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LM2598

SNVS125D –MARCH 1998–REVISED MAY 2016

®

LM2598 SIMPLE SWITCHER Power Converter 150-kHz 1-A Step-Down Voltage Regulator,

With Features

1 Features

3 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 1-A load with excellent line and load

regulation. These devices are available in fixed output

voltages of 3.3 V, 5 V, 12 V, and an adjustable output

version.

1

•

3.3-V, 5-V, 12-V, and Adjustable Output Versions

Adjustable Version Output Voltage Range, 1.2-V

to 37-V ±4% Max Over Line and Load Conditions

•

1-A Output Current

Available in 7-Pin TO-220 and DDPAK (Surface

Mount) Package

The LM2598 is a member of the LM259x family.

•

Input Voltage Range Up to 40 V

Excellent Line and Load Regulation Specifications

150 kHz Fixed Frequency Internal Oscillator

Shutdown/Soft-start

Requiring

a

minimum number of external

components, these regulators are simple to use and

include internal frequency compensation, improved

line and load specifications, fixed-frequency oscillator,

Shutdown/Soft-start, error flag delay and error flag

output.

Out of Regulation Error Flag

Error Output Delay

Low Power Standby Mode, I_Q, Typically 85 μA

High Efficiency

The LM2598 series operates at a switching frequency

of 150 kHz, thus allowing smaller sized filter

components than what would be required with lower-

Uses Readily Available Standard Inductors

Thermal Shutdown and Current Limit Protection

frequency switching regulators. Available in

a

standard 7-lead TO-220 package with several

different lead bend options, and a 7-lead DDPAK

surface mount package. Typically, for output voltages

less than 12 V, and ambient temperatures less than

50°C, no heat sink is required.

2 Applications

•

Simple High-Efficiency Step-down (Buck)

Regulator

Device Information⁽¹⁾

•

Efficient Preregulator for Linear Regulators

On-Card Switching Regulators

PART NUMBER

PACKAGE

TO-220 (7)

TO-263 (7)

BODY SIZE (NOM)

14.986 mm × 10.16 mm

10.10 mm × 8.89 mm

Positive to Negative Converter

LM2598

(1) For all available packages, see the orderable addendum at

the end of the data sheet.

Typical Application

Fixed output voltage versions

1

An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,

intellectual property matters and other important disclaimers. PRODUCTION DATA.

LM2598

SNVS125D –MARCH 1998–REVISED MAY 2016

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Table of Contents

8.1 Overview ................................................................. 11

8.2 Functional Block Diagram ....................................... 11

8.3 Feature Description................................................. 11

8.4 Device Functional Modes........................................ 16

Application and Implementation ........................ 17

9.1 Application Information............................................ 17

9.2 Typical Application .................................................. 28

1

2

3

4

5

6

7

Features.................................................................. 1

Applications ........................................................... 1

Description ............................................................. 1

Revision History..................................................... 2

Description (continued)......................................... 3

Pin Configuration and Functions......................... 3

Specifications......................................................... 4

7.1 Absolute Maximum Ratings ..................................... 4

7.2 ESD Ratings.............................................................. 4

7.3 Recommended Operating Conditions....................... 4

7.4 Thermal Information.................................................. 4

7.5 Electrical Characteristics – 3.3-V Version................. 5

7.6 Electrical Characteristics – 5-V Version.................... 5

7.7 Electrical Characteristics – 12-V Version.................. 5

9

10 Power Supply Recommendations ..................... 37

11 Layout................................................................... 37

11.1 Layout Guidelines ................................................. 37

11.2 Layout Examples................................................... 37

11.3 Thermal Considerations........................................ 38

12 Device and Documentation Support ................. 40

12.1 Community Resources.......................................... 40

12.2 Trademarks........................................................... 40

12.3 Electrostatic Discharge Caution............................ 40

12.4 Glossary................................................................ 40

7.8 Electrical Characteristics – Adjustable Voltage

Version....................................................................... 6

7.9 Electrical Characteristics – All Output Voltage

Versions ..................................................................... 6

13 Mechanical, Packaging, and Orderable

7.10 Typical Characteristics............................................ 8

Information ........................................................... 40

8

Detailed Description ............................................ 11

4 Revision History

NOTE: Page numbers for previous revisions may differ from page numbers in the current version.

Changes from Revision C (April 2013) to Revision D

Page

•

Added ESD Ratings table, Feature Description section, Device Functional Modes, Application and Implementation

section, Power Supply Recommendations section, Layout section, Device and Documentation Support section, and

Mechanical, Packaging, and Orderable Information section. ................................................................................................. 1

•

Removed all references to design software Switchers Made Simple .................................................................................... 1

Changes from Revision B (April 2013) to Revision C

Page

•

Changed layout of National Semiconductor Data Sheet to TI format .................................................................................. 38

2

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5 Description (continued)

A standard series of inductors (both through hole and surface mount types) are available from several different

manufacturers optimized for use with the LM2598. This feature greatly simplifies the design of switch-mode

power supplies.

Other features include a specified ±4% tolerance on output voltage under all conditions of input voltage and

output load conditions, and ±15% on the oscillator frequency. External shutdown is included, featuring typically

85-μA standby current. Self-protection features include a two stage current limit for the output switch and an

overtemperature shutdown for complete protection under fault conditions.

6 Pin Configuration and Functions

NDZ Package

KTW Package

7-Pin TO-220

7-Pin TO-263

Top View

Pin Functions

I/O

DESCRIPTION

NO.

NAME

Internal switch. The voltage at this pin switches between approximately (+V_IN– V_SAT) and

approximately –0.5 V, with a duty cycle of V_OUT/ V_IN. To minimize coupling to sensitive

circuitry, the PCB copper area connected to this pin must be kept to a minimum.

1

Output

O

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 required by the regulator.

2

3

+V_IN

I

Open collector output that provides a low signal (flag transistor ON) when the regulated output

voltage drops more than 5% from the nominal output voltage. 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.⁽¹⁾

Error Flag

O

4

5

6

Ground

Delay

—

O

I

Circuit ground.

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.⁽¹⁾

Feedback

Senses the regulated output voltage to complete the feedback loop.

This dual function pin provides the following features: (a) Allows the switching regulator circuit

to be shut down using logic level signals thus dropping the total input supply current to

approximately 80 μA. (b) Adding a capacitor to this pin provides a soft-start feature which

minimizes start-up current and provides a controlled ramp up of the output voltage.⁽¹⁾

Shutdown/Soft-

start

7

I

(1) If any of the above three features (Shutdown/Soft-start, Error Flag, or Delay) are not used, the respective pins must be left open.

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7 Specifications

7.1 Absolute Maximum Ratings

over operating free-air temperature range (unless otherwise noted)⁽¹⁾⁽²⁾

MIN

MAX

45

UNIT

V

Maximum supply voltage, V_IN

SD/SS pin input voltage⁽³⁾

Delay pin voltage⁽³⁾

6

V

1.5

45

V

Flag pin voltage

–0.3

V

Feedback pin voltage

25

V

Output voltage to ground (steady state)

Power dissipation

–1

V

Internally limited

215

Vapor phase (60 s)

Infrared (10 s)

KTW package

Lead temperature

245

260

150

°C

NDZ package (soldering, 10 s)

Maximum junction temperature

Storage temperature, T_stg

°C

–65

(1) Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings

only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended

Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.

(2) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and

specifications.

(3) Voltage internally clamped. If clamp voltage is exceeded, limit current to a maximum of 1 mA.

7.2 ESD Ratings

VALUE

UNIT

V_(ESD)

Electrostatic discharge

Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001⁽¹⁾⁽²⁾

±2000

V

(1) JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.

(2) The human body model is a 100-pF capacitor discharged through a 1.5k resistor into each pin.

7.3 Recommended Operating Conditions

MIN

MAX

40

UNIT

Supply voltage

Temperature

4.5

V

–25

125

°C

7.4 Thermal Information

LM2598

THERMAL METRIC⁽¹⁾

KTW (TO-263)

NDZ (TO-220)

UNIT

7 PINS

—

7 PINS

See⁽⁴⁾

50

—

2

See⁽⁵⁾

See⁽⁶⁾

See⁽⁷⁾

50

R_θJA

Junction-to-ambient thermal resistance⁽²⁾⁽³⁾

°C/W

30

20

R_θJC(top)

Junction-to-case (top) thermal resistance

2

(1) For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application

report, SPRA953.

(2) The package thermal impedance is calculated in accordance to JESD 51-7.

(3) Thermal Resistances were simulated on a 4 -layer, JEDEC board.

(4) Junction to ambient thermal resistance (no external heat sink) for the package mounted TO-220 package mounted vertically, with the

leads soldered to a printed circuit board with (1 oz.) copper area of approximately 1 in².

(5) Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 0.5 in²of (1

oz.) copper area.

(6) Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 2.5 in²of (1

oz.) copper area.

(7) Junction to ambient thermal resistance with the TO-263 package tab soldered to a double sided printed circuit board with 3 in²of (1 oz.)

copper area on the LM2598S side of the board, and approximately 16 in²of copper on the other side of the PCB.

4

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7.5 Electrical Characteristics – 3.3-V Version

Specifications are for T_J= 25°C, unless otherwise specified.

(1)

PARAMETER

TEST CONDITIONS

MIN⁽¹⁾

TYP⁽²⁾

MAX

UNIT

SYSTEM PARAMETERS⁽³⁾(see Figure 42 and Figure 45 for test circuits)

T_J= 25°C

3.168

3.135

3.3

3.432

3.465

4.75 V ≤ V_IN≤ 40 V,

0.1 A ≤ I_LOAD≤ 1 A

V_OUT

Output voltage

Efficiency

V

Over full operating

temperature range

η

V_IN= 12 V, I_LOAD= 1 A

78%

(1) All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard

Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).

(2) Typical numbers are at 25°C and represent the most likely norm.

(3) 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 42 and Figure 45, system performance is as shown in system parameters of Electrical

Characteristics.

7.6 Electrical Characteristics – 5-V Version

Specifications are for T_J= 25°C, unless otherwise specified.

PARAMETER

TEST CONDITIONS

MIN⁽¹⁾

TYP⁽²⁾

MAX⁽¹⁾

UNIT

SYSTEM PARAMETERS⁽³⁾(see Figure 42 and Figure 45 for test circuits)

T_J= 25°C

4.8

5

5.2

7 V ≤ V_IN≤ 40 V,

0.1 A ≤ I_LOAD≤ 1 A

V_OUT

Output voltage

V

Over full operating

temperature range

4.75

5.25

η

Efficiency

V_IN= 12 V, I_LOAD= 1 A

82%

(1) All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard

Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).

(2) Typical numbers are at 25°C and represent the most likely norm.

(3) 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 42 and Figure 45, system performance is as shown in system parameters of Electrical

Characteristics.

7.7 Electrical Characteristics – 12-V Version

Specifications are for T_J= 25°C, unless otherwise specified.

PARAMETER

TEST CONDITIONS

MIN⁽¹⁾

TYP⁽²⁾

MAX⁽¹⁾

UNIT

SYSTEM PARAMETERS⁽³⁾(see Figure 42 and Figure 45 for test circuits)

T_J= 25°C

11.52

11.4

12

12.48

12.6

15 V ≤ V_IN≤ 40 V,

0.1 A ≤ I_LOAD≤ 1 A

V_OUT

Output voltage

Efficiency

V

Over full operating

temperature range

η

V_IN= 25 V, I_LOAD= 1 A

90%

(1) All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard

Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).

(2) Typical numbers are at 25°C and represent the most likely norm.

(3) 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 42 and Figure 45, system performance is as shown in system parameters of Electrical

Characteristics.

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7.8 Electrical Characteristics – Adjustable Voltage Version

Specifications are for T_J= 25°C, unless otherwise specified.

PARAMETER

TEST CONDITIONS

MIN⁽¹⁾

TYP⁽²⁾

MAX⁽¹⁾UNIT

SYSTEM PARAMETERS⁽³⁾(see Figure 42 and Figure 45 for test circuits)

4.5 V ≤ V_IN≤ 40 V, 0.1 A ≤ I_LOAD≤ 1 A

1.23

T_J= 25°C

1.193

1.18

1.267

V

V_OUTprogrammed for 3 V,

circuit of Figure 42 and

Figure 45

V_FB

Feedback voltage

Efficiency

Over full operating

temperature range

1.28

η

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

78%

(1) All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard

Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).

(2) Typical numbers are at 25°C and represent the most likely norm.

(3) 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 42 and Figure 45, system performance is as shown in system parameters of Electrical

Characteristics.

7.9 Electrical Characteristics – All Output Voltage Versions

Specifications are for T_J= 25°C unless otherwise noted. Unless otherwise specified, V_IN= 12 V for the 3.3-V, 5-V, and

Adjustable version and V_IN= 24 V for the 12-V version. I_LOAD= 500 mA

PARAMETER

TEST CONDITIONS

MIN⁽¹⁾

TYP⁽²⁾MAX⁽¹⁾UNIT

DEVICE PARAMETERS

T_J= 25°C

10

150

1

50

100

173

1.2

Adjustable version only,

V_FB= 1.3 V

Feedback bias

current

I_b

nA

kHz

V

Over full operating

temperature range

T_J= 25°C

127

110

f_O

Oscillator frequency

See⁽³⁾

Over full operating

temperature range

T_J= 25°C

(4)(5)

V_SAT

DC

I_CL

Saturation voltage

I_OUT= 1 A

Over full operating

temperature range

1.3

Max duty cycle (ON) See⁽⁵⁾

100%

0%

Minimum duty cycle

See⁽⁶⁾

(OFF)

T_J= 25°C

1.2

1.5

2.4

2.6

Current limit

Peak current⁽⁴⁾⁽⁵⁾

A

Over full operating

temperature range

1.15

Output = 0 V, see^(4)(6)(7)

Output = –1 V

50

15

μA

Output leakage

current

I_L

2

5

mA

Operating quiescent

current

I_Q

SD/SS pin open⁽⁶⁾

10

200

250

mA

T_J= 25°C

85

Current standby

quiescent

I_STBY

SD/SS pin = 0 V⁽⁷⁾

μA

Over full operating

temperature range

(1) All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard

Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).

(2) Typical numbers are at 25°C and represent the most likely norm.

(3) 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.

(4) No diode, inductor or capacitor connected to output pin.

(5) Feedback pin removed from output and connected to 0 V to force the output transistor switch ON.

(6) Feedback pin removed from output and connected to 12 V for the 3.3-V, 5-V, and the Adjustable version, and 15 V for the 12-V version,

to force the output transistor switch OFF.

(7) V_IN= 40 V.

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Electrical Characteristics – All Output Voltage Versions (continued)

Specifications are for T_J= 25°C unless otherwise noted. Unless otherwise specified, V_IN= 12 V for the 3.3-V, 5-V, and

Adjustable version and V_IN= 24 V for the 12-V version. I_LOAD= 500 mA

PARAMETER

TEST CONDITIONS

MIN⁽¹⁾

TYP⁽²⁾MAX⁽¹⁾UNIT

SHUTDOWN AND SOFT-START CONTROL (see Figure 42 and Figure 45 for test circuits)

T_J= 25°C

1.3

Low, (Shutdown Mode), over full operating temperature

range

Shutdown threshold

voltage

0.6

V

V_SD

High, (Soft-start Mode), over full operating temperature

range

2

V_OUT= 20% of nominal output voltage

V_OUT= 100% of nominal output voltage

V_SHUTDOWN= 0.5 V

2

V

3

V_SS

Soft-start voltage

I_SD

I_SS

Shutdown current

Soft-start current

5

10

5

μA

V_Soft-start= 2.5 V

1.6

FLAG AND DELAY CONTROL (see Figure 42 and Figure 45 for test circuits)

Regulator dropout

detector threshold

voltage

Low (Flag ON)

I_SINK= 3 mA

92%

96%

0.3

98%

V

Voltage flag output

saturation

T_J= 25°C

0.7

1

VF_SAT

V_DELAY= 0.5 V

V_FLAG= 40 V

Over full operating

temperature range

Flag output leakage

current

IF_L

0.3

μA

1.25

V

Voltage delay pin

threshold

Low (Flag ON)

1.21

V

High (Flag OFF) and V_OUTRegulated

1.29

6

Delay pin source

current

V_DELAY= 0.5 V

3

μA

T_J= 25°C

55

350

400

Delay pin saturation

Low (Flag ON)

mV

Over full operating

temperature range

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7.10 Typical Characteristics

Circuit of Figure 45

Figure 1. Normalized Output Voltage

Figure 2. Line Regulation

Figure 4. Switch Saturation Voltage

Figure 6. Dropout Voltage

Figure 3. Efficiency

Figure 5. Switch Current Limit

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Typical Characteristics (continued)

Circuit of Figure 45

Figure 7. Operating Quiescent Current

Figure 8. Shutdown Quiescent Current

Figure 10. Feedback Pin Bias Current

Figure 12. Switching Frequency

Figure 9. Minimum Operating Supply Voltage

Figure 11. Flag Saturation Voltage

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Typical Characteristics (continued)

Circuit of Figure 45

Figure 13. Soft-start

Figure 14. Shutdown/Soft-start Current

Figure 16. Soft-start Response

Figure 15. Delay Pin Current

Figure 17. Shutdown and Soft-start Threshold Voltage

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8 Detailed Description

8.1 Overview

The LM2598 SIMPLE SWITCHER^®regulator is an easy-to-use, nonsynchronous, step-down DC-DC converter

with a wide input voltage range up to 40 V. The regulator is capable of delivering up to 1-A DC load current with

excellent line and load regulation. These devices are available in fixed output voltages of 3.3-V, 5-V, 12-V and an

adjustable output version. The family requires few external components, and the pin arrangement was designed

for simple, optimum PCB layout.

8.2 Functional Block Diagram

8.3 Feature Description

8.3.1 SHUTDOWN and Soft-Start

The circuit shown in Figure 20 is a standard buck regulator with 24-V_IN, 12-V_OUT, 280-mA load, and using a

0.068-μF soft-start capacitor. The photo in Figure 18 and Figure 19 show the effects of Soft-start on the output

voltage, the input current, with, and without a soft-start capacitor. Figure 18 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 start-up is very evident when comparing the two photos. The Soft-start feature reduces the start-up

current from 1 A down to 240 mA, and delays and slows down the output voltage rise time.

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

start-up 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.

If only the shutdown feature is required, the Soft-start capacitor can be eliminated.

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Feature Description (continued)

Figure 18. Output Voltage, Input Current, and Error Flag

Signal at Start-Up With Soft-start

Figure 19. Output Voltage and Input Current at Start-Up

Without Soft-start

Figure 20. Typical Circuit Using Shutdown/Soft-start and Error Flag Features

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Feature Description (continued)

Figure 21. Inverting –5-V Regulator With Shutdown and Soft-start

8.3.2 Inverting Regulator

The circuit in Figure 21 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 regulates it.

This example uses the LM2598-5 to generate a –5-V output, but other output voltages are possible by selecting

other output voltage versions, including the adjustable version. Because 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 22 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 40 V. In this example, when converting 20 V to –5 V, the regulator would

see 25 V between the input pin and ground pin. The LM2598 has a maximum input voltage rating of 40 V.

Figure 22. Maximum Load Current for Inverting

Regulator Circuit

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Feature Description (continued)

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 providing good closed loop stability.

A Schottky diode is recommended 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-A inductor is the best choice. Capacitor

selection can also be narrowed down to just a few values. Using the values shown in Figure 21 provides 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 1.5 A) are required 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 loaded down, may not work correctly. Because of the relatively high start-up currents

required by the inverting topology, the soft-start feature shown in Figure 21 is recommended.

Also shown in Figure 21 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.

8.3.3 Undervoltage Lockout

Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage.

Figure 23 shows an undervoltage lockout feature applied to a buck regulator, while Figure 24 and Figure 25 are

for the inverting types (only the circuitry pertaining to the undervoltage lockout is shown). Figure 23 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 or Soft-start pin low, keeping the regulator in the

shutdown mode. As the input voltage exceeds 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 1.5 V greater than the Zener voltage.

Figure 23. Undervoltage Lockout for a Buck Regulator

Figure 24 and Figure 25 apply the same feature to an inverting circuit. Figure 24 features a constant threshold

voltage for turnon and turnoff (Zener voltage plus approximately 1 V). Because the SD/SS pin has an internal 7-V

zener clamp, R2 is required to limit the current into this pin to approximately 1 mA when Q1 is on. If hysteresis is

required, the circuit in Figure 25 has a turnon voltage which is different than the turnoff voltage. The amount of

hysteresis is approximately equal to the value of the output voltage.

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Feature Description (continued)

Figure 24. Undervoltage Lockout Without

Hysteresis for an Inverting Regulator

Figure 25. Undervoltage Lockout With

Hysteresis for an Inverting Regulator

8.3.4 Negative Voltage Charge Pump

Occasionally a low current negative voltage is required 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 26. This unregulated negative voltage is approximately 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 several 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 1.5 A).

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.

Figure 26. Charge Pump for Generating a

Low Current, Negative Output Voltage

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8.4 Device Functional Modes

8.4.1 Discontinuous Mode Operation

The selection guide chooses inductor values suitable for continuous mode operation, but for low current

applications or high input voltages, a discontinuous mode design may be a better choice. Discontinuous mode

would use an inductor that would be physically smaller, and would require only one half to one third the

inductance value required for a continuous mode design. The peak switch and inductor currents is higher in a

discontinuous design, but at these low load currents (200 mA and below), the maximum switch current is still less

than the switch current limit.

Discontinuous operation can have voltage waveforms that are considerably different than a continuous design.

The output pin (switch) waveform can have some damped sinusoidal ringing present (see Figure 46) This ringing

is normal for discontinuous operation, and is not caused by feedback loop instabilities. In discontinuous

operation, there is a period of time where neither the switch nor 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 or diode parasitic capacitance causing this characteristic ringing. Normally this ringing is not a

problem, unless the amplitude becomes great enough to exceed the input voltage, and even then, there is very

little energy present to cause damage.

Different inductor types or core materials produce different amounts of this characteristic ringing. Ferrite core

inductors 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.

Figure 27. Post Ripple Filter Waveform

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9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

9.1 Application Information

9.1.1 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 operating regions (See the bottom curve in Figure 28).

1. Regulator in shutdown: When the SD/SS pin voltage is between 0 V and 1.3 V, the regulator is in shutdown,

the output voltage is zero, and the IC quiescent current is approximately 85 μA.

2. Regulator ON, but the output voltage is zero: With the SD/SS pin voltage between approximately 1.3 V and

1.8 V, the internal regulator circuitry is operating, the quiescent current rises to approximately 5 mA, but the

output voltage is still zero. Also, as the 1.3-V 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.

3. Soft-start region: When the SD/SS pin voltage is between 1.8 V and 2.8 V at 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 2.8 V. As the duty cycle increases, the output voltage

also increases at a controlled ramp up. See the center curve in Figure 28. The input supply current

requirement also starts out at a low level for the narrow pulses and ramp up in a controlled manner. This is a

very useful feature in some switcher topologies that require large start-up currents (such as the inverting

configuration) which can load down the input power supply.

Note: The lower curve shown in Figure 28 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.

4. Normal operation: Above 2.8 V, the circuit operates as a standard pulse width modulated switching regulator.

The capacitor continues to charge up until it reaches the internal clamp voltage of approximately 7 V. 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 7 V,

the voltage on the SD/SS pin tracks the input voltage and can be disturbed by a step in the voltage. To maintain

proper function under these conditions, it is strongly recommended that the SD/SS pin be clamped externally

between the 3-V maximum soft-start threshold and the 4.5-V minimum input voltage. Figure 30 is an example of

an external approximately 3.7-V clamp that prevents a line-step related glitch but does not interfere with the soft-

start behavior of the device.

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Application Information (continued)

Figure 28. Soft-start, Delay, Error, Output

Figure 29. Timing Diagram for 5-V Output

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Application Information (continued)

V_IN

[a2598

ꢀ

v1

{5/{{

C_SS

ù1

3ë

Figure 30. External 3.7-V Soft-Start Clamp

9.1.2 Delay Capacitor (C_DELAY

)

Provides delay for the error flag output. See the upper curve in Figure 28, and also refer to timing diagrams in

Figure 29. 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 capacitor resulting in a voltage ramp. When this voltage

reaches a threshold of approximately 1.3 V, the open collector 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.

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 error flag output, R_Pull

(or power OK), is the collector of a NPN transistor, with the emitter internally

Up

grounded. To use the error flag, a pullup resistor to a positive voltage is required. The error flag transistor is

rated up to a maximum of 45 V and can sink approximately 3 mA. If the error flag is not used, it can be left open.

9.1.3 Feedforward Capacitor (C_FF)

NOTE

Adjustable output voltage version only

Figure 45 shows a feedfoward capacitor across R2 which is used when the output voltage is greater than 10 V 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.

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 comparator to trigger the error flag. In this situation,

adding a resistor, R_FF, in series with the feedforward capacitor, approximately 3 times R1, attenuates the ripple

voltage at the feedback pin.

9.1.4 Input Capacitor (C_IN)

A low ESR aluminum or tantalum bypass capacitor is required between the input pin and ground pin. The

capacitor must be located near the regulator using short leads. This capacitor prevents large voltage transients

from appearing at the input, and provides the instantaneous current required 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 must be chosen for

its RMS current rating rather than its capacitance or voltage ratings, although the capacitance value and voltage

rating are directly related to the RMS current rating.

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Application Information (continued)

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 approximately 10°C above an ambient temperature of 105°C. The ability of the capacitor to dissipate

this heat to the surrounding air determines the amount of current the capacitor can safely sustain. Capacitors that

are physically large and have a large surface area typically has higher RMS current ratings. For a given capacitor

value, a higher voltage electrolytic capacitor is physically larger than a lower voltage capacitor, and thus be able

to dissipate more heat to the surrounding air, and therefore has a higher RMS current rating.

Figure 31. RMS Current Ratings for Low

ESR Electrolytic Capacitors (Typical)

Figure 32. 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.

Selecting an input capacitor requires consulting the manufacturers 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 required to satisfy the RMS current requirements.

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Application Information (continued)

Figure 31 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 electrolytic

capacitors designed for switching regulator applications. Other capacitor manufacturers offer similar types of

capacitors, 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.

Because of their small size and excellent performance, surface 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 turnon when the input voltage is suddenly

applied, and of course, higher input voltages produce higher inrush currents. Several capacitor manufacturers do

a 100% surge current testing on their products to minimize this potential problem. If high turnon currents are

expected, it may be necessary to limit this current by adding either some resistance or inductance before the

tantalum capacitor, or select a higher voltage capacitor. As with aluminum electrolytic capacitors, the RMS ripple

current rating must be sized to the load current.

9.1.5 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

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

low ESR value is required. This value is determined by the maximum 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, provides design solutions under all conditions.

If very low output ripple voltage (less than 15 mV) is required, see Output Voltage Ripple and Transients for a

post ripple filter.

An aluminum electrolytic capacitor's ESR value is related to the capacitance value and its voltage rating. In most

cases, higher voltage electrolytic capacitors have lower ESR values (see Figure 32). Often, capacitors with much

higher voltage ratings may be required to provide the low ESR values required for low output ripple voltage.

The output capacitor for many different switcher designs often can be satisfied with only three or four different

capacitor values and several different voltage ratings. See Figure 38 and Table 1 for typical capacitor values,

voltage ratings, and manufacturers capacitor types.

Electrolytic capacitors are not recommended for temperatures below –25°C. The ESR rises dramatically at cold

temperatures and typically rises 3X at –25°C and as much as 10X at –40°C. See curve shown in Figure 33.

Solid tantalum capacitors have a much better ESR specifications for cold temperatures and are recommended

for temperatures below –25°C.

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Application Information (continued)

Table 1. Output Capacitor and Feedforward Capacitor Selection Table

THROUGH-HOLE ELECTROLYTIC

SURFACE-MOUNT TANTALUM

OUTPUT

VOLTAGE

(V)

PANASONIC

NICHICON PL

SERIES

AVX TPS

SERIES

(μF/V)

SPRAGUE

FEEDFORWARD

CAPACITOR

FEEDFORWARD

CAPACITOR

HFQ SERIES

595D SERIES

(μF/V)

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

100/16

68/20

330/6.3

220/10

180/16

120/20

100/20

33/35

0

4.7 nF

3.3 nF

1.5 nF

1 nF

4.7 nF

3.3 nF

1.5 nF

220 pF

6

9

12

15

24

28

68/20

33/25

82/50

1 nF

10/35

33/35

9.1.6 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.

Because of their very fast switching speed and low forward voltage drop, Schottky diodes provide the best

performance, especially in low output voltage applications (5 V and lower). Ultra-fast recovery, or high-efficiency

rectifiers are also a good choice, but some types with an abrupt turnoff characteristic 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 must not be used.

Figure 33. Capacitor ESR Change vs Temperature

9.1.7 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 operate in the

discontinuous mode when the load current is low.

The LM2598 (or any of the Simple Switcher family) can be used for both continuous or discontinuous modes of

operation.

In many cases the preferred mode of operation is the continuous mode. This mode offers greater output power,

lower peak switch, inductor and diode currents, and can have lower output ripple voltage. However, the

continuous mode requires larger inductor values to keep the inductor current flowing continuously, especially at

low output load currents or high input voltages.

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To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Table 1

through Figure 37). This guide assumes that the regulator is operating in the continuous mode, and selects an

inductor that allows 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 34.)

Figure 34. (ΔI_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.

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, and so forth, 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; however,

because 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 causing

problems with both the switching regulator operation and nearby sensitive circuitry, and can give incorrect scope

readings because of induced voltages in the scope probe. Also seeOpen Core Inductors.

When multiple switching regulators are located on the same PCB, 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) must 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 Engineering.

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 overheating of the inductor or the LM2598. Different inductor types have different

saturation characteristics, and this must be kept in mind when selecting an inductor.

The inductor manufacturer's data sheets include current and energy limits to avoid inductor saturation.

For continuous mode operation, see the inductor selection graphs in Figure 35 through Figure 38.

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Figure 35. LM2598-3.3

Figure 36. LM2598-5.0

Figure 38. LM2598-ADJ

Figure 37. LM2598-12

Table 2. Inductor Manufacturers Part Numbers

SCHOTTKY

RENCO

PULSE ENGINEERING

COILCRAFT

INDUCTANCE CURRENT

THROUGH

HOLE

SURFACE

MOUNT

THROUGH

SURFACE

MOUNT

THROUGH

HOLE

SURFACE

MOUNT

SURFACE

MOUNT

(μH)

(A)

HOLE

L4

68

47

0.32

0.37

0.44

0.32

0.39

0.48

0.58

0.7

67143940

67148310

67148320

67143960

67143970

67143980

67143990

67144000

67148340

67148350

67148360

67144030

67144040

67144050

67144060

67144070

67144310

67148420

67148430

67144330

67144340

67144350

67144360

67144380

67148450

67148460

67148470

67144410

67144420

67144430

67144440

67144450

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-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

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

L5

L6

33

L9

220

150

100

68

L10

L11

L12

L13

L14

L15

L16

L17

L18

L19

L20

L21

RL-5470-4

RL-5470-5

RL-5470-6

47

RL-5470-7

33

0.83

0.99

1.24

0.42

0.55

0.66

0.82

0.99

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

24

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Table 2. Inductor Manufacturers Part Numbers (continued)

SCHOTTKY

RENCO

PULSE ENGINEERING

COILCRAFT

INDUCTANCE CURRENT

THROUGH

HOLE

SURFACE

MOUNT

THROUGH

SURFACE

MOUNT

THROUGH

HOLE

SURFACE

MOUNT

SURFACE

MOUNT

(μH)

(A)

HOLE

RL-5471-6

RL-5471-7

RL-1283-22-43

RL-5471-1

RL-5471-2

RL-5471-3

RL-5471-4

RL-5471-5

RL-5473-1

L22

L23

L24

L26

L27

L28

L29

L30

L35

47

33

1.17

1.4

67144080

67144090

67148370

67144100

67144110

67144120

67144130

67144140

67144170

67144460

67144470

67144480

67144490

67144500

67144510

67144520

—

PE-53822

PE-53823

PE-53824

PE-53826

PE-53827

PE-53828

PE-53829

PE-53830

PE-53935

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

DO3316-473

DO3316-333

DO3316-223

DO5022P-334

DO5022P-224

DO5022P-154

DO5022P-104

DO5022P-683

—

22

1.7

330

220

150

100

68

0.8

1

1.2

1.47

1.78

2.15

47

9.1.8 Output Voltage Ripple and Transients

The output voltage of a switching power supply operating in the continuous mode contains a sawtooth ripple

voltage at the switcher frequency, and may also contain short voltage spikes at the peaks of the sawtooth

waveform.

The output ripple voltage is a function of the inductor sawtooth 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; however, caution must be exercised when

using extremely low ESR capacitors because they can affect the loop stability, resulting in oscillation problems. If

very low output ripple voltage is required (less than 20 mV), TI recommends a post ripple filter (see Figure 45).

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 desired, because it is out of the regulator feedback loop.

Figure 27 shows a typical output ripple voltage, with and without a post ripple filter.

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, preferably at the output capacitor. This provides a very short scope ground, thus eliminating

the problems associated with the 3 inch ground lead normally provided with the probe, and provides a much

cleaner and more accurate picture of the ripple voltage waveform.

The voltage spikes are caused by the fast switching action of the output switch, the diode, the parasitic

inductance of the output filter capacitor, and its associated wiring. To minimize these voltage spikes, the output

capacitor must 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.

Figure 39. Peak-to-Peak Inductor

Ripple Current vs Load Current

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When a switching regulator is operating in the continuous mode, the inductor current waveform ranges from a

triangular to a sawtooth type of waveform (depending on the input voltage). For a given input and output voltage,

the peak-to-peak amplitude of this inductor current waveform remains constant. As the load current increases or

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 reaches zero, and

the switcher smoothly changes from a continuous to a discontinuous mode of operation. Most switcher designs

(regardless how large the inductor value is) is forced to run discontinuous if the output is lightly loaded. This is a

perfectly acceptable mode of operation.

In a switching regulator design, knowing the value of the peak-to-peak inductor ripple current (ΔI_IND) can be

useful for determining a number of other circuit parameters. Parameters such as, peak inductor or peak switch

current, minimum load current before the circuit becomes discontinuous, output ripple voltage and output

capacitor ESR can all be calculated from the peak-to-peak ΔI_IND. When the inductor nomographs shown in

Figure 35 through Figure 38 are used to select an inductor value, the peak-to-peak inductor ripple current can

immediately be determined. Figure 39 shows the range of (ΔI_IND) that can be expected for different load currents.

Figure 39 also shows how the peak-to-peak inductor ripple current (ΔI_IND) changes as the designer goes from the

lower border to the upper border (for a given load current) within an inductance region. The upper border

represents a higher input voltage, while the lower border represents a lower input voltage (see Inductor Selection

Guides).

These curves are only correct for continuous mode operation, and only if the inductor selection guides are used

to select the inductor value

Consider the following example:

V_OUT= 5 V, maximum load current of 800 mA

V_IN= 12 V, nominal, varying between 10 V and 14 V.

The selection guide in Figure 36 shows that the vertical line for a 0.8-A load current and the horizontal line for the

12-V input voltage intersect approximately midway between the upper and lower borders of the 68-μH inductance

region. A 68-μH inductor allows a peak-to-peak inductor current (ΔI_IND) to a percentage of the maximum load

current. Referring to Figure 39, follow the 0.8-A line approximately midway into the inductance region, and read

the peak-to-peak inductor ripple current (ΔI_IND) on the left hand axis (approximately 300-mA p-p).

As the input voltage increases to 14 V, it approaches the upper border of the inductance region, and the inductor

ripple current increases. Figure 39 shows that for a load current of 0.8 A, the peak-to-peak inductor ripple current

(ΔI_IND) is 300 mA with 12-V in, and can range from 340 mA at the upper border (14-V in) to 225 mA at the lower

border (10-V in).

Once the ΔI_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 discontinuous

3. Output Ripple Voltage = (ΔI_IND) × (ESR of C_OUT) = 0.3 A × 0.16 Ω = 48 mV_p-p

4. ESR of C_OUT

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9.1.9 Open Core Inductors

Another possible source of increased output ripple voltage or unstable operation is from an open core inductor.

Ferrite bobbin or stick inductors have magnetic lines of flux flowing through the air from one end of the bobbin to

the other end. These magnetic lines of flux induce a voltage into any wire or PCB copper trace that comes within

the magnetic field of the inductor. The strength of the magnetic field, the orientation and location of the PC

copper trace to the magnetic 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 PCB 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.

If unstable operation is seen, and an open core inductor is used, it is possible that the location of the inductor

with respect 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 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 torroid or E-core 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 must be minimized.

Sometimes, placing a trace directly beneath a bobbin inductor provides good results, provided it is exactly in the

center of the inductor (because the induced voltages cancel themselves out). However, problems could arise if

the trace is off center. If flux problems are present, even the direction of the inductor winding can make a

difference in some circuits.

This discussion on open core inductors is not to frighten users, but to alert them on what kind of problems to

watch out for when using them. Open core bobbin or stick inductors are an inexpensive, simple way of making a

compact, efficient inductor, and they are used by the millions in many different applications.

Circuit Data for Temperature Rise Curve TO-220 Package (T)

Capacitors

Inductor

Through hole electrolytic

Through hole, Schott, 68 μH

Diode

Through hole, 3-A, 40-V, Schottky

3 square inches single sided 2 oz. copper (0.0028″)

Printed-circuit board

Figure 40. Junction Temperature Rise, TO-220

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Circuit Data for Temperature Rise Curve DDPAK Package (S)

Capacitors

Inductor

Surface mount tantalum, molded D size

Surface mount, Schott, 68 μH

Diode

Surface mount, 3-A, 40-V, Schottky

Printed-circuit board

3 square inches single sided 2 oz. copper (0.0028″)

Figure 41. Junction Temperature Rise, DDPAK

9.2 Typical Application

9.2.1 LM2598 Fixed Output Series Buck Regulator

Component Values shown are for V_IN= 15 V, V_OUT= 5 V, I_LOAD= 1 A.

120-μF, 50-V, Aluminum Electrolytic Nichicon PL Series

120-μF, 35-V Aluminum Electrolytic, Nichicon PL Series

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

68-μH, L30

Typical Values

*C_SS: — 0.1 μF

C_DELAY

:

— 0.1 μF

R_{Pull Up}

:

— 4.7k

Figure 42. Fixed Output Voltage Version

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Typical Application (continued)

9.2.1.1 Design Requirements

Table 3 lists the design parameters of this application example.

Table 3. Design Parameters

PARAMETERS

EXAMPLE VALUE

Regulated output voltage (3.3 V, 5 V or 12 V), V_OUT

Maximum DC input voltage, V_IN(max)

Maximum load current, I_LOAD(max)

5 V

12 V

1 A

9.2.1.2 Detailed Design Procedure

9.2.1.2.1 Inductor Selection (L1)

1. Select the correct inductor value selection guide from Figure 35, Figure 36, or Figure 37 (Output voltages of

3.3 V, 5 V, or 12 V respectively.) Use the inductor selection guide for the 5-V version shown in Figure 36.

2. From the inductor value selection guide, identify the inductance 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). From the inductor value selection guide shown in Figure 36, the inductance region

intersected by the 12-V horizontal line and the 1-A vertical line is 68 μH, and the inductor code is L30.

3. Select an appropriate inductor from the four manufacturer's part numbers listed in Table 2. The inductance

value required is 68 μH. See row L30 of Table 2 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.)

9.2.1.2.2 Output Capacitor Selection (C_OUT

)

1. 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 must be located close to the IC using short capacitor leads and short copper traces. Do not use

capacitors larger than 330 μF.

For additional information, see section on output capacitors in Output Capacitor (C_OUT) section.

2. To simplify the capacitor selection procedure, see Figure 38 for quick design component selection. This table

contains different input voltages, output voltages, and load currents, and lists various inductors and output

capacitors that provide the best design solutions.

From Figure 38, locate the 5-V output voltage section. In the load current column, choose the load current

line that is closest to the current required for the application; for this example, use the 1-A line. In the

maximum input voltage column, select the line that covers the input voltage required for the application; in

this example, use the 15-V line. The rest of this line shows the recommended inductors and capacitors that

provide the best overall performance.

The capacitor list contains both through hole electrolytic and surface mount tantalum capacitors from four

different capacitor manufacturers. TI recommends using both the manufacturers and the manufacturer's

series that are listed in Figure 38.

In this example aluminum electrolytic capacitors from several different manufacturers are available with the

range of ESR numbers required.

–

220-μF, 25-V Panasonic HFQ Series

220 μF, 25-V Nichicon PL Series

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Table 4. LM2598 Fixed Voltage Quick Design Component Selection Table

OUTPUT CAPACITOR

CONDITIONS

INDUCTOR

THROUGH-HOLE

ELECTROLYTIC

SURFACE-MOUNT TANTALUM

OUTPUT

LOAD

MAX INPUT

PANASONIC

HFQ SERIES

NICHICON

PL SERIES

(μF/V)

AVX TPS

SERIES

(μF/V)

SPRAGUE

595D SERIES

(μF/V)

INDUCTANCE INDUCTOR

VOLTAGE CURRENT VOLTAGE

(μH)

(#)

(V)

(A)

(V)

(μF/V)

5

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

330/16

270/25

220/35

220/16

150/25

82/35

220/10

220/16

100/16

220/10

100/16

220/10

100/16

68/20

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

120/25

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

3. The capacitor voltage rating for electrolytic capacitors must be at least 1.5 times greater than the output

voltage, and often much higher voltage ratings are required to satisfy the low ESR requirements for low

output ripple voltage

For a 5-V output, a capacitor voltage rating at least 7.5 V or more is required. But, in this example, even a

low ESR, switching grade, 220-μF, 10-V aluminum electrolytic capacitor would exhibit approximately 225 mΩ

of ESR (see the curve in Figure 32 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) must be selected. A 16-V or 25-V capacitor reduces the ripple voltage by

approximately half.

9.2.1.2.3 Catch Diode Selection (D1)

1. 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 must 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. See Table 5. In this example, a 3-A, 20-V, 1N5820 Schottky diode provides the

best performance, and does not overstressed even for a shorted output.

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Table 5. Diode Selection Table

1-A DIODES

3-A DIODES

SURFACE MOUNT

ULTRA FAST

THROUGH HOLE

ULTRA FAST

SURFACE MOUNT

THROUGH HOLE

VR

ULTRA FAST

RECOVERY

ULTRA FAST

SCHOTTKY

RECOVERY

SK12

1N5817

SR102

IN5820

SR302

All of these

diodes are rated

to at least 50 V.

All of these

diodes are rated

to at least 50 V.

All of these

diodes are rated

to at least 50 V.

All of these

diodes are rated

to at least 50 V.

20 V

30 V

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

40 V

MBR340

31DQ04

SR305

MURS120

10BF10

11DQ04

SR105

MUR120

MURS320

30WF10

MUR320

30WF10

50 V

or

MBR150

11DQ05

MBRS360

30WQ05

MBR350

31DQ05

more

2. The reverse voltage rating of the diode must be at least 1.25 times the maximum input voltage.

3. 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 must 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 reverse recovery times of 50 ns or less. Rectifiers such as the 1N5400

must not be used because they are too slow.

9.2.1.2.4 Input Capacitor (C_IN)

A low ESR aluminum or tantalum bypass capacitor is required 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 must be

selected to be at least ½ the DC load current. The capacitor manufacturers data sheet must be checked to

assure that this current rating is not exceeded. Figure 31 shows typical RMS current ratings for several different

aluminum electrolytic capacitor values.

This capacitor must be located close to the IC using short leads and the voltage rating must be approximately 1.5

times the maximum input voltage.

If solid tantalum input capacitors are used, TI recommends they be surge current tested by the manufacturer.

Use caution when using ceramic capacitors for input bypassing, because it may cause severe ringing at the V_IN

pin.

The important parameters for the Input capacitor are the input voltage rating and the RMS current rating. With a

nominal input voltage of 12 V, an aluminum electrolytic capacitor with a voltage rating greater than 18 V

(1.5 × V_IN) is necessary. The next higher capacitor voltage rating is 25 V.

The RMS current rating requirement for the input capacitor in a buck regulator is approximately ½ the DC load

current. In this example, with a 1-A load, a capacitor with a RMS current rating of at least 500 mA is required.

Figure 31 shows curves that can be used to select an appropriate input capacitor. From the curves, locate the

25-V line and note which capacitor values have RMS current ratings greater than 500 mA. Either a 180-μF or

220-μF, 25-V capacitor could be used.

For a through-hole design, a 220-μF, 25-V electrolytic capacitor (Panasonic HFQ series or Nichicon PL series or

equivalent) 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 are recommended. The TPS series available from AVX,

and the 593D series from Sprague are both surge current tested.

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9.2.1.3 Application Curves

Load Transient Response for Continuous Mode

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

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

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

Continuous Mode Switching Waveforms

V_IN= 20 V, V_OUT= 5 V, I_LOAD= 1 A, L = 68 μH,

C_OUT= 120 μF, C_OUTESR = 100 mΩ

A: Output Pin Voltage, 10 V/div.

B: 250-mA to 750-mA Load Pulse

B: Inductor Current 0.5 A/div.

C: Output Ripple Voltage, 50 mV/div.

Figure 44. Horizontal Time Base: 100 μs/div

Figure 43. Horizontal Time Base: 2 μs/div

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9.2.2 LM2598 Adjustable Output Series Buck Regulator

where V_REF= 1.23 V

Select R₁to be approximately 1 kΩ, use a 1% resistor for best stability.

Component Values shown are for V_IN= 20 V,

V_OUT= 10 V, I_LOAD= 1 A.

C_IN— 120 μF, 35-V, Aluminum Electrolytic Nichicon PL Series

C_OUT— 120 μF, 35-V Aluminum Electrolytic, Nichicon PL Series

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

L1 —100 μH, L29

R₁—1 kΩ, 1%

R₂—7.1 kΩ, 1%

C_FF— 3.3 nF, See Feedforward Capacitor (C_FF

)

R_FF— 3 kΩ, See Feedforward Capacitor (C_FF

)

Typical Values

C_SS—0.1 μF

C_DELAY—0.1 μF

R_{PULL UP}—4.7 kΩ

Figure 45. Adjustable Output Voltage Version

9.2.2.1 Design Requirements

Table 6 lists the design parameters for this application example.

Table 6. Design Parameters

PARAMETERS

Regulated output voltage (3.3 V, 5 V or 12 V), V_OUT

Maximum DC input voltage, V_IN(max)

Maximum load current, I_LOAD(max)

Switching frequency, F

EXAMPLE VALUE

20 V

28 V

1 A

Fixed at a nominal 150 kHz

9.2.2.2 Detailed Design Procedure

9.2.2.2.1 Programming Output Voltage

Select R₁and R₂, as shown in Figure 45.

Use Equation 1 to select the appropriate resistor values.

(1)

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Select a value for R₁with Equation 2 between 240 Ω and 1.5 kΩ. The lower resistor values minimize noise

pickup in the sensitive feedback pin. (For the lowest temperature coefficient and the best stability with time, use

1% metal film resistors.)

(2)

Select R₁with Equation 3 to be 1 kΩ, 1%. Solve for R₂.

(3)

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

R₂= 15.4 kΩ.

9.2.2.2.2 Inductor Selection (L1)

1. Calculate the inductor Volt • microsecond constant E • T (V • μs) with Equation 4.

where

•

V_SAT= internal switch saturation voltage = 1 V

V_D= diode forward voltage drop = 0.5 V

(4)

(5)

Calculate the inductor Volt • microsecond constant (E • T) with Equation 5.

2. Use the E • T value from the previous formula and match it with the E • T number on the vertical axis of the

see the inductor selection graphs in Figure 35 through Figure 38.

E • T = 34.8 (V • μs)

3. On the horizontal axis, select the maximum load current.

I_LOAD(max) = 1 A

4. 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).

From the inductor selection graphs in Figure 35 through Figure 38, the inductance region intersected by the

35 (V • μs) horizontal line and the 1-A vertical line is 100 μH, and the inductor code is L29.

5. Select an appropriate inductor from the four manufacturer's part numbers listed in Table 2.

From the table in Table 2, locate line L29, and select an inductor part number from the list of manufacturers'

part numbers.

9.2.2.2.3 Output Capacitor Selection (C_OUT

)

1. In the majority of applications, low ESR electrolytic or solid tantalum capacitors between 82 μF and 220 μF

provide the best results. This capacitor must be located close to the IC using short capacitor leads and short

copper traces. Do not use capacitors larger than 220 μF. For additional information, see Output Capacitor

(C_OUT).

2. To simplify the capacitor selection procedure, see Table 1 for a quick design guide. This table contains

different output voltages, and lists various output capacitors that provide the best design solutions.

From Table 1, 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 24-V line. Under the Output Capacitor (C_OUT

)

section, select a capacitor from the list of through hole electrolytic or surface mount tantalum types from four

different capacitor manufacturers. TI recommends that both the manufacturers and the manufacturers series

that are listed in Table 1 be used.

In this example, through hole aluminum electrolytic capacitors from several different manufacturers are

available:

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–

82-μF, 35-V Panasonic HFQ Series

82-μF, 35-V Nichicon PL Series

3. The capacitor voltage rating must be at least 1.5 times greater than the output voltage, and often much

higher voltage ratings are required to satisfy the low ESR requirements required for low output ripple voltage.

For a 20-V output, a capacitor rating of at least 30 V or more is required. In this example, either a 35-V or

50-V capacitor would work. A 35-V rating was chosen although a 50-V rating could also be used if a lower

output ripple voltage is required.

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 Table 1. Refer to the capacitor manufacturers

data sheet for this information.

9.2.2.2.4 Feedforward Capacitor (C_FF

)

For output voltages greater than approximately 10 V, an additional capacitor is required (use Equation 6; see

Figure 45). 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 additional stability for high output voltages, low input or output

voltages, or very low ESR output capacitors, such as solid tantalum capacitors.

(6)

This capacitor type can be ceramic, plastic, silver mica, etc. (Because of the unstable characteristics of ceramic

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

The table shown in Table 1 contains feedforward capacitor values for various output voltages. In this example, a

1-nF capacitor is required.

9.2.2.2.5 Catch Diode Selection (D1)

1. 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 must 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.

See Table 5. Schottky diodes provide the best performance, and in this example a 3-A, 40-V, 1N5822

Schottky diode is a good choice. The 3-A diode rating is more than adequate and does not overstressed

even for a shorted output.

2. The reverse voltage rating of the diode must be at least 1.25 times the maximum input voltage.

3. This diode must be fast (short reverse recovery time) and must be placed 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 must be the first choice, especially in low

output voltage applications. Ultra-fast recovery or high-efficiency rectifiers are also good choices, but some

types with an abrupt turnoff characteristic may cause instability or EMl problems. Ultra-fast recovery diodes

typically have reverse recovery times of 50 ns or less. Rectifiers such as the 1N4001 series must not be

used because they are too slow.

9.2.2.2.6 Input Capacitor (C_IN)

A low ESR aluminum or tantalum bypass capacitor is required 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 must be

selected to be at least ½ the DC load current. The capacitor manufacturers data sheet must be checked to

assure that this current rating is not exceeded. Figure 31 shows typical RMS current ratings for several different

aluminum electrolytic capacitor values.

This capacitor must be located close to the IC using short leads and the voltage rating must 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.

Use caution when using a high dielectric constant ceramic capacitor for input bypassing, because it may cause

severe ringing at the V_INpin.

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The important parameters for the input capacitor are the input voltage rating and the RMS current rating. With a

nominal input voltage of 28 V, an aluminum electrolytic aluminum electrolytic capacitor with a voltage rating

greater than 42 V (1.5 × V_IN) is required. Because the the next higher capacitor voltage rating is 50 V, a 50-V

capacitor must be used. The capacitor voltage rating of (1.5 × V_IN) is a conservative guideline, and can be

modified somewhat if desired.

The RMS current rating requirement for the input capacitor of a buck regulator is approximately ½ the DC load

current. In this example, with a 1-A load, a capacitor with a RMS current rating of at least 500 mA is required.

Figure 31 shows curves that can be used to select an appropriate input capacitor. From the curves, locate the

50-V line and note which capacitor values have RMS current ratings greater than 500 mA. Either a 100-μF or

120-μF, 50-V capacitor could be used.

For a through-hole design, a 120-μF, 50-V electrolytic capacitor (Panasonic HFQ series or Nichicon PL series or

equivalent) 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 Input Capacitor (C_IN)). The TPS series available from AVX, and the 593D

series from Sprague are both surge current tested.

9.2.2.3 Application Curves

Load Transient Response for Discontinuous Mode

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

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

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

Discontinuous Mode Switching Waveforms

B: 250-mA to 750-mA Load Pulse

V_IN= 20 V,V_OUT= 5 V, I_LOAD= 600 mA, L = 22 μH,

C_OUT= 220 μF,C_OUTESR = 50 mΩ

A: Output Pin Voltage, 10 V/div.

B: Inductor Current 0.5 A/div.

C: Output Ripple Voltage, 50 mV/div.

Figure 47. Horizontal Time Base: 200 μs/div

Figure 46. Horizontal Time Base: 2 μs/div

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10 Power Supply Recommendations

The LM2598 is designed to operate from an input voltage supply up to 40 V. This input supply must be well

regulated and able to withstand maximum input current and maintain a stable voltage.

11 Layout

11.1 Layout Guidelines

As in any switching regulator, layout is very important. Rapidly 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 must be wide printed circuit traces and must be kept as short as

possible. For best results, external components must be placed as close to the switcher lC as possible using

ground plane construction or single point grounding.

If open core inductors are used, take special care regarding the location and positioning of this type of inductor.

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 associated wiring. Physically place both resistors near the IC, and route the wiring away from the inductor,

especially an open core type of inductor (see Open Core Inductors for more information).

11.2 Layout Examples

C_IN—150-μF, 50-V Aluminum Electrolytic, Panasonic HFQ series

C_OUT—120-μF, 25-V Aluminum Electrolytic, Panasonic HFQ series

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

L1 — 68-μH, L30, Renco, Through hole

R_PULL-UP— 10 kΩ

C_DELAY— 0.1 μF

C_SD/SS— 0.1 μF

Figure 48. Typical Through-Hole PCB Layout, Fixed Output (1x Size), Double-Sided, Through-Hole Plated

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Layout Examples (continued)

C_IN— 150-μF, 50-V, Aluminum Electrolytic, Panasonic HFQ series

C_OUT— 120-μF, 25-V Aluminum Electrolytic, Panasonic HFQ series

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

L1 — 68-μH, L30, Renco, Through hole

R1 — 1 kΩ, 1%

R2—Use formula in Design Procedure

C_FF—See Feedforward Capacitor (C_FF).

R_FF—See Feedforward Capacitor (C_FF).

R_PULL-UP—10 kΩ

C_DELAY— 0.1-μF

C_SD/SS— 0.1 μF

Figure 49. Typical Through-Hole PCB Layout, Adjustable Output (1x Size), Double-Sided, Through-Hole

Plated

11.3 Thermal Considerations

The LM2598 is available in two packages: a 7-pin TO-220 (T) and a 7-pin surface mount DDPAK (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). Figure 40 shows 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 temperatures require some heat sinking, either to the PCB or a small external heat

sink.

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Thermal Considerations (continued)

The DDPAK surface mount package tab is designed to be soldered to the copper on a printed-circuit board

(PCB). 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 PCB copper area that the package is soldered to must be at least 0.4

in², and ideally must 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 further thermal improvements are required, TI recommends double-sided or

multilayer PCB with large copper areas.

Figure 41 shows the LM2598S (DDPAK package) junction temperature rise above ambient temperature with a 1-

A 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 PCB 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.

For the best thermal performance, wide copper traces and generous amounts of PCB copper must be used in

the board layout. (One exception to this is the output (switch) pin, which must 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 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 PCB to

dissipate heat also depends on the size, quantity, and spacing of other components on the board, as well as

whether the surrounding air is still or moving. Furthermore, some of these components such as the catch diode

adds heat to the PCB 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.

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12 Device and Documentation Support

12.1 Community Resources

The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective

contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of

Use.

TI E2E™ Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration

among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help

solve problems with fellow engineers.

Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and

contact information for technical support.

12.2 Trademarks

E2E is a trademark of Texas Instruments.

SIMPLE SWITCHER is a registered trademark of Texas Instruments.

All other trademarks are the property of their respective owners.

12.3 Electrostatic Discharge Caution

These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam

during storage or handling to prevent electrostatic damage to the MOS gates.

12.4 Glossary

SLYZ022 — TI Glossary.

This glossary lists and explains terms, acronyms, and definitions.

13 Mechanical, Packaging, and Orderable Information

The following pages include mechanical, packaging, and orderable information. This information is the most

current data available for the designated devices. This data is subject to change without notice and revision of

this document. For browser-based versions of this data sheet, refer to the left-hand navigation.

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MECHANICAL DATA

KTW0007B

TS7B (Rev E)

BOTTOM SIDE OF PACKAGE

www.ti.com

MECHANICAL DATA

NDZ0007B

TA07B (Rev E)

www.ti.com

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型号：	LM2598T-ADJ/NOPB
厂家：	TEXAS INSTRUMENTS
描述：	4.5V 至 40V、1A SIMPLE SWITCHER® 降压转换器 \| NDZ \| 7 \| -40 to 125 局域网开关转换器
文件：	总48页 (文件大小：2333K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LM2598T-ADJ/NOPB [TI]

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