA(max)

FLAG/DELAY CONTROL Test Circuit of Figure 1

Regulator Dropout Detector Low (Flag ON)

Threshold Voltage

96

%

%(min)

%(max)

V

92

98

VF_SAT

IF_L

Flag Output Saturation

Voltage

I_SINK= 3 mA

V_DELAY= 0.5V

V_FLAG= 40V

0.3

0.7/1.0

V(max)

µA

Flag Output Leakage

Current

0.3

Delay Pin Threshold

Voltage

1.25

V

Low (Flag ON)

1.21

1.29

V(min)

V(max)

µA

High (Flag OFF) and V_OUTRegulated

V_DELAY= 0.5V

Delay Pin Source Current

Delay Pin Saturation

3

6

µA(max)

mV

Low (Flag ON)

55

350/400

mV(max)

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is

intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.

Note 2: Voltage internally clamped. If clamp voltage is exceeded, limit current to a maximum of 1 mA.

Note 3: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.

Note 4: Typical numbers are at 25˚C and represent the most likely norm.

Note 5: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%

production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used

to calculate Average Outgoing Quality Level (AOQL).

Note 6: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2598

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

Note 7: The switching frequency is reduced when the second stage current limit is activated. The amount of reduction is determined by the severity of current

overload.

Note 8: No diode, inductor or capacitor connected to output pin.

Note 9: Feedback pin removed from output and connected to 0V to force the output transistor switch ON.

Note 10: Feedback pin removed from output and connected to 12V for the 3.3V, 5V, and the ADJ. version, and 15V for the 12V version, to force the output transistor

switch OFF.

Note 11: V = 40V.

IN

Note 12: 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 13: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 0.5 in of (1 oz.) copper area.

2

Note 14: 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 15: 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 LM2598S side of the board, and approximately 16 in of copper on the other side of the p-c board. See application hints in this data sheet and the thermal model

in Switchers Made Simple^®version 4.2 software.

5

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Typical Performance Characteristics (Circuit of Figure 1)

Normalized

Output Voltage

Efficiency

Line Regulation

DS012593-14

DS012593-3

DS012593-2

Switch Saturation

Voltage

Switch Current Limit

Dropout Voltage

DS012593-16

DS012593-17

DS012593-15

Operating

Quiescent Current

Shutdown

Quiescent Current

Minimum Operating

Supply Voltage

DS012593-4

DS012593-5

DS012593-6

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6

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

Feedback Pin

Bias Current

Flag Saturation

Voltage

Switching Frequency

DS012593-8

DS012593-49

DS012593-7

Soft-start

Shutdown/Soft-start

Current

Delay Pin Current

DS012593-9

DS012593-11

DS012593-10

Soft-start Response

Shutdown/Soft-start

Threshold Voltage

DS012593-12

DS012593-13

7

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Typical Performance Characteristics (Circuit of Figure 1)

Continuous Mode Switching Waveforms

V_IN= 20V, V_OUT= 5V, I_LOAD= 1A

L = 68 µH, C_OUT= 120 µF, C_OUTESR = 100 mΩ

Discontinuous Mode Switching Waveforms

V_IN= 20V, V_OUT= 5V, I_LOAD= 600 mA

L = 22 µH, C_OUT= 220 µF, C_OUTESR = 50 mΩ

DS012593-18

DS012593-19

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

V_IN= 20V, V_OUT= 5V, I_LOAD= 250 mA to 750 mA

L = 68 µH, C_OUT= 120 µF, C_OUTESR = 100 mΩ

Load Transient Response for Discontinuous Mode

V_IN= 20V, V_OUT= 5V, I_LOAD= 250 mA to 750 mA

L = 22 µH, C_OUT= 220 µF, C_OUTESR = 50 mΩ

DS012593-20

DS012593-21

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.

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8

Test Circuit and Layout Guidelines

Fixed Output Voltage Versions

DS012593-23

Component Values shown are for V = 15V, V

= 5V, I

= 1A.

LOAD

IN

OUT

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

120 µF, 35V Aluminum Electrolytic, Nichicon “PL Series”

3A, 40V Schottky Rectifier, 1N5822

68 µH, L30

Typical Values

*C

:

— 0.1 µF

— 4.7k

SS

C

R

:

DELAY

Pull Up

Adjustable Output Voltage Versions

DS012593-24

where V

= 1.23V

REF

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

1

Component Values shown are for V = 20V,

IN

V

OUT

= 10V, I

= 1A.

LOAD

C

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

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

IN

OUT

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

L1 — 100 µH, L29

R

C

R

— 1 kΩ, 1%

— 7.15k, 1%

— 3.3 nF, See Application Information Section

— 3 kΩ, See Application Information Section

1

2

FF

Typical Values

C

R

— 0.1 µF

SS

— 0.1 µF

DELAY

— 4.7k

PULL UP

FIGURE 1. Standard Test Circuits and Layout Guides

9

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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.

Test Circuit and Layout Guidelines

(Continued)

As in any switching regulator, layout is very important. Rap-

idly switching currents associated with wiring inductance can

generate voltage transients which can cause problems. For

minimal inductance and ground loops, the wires indicated by

heavy lines should be wide printed circuit traces and

should be kept as short as possible. For best results,

external components should be located as close to the

switcher lC as possible using ground plane construction or

single point grounding.

When using the adjustable version, special care must be

taken as to the location of the feedback resistors and the

associated wiring. Physically locate both resistors near the

IC, and route the wiring away from the inductor, especially an

open core type of inductor. (See application section for more

information.)

LM2598 Series Buck Regulator Design Procedure (Fixed Output)

PROCEDURE (Fixed Output Voltage Version)

Given:

EXAMPLE (Fixed Output Voltage Version)

Given:

V_OUT= 5V

_IN(max) = 12V

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

V_IN(max) = Maximum DC Input Voltage

V

I_LOAD(max) = Maximum Load Current

I_LOAD(max) = 1A

1. Inductor Selection (L1)

A. Select the correct inductor value selection guide from

Figures Figure 4, Figure 5, or Figure 6 (Output voltages of

3.3V, 5V, or 12V respectively.) For all other voltages, see the

design procedure for the adjustable version.

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

in Figure 5.

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

ductance region intersected by the Maximum Input Voltage

line and the Maximum Load Current line. Each region is

identified by an inductance value and an inductor code

(LXX).

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

the inductance region intersected by the 12V horizontal line

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

L30.

C. Select an appropriate inductor from the four manufactur-

er’s part numbers listed in Figure 8.

C. The inductance value required is 68 µH. From the table in

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

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

instance, both through hole and surface mount inductors are

available.)

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.

For additional information, see section on output capaci-

tors in application information section.

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10

LM2598 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

PROCEDURE (Fixed Output Voltage Version)

EXAMPLE (Fixed Output Voltage Version)

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

capacitors that will provide the best design solutions.

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.

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.

In this example aluminum electrolytic capacitors from several

different manufacturers are available with the range of ESR

numbers needed.

220 µF 25V Panasonic HFQ Series

220 µF 25V Nichicon PL Series

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 .

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 17 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

capacitor with a higher voltage rating (lower ESR) should be

selected. A 16V or 25V capacitor will reduce the ripple volt-

age by approximately half.

D. For computer aided design software, see Switchers Made

™

Simple (version 4.2 or later).

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 LM2598. 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

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 LM2598 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.

11

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LM2598 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

PROCEDURE (Fixed Output Voltage Version)

4. Input Capacitor (C_IN

EXAMPLE (Fixed Output Voltage Version)

)

4. 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

input voltage rating and the RMS current rating. With a

nominal input voltage of 12V, an aluminum electrolytic ca-

pacitor with a voltage rating greater than 18V (1.5 x V_IN

)

at least ¹⁄

2

the DC load current. The capacitor manufacturers

would be needed. The next higher capacitor voltage rating is

25V.

data sheet must be checked to assure that this current rating

is not exceeded. The curve shown in Figure 16 shows typical

RMS current ratings for several different aluminum electro-

lytic capacitor values.

The RMS current rating requirement for the input capacitor in

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

Figure 16 can be used to select an appropriate input capaci-

tor. From the curves, locate the 25V line and note which

capacitor values have RMS current ratings greater than 500

mA. Either a 180 µF or 220 µF, 25V capacitor could be used.

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.

If solid tantalum input capacitors are used, it is recomended

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

ratings are adequate.

Use caution when using ceramic capacitors for input bypass-

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

For additional information, see section on input capaci-

tors in Application Information section.

For surface mount designs, solid tantalum capacitors are

recommended. The TPS series available from AVX, and the

593D series from Sprague are both surge current tested.

Conditions

Load

Inductor

Output Capacitor

Through Hole Electrolytic

Surface Mount Tantalum

Output

Max Input Inductance Inductor

Panasonic

HFQ Series

(µF/V)

Nichicon

PL Series

(µF/V)

330/16

270/25

220/35

220/16

150/25

82/35

AVX TPS

Series

(µF/V)

220/10

220/16

100/16

220/10

100/16

220/10

100/16

68/20

Sprague

595D Series

(µF/V)

#

Voltage Current

Voltage

(V)

5

(µH)

( )

(V)

3.3

(A)

1

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

10

40

6

47

68

47

0.5

1

10

40

8

68

100

33

5

330/16

220/25

180/35

120/35

180/16

120/25

100/25

220/25

120/25

82/25

10

15

40

9

47

68

100

68

0.5

1

20

40

15

18

30

40

15

20

40

150

47

12

68/20

120/20

100/20

68/25

68

68/20

150

220

68

68/20

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. LM2598 Fixed Voltage Quick Design Component Selection Table

www.national.com

12

LM2598 Series Buck Regulator Design Procedure (Adjustable Output)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

Given:

V_OUT= Regulated Output Voltage

V_OUT= 20V

V_IN(max) = Maximum Input Voltage

V_IN(max) = 28V

I

_LOAD(max) = Maximum Load Current

I_LOAD(max) = 1A

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

1. Programming Output Voltage (Selecting R₁and R₂, as

shown in Figure 1)

Use the following formula to select the appropriate resistor

values.

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

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

R₂= 15.4 kΩ.

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

resistor values minimize noise pickup in the sensitive feed-

back pin. (For the lowest temperature coefficient and the best

stability with time, use 1% metal film resistors.)

2. Inductor Selection (L1)

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

A. Calculate the inductor Volt • microsecond constant

(V • µs), from the following formula:

(E • T),

where V_SAT= internal switch saturation voltage = 1V

and V_D= diode forward voltage drop = 0.5V

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

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

Value Selection Guide shown in Figure 7.

B. E • T = 34.8 (V • µs)

C. I_LOAD(max) = 1A

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).

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.

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

inductor part number from the list of manufacturers part

numbers.

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 82 µF and 220 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 220 µF. For additional informa-

tion, see section on output capacitors in application

information section.

A. See section on C_OUTin Application Information section.

13

www.national.com

LM2598 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

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

quick design table shown in Figure 3. This table contains

different output voltages, and lists various output capacitors

that will provide the best design solutions.

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.

In this example, through hole aluminum electrolytic capaci-

tors from several different manufacturers are available.

82 µF 35V Panasonic HFQ Series

82 µF 35V Nichicon PL Series

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.

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

rating could also be used if a lower output ripple voltage is

needed.

Other manufacturers or other types of capacitors may also

be used, provided the capacitor specifications (especially the

100 kHz ESR) closely match the types listed in the table.

Refer to the capacitor manufacturers data sheet for this

information.

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

4. Feedforward Capacitor (C_FF)

For output voltages greater than approximately 10V, an 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.)

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 LM2598. The most stressful condition for

this diode is an overload or shorted output condition.

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

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

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

diode 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 LM2598 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.

www.national.com

14

LM2598 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

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

input voltage rating and the RMS current rating. With a

nominal input voltage of 28V, an aluminum electrolytic alumi-

num electrolytic capacitor with a voltage rating greater than

42V (1.5 x V_IN) would be needed. Since the the next higher

capacitor voltage rating is 50V, a 50V capacitor should be

used. The capacitor voltage rating of (1.5 x V_IN) is a conser-

vative guideline, and can be modified somewhat if desired.

at least ¹⁄

the DC load current. The capacitor manufacturers

2

data sheet must be checked to assure that this current rating

is not exceeded. The curve shown in Figure 16 shows typical

RMS current ratings for several different aluminum electro-

lytic capacitor values.

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 16 can be used to select an

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

line and note which capacitor values have RMS current

ratings greater than 500 mA. Either a 100 µF or 120 µF, 50V

capacitor could be used.

Use caution when using a high dielectric constant ceramic

capacitor for input bypassing, because it may cause severe

ringing at the V_INpin.

For a through hole design, a 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

ratings are adequate.

For surface mount designs, solid tantalum capacitors can be

used, but caution must be exercised with regard to the

capacitor surge current rating (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

current tested.

For additional information, see section on input capaci-

tor in application information section.

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

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

15

www.national.com

LM2598 Series Buck Regulator Design Procedure

INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)

DS012593-25

DS012593-27

FIGURE 4. LM2598-3.3

FIGURE 6. LM2598-12

DS012593-28

DS012593-26

FIGURE 7. LM2598-ADJ

FIGURE 5. LM2598-5.0

www.national.com

16

LM2598 Series Buck Regulator Design Procedure (Continued)

Inductance

(µH)

Current

(A)

Schott

Through

Renco

Pulse Engineering

Coilcraft

Surface

Mount

Through

Hole

Surface

Mount

Through

Surface

Hole

Mount

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

67143940

67148310

67148320

67143960

67143970

67143980

67143990

67144000

67148340

67148350

67148360

67144030

67144040

67144050

67144060

67144070

67144080

67144090

67148370

67144100

67144110

67144120

67144130

67144140

67144170

67144310

67148420

67148430

67144330

67144340

67144350

67144360

67144380

67148450

67148460

67148470

67144410

67144420

67144430

67144440

67144450

67144460

67144470

67144480

67144490

67144500

67144510

67144520

—

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

17

www.national.com

LM2598 Series Buck Regulator Design Procedure (Continued)

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

Phone +353 93 24 107

FAX +353 93 24 459

Phone (800) 645-5828

FAX (516) 586-5562

Phone (612) 475-1173

FAX (612) 475-1786

FIGURE 9. Inductor Manufacturers Phone Numbers

Pulse Engineering Inc.,

Europe

Renco Electronics Inc.

Schott Corp.

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

1A Diodes

VR

3A Diodes

Surface Mount

Through Hole

Surface Mount

Through Hole

Schottky

Ultra Fast

Schottky

Ultra Fast

Schottky

Ultra Fast

Schottky

Ultra Fast

Recovery

All of these

diodes are

rated to at

least 50V.

Recovery

All of these

diodes are

rated to at

least 50V.

Recovery

All of these

diodes are

rated to at

least 50V.

Recovery

All of these

diodes are

rated to at

least 50V.

20V

30V

40V

SK12

1N5817

SR102

IN5820

SR302

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

more

MBR150

11DQ05

MBRS360

30WQ05

MBR350

31DQ05

FIGURE 11. Diode Selection Table

www.national.com

18

Block Diagram

DS012593-29

FIGURE 12.

Special Note If any of the above three features (Shutdown

/Soft-start, Error Flag, or Delay) are not used, the respective

pins should be left open.

Application Information

PIN FUNCTIONS

+V_IN(Pin 2)—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.

EXTERNAL COMPONENTS

SOFT-START CAPACITOR

C_SS—A capacitor on this pin provides the regulator with a

Soft-start feature (slow start-up). When the DC input voltage

is first applied to the regulator, or when the Shutdown

/Soft-start pin is allowed to go high, a constant current

(approximately 5 µA begins charging this capacitor). As the

capacitor voltage rises, the regulator goes through four op-

erating regions (See the bottom curve in Figure 13).

Ground (Pin 4)—Circuit ground.

Output (Pin 1)—Internal switch. The voltage at this pin

switches between approximately (+V_IN− V_SAT) and approxi-

mately −0.5V, with a duty cycle of 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 (Pin 6)—Senses the regulated output voltage to

complete the feedback loop.

1. Regulator in Shutdown. When the SD /SS pin voltage is

between 0V and 1.3V, the regulator is in shutdown, the

output voltage is zero, and the IC quiescent current is ap-

proximately 85 µA.

Shutdown /Soft-start (Pin 7)—This dual function pin pro-

vides the following features: (a) Allows the switching regula-

tor circuit to be shut down using logic level signals thus

dropping the total input supply current to approximately

85 µA. (b) Adding a capacitor to this pin provides a soft-start

feature which minimizes startup current and provides a con-

trolled ramp up of the output voltage.

2. Regulator ON, but the output voltage is zero. With the

SD /SS pin voltage between approximately 1.3V and 1.8V,

the internal regulator circuitry is operating, the quiescent

current rises to approximately 5 mA, but the output voltage is

still zero. Also, as the 1.3V threshold is exceeded, the

Soft-start capacitor charging current decreases from 5 µA

down to approximately 1.6 µA. This decreases the slope of

capacitor voltage ramp.

Error Flag (Pin 3)—Open collector output that provides a

low signal (flag transistor ON) when the regulated output

voltage drops more than 5% from the nominal output volt-

age. On start up, Error Flag is low until V_OUTreaches 95% of

the nominal output voltage and a delay time determined by

the Delay pin capacitor. This signal can be used as a reset to

a microprocessor on power-up.

3. Soft-start Region. When the SD /SS pin voltage is be-

@

tween 1.8V and 2.8V ( 25˚C), the regulator is in a Soft-start

condition. The switch (Pin 1) duty cycle initially starts out

very low, with narrow pulses and gradually get wider as the

capacitor SD /SS pin ramps up towards 2.8V. As the duty

cycle increases, the output voltage also increases at a con-

trolled ramp up. See the center curve in Figure 13. The input

supply current requirement also starts out at a low level for

Delay (Pin 5)—At power-up, this pin can be used to provide

a time delay between the time the regulated output voltage

reaches 95% of the nominal output voltage, and the time the

error flag output goes high.

19

www.national.com

Application Information (Continued)

the narrow pulses and ramp up in a controlled manner. This

is a very useful feature in some switcher topologies that

require large startup currents (such as the inverting configu-

ration) which can load down the input power supply.

Note: The lower curve shown in Figure 13 shows the Soft-start region from

0% to 100%. This is not the duty cycle percentage, but the output

voltage percentage. Also, the Soft-start voltage range has a negative

temperature coefficient associated with it. See the Soft-start curve in

the electrical characteristics section.

4. Normal operation. Above 2.8V, the circuit operates as a

standard Pulse Width Modulated switching regulator. The

capacitor will continue to charge up until it reaches the

internal clamp voltage of approximately 7V. If this pin is

driven from a voltage source, the current must be limited to

about 1 mA.

If the part is operated with an input voltage at or below the

internal soft-start clamp voltage of approximately 7V, the

voltage on the SD/SS pin tracks the input voltage and can be

disturbed by a step in the voltage. To maintain proper func-

tion under these conditions, it is strongly recommended that

the SD/SS pin be clamped externally between the 3V maxi-

mum soft-start threshold and the 4.5V minimum input volt-

age. Figure 15 is an example of an external 3.7V (approx.)

clamp that prevents a line-step related glitch but does not

interfere with the soft-start behavior of the device.

DS012593-30

FIGURE 13. Soft-start, Delay, Error, Output

DS012593-31

FIGURE 14. Timing Diagram for 5V Output

www.national.com

20

Application Information (Continued)

DS012593-65

FIGURE 15. External 3.7V Soft-Start Clamp

DELAY CAPACITOR

INPUT CAPACITOR

C_DELAY—Provides delay for the error flag output. See the

upper curve in Figure 13, and also refer to timing diagrams in

Figure 14. A capacitor on this pin provides a time delay

between the time the regulated output voltage (when it is

increasing in value) reaches 95% of the nominal output

voltage, and the time the error flag output goes high. A 3 µA

constant current from the delay pin charges the delay ca-

pacitor resulting in a voltage ramp. When this voltage

reaches a threshold of approximately 1.3V, the open collec-

tor error flag output (or power OK) goes high. This signal can

be used to indicate that the regulated output has reached the

correct voltage and has stabilized.

C_IN—A low ESR aluminum or tantalum bypass capacitor is

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

located near the regulator using short leads. This capacitor

prevents large voltage transients from appearing at the in-

put, and provides the instantaneous current needed each

time the switch turns on.

The important parameters for the Input capacitor are the

voltage rating and the RMS current rating. Because of the

relatively high RMS currents flowing in a buck regulator’s

input capacitor, this capacitor should be chosen for its RMS

current rating rather than its capacitance or voltage ratings,

although the capacitance value and voltage rating are di-

rectly related to the RMS current rating.

If, for any reason, the regulated output voltage drops by 5%

or more, the error output flag (Pin 3) immediately goes low

(internal transistor turns on). The delay capacitor provides

very little delay if the regulated output is dropping out of

regulation. The delay time for an output that is decreasing is

approximately a 1000 times less than the delay for the rising

output. For a 0.1 µF delay capacitor, the delay time would be

approximately 50 ms when the output is rising and passes

through the 95% threshold, but the delay for the output

dropping would only be approximately 50 µs.

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

capacitor’s power rating. The RMS current flowing through

the capacitors internal ESR produces power which causes

the internal temperature of the capacitor to rise. The RMS

current rating of a capacitor is determined by the amount of

current required to raise the internal temperature approxi-

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

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

ing air will determine the amount of current the capacitor can

safely sustain. Capacitors that are physically large and have

a large surface area will typically have higher RMS current

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

trolytic capacitor will be physically larger than a lower voltage

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

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

rent rating.

R_{Pull Up}—The error flag output, (or power OK) is the col-

lector of

a NPN transistor, with the emitter internally

grounded. To use the error flag, a pullup resistor to a positive

voltage is needed. The error flag transistor is rated up to a

maximum of 45V and can sink approximately 3 mA. If the

error flag is not used, it can be left open.

FEEDFORWARD CAPACITOR

(Adjustable Output Voltage Version)

C_FF— A Feedforward Capacitor C_FF, shown across R2 in

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

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

compensation to the feedback loop and increases the phase

margin for better loop stability. For C_FFselection, see the

design procedure section.

>

If the output ripple is large ( 5% of the nominal output

voltage), this ripple can be coupled to the feedback pin

through the feedforward capacitor and cause the error com-

parator to trigger the error flag. In this situation, adding a

resistor, R_FF, in series with the feedforward capacitor, ap-

proximately 3 times R1, will attenuate the ripple voltage at

the feedback pin.

21

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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.

Application Information (Continued)

OUTPUT CAPACITOR

C_OUT—An output capacitor is required to filter the output

and provide regulator loop stability. Low impedance or low

ESR Electrolytic or solid tantalum capacitors designed for

switching regulator applications must be used. When select-

ing an output capacitor, the important capacitor parameters

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

RMS ripple current rating, voltage rating, and capacitance

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

important parameter.

DS012593-32

FIGURE 16. RMS Current Ratings for Low

ESR Electrolytic Capacitors (Typical)

The output capacitor requires an ESR value that has an

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

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

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

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

extremely low, there is a possibility of an unstable feedback

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

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

design solutions under all conditions.

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.

DS012593-33

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 17). Often, capacitors with much higher voltage

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

quired for low output ripple voltage.

FIGURE 17. Capacitor ESR vs Capacitor Voltage Rating

(Typical Low ESR Electrolytic Capacitor)

The consequences of operating an electrolytic capacitor

above the RMS current rating is a shortened operating life.

The higher temperature speeds up the evaporation of the

capacitor’s electrolyte, resulting in eventual failure.

The output capacitor for many different switcher designs

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

tor values and several different voltage ratings. See the

quick design component selection tables in Figure 2 and

Figure 3 for typical capacitor values, voltage ratings, and

manufacturers capacitor types.

Selecting an input capacitor requires consulting the manu-

facturers data sheet for maximum allowable RMS ripple

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

general guideline would be to select a capacitor with a ripple

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

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

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

conservative design. The capacitor voltage rating must be at

least 1.25 times greater than the maximum input voltage,

and often a much higher voltage capacitor is needed to

satisfy the RMS current requirements.

Electrolytic capacitors are not recommended for tempera-

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

@

peratures and typically rises 3X

−25˚C and as much as

10X at −40˚C. See curve shown in Figure 18.

Solid tantalum capacitors have a much better ESR spec for

cold temperatures and are recommended for temperatures

below −25˚C.

A graph shown in Figure 16 shows the relationship between

an electrolytic capacitor value, its voltage rating, and the

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

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

lytic capacitors designed for switching regulator applications.

Other capacitor manufacturers offer similar types of capaci-

tors, but always check the capacitor data sheet.

CATCH DIODE

Buck regulators require a diode to provide a return path for

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

a fast diode and must be located close to the LM2598 using

short leads and short printed circuit traces.

“Standard” electrolytic capacitors typically have much higher

ESR numbers, lower RMS current ratings and typically have

a shorter operating lifetime.

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-

Because of their small size and excellent performance, sur-

face mount solid tantalum capacitors are often used for input

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22

Application Information (Continued)

teristic may cause instability or EMI problems. Ultra-fast

recovery diodes typically have reverse recovery times of 50

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

too slow and should not be used.

DS012593-35

FIGURE 19. (∆I_IND) Peak-to-Peak Inductor

Ripple Current (as a Percentage of the

Load Current) vs Load Current

By allowing the percentage of inductor ripple current to

increase for low load currents, the inductor value and size

can be kept relatively low.

DS012593-34

FIGURE 18. Capacitor ESR Change vs Temperature

When operating in the continuous mode, the inductor current

waveform ranges from a triangular to a sawtooth type of

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

value of this current waveform equal to the DC output load

current.

INDUCTOR SELECTION

All switching regulators have two basic modes of operation;

continuous and discontinuous. The difference between the

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

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

normal switching cycle. Each mode has distinctively different

operating characteristics, which can affect the regulators

performance and requirements. Most switcher designs will

operate in the discontinuous mode when the load current is

low.

Inductors are available in different styles such as pot core,

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

materials, such as ferrites and powdered iron. The least

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

wound on a ferrite bobbin. This type of construction makes

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

completely contained within the core, it generates more

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

induce voltages into nearby printed circuit traces, thus caus-

ing problems with both the switching regulator operation and

nearby sensitive circuitry, and can give incorrect scope read-

ings because of induced voltages in the scope probe. Also

see section on Open Core Inductors.

The LM2598 (or any of the Simple Switcher family) can be

used for both continuous or discontinuous modes of opera-

tion.

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

tinuous mode. It offers greater output power, lower peak

switch, inductor and diode currents, and can have lower

output ripple voltage. But it does require larger inductor

values to keep the inductor current flowing continuously,

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

ages.

When multiple switching regulators are located on the same

PC board, open core magnetics can cause interference

between two or more of the regulator circuits, especially at

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

structure) should be used in these situations.

To simplify the inductor selection process, an inductor selec-

tion guide (nomograph) was designed (see Figure 3 through

Figure 6). This guide assumes that the regulator is operating

in the continuous mode, and selects an inductor that will

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

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 19.)

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 LM2598. 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.

23

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ESR capacitors because they can affect the loop stability,

resulting in oscillation problems. If very low output ripple

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

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

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

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

pacitor is also required to assure good dynamic load re-

sponse and ripple reduction. The ESR of this capacitor may

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

feedback loop. The photo shown in Figure 20 shows a

typical output ripple voltage, with and without a post ripple

filter.

Application Information (Continued)

DISCONTINUOUS MODE OPERATION

The selection guide chooses inductor values suitable for

continuous mode operation, but for low current applications

and/or high input voltages, a discontinuous mode design

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

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

third the inductance value needed for a continuous mode

design. The peak switch and inductor currents will be higher

in a discontinuous design, but at these low load currents

(200 mA and below), the maximum switch current will still be

less than the switch current limit.

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.

Discontinuous operation can have voltage waveforms that

are considerable different than a continuous design. The

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

soidal ringing present. (See Typical Perfomance Character-

istics photo titled Discontinuous Mode Switching Wave-

forms) This ringing is normal for discontinuous operation,

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

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

switch or the diode are conducting, and the inductor current

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

energy can circulate between the inductor and the switch/

diode parasitic capacitance causing this characteristic ring-

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

tude becomes great enough to exceed the input voltage, and

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

age.

The voltage spikes are caused by the fast switching action of

the output switch, the diode, and the parasitic inductance of

the output filter capacitor, and its associated wiring. To mini-

mize 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 evaluate

these transients, all contribute to the amplitude of these

spikes.

Different inductor types and/or core materials produce differ-

ent amounts of this characteristic ringing. Ferrite core induc-

tors have very little core loss and therefore produce the most

ringing. The higher core loss of powdered iron inductors

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

in parallel with the inductor to dampen the ringing. The

computer aided design software Switchers Made Simple

(version 4.2) will provide all component values for continu-

ous and discontinuous modes of operation.

DS012593-37

FIGURE 21. Peak-to-Peak Inductor

Ripple Current vs Load Current

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

DS012593-36

voltage). For

a given input and output voltage, the

FIGURE 20. Post Ripple Filter Waveform

peak-to-peak amplitude of this inductor current waveform

remains constant. As the load current increases or de-

creases, the entire sawtooth current waveform also rises

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

waveform is equal to the DC load current.

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.

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

the sawtooth current waveform will reach zero, and the

switcher will smoothly change from a continuous to a discon-

tinuous mode of operation. Most switcher designs (irregard-

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

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

fectly acceptable mode of operation.

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

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

ever, caution must be exercised when using extremely low

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24

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.

In a switching regulator design, knowing the value of the

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

determining a number of other circuit parameters. Param-

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

mum load current before the circuit becomes discontinuous,

output ripple voltage and output capacitor ESR can all be

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

nomographs shown in Figure 4 through 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 21 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).

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

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

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

mine if this is the problem, temporarily raise the inductor

away from the board by several inches and then check

circuit operation. If the circuit now operates correctly, then

the magnetic flux from the open core inductor is causing the

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

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

PC layout may be necessary. Magnetic flux cutting the IC

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

tive traces of the output capacitor should be minimized.

These curves are only correct for continuous mode opera-

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

select the inductor value

Consider the following example:

V_OUT= 5V, maximum load current of 800 mA

V_IN= 12V, 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

input voltage intersect approximately midway between the

upper 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

current. Referring to Figure 21, 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 in-

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

center of the inductor (because the induced voltages cancel

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

other, then problems could arise. If flux problems are

present, even the direction of the inductor winding can make

a difference in some circuits.

As the input voltage increases to 14V, it approaches the

upper border of the inductance region, and the inductor

ripple current increases. Referring to the curve in Figure 21,

This discussion on open core inductors is not to frighten the

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

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

tors are an inexpensive, simple way of making a compact

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

different applications.

it can be seen that for

a load current of 0.8A, the

peak-to-peak inductor 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).

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

2. Minimum load current before the circuit becomes dis-

continuous

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

= 0.3A x 0.16Ω=48 mV p-p

)

4. ESR of C_OUT

DS012593-38

25

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

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

Additional copper area improves the thermal characteristics,

but with copper areas greater than approximately 3 in², only

small improvements in heat dissipation are realized. If fur-

ther thermal improvements are needed, double sided or

multilayer PC-board with large copper areas are recom-

mended.

Application Information (Continued)

Circuit Data for Temperature Rise Curve TO-220

Package (T)

Capacitors Through hole electrolytic

Inductor

Diode

Through hole, Schott, 68 µH

The curves shown in Figure 23 show the LM2598S (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

junction temperature.

Through hole, 3A 40V, Schottky

PC board

3 square inches single sided 2 oz. copper

(0.0028")

FIGURE 22. Junction Temperature Rise, TO-220

For the best thermal performance, wide copper traces and

generous amounts of printed circuit board copper should be

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

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

Large areas of copper provide the best transfer of heat

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

air lowers the thermal resistance even further.

Package thermal resistance and junction temperature rise

numbers are all approximate, and there are many factors

that will affect these numbers. Some of these factors include

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

board temperature. Other factors are, trace width, total

printed circuit copper area, copper thickness, single- or

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

the board. The effectiveness of the PC board to dissipate

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

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

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

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

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

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

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

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

the board.

DS012593-39

Circuit Data for Temperature Rise Curve TO-263

Package (S)

Capacitors Surface mount tantalum, molded “D” size

Inductor

Diode

Surface mount, Schott, 68 µH

Surface mount, 3A 40V, Schottky

PC board

3 square inches single sided 2 oz. copper

(0.0028")

SHUTDOWN /SOFT-START

FIGURE 23. Junction Temperature Rise, TO-263

The circuit shown in Figure 26 is a standard buck regulator

with 24V in, 12V out, 280 mA load, and using a 0.068 µF

Soft-start capacitor. The photo in Figure 24 and Figure 25

show the effects of Soft-start on the output voltage, the input

current, with, and without a Soft-start capacitor. Figure 24

also shows the error flag output going high when the output

voltage reaches 95% of the nominal output voltage. The

reduced input current required at startup is very evident

when comparing the two photos. The Soft-start feature re-

duces the startup current from 1A down to 240 mA, and

delays and slows down the output voltage rise time.

THERMAL CONSIDERATIONS

The LM2598 is available in two packages, a 7-pin TO-220

(T) and a 7-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

22 show the LM2598T junction temperature rises above

ambient temperature for different input and output voltages.

The data for these curves was taken with the LM2598T

(TO-220 package) operating as a switching regulator in an

ambient temperature of 25˚C (still air). These temperature

rise numbers are all approximate and there are many factors

that can affect these temperatures. Higher ambient tempera-

tures require some heat sinking, either to the PC board or a

small external heat sink.

This reduction in start up current is useful in situations where

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

can deliver. In some applications Soft-start can be used to

replace undervoltage lockout or delayed startup functions.

If a very slow output voltage ramp is desired, the Soft-start

capacitor can be made much larger. Many seconds or even

minutes are possible.

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

If only the shutdown feature is needed, the Soft-start capaci-

tor can be eliminated.

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26

Application Information (Continued)

DS012593-41

DS012593-40

FIGURE 25. Output Voltage, Input Current, at Start-Up,

WITHOUT Soft-start

FIGURE 24. Output Voltage, Input Current, Error Flag

Signal, at Start-Up, WITH Soft-start

DS012593-42

FIGURE 26. Typical Circuit Using Shutdown /Soft-start and Error Flag Features

DS012593-43

FIGURE 27. Inverting −5V Regulator With Shutdown and Soft-start

27

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

tively high startup currents required by the inverting topology,

the Soft-start feature shown in Figure 27 is recommended.

Application Information (Continued)

lNVERTING REGULATOR

The circuit in Figure 27 converts a positive input voltage to a

negative output voltage with a common ground. The circuit

operates by bootstrapping the regulators ground pin to the

negative output voltage, then grounding the feedback pin,

the regulator senses the inverted output voltage and regu-

lates it.

Also shown in Figure 27 are several shutdown methods for

the inverting configuration. With the inverting configuration,

some level shifting is required, because the ground pin of the

regulator is no longer at ground, but is now at the negative

output voltage. The shutdown methods shown accept

ground referenced shutdown signals.

This example uses the LM2598-5 to generate a −5V output,

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 28 provides a

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

different input and output voltage conditions.

UNDERVOLTAGE LOCKOUT

Some applications require the regulator to remain off until

the input voltage reaches a predetermined voltage. Figure

29 contains a undervoltage lockout circuit for a buck configu-

ration, while Figure 30 and Figure 31 are for the inverting

types (only the circuitry pertaining to the undervoltage lock-

out is shown). Figure 29 uses a zener diode to establish the

threshold voltage when the switcher begins operating. When

the input voltage is less than the zener voltage, resistors R1

and R2 hold the Shutdown /Soft-start pin low, keeping the

regulator in the shutdown mode. As the input voltage ex-

ceeds the zener voltage, the zener conducts, pulling the

Shutdown /Soft-start pin high, allowing the regulator to begin

switching. The threshold voltage for the undervoltage lockout

feature is approximately 1.5V greater than the zener voltage.

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. In this example, when

converting +20V to −5V, the regulator would see 25V be-

tween the input pin and ground pin. The LM2598 has a

maximum input voltage rating of 40V.

DS012593-45

FIGURE 29. Undervoltage Lockout for a Buck

Regulator

DS012593-44

FIGURE 28. Maximum Load Current for Inverting

Regulator Circuit

Figure 30 and Figure 31 apply the same feature to an

inverting circuit. Figure 30 features a constant threshold

voltage for turn on and turn off (zener voltage plus approxi-

mately one volt). Since the SD /SS pin has an internal 7V

zener clamp, R2 is needed to limit the current into this pin to

approximately 1 mA when Q1 is on. If hysteresis is needed,

the circuit in Figure 31 has a turn ON voltage which is

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

esis is approximately equal to the value of the output

voltage.

An additional diode is 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 closely 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 1N5400 diode could be

used.

Because of differences in the operation of the inverting

regulator, the standard design procedure is not used to

select the inductor value. In the majority of designs, a 68 µH,

1.5 Amp inductor is the best choice. Capacitor selection can

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

shown in Figure 27 will provide good results in the majority of

inverting designs.

This type of inverting regulator can require relatively large

amounts of input current when starting up, even with light

loads. Input currents as high as the LM2598 current limit

(approximately 1.5A) are needed for 2 ms or more, until the

output reaches its nominal output voltage. The actual time

depends on the output voltage and the size of the output

capacitor. Input power sources that are current limited or

sources that can not deliver these currents without getting

DS012593-47

FIGURE 30. Undervoltage Lockout Without

Hysteresis for an Inverting Regulator

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28

Application Information (Continued)

DS012593-46

FIGURE 31. Undervoltage Lockout With

Hysteresis for an Inverting Regulator

NEGATIVE VOLTAGE CHARGE PUMP

Occasionally a low current negative voltage is needed for

biasing parts of a circuit. A simple method of generating a

negative voltage using a charge pump technique and the

switching waveform present at the OUT pin, is shown in

Figure 32. This unregulated negative voltage is approxi-

mately equal to the positive input voltage (minus a few volts),

and can supply up to a 200 mA of output current. There is a

requirement however, that there be a minimum load of sev-

eral hundred mA on the regulated positive output for the

charge pump to work correctly. Also, resistor R1 is required

to limit the charging current of C1 to some value less than

the LM2598 current limit (typically 1.5A).

This method of generating a negative output voltage without

an additional inductor can be used with other members of

the Simple Switcher Family, using either the buck or boost

topology.

DS012593-48

FIGURE 32. Charge Pump for Generating a

Low Current, Negative Output Voltage

29

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Application Information (Continued)

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

PLATED

DS012593-51

C_IN— 150 µF/50V Aluminum Electrolytic, Panasonic “HFQ series”

C_OUT— 120 µF/25V Aluminum Electrolytic, Panasonic “HFQ series”

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

R_PULL-UP— 10 kΩ

C_DELAY— 0.1 µF

C_SD/SS— 0.1 µF

L1 — 68 µH, L30, Renco, Through hole

FIGURE 33. Fixed Output PC Board Layout

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30

Application Information (Continued)

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

THROUGH HOLE PLATED

DS012593-52

C_IN— 150 µF/50V, Aluminum Electrolytic, Panasonic “HFQ series”

C_OUT— 120 µF/25V Aluminum Electrolytic, Panasonic “HFQ series”

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

L1 — 68 µH, L30, Renco, Through hole

C_FF— See Figure 4.

R_FF— See Application Information Section (C_FFSection)

R_PULL-UP— 10 kΩ

C_DELAY— 0.1 µF

R1 — 1 kΩ, 1%

C_SD/SS— 0.1 µF

R2 — Use formula in Design Procedure

FIGURE 34. Adjustable Output PC Board Layout

31

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Physical Dimensions inches (millimeters) unless otherwise noted

7-Lead TO-220 (T)

Order Number LM2598T-3.3, LM2598T-5.0,

LM2598T-12 or LM2598T-ADJ

NS Package Number TA07B

www.national.com

32

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

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

Order Number LM2598S-3.3, LM2598S-5.0, LM2598S-12 or LM2598S-ADJ

NS Package Number TS7B

LIFE SUPPORT POLICY

NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT

DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL

COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or

systems which, (a) are intended for surgical implant

into the body, or (b) support or sustain life, and

whose failure to perform when properly used in

accordance with instructions for use provided in the

labeling, can be reasonably expected to result in a

significant injury to the user.

2. A critical component is any component of a life

support device or system whose failure to perform

can be reasonably expected to cause the failure of

the life support device or system, or to affect its

safety or effectiveness.

National Semiconductor

Corporation

Americas

Tel: 1-800-272-9959

Fax: 1-800-737-7018

Email: support@nsc.com

National Semiconductor

Europe

National Semiconductor

Asia Pacific Customer

Response Group

Tel: 65-2544466

Fax: 65-2504466

National Semiconductor

Japan Ltd.

Tel: 81-3-5639-7560

Fax: 81-3-5639-7507

Fax: +49 (0) 180-530 85 86

Email: europe.support@nsc.com

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Email: ap.support@nsc.com

www.national.com

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：	LM2598T-ADJ
厂家：	National Semiconductor
描述：	SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator, with Features SIMPLE SWITCHER系列电源转换器150千赫1A降压型稳压器，具有特色转换器稳压器开关式稳压器或控制器电源电路开关式控制器局域网
文件：	总33页 (文件大小：849K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LM2598T-ADJ [NSC]

相关型号：

LM2598T-ADJ/NOPB

LM2599

LM2599

LM2599S-12

LM2599S-12/NOPB

LM2599S-3.3

LM2599S-3.3

LM2599S-3.3/NOPB

LM2599S-5.0

LM2599S-5.0

LM2599S-5.0/NOPB

LM2599S-ADJ