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LM2599

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®

LM2599 SIMPLE SWITCHER Power Converter 150 kHz 3A Step-Down Voltage Regulator,

with Features

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FEATURES

DESCRIPTION

The LM2599 series of regulators are monolithic

integrated circuits that provide all the active functions

for a step-down (buck) switching regulator, capable of

driving a 3A load with excellent line and load

regulation. These devices are available in fixed output

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

version.

23

•

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

•

Adjustable Version Output Voltage Range,

1.2V to 37V ±4% Max Over Line and Load

Conditions

•

Ensured 3A Output Current

Available in 7-Pin TO-220 and TO-263 (Surface

Mount) Package

This series of switching regulators is similar to the

LM2596 series, with additional supervisory and

performance features added.

•

Input Voltage Range Up to 40V

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_QTypically 80 μA

High Efficiency

The LM2599 series operates at a switching frequency

of 150 kHz thus allowing smaller sized filter

components than what would be needed 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 TO-263

Surface mount package.

APPLICATIONS

•

Simple High-Efficiency Step-down (Buck)

Regulator

A standard series of inductors (both through hole and

surface mount types) are available from several

different manufacturers optimized for use with the

LM2599 series. This feature greatly simplifies the

design of switch-mode power supplies.

•

Efficient Pre-Regulator for Linear Regulators

On-Card Switching Regulators

Positive to Negative Converter

Other features include an ensured ±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 80 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

(1)

conditions.

(1) † Patent Number 5,382,918.

1

Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of

Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.

2

3

SIMPLE SWITCHER is a registered trademark of Texas Instruments.

All other trademarks are the property of their respective owners.

PRODUCTION DATA information is current as of publication date.

Products conform to specifications per the terms of the Texas

Instruments standard warranty. Production processing does not

necessarily include testing of all parameters.

LM2599

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Typical Application

(Fixed Output Voltage Versions)

Connection Diagrams

Figure 1. Bent and Staggered Leads, Through

Hole Package

Figure 2. Surface Mount Package

7-Lead TO-263 (KTW)

7-Lead TO-220 (NDZ)

Package Number KTW0007B

Package Number NDZ0007B

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.

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(1)(2)

Absolute Maximum Ratings

Maximum Supply Voltage (V_IN

)

45V

6V

(3)

SD /SS Pin Input Voltage

(3)

Delay Pin Voltage

1.5V

Flag Pin Voltage

−0.3 ≤ V ≤45V

−0.3 ≤ V ≤+25V

Feedback Pin Voltage

Output Voltage to Ground

(Steady State)

−1V

Internally limited

−65°C to +150°C

Power Dissipation

Storage Temperature Range

ESD Susceptibility

(4)

Human Body Model

2 kV

Lead Temperature

KTW Package

Vapor Phase (60 sec.)

Infrared (10 sec.)

+215°C

+245°C

+260°C

+150°C

NDZ Package (Soldering, 10 sec.)

Maximum Junction Temperature

(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 ensure specific performance limits. For ensured specifications and test

conditions, see the Electrical Characteristics section.

(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.

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

Operating Conditions

Temperature Range

−40°C ≤ T_J≤ +125°C

Supply Voltage

4.5V to 40V

LM2599-3.3 Electrical Characteristics

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

Temperature Range.

Symbol

Parameter

Conditions

LM2599-3.3

Units

(Limits)

Typ

Limit

(1)

(2)

SYSTEM PARAMETERS ⁽³⁾Test Circuit Figure 24

V_OUT

Output Voltage

4.75V ≤ V_IN≤ 40V, 0.2A ≤ I_LOAD≤ 3A

3.3

73

V

3.168/3.135

3.432/3.465

V(min)

V(max)

%

η

Efficiency

V_IN= 12V, I_LOAD= 3A

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

(2) All limits ensured 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 specified via correlation using standard Statistical Quality Control

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

(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.

When the LM2599 is used as shown in the Figure 24 test circuit, system performance will be as shown in system parameters of

Electrical Characteristics section.

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LM2599-5.0 Electrical Characteristics

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

Temperature Range.

Symbol

Parameter

Conditions

LM2599-5.0

Units

(Limits)

Typ

Limit

(1)

(2)

SYSTEM PARAMETERS ⁽³⁾Test Circuit Figure 24

V_OUT

Output Voltage

7V ≤ V_IN≤ 40V, 0.2A ≤ I_LOAD≤ 3A

5

V

4.800/4.750

5.200/5.250

V(min)

V(max)

%

η

Efficiency

V_IN= 12V, I_LOAD= 3A

80

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

(2) All limits ensured 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 specified via correlation using standard Statistical Quality Control

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

(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.

When the LM2599 is used as shown in the Figure 24 test circuit, system performance will be as shown in system parameters of

Electrical Characteristics section.

LM2599-12 Electrical Characteristics

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

Temperature Range.

Symbol

Parameter

Conditions

LM2599-12

Units

(Limits)

Typ

Limit

(1)

(2)

SYSTEM PARAMETERS ⁽³⁾Test Circuit Figure 24

V_OUT

Output Voltage

15V ≤ V_IN≤ 40V, 0.2A ≤ I_LOAD≤ 3A

12

90

V

11.52/11.40

12.48/12.60

V(min)

V(max)

%

η

Efficiency

V_IN= 25V, I_LOAD= 3A

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

(2) All limits ensured 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 specified via correlation using standard Statistical Quality Control

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

(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.

When the LM2599 is used as shown in the Figure 24 test circuit, system performance will be as shown in system parameters of

Electrical Characteristics section.

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LM2599-ADJ Electrical Characteristics

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

Temperature Range.

Symbol

Parameter

Conditions

LM2599-ADJ

Units

(Limits)

Typ

Limit

(1)

(2)

SYSTEM PARAMETERS ⁽³⁾Test Circuit Figure 24

V_FB

Feedback Voltage

4.5V ≤ V_IN≤ 40V, 0.2A ≤ I_LOAD≤ 3A

1.230

73

V

V_OUTprogrammed for 3V. Circuit of Figure 24.

1.193/1.180

1.267/1.280

V(min)

V(max)

%

η

Efficiency

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

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

(2) All limits ensured 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 specified via correlation using standard Statistical Quality Control

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

(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.

When the LM2599 is used as shown in the Figure 24 test circuit, system performance will be as shown in system parameters of

Electrical Characteristics section.

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

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

12V version. I_LOAD= 500 mA

Symbol

Parameter

Conditions

LM2599-XX

Units

(Limits)

Typ

Limit

(1)

(2)

DEVICE PARAMETERS

I_b

Feedback Bias Current

Adjustable Version Only, V_FB= 1.3V

10

nA

nA (max)

kHz

50/100

(3)

f_O

Oscillator Frequency

Saturation Voltage

See

150

127/110

173/173

kHz(min)

kHz(max)

V

(4) (5)

V_SAT

DC

I_OUT= 3A

1.16

1.4/1.5

V(max)

%

(5)

Max Duty Cycle (ON)

Min Duty Cycle (OFF)

Current Limit

See

100

0

(6)

See

(4) (5)

I_CL

Peak Current,

4.5

A

3.6/3.4

6.9/7.5

50

A(min)

A(max)

μA(max)

mA

(4) (6) (7)

I_L

Output Leakage Current

See

Output = 0V

Output = −1V

2

5

30

10

mA(max)

mA

(6)

I_Q

Operating Quiescent

Current

SD /SS Pin Open

mA(max)

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

(2) All limits ensured 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 specified via correlation using standard Statistical Quality Control

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

(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 0V to force the output transistor switch ON.

(6) 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.

(7) V_IN= 40V.

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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

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

12V version. I_LOAD= 500 mA

Symbol

Parameter

Conditions

LM2599-XX

Units

(Limits)

Typ

Limit

(1)

(2)

(7)

I_STBY

Standby Quiescent

SD /SS pin = 0V

80

μA

μA(max)

°C/W

Current

200/250

θ_JC

θ_JA

Thermal Resistance

TO220 or TO263 Package, Junction to Case

2

(8)

TO220 Package, Juncton to Ambient

50

30

20

(9)

TO263 Package, Juncton to Ambient

(10)

TO263 Package, Juncton to Ambient

(11)

TO263 Package, Juncton to Ambient

SHUTDOWN/SOFT-START CONTROL Test Circuit of Figure 24

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

I_SD

I_SS

Soft-start Voltage

Shutdown Current

Soft-start Current

V_OUT= 20% of Nominal Output Voltage

V_OUT= 100% of Nominal Output Voltage

V_SHUTDOWN= 0.5V

2

3

5

μA

10

5

μA(max)

μA

V_Soft-start= 2.5V

1.6

96

μA(max)

FLAG/DELAY CONTROL Test Circuit of Figure 24

Regulator Dropout Detector

Threshold Voltage

Low (Flag ON)

%

%(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

Delay Pin Threshold

Voltage

0.3

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)

(8) 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.

(9) 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.

(10) 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.

(11) 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 LM2599S side of the board, and approximately 16 in²of copper on the other side of the p-c board. See Application

Information in this data sheet and the thermal model in Switchers Made Simple version 4.2.1 (or later) software.

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Typical Performance Characteristics

(Circuit of Figure 24)

Normalized

Output Voltage

Line Regulation

Figure 3.

Figure 4.

Switch Saturation

Voltage

Efficiency

Figure 5.

Figure 6.

Switch Current Limit

Dropout Voltage

Figure 7.

Figure 8.

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Typical Performance Characteristics (continued)

(Circuit of Figure 24)

Operating

Quiescent Current

Shutdown

Quiescent Current

Figure 9.

Figure 10.

Minimum Operating

Supply Voltage

Feedback Pin

Bias Current

Figure 11.

Figure 12.

Flag Saturation

Voltage

Switching Frequency

Figure 13.

Figure 14.

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Typical Performance Characteristics (continued)

(Circuit of Figure 24)

Shutdown /Soft-start

Current

Soft-start

Figure 15.

Figure 16.

Daisy Pin Current

Soft-start Response

Figure 17.

Figure 18.

Continuous Mode Switching Waveforms

V_IN= 20V, V_OUT= 5V, I_LOAD= 2A

L = 32 μH, C_OUT= 220 μF, C_OUTESR = 50 mΩ

Shutdown/Soft-start

Threshold Voltage

A: Output Pin Voltage, 10V/div.

B: Inductor Current 1A/div.

C: Output Ripple Voltage, 50 mV/div.

Figure 19.

Figure 20. Horizontal Time Base: 2 μs/div.

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Typical Performance Characteristics (continued)

(Circuit of Figure 24)

Discontinuous Mode Switching Waveforms

Load Transient Response for Continuous Mode

V_IN= 20V, V_OUT= 5V, I_LOAD= 500 mA

V_IN= 20V, V_OUT= 5V, I_LOAD= 500 mA to 2A

L = 10 μH, C_OUT= 330 μF, C_OUTESR = 45 mΩ

L = 32 μH, C_OUT= 220 μF, C_OUTESR = 50 mΩ

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

B: 500 mA to 2A Load Pulse

A: Output Pin Voltage, 10V/div.

B: Inductor Current 0.5A/div.

C: Output Ripple Voltage, 100 mV/div.

Figure 21. Horizontal Time Base: 2 μs/div.

Figure 22. Horizontal Time Base: 50 μs/div.

Load Transient Response for Discontinuous Mode

V_IN= 20V, V_OUT= 5V, I_LOAD= 500 mA to 2A

L = 10 μH, C_OUT= 330 μF, C_OUTESR = 45 mΩ

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

B: 500 mA to 2A Load Pulse

Figure 23. Horizontal Time Base: 200 μs/div.

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Test Circuit and Layout Guidelines

Fixed Output Voltage Versions

Component Values shown are for V_IN= 15V,

V_OUT= 5V, I_LOAD= 3A.

C_IN

—

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

C_OUT

D1

—

220 μF, 25V Aluminum Electrolytic, Nichicon “PL Series”

5A, 40V Schottky Rectifier, 1N5825

68 μH, L38

—

L1

Typical Values

C_SS0.1 μF

—

C_DELAY

R_{Pull Up}

—

0.1 μF

4.7k

Adjustable Output Voltage Versions

where V_REF= 1.23V

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

Component Values shown are for V_IN= 20V,

V_OUT= 10V, I_LOAD= 3A.

C_IN

C_OUT

:

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

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

:

D1 — 5A, 30V Schottky Rectifier, 1N5824

L1 — 68 μH, L38

R₁— 1 kΩ, 1%

R₂— 7.15k, 1%

C_FF— 3.3 nF, See Application Information Section

R_FF— 3 kΩ, See Application Information Section

Typical Values

C_SS—0.1 μF

C_DELAY—0.1 μF

R_{PULL UP}—4.7k

Figure 24. Standard Test Circuits and Layout Guides

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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 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.

If open core inductors are used, special care must be taken as to 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 locate both resistors near the IC, and route the wiring away from the inductor,

especially an open core type of inductor. (See Application Information section for more information.)

Table 1. LM2599 Series Buck Regulator Design Procedure (Fixed Output)

PROCEDURE (Fixed Output Voltage Version)

EXAMPLE (Fixed Output Voltage Version)

Given:

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

V_IN(max) = Maximum DC Input Voltage

I_LOAD(max) = Maximum Load Current

V_OUT= 5V

V_IN(max) = 12V

I_LOAD(max) = 3A

1. Inductor Selection (L1)

A. Select the correct inductor value selection guide from Figure 25, A. Use the inductor selection guide for the 5V version shown in

Figure 26, or 6. (Output voltages of 3.3V, 5V, or 12V respectively.) Figure 26.

For all other voltages, see the Design Procedure for the adjustable

version.

B. From the inductor value selection guide shown in Figure 26, the

inductance region intersected by the 12V horizontal line and the 3A

B. From the inductor value selection guide, identify the inductance vertical line is 33 μH, and the inductor code is L40.

region intersected by the Maximum Input Voltage line and the

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

Maximum Load Current line. Each region is identified by an

Table 5, go to the L40 line and choose an inductor part number from

inductance value and an inductor code (LXX).

any of the four manufacturers shown. (In most instance, both

C. Select an appropriate inductor from the four manufacturer's part through hole and surface mount inductors are available.)

numbers listed in Table 5.

2. Output Capacitor Selection (C_OUT

)

2. Output Capacitor Selection (C_OUT)

A. In the majority of applications, low ESR (Equivalent Series A. See section on output capacitors in Application Information

Resistance) electrolytic capacitors between 82 μF and 820 μF and section.

low ESR solid tantalum capacitors between 10 μF and 470 μF

B. From the quick design component selection table shown in

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

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

the IC using short capacitor leads and short copper traces. Do not

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

use capacitors larger than 820 μF.

needed in your application, for this example, use the 3A line. In the

For additional information, see section on output capacitors in maximum input voltage column, select the line that covers the input

Application Information section.

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.

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

design component selection table shown in Table 2. This table

contains different input voltages, output voltages, and load currents, The capacitor list contains both through hole electrolytic and surface

and lists various inductors and output capacitors that will provide the mount tantalum capacitors from four different capacitor

best design solutions.

manufacturers. It is recommended that both the manufacturers and

the manufacturer's series that are listed in the table be used.

C. The capacitor voltage rating for electrolytic capacitors should be

at least 1.5 times greater than the output voltage, and often much In this example aluminum electrolytic capacitors from several

higher voltage ratings are needed to satisfy the low ESR different manufacturers are available with the range of ESR numbers

requirements for low output ripple voltage.

needed.

D. For computer aided design software, see Switchers Made

Simple (version 4.2.1 or later).

330 μF 35V Panasonic HFQ Series

330 μF 35V Nichicon PL Series

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

is needed. But even a low ESR, switching grade, 220 μF 10V

aluminum electrolytic capacitor would exhibit approximately 225 mΩ

of ESR (see the curve in Figure 33 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 value or with a higher voltage rating (lower

ESR) should be selected. A 16V or 25V capacitor will reduce the

ripple voltage by approximately half.

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Table 1. LM2599 Series Buck Regulator Design Procedure (Fixed Output) (continued)

PROCEDURE (Fixed Output Voltage Version)

EXAMPLE (Fixed Output Voltage Version)

3. Catch Diode Selection (D1)

A. The catch diode current rating must be at least 1.3 times greater A. Refer to the table shown in Table 8. In this example, a 5A, 20V,

than the maximum load current. Also, if the power supply design 1N5823 Schottky diode will provide the best performance, and will

must withstand a continuous output short, the diode should have a not be overstressed even for a shorted output.

current rating equal to the maximum current limit of the LM2599. The

most stressful condition for this diode is an overload or shorted

output condition.

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 LM2599 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

reverse recovery times of 50 ns or less. Rectifiers such as the

IN5400 series are much too slow and should not be used.

4. Input Capacitor (C_IN

)

4. Input Capacitor (C_IN)

A low ESR aluminum or tantalum bypass capacitor is needed The important parameters for the Input capacitor are the input

between the input pin and ground to prevent large voltage transients voltage rating and the RMS current rating. With a nominal input

from appearing at the input. In addition, the RMS current rating of voltage of 12V, an aluminum electrolytic capacitor with a voltage

the input capacitor should be selected to be at least ½ the DC load rating greater than 18V (1.5 × V_IN) would be needed. The next

current. The capacitor manufacturers data sheet must be checked to higher capacitor voltage rating is 25V.

assure that this current rating is not exceeded. The curve shown in

The RMS current rating requirement for the input capacitor in a buck

Figure 32 shows typical RMS current ratings for several different

regulator is approximately ½ the DC load current. In this example,

aluminum electrolytic capacitor values.

with a 3A load, a capacitor with a RMS current rating of at least 1.5A

This capacitor should be located close to the IC using short leads is needed. The curves shown in Figure 32 can be used to select an

and the voltage rating should be approximately 1.5 times the appropriate input capacitor. From the curves, locate the 35V line and

maximum input voltage.

note which capacitor values have RMS current ratings greater than

1.5A. A 680 μF, 35V capacitor could be used.

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 680 μF/35V 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.

Use caution when using ceramic capacitors for input bypassing,

because it may cause severe ringing at the V_INpin.

For additional information, see section on input capacitors 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.

Table 2. LM2599 Fixed Voltage Quick Design Component Selection Table

Conditions

Inductor

Output Capacitor

Through Hole Electrolytic Surface Mount Tantalum

Output

Voltage

(V)

Load

Current

(A)

Max Input

Voltage (V)

Panasonic

HFQ Series

(μF/V)

Nichicon

AVX TPS

Series

Sprague

595D Series

(μF/V)

Inductance

Inductor

(#)

(μH)

PL Series

(μF/V)

5

7

22

33

22

33

47

L41

L40

L33

L32

L39

470/25

560/16

560/35

680/35

470/35

330/35

270/50

330/6.3

220/10

390/6.3

330/10

560/35

3

2

10

40

6

680/35

3.3

560/35

470/25

10

40

330/35

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Table 2. LM2599 Fixed Voltage Quick Design Component Selection Table (continued)

Conditions

Inductor

Output Capacitor

Through Hole Electrolytic Surface Mount Tantalum

Output

Voltage

(V)

Load

Current

(A)

Max Input

Voltage (V)

Panasonic

HFQ Series

(μF/V)

Nichicon

AVX TPS

Series

(μF/V)

220/10

100/10

100/16

68/20

Sprague

595D Series

(μF/V)

Inductance

Inductor

(#)

(μH)

PL Series

(μF/V)

560/16

560/25

330/35

270/35

560/16

180/35

470/25

330/25

180/25

180/35

330/25

180/25

82/25

8

22

33

47

22

68

22

33

68

33

68

150

L41

L40

L39

L33

L38

L41

L40

L44

L32

L38

L42

470/25

560/25

330/35

470/25

180/35

470/25

330/25

180/25

180/35

330/25

180/25

82/25

330/10

270/10

180/16

120/20

180/16

120/20

68/25

10

15

40

9

3

2

3

2

5

20

40

15

18

30

40

15

20

40

12

Table 3. LM2599 Series Buck Regulator Design Procedure (Adjustable Output)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

Given:

V_OUT= Regulated Output Voltage

V_IN(max) = Maximum Input Voltage

I_LOAD(max) = Maximum Load Current

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

V_OUT= 20V

V_IN(max) = 28V

I_LOAD(max) = 3A

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

1. Programming Output Voltage (Selecting R₁and R₂, as shown in 1. Programming Output Voltage (Selecting R₁and R₂, as shown in

Figure 24)

Use the following formula to select the appropriate resistor values.

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

(1)

(3)

Select a value for R₁between 240Ω 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.)

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

R₂= 15.4 kΩ.

(2)

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Table 3. LM2599 Series Buck Regulator Design Procedure (Adjustable Output) (continued)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

2. Inductor Selection (L1)

A. Calculate the inductor Volt • microsecond constant E • T (V • μs), A. Calculate the inductor Volt • microsecond constant (E • T),

from the following formula:

(4)

(5)

where V_SAT= internal switch saturation voltage = 1.16V and V_D

diode forward voltage drop = 0.5V

=

B. E • T = 34.2 (V • μs)

C. I_LOAD(max) = 3A

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 28.

D. From the inductor value selection guide shown in Figure 28, the

inductance region intersected by the 34 (V • μs) horizontal line and

the 3A vertical line is 47 μH, and the inductor code is L39.

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

E. From the table in Table 5, locate line L39, and select an inductor

part number from the list of manufacturers part numbers.

D. Identify the inductance region intersected by the E • T value and

the Maximum Load Current value. Each region is identified by an

inductance value and an inductor code (LXX).

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

numbers listed in Table 5.

3.Output Capacitor Selection (C_OUT

)

3. Output Capacitor SeIection (C_OUT)

A. In the majority of applications, low ESR electrolytic or solid A. See section on C_OUTin Application Information section.

tantalum capacitors between 82 μF and 820 μF provide the best

B. From the quick design table shown in Table 4, locate the output

results. This capacitor should be located close to the IC using short

voltage column. From that column, locate the output voltage closest

capacitor leads and short copper traces. Do not use capacitors

to the output voltage in your application. In this example, select the

larger than 820 μF. For additional information, see section on

output capacitors in Application Information section.

24V line. Under the OUTPUT CAPACITOR section, select

a

capacitor from the list of through hole electrolytic or surface mount

B. To simplify the capacitor selection procedure, refer to the quick tantalum types from four different capacitor manufacturers. It is

design table shown in Table 4. This table contains different output recommended that both the manufacturers and the manufacturers

voltages, and lists various output capacitors that will provide the best series that are listed in the table be used.

design solutions.

In this example, through hole aluminum electrolytic capacitors from

C. The capacitor voltage rating should be at least 1.5 times greater several different manufacturers are available.

than the output voltage, and often much higher voltage ratings are

needed to satisfy the low ESR requirements needed for low output

ripple voltage.

220/35 Panasonic HFQ Series

150/35 Nichicon PL Series

C. For a 20V output, a capacitor rating of at least 30V or more is

needed. In this example, either a 35V or 50V capacitor would work.

A 50V rating was chosen because it has a lower ESR which

provides a lower output ripple voltage.

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 24)

4. Feedforward Capacitor (C_FF)

For output voltages greater than approximately 10V, an additional The table shown in Table 4 contains feed forward capacitor values

capacitor is required. The compensation capacitor is typically for various output voltages. In this example, a 560 pF capacitor is

between 100 pF and 33 nF, and is wired in parallel with the output needed.

voltage setting resistor, R₂. It provides additional stability for high

output voltages, low input-output voltages, and/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.)

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Table 3. LM2599 Series Buck Regulator Design Procedure (Adjustable Output) (continued)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

5. Catch Diode Selection (D1)

A. The catch diode current rating must be at least 1.3 times greater A. Refer to the table shown in Table 8. Schottky diodes provide the

than the maximum load current. Also, if the power supply design best performance, and in this example a 3A, 40V, 1N5825 Schottky

must withstand a continuous output short, the diode should have a diode would be a good choice. The 3A diode rating is more than

current rating equal to the maximum current limit of the LM2599. The adequate and will not be overstressed even for a shorted output.

most stressful condition for this diode is an overload or shorted

output condition.

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 LM2599 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

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 are much too slow and

should not be used.

6. Input Capacitor (C_IN

)

6. Input Capacitor (C_IN)

A low ESR aluminum or tantalum bypass capacitor is needed The important parameters for the Input capacitor are the input

between the input pin and ground to prevent large voltage transients voltage rating and the RMS current rating. With a nominal input

from appearing at the input. In addition, the RMS current rating of voltage of 28V, an aluminum electrolytic aluminum electrolytic

the input capacitor should be selected to be at least ½ the DC load capacitor with a voltage rating greater than 42V (1.5 × V_IN) would be

current. The capacitor manufacturers data sheet must be checked to needed. Since the the next higher capacitor voltage rating is 50V, a

assure that this current rating is not exceeded. The curve shown in 50V capacitor should be used. The capacitor voltage rating of (1.5 ×

Figure 32 shows typical RMS current ratings for several different V_IN) is a conservative guideline, and can be modified somewhat if

aluminum electrolytic capacitor values.

desired.

This capacitor should be located close to the IC using short leads The RMS current rating requirement for the input capacitor of a buck

and the voltage rating should be approximately 1.5 times the regulator is approximately ½ the DC load current. In this example,

maximum input voltage.

with a 3A load, a capacitor with a RMS current rating of at least 1.5A

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 32 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 1.5A. Either

a 470 μF or 680 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_IN

pin.

For a through hole design, a 680 μF/50V 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 additional information, see section on input capacitor in

Application Information section.

For surface mount designs, solid tantalum capacitors can be used,

but caution must be exercised with regard to the capacitor sure

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.

To further simplify the buck regulator design procedure, Texas

Instruments is making available computer design software to be

used with the Simple Switcher line ot switching regulators.Switchers

Made Simple (version 4.2.1 or later) is available on a 3½″ diskette

for IBM compatible computers.

Table 4. Output Capacitor and Feedforward Capacitor Selection Table

Output

Voltage

(V)

Through Hole Output Capacitor

Surface Mount Output Capacitor

Panasonic

HFQ Series

(μF/V)

Nichicon PL

Series

Feedforward

AVX TPS

Series

Sprague

595D Series

(μF/V)

Feedforward

Capacitor

(μF/V)

2

4

6

820/35

33 nF

10 nF

3.3 nF

330/6.3

220/10

470/4

33 nF

10 nF

3.3 nF

560/35

470/35

390/6.3

470/25

330/10

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Table 4. Output Capacitor and Feedforward Capacitor Selection Table (continued)

Output

Voltage

(V)

Through Hole Output Capacitor

Surface Mount Output Capacitor

Panasonic

HFQ Series

(μF/V)

Nichicon PL

Series

Feedforward

AVX TPS

Series

(μF/V)

100/16

68/20

Sprague

595D Series

(μF/V)

Feedforward

Capacitor

(μF/V)

9

330/25

1.5 nF

1 nF

180/16

1.5 nF

1 nF

1 2

1 5

2 4

2 8

330/25

180/16

220/35

680 pF

560 pF

390 pF

120/20

680 pF

220 pF

220/35

150/35

33/25

100/50

10/35

15/50

LM2599 Series Buck Regulator Design Procedure

INDUCTOR VALUE SELECTION GUIDES

(For Continuous Mode Operation)

Figure 25. LM2599-3.3

Figure 26. LM2599-5.0

Figure 27. LM2599-12

Figure 28. LM2599-ADJ

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Coilcraft

Table 5. Inductor Manufacturers Part Numbers

Inductance

Current

(A)

Schott

Renco

Through

Hole

Pulse Engineering

(μH)

Through

Hole

Surface

Mount

Surface

Mount

Through

Hole

Surface

Mount

Surface

Mount

L15

L21

L22

L23

L24

L25

L26

L27

L28

L29

L30

L31

L32

L33

L34

L35

L36

L37

L38

L39

L40

L41

L42

L43

L44

22

68

0.99

1.17

1.40

1.70

2.1

67148350 67148460 RL-1284-22-43

67144070 67144450 RL-5471-5

67144080 67144460 RL-5471-6

67144090 67144470 RL-5471-7

67148370 67148480 RL-1283-22-43

67148380 67148490 RL-1283-15-43

67144100 67144480 RL-5471-1

67144110 67144490 RL-5471-2

67144120 67144500 RL-5471-3

67144130 67144510 RL-5471-4

67144140 67144520 RL-5471-5

67144150 67144530 RL-5471-6

67144160 67144540 RL-5471-7

67148390 67148500 RL-1283-22-43

67148400 67148790 RL-1283-15-43

RL1500-22 PE-53815 PE-53815-S

RL1500-68 PE-53821 PE-53821-S

DO3308-223

DO3316-683

DO3316-473

DO3316-333

DO3316-223

DO3316-153

DOS022P-334

DOS022P-224

DOS022P-154

DOS022P-104

DOS022P-683

DOS022P-473

DOS022P-333

DOS022P-223

DOS022P-153

—

47

—

PE-53822 PE-53822-S

PE-53823 PE-53823-S

PE-53824 PE-53825-S

PE-53825 PE-53824-S

PE-53826 PE-53826-S

PE-53827 PE-53827-S

PE-53828 PE-53828-S

PE-53829 PE-53829-S

PE-53830 PE-53830-S

PE-53831 PE-53831-S

PE-53932 PE-53932-S

PE-53933 PE-53933-S

PE-53934 PE-53934-S

PE-53935 PE-53935-S

PE-54036 PE-54036-S

PE-54037 PE-54037-S

PE-54038 PE-54038-S

PE-54039 PE-54039-S

PE-54040 PE-54040-S

PE-54041 PE-54041-S

PE-54042 PE-54042-S

33

22

15

330

220

150

100

68

0.80

1.00

1.20

1.47

1.78

2.2

47

33

2.5

22

3.1

15

3.4

220

150

100

68

1.70

2.1

67144170

67144180

67144190

67144200

67144210

—

RL-5473-1

RL-5473-4

RL-5472-1

RL-5472-2

RL-5472-3

—

2.5

—

3.1

—

47

3.5

—

33

3.5

67144220 67148290 RL-5472-4

67144230 67148300 RL-5472-5

—

22

3.5

—

150

100

68

2.7

67148410

67144240

67144250

—

RL-5473-4

RL-5473-2

RL-5473-3

—

3.4

PE-54043

PE-54044

—

3.4

—

Table 6. Inductor Manufacturers Phone Numbers

Coilcraft Inc.

Phone

FAX

(800) 322-2645

(708) 639-1469

+11 1236 730 595

+44 1236 730 627

(619) 674-8100

(619) 674-8262

+353 93 24 107

+353 93 24 459

(800) 645-5828

(516) 586-5562

(612) 475-1173

(612) 475-1786

Coilcraft Inc., Europe

Pulse Engineering Inc.

Phone

FAX

Phone

FAX

Pulse Engineering Inc.,

Europe

Phone

FAX

Renco Electronics Inc.

Phone

FAX

Schott Corp.

Phone

FAX

Table 7. Capacitor Manufacturers Phone Numbers

Nichicon Corp.

Panasonic

Phone

FAX

(708) 843-7500

(708) 843-2798

(714) 373-7857

(714) 373-7102

Phone

FAX

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Table 7. Capacitor Manufacturers Phone Numbers (continued)

AVX Corp.

Phone

FAX

(803) 448-9411

(803) 448-1943

(207) 324-4140

(207) 324-7223

Sprague/Vishay

Phone

FAX

Table 8. Diode Selection Table

3 Amp Diodes

4 to 6 Amp Diodes

Surface Mount Through Hole

Surface Mount

Schottky Ultra Fast

Through Hole

VR

Schottky

Ultra Fast

Schottky

Ultra Fast

Recovery

All of

Schottky

Ultra Fast

Recovery

All of

Recovery

All of

Recovery

All of

1N5820

SR302

SR502

1N5823

SB520

20V

30V

SK32

these

diodes

are rated

to at

MBR320

1N5821

MBR330

31DQ03

1N5822

SR304

diodes

are rated

to at

diodes

are rated

to at

diodes

are rated

to at

30WQ03

SK33

50WQ03

50WQ04

SR503

1N5824

SB530

SR504

1N5825

SB540

least

50V.

SK34

MBRS340

30WQ04

SK35

40V

MBR340

31DQ04

SR305

MURS320

30WF10

MUR320

MURS620

50WF10

MUR620

HER601

50V

or

more

MBRS360

30WQ05

MBR350

31DQ05

50WQ05

SB550

50SQ080

Block Diagram

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APPLICATION INFORMATION

PIN DESCRIPTIONS

+V_IN(Pin 1)This is the positive input supply for the IC switching regulator. A suitable input bypass capacitor must

be present at this pin to minimize voltage transients and to supply the switching currents needed by the

regulator.

Ground (Pin 4)Circuit ground.

Output (Pin 2)Internal switch. The voltage at this pin switches between approximately (+V_IN− V_SAT) and

approximately −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.

Shutdown /Soft-start (Pin 7)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

startup current and provides a controlled ramp up of the output voltage.

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 voltage. On start up, Error Flag is low until V_OUT

reaches 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.

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.

Special NoteIf any of the above three features (Shutdown /Soft-start, Error Flag, or Delay) are not used, the

respective pins should be left open.

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 operating regions (See the bottom curve in Figure 29).

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 approximately 85 μA.

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.

3. Soft-start Region. When the SD /SS pin voltage is between 1.8V and 2.8V (@ 25°C), the regulator is in a

Soft-start condition. The switch (Pin 2) 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 controlled ramp up. See the center curve in Figure 29. 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 startup currents (such as the

inverting configuration) which can load down the input power supply.

Note: The lower curve shown in Figure 29 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.

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

between the 3V maximum soft-start threshold and the 4.5V minimum input voltage. Figure 31 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.

Figure 29. Soft-start, Delay, Error, Output

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Figure 30. Timing Diagram for 5V Output

V_IN

LM2599

5

Q1

SD/SS

C_SS

Z1

3V

Figure 31. External 3.7V Soft-Start Clamp

DELAY CAPACITOR

C_DELAY— Provides delay for the error flag output. See the upper curve in Figure 29, and also refer to timing

diagrams in Figure 30. 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 1.3V, 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.

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

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C_FF— A Feedforward Capacitor C_FF, shown across R2 in Figure 24 is used when the output voltage is greater

than 10V or when C_OUThas a very low ESR. This capacitor adds lead compensation to the feedback loop and

increases the phase margin for better loop stability. For C_FFselection, see the 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 comparator to trigger the error flag. In this situation,

adding a resistor, R_FF, in series with the feedforward capacitor, approximately 3 times R1, will attenuate the

ripple voltage at the feedback pin.

INPUT CAPACITOR

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 input, 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 directly related to the RMS current rating.

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 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 electrolytic 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 current rating.

Figure 32. RMS Current Ratings for Low

ESR Electrolytic Capacitors (Typical)

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Figure 33. 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 needed to satisfy the RMS current requirements.

A graph shown in Figure 32 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 turn on 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 turn on 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.

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 needed. 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, will provide design solutions under all conditions.

If very low output ripple voltage (less than 15 mV) is required, refer to the section on OUTPUT VOLTAGE

RIPPLE AND TRANSIENTS for a post ripple filter.

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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 33). Often, capacitors with much

higher voltage ratings may be needed 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 the quick design component selection tables in

Table 2 and Table 4 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 @ −25°C and as much as 10X at −40°C. See curve shown in Figure 34.

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

temperatures below −25°C.

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 LM2599 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 (5V 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 IN5400 series are much too slow and should not be used.

Figure 34. Capacitor ESR Change vs Temperature

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.

The LM2599 (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. 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 voltages.

To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Figure 25

through 7). 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 35).

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Figure 35. (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, 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 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 see section on OPEN CORE INDUCTORS.

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.

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 and/or the LM2599. 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.

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 (1A and below), the maximum switch current will still be less than the switch

current limit.

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

The output pin (switch) waveform can have some damped sinusoidal ringing present. (See Typical Performance

Characteristics photo titled Discontinuous Mode Switching Waveforms) 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 or the diode are conducting, and the inductor current has dropped to zero. During this

time, a small amount of energy can circulate between the inductor and the switch/diode parasitic capacitance

causing this characteristic ringing. 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 and/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. The computer aided design software Switchers Made Simple (version 4.3) will provide all component

values for continuous and discontinuous modes of operation.

Figure 36. Post Ripple Filter Waveform

OUTPUT VOLTAGE RIPPLE AND TRANSIENTS

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

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

waveform.

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 needed (less than 20 mV), a post ripple filter is recommended (See Figure 24).

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.

The photo shown in Figure 36 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, 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.

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 minimize these voltage spikes, the output

capacitor should be designed for switching regulator applications, and the lead lengths must be kept very short.

Wiring inductance, stray capacitance, as well as the scope probe used to evaluate these transients, all contribute

to the amplitude of these spikes.

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Figure 37. 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

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 will reach zero, and

the switcher will smoothly change from a continuous to a discontinuous mode of operation. Most switcher

designs (irregardless how large the inductor value is) will be 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 25 through 7 are used to select an inductor value, the peak-to-peak inductor ripple current can

immediately be determined. The curve shown in Figure 37 shows the range of (ΔI_IND) that can be expected for

different load currents. The curve also shows how the peak-to-peak inductor ripple current (ΔI_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 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= 5V, maximum load current of 2.5A

V_IN= 12V, nominal, varying between 10V and 16V.

The selection guide in Figure 26 shows that the vertical line for a 2.5A load current, and the horizontal line for the

12V input voltage intersect approximately midway between the upper and lower borders of the 33 μH inductance

region. A 33 μH inductor will allow a peak-to-peak inductor current (ΔI_IND) to flow that will be a percentage of the

maximum load current. Referring to Figure 37, follow the 2.5A line approximately midway into the inductance

region, and read the peak-to-peak inductor ripple current (ΔI_IND) on the left hand axis (approximately 620 mA p-

p).

As the input voltage increases to 16V, it approaches the upper border of the inductance region, and the inductor

ripple current increases. Referring to the curve in Figure 37, it can be seen that for a load current of 2.5A, the

peak-to-peak inductor ripple current (ΔI_IND) is 620 mA with 12V in, and can range from 740 mA at the upper

border (16V in) to 500 mA at the lower border (10V in).

Once the ΔI_INDvalue is known, the following formulas can be used to calculate additional information about the

switching regulator circuit.

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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

= 0.62A×0.1Ω=62 mV p-p

)

–

4. added

for

line

break

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 will induce a voltage into any wire or PC board copper trace that

comes within the inductor's magnetic field. 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 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.

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 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 will correct the problem, or re-arranging the PC layout may be necessary. Magnetic

flux cutting the IC device ground trace, feedback trace, or the positive or negative traces of the output capacitor

should be minimized.

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.

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” inductors are an inexpensive, simple way of

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

THERMAL CONSIDERATIONS

The LM2599 is available in two packages, a 7-pin TO-220 (NDZ) and a 7-pin surface mount TO-263 (KTW).

The TO-220 package needs a heat sink under most conditions. The size of the heat sink depends on the input

voltage, the output voltage, the load current and the ambient temperature. The curves in Figure 38 show the

LM2599T junction temperature rises above ambient temperature for a 3A load and different input and output

voltages. The data for these curves was taken with the LM2599T (TO-220 package) operating as a buck

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

more heat sinking.

The TO-263 surface mount package tab is designed to be soldered to the copper on a printed circuit board. The

copper and the board are the heat sink for this package and the other heat producing components, such as the

catch diode and inductor. The pc board copper area that the package is soldered to should be at least 0.4 in²,

and ideally should have 2 or more square inches of 2 oz. (0.0028 in) copper. Additional copper area improves

the thermal characteristics, but with copper areas greater than approximately 6 in², only small improvements in

heat dissipation are realized. If further thermal improvements are needed, double sided, multilayer pc-board with

large copper areas and/or airflow are recommended.

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The curves shown in Figure 39 show the LM2599S (TO-263 package) junction temperature rise above ambient

temperature with a 2A 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. When load currents higher than 2A are used, double sided or multilayer pc-boards with large

copper areas and/or airflow might be needed, especially for high ambient temperatures and high output voltages.

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

Capacitors

Inductor

Diode

Through hole electrolytic

Through hole Renco

Through hole, 5A 40V, Schottky

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

PC board

Figure 38. Junction Temperature Rise, TO-220

Circuit Data for Temperature Rise Curve TO-263 Package (KTW)

Surface mount tantalum, molded “D” size

Capacitors

Inductor

Diode

Surface mount, Pulse engineering, 68 μH

Surface mount, 5A 40V, Schottky

PC board

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

Figure 39. Junction Temperature Rise, TO-263

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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 surrounding air is still or moving. Furthermore, some of these components 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.

SHUTDOWN /SOFT-START

The circuit shown in Figure 42 is a standard buck regulator with 20V in, 12V out, 1A load, and using a 0.068 μF

Soft-start capacitor. The photo in Figure 40 Figure 41 show the effects of Soft-start on the output voltage, the

input current, with, and without a Soft-start capacitor. The reduced input current required at startup is very

evident when comparing the two photos. The Soft-start feature reduces the startup current from 2.6A down to

650 mA, and delays and slows down the output voltage rise time.

Figure 40. Output Voltage, Input Current,

at Start-Up, WITH Soft-start

Figure 41. Output Voltage, Input Current,

at Start-Up, WITHOUT Soft-start

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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.

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

Figure 42. Typical Circuit Using Shutdown /Soft-start and Error Flag Features

Figure 43. Inverting −5V Regulator With Shutdown and Soft-start

lNVERTING REGULATOR

The circuit in Figure 43 converts a positive input voltage to a negative output voltage with a common ground. The

circuit operates by bootstrapping the regulator's ground pin to the negative output voltage, then grounding the

feedback pin, the regulator senses the inverted output voltage and regulates it.

This example uses the LM2599-5 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 45 provides a guide as to the amount

of output load current possible for the different input and output voltage conditions.

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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 between the input pin and ground pin. The LM2599 has a maximum input voltage rating of 40V.

Figure 44. Inverting Regulator

Figure 45. Maximum Load Current for

Inverting Regulator Circuit

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 IN5400 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 33 μH, 3.5A inductor is the best choice. Capacitor

selection can also be narrowed down to just a few values. Using the values shown in Figure 43 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 LM2599 current limit (approximately 4.5A) 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 loaded down, may not work correctly. Because of the relatively high startup currents required by

the inverting topology, the Soft-start feature shown in Figure 43 is recommended.

Also shown in Figure 43 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.

UNDERVOLTAGE LOCKOUT

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

Figure 46 contains a undervoltage lockout circuit for a buck configuration, while Figure 47 and 30 are for the

inverting types (only the circuitry pertaining to the undervoltage lockout is shown). Figure 46 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 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 1.5V greater than the zener voltage.

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SNVS123C –APRIL 1998–REVISED APRIL 2013

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Figure 46. Undervoltage Lockout for a Buck Regulator

Figure 47 and 30 apply the same feature to an inverting circuit. Figure 47 features a constant threshold voltage

for turn on and turn off (zener voltage plus approximately one volt). If hysteresis is needed, the circuit in

Figure 48 has a turn ON voltage which is different than the turn OFF voltage. The amount of hysteresis is

approximately equal to the value of the output voltage. 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.

Figure 47. Undervoltage Lockout Without

Hysteresis for an Inverting Regulator

Figure 48. 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 49. This unregulated negative voltage is approximately equal to the positive input voltage

(minus a few volts), and can supply up to a 600 mA of output current. There is a requirement however, that there

be a minimum load of 1.2A 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 LM2599 current limit (typically

4.5A).

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.

34

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SNVS123C –APRIL 1998–REVISED APRIL 2013

Figure 49. Charge Pump for Generating a

Low Current, Negative Output Voltage

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

C_IN

C_OUT

:

— 470 μF, 50V, Aluminum Electrolytic Panasonic, “HFQ Series”

— 330 μF, 35V, Aluminum Electrolytic Panasonic, “HFQ Series”

:

D1: — 5A, 40V Schottky Rectifier, 1N5825

L1: — 47 μH, L39, Renco, Through Hole

R_{PULL UP}

C_DELAY

C_SD/SS

:

— 10k

— 0.1 μF

:

Thermalloy Heat Sink #7020

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

C_IN

C_OUT

:

— 470 μF, 50V, Aluminum Electrolytic Panasonic, “HFQ Series”

— 220 μF, 35V Aluminum Electrolytic Panasonic, “HFQ Series”

:

D1: — 5A, 40V Schottky Rectifier, 1N5825

L1: — 47 μH, L39, Renco, Through Hole

R₁: — 1 kΩ, 1%

R₂: — Use formula in Design Procedure

C_FF

R_FF

:

— See Figure 25.

— See Application Information Section (C_FFSection)

R_{PULL UP}

C_DELAY

C_SD/SS

:

— 10k

— 0.1 μF

:

Thermalloy Heat Sink #7020

Figure 50. PC Board Layout

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SNVS123C –APRIL 1998–REVISED APRIL 2013

REVISION HISTORY

Changes from Revision B (April 2013) to Revision C

Page

•

Changed layout of National Data Sheet to TI format .......................................................................................................... 36

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PACKAGE MATERIALS INFORMATION

www.ti.com

23-Sep-2013

TAPE AND REEL INFORMATION

*All dimensions are nominal

Device

Package Package Pins

Type Drawing

SPQ

Reel

A0

B0

K0

P1

W

Pin1

Diameter Width (mm) (mm) (mm) (mm) (mm) Quadrant

(mm) W1 (mm)

LM2599SX-12/NOPB

LM2599SX-3.3/NOPB

LM2599SX-5.0/NOPB

DDPAK/

TO-263

KTW

7

500

330.0

24.4

10.75 14.85

5.0

16.0

24.0

Q2

DDPAK/

TO-263

DDPAK/

TO-263

LM2599SX-ADJ/NOPB DDPAK/

TO-263

Pack Materials-Page 1

PACKAGE MATERIALS INFORMATION

www.ti.com

23-Sep-2013

*All dimensions are nominal

Device

Package Type Package Drawing Pins

SPQ

Length (mm) Width (mm) Height (mm)

LM2599SX-12/NOPB

LM2599SX-3.3/NOPB

LM2599SX-5.0/NOPB

LM2599SX-ADJ/NOPB

DDPAK/TO-263

KTW

7

500

367.0

45.0

Pack Materials-Page 2

MECHANICAL DATA

NDZ0007B

TA07B (Rev E)

www.ti.com

MECHANICAL DATA

KTW0007B

TS7B (Rev E)

BOTTOM SIDE OF PACKAGE

www.ti.com

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型号：	LM2599
厂家：	TEXAS INSTRUMENTS
描述：	具有多种特性的 SIMPLE SWITCHER 电源转换器 150 KHz 3A 降压电压稳压器开关控制器开关式稳压器开关式控制器电源电路转换器开关式稳压器或控制器
文件：	总44页 (文件大小：2216K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LM2599 [TI]

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