LM2599_技术文档

April 1998

LM2599

SIMPLE SWITCHER^®Power Converter 150 kHz 3A

Step-Down Voltage Regulator, with Features

standby current. Self protection features include a two stage

current limit for the output switch and an over temperature

shutdown for complete protection under fault conditions.

General Description

The 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 avail-

able in fixed output voltages of 3.3V, 5V, 12V, and an adjust-

able output version.

Features

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

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

±

%

4

max over line and load conditions

This series of switching regulators is similar to the LM2596

series, with additional supervisory and performance features

added.

n Guaranteed 3A output current

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

Package

n Input voltage range up to 40V

n 150 kHz fixed frequency internal oscillator

n 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 de-

lay and error flag output.

n Out of regulation error flag

The LM2599 series operates at a switching frequency of 150

kHz thus allowing smaller sized filter components than what

would be needed with lower frequency switching regulators.

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

different lead bend options, and a 7-lead TO-263 Surface

mount package.

n Error output delay

n Low power standby mode, I_Qtypically 80 µA

n High Efficiency

n Uses readily available standard inductors

n Thermal shutdown and current limit protection

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

face mount types) are available from several different manu-

facturers optimized for use with the LM2599 series. This fea-

ture greatly simplifies the design of switch-mode power

supplies.

Applications

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

n Efficient pre-regulator for linear regulators

n On-card switching regulators

n Positive to Negative converter

±

Other features include a guaranteed 4% tolerance on out-

put voltage under all conditions of input voltage and output

†

Note: Patent Number 5,382,918.

±

%

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

ternal shutdown is included, featuring typically 80 µA

Typical Application (Fixed Output Voltage Versions)

DS012582-1

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

DS012582

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Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

ESD Susceptibility

Human Body Model (Note 3)

Lead Temperature

2 kV

S Package

Vapor Phase (60 sec.)

Infrared (10 sec.)

+215˚C

+245˚C

+260˚C

+150˚C

Maximum Supply Voltage (V_IN

)

45V

6V

SD /SS Pin Input Voltage (Note 2)

Delay Pin Voltage (Note 2)

Flag Pin Voltage

T Package (Soldering, 10 sec.)

Maximum Junction Temperature

1.5V

−0.3 ≤ V ≤45V

−0.3 ≤ V ≤+25V

Feedback Pin Voltage

Output Voltage to Ground

(Steady State)

Operating Conditions

Temperature Range

Supply Voltage

−1V

Internally limited

−65˚C to +150˚C

−40˚C ≤ T_J≤ +125˚C

Power Dissipation

4.5V to 40V

Storage Temperature Range

LM2599-3.3

Electrical Characteristics

=

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

ture Range.

Symbol

Parameter

Conditions

LM2599-3.3

Limit

Units

(Limits)

Typ

(Note 4)

(Note 5)

SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1

V_OUT

Output Voltage

4.75V ≤ V_IN≤ 40V, 0.2A ≤ I_LOAD≤ 3A

3.3

73

V

3.168/3.135

3.432/3.465

V(min)

V(max)

%

= =

V_IN12V, I_LOAD3A

η

Efficiency

LM2599-5.0

Electrical Characteristics

=

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

ture Range.

Symbol

Parameter

Conditions

LM2599-5.0

Limit

Units

(Limits)

Typ

(Note 4)

(Note 5)

SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1

V_OUT

Output Voltage

7V ≤ V_IN≤ 40V, 0.2A ≤ I_LOAD≤ 3A

5

V

4.800/4.750

5.200/5.250

V(min)

V(max)

%

=

η

Efficiency

V_IN12V, I_LOAD3A

80

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2

LM2599-12

Electrical Characteristics

=

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

ture Range.

Symbol

Parameter

Conditions

LM2599-12

Units

(Limits)

Typ

Limit

(Note 4)

(Note 5)

SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1

V_OUT

Output Voltage

15V ≤ V_IN≤ 40V, 0.2A ≤ I_LOAD≤ 3A

12

90

V

11.52/11.40

12.48/12.60

V(min)

V(max)

%

= =

V_IN25V, I_LOAD3A

η

Efficiency

LM2599-ADJ

Electrical Characteristics

=

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

Range.

Symbol

Parameter

Conditions

LM2599-ADJ

Units

(Limits)

Typ

Limit

(Note 4)

1.230

(Note 5)

SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1

V_FB

Feedback Voltage

4.5V ≤ V_IN≤ 40V, 0.2A ≤ I_LOAD≤ 3A

V

V_OUTprogrammed for 3V. Circuit of Figure 1.

1.193/1.180

1.267/1.280

V(min)

V(max)

%

=

η

Efficiency

V_IN12V, V_OUT3V, I_LOAD3A

73

All Output Voltage Versions

Electrical Characteristics

=

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

=

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

=

sion. I_LOAD500 mA

Symbol

Parameter

Conditions

LM2599-XX

Units

(Limits)

Typ

Limit

(Note

4)

(Note 5)

DEVICE PARAMETERS

=

I_b

Feedback Bias Current

Adjustable Version Only, V_FB1.3V

10

nA

50/100

nA (max)

kHz

f_O

Oscillator Frequency

Saturation Voltage

(Note 7)

150

127/110

kHz(min)

173/173 kHz(max)

=

V_SAT

DC

I_OUT3A (Note 8) (Note 9)

1.16

V

1.4/1.5

V(max)

%

Max Duty Cycle (ON)

Min Duty Cycle (OFF)

Current Limit

(Note 9)

100

0

(Note 10)

I_CL

Peak Current, (Note 8) (Note 9)

4.5

A

3.6/3.4

6.9/7.5

50

A(min)

A(max)

µA(max)

mA

=

Output 0V

I_L

Output Leakage Current

(Note 8) (Note 10) (Note 11)

=

Output −1V

2

5

30

10

mA(max)

mA

I_Q

Operating Quiescent

Current

SD /SS Pin Open (Note 10)

mA(max)

3

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All Output Voltage Versions

Electrical Characteristics ^(Continued)

=

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

=

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

=

sion. I_LOAD500 mA

Symbol

Parameter

Conditions

LM2599-XX

Units

(Limits)

Typ

Limit

(Note

4)

(Note 5)

DEVICE PARAMETERS

=

I_STBY

Standby Quiescent

SD /SS pin 0V

(Note 11)

80

µA

µA(max)

˚C/W

Current

200/250

θ_JC

θ_JA

Thermal Resistance

TO220 or TO263 Package, Junction to Case

TO220 Package, Juncton to Ambient (Note 12)

TO263 Package, Juncton to Ambient (Note 13)

TO263 Package, Juncton to Ambient (Note 14)

TO263 Package, Juncton to Ambient (Note 15)

2

50

30

20

˚C/W

SHUTDOWN/SOFT-START CONTROL Test Circuit of Figure 1

V_SD

Shutdown Threshold

Voltage

1.3

V

Low, (Shutdown Mode)

High, (Soft-start Mode)

0.6

2

V(max)

V(min)

V

=

%

V_SS

I_SD

I_SS

Soft-start Voltage

Shutdown Current

Soft-start Current

V_OUT20 of Nominal Output Voltage

2

3

5

=

%

V_OUT100 of Nominal Output Voltage

=

V_SHUTDOWN0.5V

µA

10

5

µA(max)

µA

=

V_Soft-start2.5V

1.6

96

µA(max)

FLAG/DELAY CONTROL Test Circuit of Figure 1

%

Regulator Dropout Detector

Threshold Voltage

Low (Flag ON)

%

92

98

(min)

(max)

V

%

=

VF_SAT

IF_L

Flag Output Saturation

Voltage

I_SINK3 mA

0.3

=

V_DELAY0.5V

0.7/1.0

V(max)

µA

=

Flag Output Leakage Current

Delay Pin Threshold

Voltage

V_FLAG40V

0.3

1.25

V

Low (Flag ON)

1.21

1.29

V(min)

V(max)

µA

High (Flag OFF) and V_OUTRegulated

=

Delay Pin Source Current

Delay Pin Saturation

V_DELAY0.5V

3

6

µA(max)

mV

Low (Flag ON)

55

350/400

mV(max)

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

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

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

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

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

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

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

culate Average Outgoing Quality Level (AOQL).

Note 6: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2599

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

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

load.

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

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

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4

All Output Voltage Versions

Electrical Characteristics ^(Continued)

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

switch OFF.

=

Note 11: V

40V.

IN

Note 12: Junction to ambient thermal resistance (no external heat sink) for the package mounted TO-220 package mounted vertically, with the leads soldered to a

2

printed circuit board with (1 oz.) copper area of approximately 1 in .

2

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

2

Note 14: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 2.5 in of (1 oz.) copper area.

2

Note 15: Junction to ambient thermal resistance with the TO-263 package tab soldered to a double sided printed circuit board with 3 in of (1 oz.) copper area on

2

the LM2599S side of the board, and approximately 16 in of copper on the other side of the p-c board. See application hints in this data sheet and the thermal model

in Switchers Made Simple version 4.2.1 (or later) software.

Typical Performance Characteristics (Circuit of Figure 1)

Normalized

Output Voltage

Line Regulation

Efficiency

DS012582-3

DS012582-4

DS012582-2

DS012582-5

DS012582-8

Switch Saturation

Voltage

Switch Current Limit

Dropout Voltage

DS012582-6

DS012582-7

Operating

Quiescent Current

Shutdown

Quiescent Current

Minimum Operating

Supply Voltage

DS012582-9

DS012582-10

5

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

Feedback Pin

Bias Current

Flag Saturation

Voltage

Switching Frequency

DS012582-13

DS012582-11

DS012582-12

Soft-start

Shutdown /Soft-start

Current

Daisy Pin Current

DS012582-14

DS012582-16

DS012582-15

Soft-start Response

Shutdown/Soft-start

Threshold Voltage

DS012582-18

DS012582-53

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6

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

Continuous Mode Switching Waveforms

=

V_IN20V, V_OUT5V, I_LOAD2A

Discontinuous Mode Switching Waveforms

=

= =

32 µH, C_OUT220 µF, C_OUTESR 50 mΩ

L

=

V_IN20V, V_OUT5V, I_LOAD500 mA

=

= =

10 µH, C_OUT330 µF, C_OUTESR 45 mΩ

L

DS012582-20

A: Output Pin Voltage, 10V/div.

B: Inductor Current 1A/div.

C: Output Ripple Voltage, 50 mV/div.

DS012582-19

A: Output Pin Voltage, 10V/div.

B: Inductor Current 0.5A/div.

C: Output Ripple Voltage, 100 mV/div.

Horizontal Time Base: 2 µs/div.

Load Transient Response for Continuous Mode

Load Transient Response for Discontinuous Mode

=

V_IN20V, V_OUT5V, I_LOAD500 mA to 2A

=

V_IN20V, V_OUT5V, I_LOAD500 mA to 2A

=

= =

32 µH, C_OUT220 µF, C_OUTESR 50 mΩ

L

=

= =

10 µH, C_OUT330 µF, C_OUTESR 45 mΩ

L

DS012582-22

DS012582-21

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

B: 500 mA to 2A Load Pulse

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

B: 500 mA to 2A Load Pulse

Horizontal Time Base: 200 µs/div.

Horizontal Time Base: 50 µs/div.

Connection Diagrams and Order Information

Bent and Staggered Leads, Through Hole Package

7-Lead TO-220 (T)

Surface Mount Package

7-Lead TO-263 (S)

DS012582-50

DS012582-23

Order Number LM2599T-3.3, LM2599T-5.0,

Order Number LM2599S-3.3, LM2599S-5.0,

LM2599S-12 or LM2599S-ADJ

LM2599T-12 or LM2599T-ADJ

See NS Package Number TA07B

See NS Package Number TS7B

7

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Test Circuit and Layout Guidelines

Fixed Output Voltage Versions

DS012582-24

=

Component Values shown are for V

15V,

IN

=

3A.

V

OUT

5V, I

LOAD

C

D1

L1

—

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

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

5A, 40V Schottky Rectifier, 1N5825

IN

OUT

68 µH, L38

Typical Values

C

R

—

0.1 µF

SS

—

0.1 µF

4.7k

DELAY

Pull Up

—

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8

Test Circuit and Layout Guidelines (Continued)

Adjustable Output Voltage Versions

DS012582-25

=

where V

1.23V

REF

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

1

=

Component Values shown are for V

20V,

IN

=

3A.

V

OUT

10V, I

LOAD

C

:

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

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

IN

:

OUT

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

L1 — 68 µH, L38

R

C

R

— 1 kΩ, 1%

— 7.15k, 1%

1

2

— 3.3 nF, See Application Information Section

— 3 kΩ, See Application Information Section

FF

Typical Values

C

R

— 0.1 µF

SS

— 0.1 µF

DELAY

— 4.7k

PULL UP

FIGURE 1. Standard Test Circuits and Layout Guides

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

idly switching currents associated with wiring inductance can

generate voltage transients which can cause problems. For

minimal inductance and ground loops, the wires indicated by

heavy lines should be wide printed circuit traces and

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

ternal components should be located as close to the

switcher lC as possible using ground plane construction or

single point grounding.

If open core inductors are used, special care must be

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

tor. Allowing the inductor flux to intersect sensitive feedback,

lC groundpath and C_OUTwiring can cause problems.

When using the adjustable version, special care must be

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

sociated wiring. Physically locate both resistors near the IC,

and route the wiring away from the inductor, especially an

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

information.)

9

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LM2599 Series Buck Regulator Design Procedure (Fixed Output)

PROCEDURE (Fixed Output Voltage Version)

EXAMPLE (Fixed Output Voltage Version)

Given:

=

V_OUT5V

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

=

V_IN(max) 12V

V_IN(max) Maximum DC Input Voltage

=

I_LOAD(max) 3A

I

_LOAD(max) Maximum Load Current

1. Inductor Selection (L1)

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

ure 4, Figure 5, or 6. (Output voltages of 3.3V, 5V, or 12V re-

spectively.) For all other voltages, see the design procedure

for the adjustable version.

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

in Figure 5.

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

the inductance region intersected by the 12V horizontal line

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

L40.

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

tance region intersected by the Maximum Input Voltage line

and the Maximum Load Current line. Each region is identified

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

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

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

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

stance, both through hole and surface mount inductors are

available.)

C. Select an appropriate inductor from the four manufactur-

er’s part numbers listed in Figure 8.

2. Output Capacitor Selection (C_OUT

)

2. Output Capacitor Selection (C_OUT)

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

Resistance) electrolytic capacitors between 82 µF and 820

µF and low ESR solid tantalum capacitors between 10 µF

and 470 µF provide the best results. This capacitor should be

located close to the IC using short capacitor leads and short

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

A. See section on output capacitors in application infor-

mation section.

B. From the quick design component selection table shown

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

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

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

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

line that covers the input voltage needed in your application,

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

recommended inductors and capacitors that will provide the

best overall performance.

For additional information, see section on output capaci-

tors in application information section.

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

quick design component selection table shown in Figure 2.

This table contains different input voltages, output voltages,

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

pacitors that will provide the best design solutions.

The capacitor list contains both through hole electrolytic and

surface mount tantalum capacitors from four different capaci-

tor manufacturers. It is recommended that both the manufac-

turers and the manufacturer’s series that are listed in the

table be used.

C. The capacitor voltage rating for electrolytic capacitors

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

and often much higher voltage ratings are needed to satisfy

the low ESR requirements for low output ripple voltage.

In this example aluminum electrolytic capacitors from several

different manufacturers are available with the range of ESR

numbers needed.

D. For computer aided design software, see Switchers

Made Simple (version 4.2.1 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 ap-

proximately 225 mΩ of ESR (see the curve in Figure 16 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 volt-

age by approximately half.

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10

LM2599 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

PROCEDURE (Fixed Output Voltage Version)

3. Catch Diode Selection (D1)

EXAMPLE (Fixed Output Voltage Version)

3. Catch Diode Selection (D1)

A. The catch diode current rating must be at least 1.3 times

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

supply design must withstand a continuous output short, the

diode should have a current rating equal to the maximum

current limit of the LM2599. The most stressful condition for

this diode is an overload or shorted output condition.

A. Refer to the table shown in Figure 11. In this example, a

5A, 20V, 1N5823 Schottky diode will provide the best perfor-

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

put.

B. The reverse voltage rating of the diode should be at least

1.25 times the maximum input voltage.

C. This diode must be fast (short reverse recovery time) and

must be located close to the 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 re-

verse 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

between the input pin and ground to prevent large voltage

transients from appearing at the input. In addition, the RMS

current rating of the input capacitor should be selected to be

The important parameters for the Input capacitor are the in-

put voltage rating and the RMS current rating. With a nominal

input voltage of 12V, an aluminum electrolytic capacitor with

a voltage rating greater than 18V (1.5 x V_IN) would be

needed. The next higher capacitor voltage rating is 25V.

at least ¹⁄

the DC load current. The capacitor manufacturers

2

data sheet must be checked to assure that this current rating

is not exceeded. The curve shown in Figure 15 shows typical

RMS current ratings for several different aluminum electro-

lytic capacitor values.

The RMS current rating requirement for the input capacitor in

1

a buck regulator is approximately ⁄

2

the DC load current. In

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

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

15 can be used to select an appropriate input capacitor.

From the curves, locate the 35V line and note which capaci-

tor values have RMS current ratings greater than 1.5A. A

680 µF, 35V capacitor could be used.

This capacitor should be located close to the IC using short

leads and the voltage rating should be approximately 1.5

times the maximum input voltage.

If solid tantalum input capacitors are used, it is recomended

that they be surge current tested by the manufacturer.

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

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

lent) would be adequate. other types or other manufacturers

capacitors can be used provided the RMS ripple current rat-

ings are adequate.

Use caution when using ceramic capacitors for input bypass-

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

For additional information, see section on input capaci-

tors in Application Information section.

For surface mount designs, solid tantalum capacitors are

recommended. The TPS series available from AVX, and the

593D series from Sprague are both surge current tested.

11

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LM2599 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

Conditions

Inductor

Output Capacitor

Through Hole Electrolytic Surface Mount Tantalum

Output

Load

Max Input

Inductance Inductor

Panasonic

HFQ Series

(µF/V)

Nichicon

AVX TPS

Series

(µF/V)

Sprague

595D Series

(µF/V)

#

( )

Voltage Current

Voltage

(V)

5

(µH)

PL Series

(µF/V)

560/16

560/35

680/35

470/35

330/35

270/50

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

(V)

(A)

22

33

22

33

47

22

33

47

22

68

22

33

68

33

68

150

L41

L40

L33

L32

L39

L41

L40

L39

L33

L38

L41

L40

L44

L32

L38

L42

470/25

560/35

680/35

560/35

470/25

330/35

470/25

560/25

330/35

470/25

180/35

470/25

330/25

180/25

180/35

330/25

180/25

82/25

330/6.3

220/10

100/10

100/16

68/20

390/6.3

330/10

270/10

180/16

120/20

180/16

120/20

68/25

7

3

10

40

6

3.3

2

3

2

3

2

10

40

8

10

15

40

9

5

20

40

15

18

30

40

15

20

40

12

FIGURE 2. LM2599 Fixed Voltage Quick Design Component Selection Table

LM2599 Series Buck Regulator Design Procedure (Adjustable Output)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

Given:

=

V_OUT20V

V_OUTRegulated Output Voltage

=

V_IN(max) 28V

V_IN(max) Maximum Input Voltage

=

_LOAD(max) 3A

I

_LOAD(max) Maximum Load Current

I

=

Switching Frequency (Fixed at a nominal 150 kHz).

F

Switching Frequency (Fixed at a nominal 150 kHz).

F

1. Programming Output Voltage (Selecting R₁and R₂, as

shown in Figure 1)

%

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

Use the following formula to select the appropriate resistor

values.

=

%

R₂1k (16.26 − 1) 15.26k, closest 1 value is 15.4 kΩ.

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

resistor values minimize noise pickup in the sensitive feed-

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

=

R₂15.4 kΩ.

%

stability with time, use 1 metal film resistors.)

www.national.com

12

LM2599 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

2. Inductor Selection (L1)

EXAMPLE (Adjustable Output Voltage Version)

2. Inductor Selection (L1)

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

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

(V • µs), from the following formula:

T),

=

where V_SATinternal switch saturation voltage 1.16V and

=

V_Ddiode forward voltage drop 0.5V

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

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

Value Selection Guide shown in Figure 7.

=

B. E • T 34.2 (V • µs)

=

C. I_LOAD(max) 3A

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

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

the inductance region intersected by the 34 (V • µs) horizon-

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

code is L39.

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

value and the Maximum Load Current value. Each region is

identified by an inductance value and an inductor code

(LXX).

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

inductor part number from the list of manufacturers part num-

bers.

E. Select an appropriate inductor from the four manufactur-

er’s part numbers listed in Figure 8.

3. Output Capacitor Selection (C_OUT

)

3. Output Capacitor SeIection (C_OUT)

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

tantalum capacitors between 82 µF and 820 µF provide the

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

using short capacitor leads and short copper traces. Do not

use capacitors larger than 820 µF. For additional informa-

tion, see section on output capacitors in application in-

formation section.

A. See section on C_OUTin Application Information section.

B. From the quick design table shown in Figure 3, locate the

output voltage column. From that column, locate the output

voltage closest to the output voltage in your application. In

this example, select the 24V line. Under the output capacitor

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

trolytic or surface mount tantalum types from four different

capacitor manufacturers. It is recommended that both the

manufacturers and the manufacturers series that are listed in

the table be used.

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

quick design table shown in Figure 3. This table contains dif-

ferent output voltages, and lists various output capacitors

that will provide the best design solutions.

In this example, through hole aluminum electrolytic capaci-

tors from several different manufacturers are available.

C. The capacitor voltage rating should be at least 1.5 times

greater than the output voltage, and often much higher volt-

age ratings are needed to satisfy the low ESR requirements

needed for low output ripple voltage.

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 capaci-

tor 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. Re-

fer to the capacitor manufacturers data sheet for this informa-

tion.

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

4. Feedforward Capacitor (C_FF)

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

tional capacitor is required. The compensation capacitor is

typically between 100 pF and 33 nF, and is wired in parallel

with the output voltage setting resistor, R₂. It provides addi-

tional stability for high output voltages, low input-output volt-

ages, and/or very low ESR output capacitors, such as solid

tantalum capacitors.

The table shown in Figure 3 contains feed forward capacitor

values for various output voltages. In this example, a 560 pF

capacitor is needed.

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

(Because of the unstable characteristics of ceramic capaci-

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

13

www.national.com

LM2599 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

5. Catch Diode Selection (D1)

EXAMPLE (Adjustable Output Voltage Version)

5. Catch Diode Selection (D1)

A. The catch diode current rating must be at least 1.3 times

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

supply design must withstand a continuous output short, the

diode should have a current rating equal to the maximum

current limit of the LM2599. The most stressful condition for

this diode is an overload or shorted output condition.

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

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

1N5825 Schottky diode would be a good choice. The 3A di-

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

stressed even for a shorted output.

B. The reverse voltage rating of the diode should be at least

1.25 times the maximum input voltage.

C. This diode must be fast (short reverse recovery time) and

must be located close to the 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 charac-

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

covery diodes typically have reverse recovery times of 50 ns

or less. Rectifiers such as the 1N4001 series are much too

slow and should not be used.

6. Input Capacitor (C_IN

)

6. Input Capacitor (C_IN)

A low ESR aluminum or tantalum bypass capacitor is needed

between the input pin and ground to prevent large voltage

transients from appearing at the input. In addition, the RMS

current rating of the input capacitor should be selected to be

The important parameters for the Input capacitor are the in-

put voltage rating and the RMS current rating. With a nominal

input voltage of 28V, an aluminum electrolytic aluminum elec-

trolytic capacitor with a voltage rating greater than 42V (1.5 x

at least ¹⁄

2

the DC load current. The capacitor manufacturers

V_IN) would be needed. Since the the next higher capacitor

data sheet must be checked to assure that this current rating

is not exceeded. The curve shown in Figure 15 shows typical

RMS current ratings for several different aluminum electro-

lytic capacitor values.

voltage rating is 50V, a 50V capacitor should be used. The

capacitor voltage rating of (1.5 x V_IN) is a conservative guide-

line, and can be modified somewhat if desired.

The RMS current rating requirement for the input capacitor of

1

This capacitor should be located close to the IC using short

leads and the voltage rating should be approximately 1.5

times the maximum input voltage.

a buck regulator is approximately ⁄

2

the DC load current. In

this example, with a 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 15 can be used to select an ap-

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

line and note which capacitor values have RMS current rat-

ings greater than 1.5A. Either a 470 µF or 680 µF, 50V ca-

pacitor could be used.

Use caution when using a high dielectric constant ceramic

capacitor for input bypassing, because it may cause severe

ringing at the V_INpin.

For a through hole design, a 680 µF/50V electrolytic capaci-

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

lent) would be adequate. Other types or other manufacturers

capacitors can be used provided the RMS ripple current rat-

ings are adequate.

For additional information, see section on input capaci-

tor in application information section.

For surface mount designs, solid tantalum capacitors can be

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

pacitor sure current rating (see Application Information or in-

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

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

current tested.

To further simplify the buck regulator design procedure, Na-

tional Semiconductor is making available computer design

software to be used with the Simple Switcher line ot switch-

ing regulators. Switchers Made Simple (version 4.2.1 or

later) is available on a 3¹⁄

computers.

2" diskette for IBM compatible

www.national.com

14

LM2599 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

Output

Voltage

(V)

Through Hole Output Capacitor

Surface Mount Output Capacitor

Panasonic

Nichicon PL

Series

(µF/V)

Feedforward

AVX TPS

Sprague

595D Series

(µF/V)

Feedforward

HFQ Series

(µF/V)

Capacitor

Series

(µF/V)

330/6.3

220/10

100/16

68/20

Capacitor

2

4

820/35

560/35

470/25

330/25

220/35

100/50

820/35

470/35

470/25

330/25

220/35

150/35

100/50

33 nF

10 nF

3.3 nF

1.5 nF

1 nF

470/4

33 nF

10 nF

3.3 nF

1.5 nF

1 nF

390/6.3

330/10

180/16

120/20

33/25

6

9

1 2

1 5

2 4

2 8

680 pF

560 pF

390 pF

680 pF

220 pF

33/25

10/35

15/50

FIGURE 3. Output Capacitor and Feedforward Capacitor Selection Table

LM2599 Series Buck Regulator Design Procedure

INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)

DS012582-28

DS012582-26

FIGURE 6. LM2599-12

FIGURE 4. LM2599-3.3

DS012582-27

DS012582-29

FIGURE 5. LM2599-5.0

FIGURE 7. LM2599-ADJ

15

www.national.com

LM2599 Series Buck Regulator Design Procedure (Continued)

Inductance

(µH)

Current

(A)

Schott

Through

Renco

Through

Pulse Engineering

Coilcraft

Surface

Mount

Surface

Through

Surface

Mount

Hole

RL-1284-22-43

RL-5471-5

RL-5471-6

RL-5471-7

RL-1283-22-43

RL-1283-15-43

RL-5471-1

RL-5471-2

RL-5471-3

RL-5471-4

RL-5471-5

RL-5471-6

RL-5471-7

RL-1283-22-43

RL-1283-15-43

RL-5473-1

RL-5473-4

RL-5472-1

RL-5472-2

RL-5472-3

RL-5472-4

RL-5472-5

RL-5473-4

RL-5473-2

RL-5473-3

Mount

Hole

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

67144070

67144080

67144090

67148370

67148380

67144100

67144110

67144120

67144130

67144140

67144150

67144160

67148390

67148400

67144170

67144180

67144190

67144200

67144210

67144220

67144230

67148410

67144240

67144250

67148460

67144450

67144460

67144470

67148480

67148490

67144480

67144490

67144500

67144510

67144520

67144530

67144540

67148500

67148790

—

RL1500-22

PE-53815

PE-53821

PE-53822

PE-53823

PE-53824

PE-53825

PE-53826

PE-53827

PE-53828

PE-53829

PE-53830

PE-53831

PE-53932

PE-53933

PE-53934

PE-53935

PE-54036

PE-54037

PE-54038

PE-54039

PE-54040

PE-54041

PE-54042

PE-54043

PE-54044

PE-53815-S

PE-53821-S

PE-53822-S

PE-53823-S

PE-53825-S

PE-53824-S

PE-53826-S

PE-53827-S

PE-53828-S

PE-53829-S

PE-53830-S

PE-53831-S

PE-53932-S

PE-53933-S

PE-53934-S

PE-53935-S

PE-54036-S

PE-54037-S

PE-54038-S

PE-54039-S

PE-54040-S

PE-54041-S

PE-54042-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

—

RL1500-68

47

—

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

—

2.5

—

3.1

—

47

3.5

—

33

3.5

67148290

67148300

—

22

3.5

—

150

100

68

2.7

—

3.4

—

3.4

—

FIGURE 8. Inductor Manufacturers Part Numbers

Coilcraft Inc.

Phone (800) 322-2645

FAX (708) 639-1469

Phone +11 1236 730 595

FAX +44 1236 730 627

Phone (619) 674-8100

FAX (619) 674-8262

Phone +353 93 24 107

FAX +353 93 24 459

Phone (800) 645-5828

FAX (516) 586-5562

Phone (612) 475-1173

FAX (612) 475-1786

Coilcraft Inc., Europe

Pulse Engineering Inc.

Pulse Engineering Inc.,

Europe

Renco Electronics Inc.

Schott Corp.

FIGURE 9. Inductor Manufacturers Phone Numbers

www.national.com

16

LM2599 Series Buck Regulator Design Procedure (Continued)

Nichicon Corp.

Panasonic

Phone

FAX

(708) 843-7500

(708) 843-2798

(714) 373-7857

(714) 373-7102

(803) 448-9411

(803) 448-1943

(207) 324-4140

(207) 324-7223

Phone

FAX

AVX Corp.

Phone

FAX

Sprague/Vishay

Phone

FAX

FIGURE 10. Capacitor Manufacturers Phone Numbers

3 Amp Diodes

Surface Mount Through Hole

Schot- Ultra Fast

4 to 6 Amp Diodes

Surface Mount Through Hole

VR

Ultra Fast

Schot-

Ultra Fast

Schot-

Ultra Fast

Schottky

tky

Recovery

All of

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

FIGURE 11. Diode Selection Table

17

www.national.com

Block Diagram

DS012582-30

FIGURE 12.

Special Note If any of the above three features (Shutdown

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

pins should be left open.

Application Information

PIN FUNCTIONS

+V_IN(Pin 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.

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 (ap-

proximately 5 µA begins charging this capacitor). As the ca-

pacitor voltage rises, the regulator goes through four operat-

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

Ground (Pin 4) — Circuit ground.

Output (Pin 2) — Internal switch. The voltage at this pin

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

mately −0.5V, with a duty cycle of V_OUT/V_IN. To minimize

coupling to sensitive circuitry, the PC board copper area con-

nected to this pin should be kept to a minimum.

Feedback (Pin 6) — Senses the regulated output voltage to

complete the feedback loop.

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

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

put voltage is zero, and the IC quiescent current is approxi-

mately 85 µA.

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

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

tor circuit to be shut down using logic level signals thus drop-

ping 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.

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 cur-

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

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

Soft-start capacitor charging current decreases from 5 µA

down to approximately 1.6 µA. This decreases the slope of

capacitor voltage ramp.

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

low signal (flag transistor ON) when the regulated output

%

voltage drops more than 5 from the nominal output volt-

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

the nominal output voltage and a delay time determined by

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

a microprocessor on power-up.

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

@

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

condition. The switch (Pin 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 con-

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

supply current requirement also starts out at a low level for

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

a time delay between the time the regulated output voltage

%

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

error flag output goes high.

www.national.com

18

Application Information (Continued)

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

is a very useful feature in some switcher topologies that re-

quire large startup currents (such as the inverting configura-

tion) which can load down the input power supply.

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

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

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

perature 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 inter-

nal clamp voltage of approximately 7V. If this pin is driven

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

1 mA.

DS012582-31

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

DS012582-32

FIGURE 14. Timing Diagram for 5V Output

19

www.national.com

the internal temperature of the capacitor to rise. The RMS

current rating of a capacitor is determined by the amount of

current required to raise the internal temperature approxi-

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

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

ing air will determine the amount of current the capacitor can

safely sustain. Capacitors that are physically large and have

a large surface area will typically have higher RMS current

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

lytic capacitor will be physically larger than a lower voltage

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

rounding air, and therefore will have a higher RMS current

rating.

Application Information (Continued)

DELAY CAPACITOR

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

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

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

tween the time the regulated output voltage (when it is in-

%

creasing in value) reaches 95 of the nominal output volt-

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

constant current from the delay pin charges the delay ca-

pacitor resulting in a voltage ramp. When this voltage

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

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

be used to indicate that the regulated output has reached the

correct voltage and has stabilized.

%

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 drop-

ping would only be approximately 50 µs.

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

tor 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.

DS012582-33

FIGURE 15. RMS Current Ratings for Low

ESR Electrolytic Capacitors (Typical)

FEEDFORWARD CAPACITOR

(Adjustable Output Voltage Version)

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

ure 1 is used when the output voltage is greater than 10V or

when C_OUThas a very low ESR. This capacitor adds lead

compensation to the feedback loop and increases the phase

margin for better loop stability. For C_FFselection, see the de-

sign procedure section.

>

%

5 of the nominal output volt-

If the output ripple is large (

age), 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.

DS012582-34

FIGURE 16. Capacitor ESR vs Capacitor Voltage Rating

(Typical Low ESR Electrolytic Capacitor)

INPUT 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 ca-

pacitor’s electrolyte, resulting in eventual failure.

C_IN— A low ESR aluminum or tantalum bypass capacitor is

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

cated near the regulator using short leads. This capacitor

prevents large voltage transients from appearing at the in-

put, and provides the instantaneous current needed each

time the switch turns on.

Selecting an input capacitor requires consulting the manu-

facturers data sheet for maximum allowable RMS ripple cur-

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

eral guideline would be to select a capacitor with a ripple

The important parameters for the Input capacitor are the

voltage rating and the RMS current rating. Because of the

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

put capacitor, this capacitor should be chosen for its RMS

current rating rather than its capacitance or voltage ratings,

although the capacitance value and voltage rating are di-

rectly related to the RMS current rating.

%

current rating of approximately 50 of the DC load current.

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

%

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

conservative design. The capacitor voltage rating must be at

least 1.25 times greater than the maximum input voltage,

and often a much higher voltage capacitor is needed to sat-

isfy the RMS current requirements.

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

www.national.com

20

Solid tantalum capacitors have a much better ESR spec for

cold temperatures and are recommended for temperatures

below −25˚C.

Application Information (Continued)

A graph shown in Figure 15 shows the relationship between

an electrolytic capacitor value, its voltage rating, and the

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

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

lytic capacitors designed for switching regulator applications.

Other capacitor manufacturers offer similar types of capaci-

tors, but always check the capacitor data sheet.

CATCH DIODE

Buck regulators require a diode to provide a return path for

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

a fast diode and must be located close to the LM2599 using

short leads and short printed circuit traces.

“Standard” electrolytic capacitors typically have much higher

ESR numbers, lower RMS current ratings and typically have

a shorter operating lifetime.

Because of their very fast switching speed and low forward

voltage drop, Schottky diodes provide the best performance,

especially in low output voltage applications (5V and lower).

Ultra-fast recovery, or High-Efficiency rectifiers are also a

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

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

covery diodes typically have reverse recovery times of 50 ns

or less. Rectifiers such as the IN5400 series are much too

slow and should not be used.

Because of their small size and excellent performance, sur-

face mount solid tantalum capacitors are often used for input

bypassing, but several precautions must be observed. A

small percentage of solid tantalum capacitors can short if the

inrush current rating is exceeded. This can happen at turn on

when the input voltage is suddenly applied, and of course,

higher input voltages produce higher inrush currents. Sev-

%

eral capacitor manufacturers do a 100 surge current test-

ing on their products to minimize this potential problem. If

high turn on currents are expected, it may be necessary to

limit this current by adding either some resistance or induc-

tance before the tantalum capacitor, or select a higher volt-

age capacitor. As with aluminum electrolytic capacitors, the

RMS ripple current rating must be sized to the load current.

OUTPUT CAPACITOR

C_OUT— An output capacitor is required to filter the output

and provide regulator loop stability. Low impedance or low

ESR Electrolytic or solid tantalum capacitors designed for

switching regulator applications must be used. When select-

ing an output capacitor, the important capacitor parameters

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

RMS ripple current rating, voltage rating, and capacitance

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

important parameter.

DS012582-35

FIGURE 17. Capacitor ESR Change vs Temperature

INDUCTOR SELECTION

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

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

value is needed. This value is determined by the maximum

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.

%

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

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

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

sulting in an oscillation at the output. Using the capacitors

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

tions under all conditions.

If very low output ripple voltage (less than 15 mV) is re-

quired, refer to the section on Output Voltage Ripple and

Transients for a post ripple filter.

The LM2599 (or any of the Simple Switcher family) can be

used for both continuous or discontinuous modes of opera-

tion.

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 16). Often, capacitors with much higher voltage

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

quired for low output ripple voltage.

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

tinuous mode. It offers greater output power, lower peak

switch, inductor and diode currents, and can have lower out-

put ripple voltage. But it does require larger inductor values

to keep the inductor current flowing continuously, especially

at low output load currents and/or high input voltages.

The output capacitor for many different switcher designs of-

ten can be satisfied with only three or four different capacitor

values and several different voltage ratings. See the quick

design component selection tables in Figure 2 and 3 for typi-

cal capacitor values, voltage ratings, and manufacturers ca-

pacitor types.

To simplify the inductor selection process, an inductor selec-

tion guide (nomograph) was designed (see Figure 4 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 percent-

age 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 18).

Electrolytic capacitors are not recommended for tempera-

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

@

peratures and typically rises 3X

−25˚C and as much as

10X at −40˚C. See curve shown in Figure 17.

21

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DISCONTINUOUS MODE OPERATION

Application Information (Continued)

The selection guide chooses inductor values suitable for

continuous mode operation, but for low current applications

and/or high input voltages, a discontinuous mode design

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

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

third the inductance value needed for a continuous mode de-

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

a discontinuous design, but at these low load currents (1A

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

the switch current limit.

Discontinuous operation can have voltage waveforms that

are considerable different than a continuous design. The out-

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

dal ringing present. (See Typical Performance Characteris-

tics photo titled Discontinuous Mode Switching Waveforms)

This ringing is normal for discontinuous operation, and is not

caused by feedback loop instabilities. In discontinuous op-

eration, there is a period of time where neither the switch or

the diode are conducting, and the inductor current has

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

can circulate between the inductor and the switch/diode

parasitic capacitance causing this characteristic ringing. Nor-

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

comes great enough to exceed the input voltage, and even

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

DS012582-36

FIGURE 18. (∆I_IND) Peak-to-Peak Inductor Ripple

Current (as a Percentage of the

Load Current) vs Load Current

By allowing the percentage of inductor ripple current to in-

crease 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.

Different inductor types and/or core materials produce differ-

ent amounts of this characteristic ringing. Ferrite core induc-

tors have very little core loss and therefore produce the most

ringing. The higher core loss of powdered iron inductors pro-

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

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

Inductors are available in different styles such as pot core,

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

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

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

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

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

pletely contained within the core, it generates more

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

induce voltages into nearby printed circuit traces, thus caus-

ing problems with both the switching regulator operation and

nearby sensitive circuitry, and can give incorrect scope read-

ings because of induced voltages in the scope probe. Also

see section on Open Core Inductors.

puter aided design software

Switchers Made Simple (ver-

sion 4.3) will provide all component values for continuous

and discontinuous modes of operation.

When multiple switching regulators are located on the same

PC board, open core magnetics can cause interference be-

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

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

ture) should be used in these situations.

DS012582-37

The inductors listed in the selection chart include ferrite

E-core construction for Schott, ferrite bobbin core for Renco

and Coilcraft, and powdered iron toroid for Pulse Engineer-

ing.

FIGURE 19. Post Ripple Filter Waveform

OUTPUT VOLTAGE RIPPLE AND TRANSIENTS

Exceeding an inductor’s maximum current rating may cause

the inductor to overheat because of the copper wire losses,

or the core may saturate. If the inductor begins to saturate,

the inductance decreases rapidly and the inductor begins to

look mainly resistive (the DC resistance of the winding). This

can cause the switch current to rise very rapidly and force

the switch into a cycle-by-cycle current limit, thus reducing

the DC output load current. This can also result in overheat-

ing of the inductor and/or the LM2599. Different inductor

types have different saturation characteristics, and this

should be kept in mind when selecting an inductor.

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 saw-

tooth ripple current and the ESR of the output capacitor. A

typical output ripple voltage can range from approximately

%

0.5 to 3 of the output voltage. To obtain low ripple volt-

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

caution must be exercised when using extremely low ESR

capacitors because they can affect the loop stability, result-

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

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

The inductor manufacturer’s data sheets include current and

energy limits to avoid inductor saturation.

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22

output ripple voltage and output capacitor ESR can all be

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

nomographs shown in Figure 4 through 7 are used to select

an inductor value, the peak-to-peak inductor ripple current

can immediately be determined. The curve shown in Figure

20 shows the range of (∆I_IND) that can be expected for differ-

ent load currents. The curve also shows how the

peak-to-peak inductor ripple current (∆I_IND) changes as you

go from the lower border to the upper border (for a given load

current) within an inductance region. The upper border rep-

resents a higher input voltage, while the lower border repre-

sents a lower input voltage (see Inductor Selection Guides).

Application Information (Continued)

mended (See Figure 1). The inductance required is typically

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

good load regulation. A low ESR output filter capacitor is also

required to assure good dynamic load response and ripple

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

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

photo shown in Figure 19 shows a typical output ripple volt-

age, 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.

These curves are only correct for continuous mode opera-

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

lect the inductor value

Consider the following example:

=

V_OUT5V, maximum load current of 2.5A

=

V_IN12V, nominal, varying between 10V and 16V.

The selection guide in Figure 5 shows that the vertical line

for a 2.5A load current, and the horizontal line for the 12V in-

put voltage intersect approximately midway between the up-

per and lower borders of the 33 µH inductance region. A 33

The voltage spikes are caused by the fast switching action of

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

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

mize these voltage spikes, the output capacitor should be

designed for switching regulator applications, and the lead

lengths must be kept very short. Wiring inductance, stray ca-

pacitance, as well as the scope probe used to evaluate these

transients, all contribute to the amplitude of these spikes.

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

)

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

rent. Referring to Figure 20, follow the 2.5A line approxi-

mately midway into the inductance region, and read the

peak-to-peak inductor ripple current (∆I_IND) on the left hand

axis (approximately 620 mA p-p).

As the input voltage increases to 16V, it approaches the up-

per border of the inductance region, and the inductor ripple

current increases. Referring to the curve in Figure 20, it can

be seen that for a load current of 2.5A, the peak-to-peak in-

ductor ripple current (∆I_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 ∆I_INDvalue is known, the following formulas can be

used to calculate additional information about the switching

regulator circuit.

1. Peak Inductor or peak switch current

DS012582-49

FIGURE 20. Peak-to-Peak Inductor

Ripple Current vs Load Current

2. Minimum load current before the circuit becomes dis-

continuous

When a switching regulator is operating in the continuous

mode, the inductor current waveform ranges from a triangu-

lar to a sawtooth type of waveform (depending on the input

voltage). For

a given input and output voltage, the

peak-to-peak amplitude of this inductor current waveform re-

mains constant. As the load current increases or decreases,

the entire sawtooth current waveform also rises and falls.

The average value (or the center) of this current waveform is

equal to the DC load current.

=

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

)

=

0.62Ax0.1Ω 62 mV p-p

4.

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

the sawtooth current waveform will reach zero, and the

switcher will smoothly change from a continuous to a discon-

tinuous mode of operation. Most switcher designs (irregard-

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

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

acceptable mode of operation.

OPEN CORE INDUCTORS

In a switching regulator design, knowing the value of the

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

determining a number of other circuit parameters. Param-

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

mum load current before the circuit becomes discontinuous,

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.

23

www.national.com

ther thermal improvements are needed, double sided, multi-

layer pc-board with large copper areas and/or airflow are

recommended.

Application Information (Continued)

These magnetic lines of flux will induce a voltage into any

wire or PC board copper trace that comes within the induc-

tor’s magnetic field. The strength of the magnetic field, the

orientation and location of the PC copper trace to the mag-

netic field, and the distance between the copper trace and

the inductor, determine the amount of voltage generated in

the copper trace. Another way of looking at this inductive

coupling is to consider the PC board copper trace as one

turn of a transformer (secondary) with the inductor winding

as the primary. Many millivolts can be generated in a copper

trace located near an open core inductor which can cause

stability problems or high output ripple voltage problems.

The curves shown in Figure 22 show the LM2599S (TO-263

package) junction temperature rise above ambient tempera-

ture 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 junc-

tion 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.

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

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

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

if this is the problem, temporarily raise the inductor away

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

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

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

Substituting a closed core inductor such as a torroid or

E-core will correct the problem, or re-arranging the PC layout

may be necessary. Magnetic flux cutting the IC device

ground trace, feedback trace, or the positive or negative

traces of the output capacitor should be minimized.

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.

DS012582-38

This discussion on open core inductors is not to frighten the

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

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

tors are an inexpensive, simple way of making a compact ef-

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

ferent applications.

Circuit Data for Temperature Rise Curve TO-220

Package (T)

Capacitors Through hole electrolytic

Inductor

Diode

Through hole Renco

THERMAL CONSIDERATIONS

Through hole, 5A 40V, Schottky

The LM2599 is available in two packages, a 7-pin TO-220

(T) and a 7-pin surface mount TO-263 (S).

PC board

3 square inches single sided 2 oz. copper

(0.0028")

The TO-220 package needs a heat sink under most condi-

tions. The size of the heat sink depends on the input voltage,

the output voltage, the load current and the ambient tem-

perature. The curves in Figure 21 show the LM2599T junc-

tion 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 pack-

age) operating as a buck switching regulator in an ambient

temperature of 25˚C (still air). These temperature rise num-

bers are all approximate and there are many factors that can

affect these temperatures. Higher ambient temperatures re-

quire more heat sinking.

FIGURE 21. Junction Temperature Rise, TO-220

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 fur-

DS012582-39

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24

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.

Application Information (Continued)

Circuit Data for Temperature Rise Curve TO-263

Package (S)

Capacitors Surface mount tantalum, molded “D” size

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 22. Junction Temperature Rise, TO-263

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.

DS012582-40

FIGURE 23. Output Voltage, Input Current,

at Start-Up, WITH Soft-start

Package thermal resistance and junction temperature rise

numbers are all approximate, and there are many factors

that will affect these numbers. Some of these factors include

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

board temperature. Other factors are, trace width, total

printed circuit copper area, copper thickness, single- or

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

the board. The effectiveness of the pc board to dissipate

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

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

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

nents such as the catch diode will add heat to the pc board

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

inductor, depending on the physical size, type of core mate-

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

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

board.

DS012582-41

FIGURE 24. Output Voltage, Input Current,

at Start-Up, WITHOUT Soft-start

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.

SHUTDOWN /SOFT-START

The circuit shown in Figure 25 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 23 Figure 24 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

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 capaci-

tor can be eliminated.

25

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Application Information (Continued)

DS012582-42

FIGURE 25. Typical Circuit Using Shutdown /Soft-start and Error Flag Features

DS012582-43

FIGURE 26. Inverting −5V Regulator With Shutdown and Soft-start

lNVERTING REGULATOR

verting +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.

The circuit in Figure 26 converts a positive input voltage to a

negative output voltage with a common ground. The circuit

operates by bootstrapping the regulator’s ground pin to the

negative output voltage, then grounding the feedback pin,

the regulator senses the inverted output voltage and regu-

lates it.

This example uses the LM2599-5 to generate a −5V output,

but other output voltages are possible by selecting other out-

put 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 maxi-

mum output current greatly depends on both the input and

output voltage. The curve shown in Figure 27 provides a

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

different input and output voltage conditions.

The maximum voltage appearing across the regulator is the

absolute sum of the input and output voltage, and this must

be limited to a maximum of 40V. In this example, when con-

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26

ceeds the zener voltage, the zener conducts, pulling the

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

switching. The threshold voltage for the undervoltage lockout

feature is approximately 1.5V greater than the zener voltage.

Application Information (Continued)

Inverting Regulator

DS012582-45

FIGURE 28. Undervoltage Lockout for a Buck

Regulator

DS012582-44

FIGURE 27. Maximum Load Current for

Inverting Regulator Circuit

Figure 29 and 30 apply the same feature to an inverting cir-

cuit. Figure 29 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 30 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.

An additional diode is required in this regulator configuration.

Diode D1 is used to isolate input voltage ripple or noise from

coupling through the C_INcapacitor to the output, under light

or no load conditions. Also, this diode isolation changes the

topology to closely resemble a buck configuration thus pro-

viding good closed loop stability. A Schottky diode is recom-

mended for low input voltages, (because of its lower voltage

drop) but for higher input voltages, a IN5400 diode could be

used.

Because of differences in the operation of the inverting regu-

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

inductor value. In the majority of designs, a 33 µH, 3.5A in-

ductor is the best choice. Capacitor selection can also be

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

in Figure 26 will provide good results in the majority of invert-

ing 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 (ap-

proximately 4.5A) are needed for 2 ms or more, until the out-

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

pends on the output voltage and the size of the output

capacitor. Input power sources that are current limited or

sources that can not deliver these currents without getting

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

tively high startup currents required by the inverting topology,

the Soft-start feature shown in Figure 26 is recommended.

DS012582-47

FIGURE 29. Undervoltage Lockout Without

Hysteresis for an Inverting Regulator

Also shown in Figure 26 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.

DS012582-46

UNDERVOLTAGE LOCKOUT

FIGURE 30. Undervoltage Lockout With

Hysteresis for an Inverting Regulator

Some applications require the regulator to remain off until

the input voltage reaches a predetermined voltage. Figure

28 contains a undervoltage lockout circuit for a buck configu-

ration, while Figure 29 and 30 are for the inverting types

(only the circuitry pertaining to the undervoltage lockout is

shown). Figure 28 uses a zener diode to establish the

threshold voltage when the switcher begins operating. When

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

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

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

NEGATIVE VOLTAGE CHARGE PUMP

Occasionally a low current negative voltage is needed for bi-

asing 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 Fig-

ure 31. 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 re-

27

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Application Information (Continued)

quirement 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.

DS012582-48

FIGURE 31. Charge Pump for Generating a

Low Current, Negative Output Voltage

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

DS012582-51

C

:

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

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

IN

:

OUT

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

L1: — 47 µH, L39, Renco, Through Hole

R

C

:

— 10k

— 0.1 µF

PULL UP

:

DELAY

:

SD/SS

Thermalloy Heat Sink #7020

www.national.com

28

Application Information (Continued)

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

DS012582-52

C

:

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

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

IN

:

OUT

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

L1: — 47 µH, L39, Renco, Through Hole

R : — 1 kΩ, 1%

1

R : — Use formula in Design Procedure

2

C

R

C

:

— See Figure 4.

— See Application Information Section (C Section)

FF

:

— 10k

— 0.1 µF

PULL UP

:

DELAY

SD/SS

Thermalloy Heat Sink #7020

FIGURE 32. PC Board Layout

29

www.national.com

Physical Dimensions inches (millimeters) unless otherwise noted

7-Lead TO-220 Bent and Staggered Package

Order Number LM2599T-3.3, LM2599T-5.0,

LM2599T-12 or LM2599T-ADJ

NS Package Number TA07B

www.national.com

30

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

7-Lead TO-263 Bent and Formed Package

Order Number LM2599S-3.3, LM2599S-5.0, LM2599S-12 or LM2599S-ADJ

NS Package Number TS7B

LIFE SUPPORT POLICY

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

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

COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or

systems which, (a) are intended for surgical implant

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

whose failure to perform when properly used in

accordance with instructions for use provided in the

labeling, can be reasonably expected to result in a

significant injury to the user.

2. A critical component is any component of a life

support device or system whose failure to perform

can be reasonably expected to cause the failure of

the life support device or system, or to affect its

safety or effectiveness.

National Semiconductor

Corporation

Americas

Tel: 1-800-272-9959

Fax: 1-800-737-7018

Email: support@nsc.com

National Semiconductor

Europe

National Semiconductor

Asia Pacific Customer

Response Group

Tel: 65-2544466

Fax: 65-2504466

National Semiconductor

Japan Ltd.

Tel: 81-3-5639-7560

Fax: 81-3-5639-7507

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

Email: europe.support@nsc.com

Deutsch Tel: +49 (0) 69 9508 6208

English Tel: +44 (0) 870 24 0 2171

Français Tel: +33 (0) 1 41 91 8790

Email: ap.support@nsc.com

www.national.com

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.


型号：	LM2599
厂家：	National Semiconductor
描述：	SIMPLE SWITCHER Power Converter 150 kHz 3A Step-Down Voltage Regulator, with Features SIMPLE SWITCHER系列电源转换器150千赫3A降压型稳压器，具有特色转换器稳压器
文件：	总31页 (文件大小：814K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LM2599 [NSC]

相关型号：

LM2599S-12

LM2599S-12/NOPB

LM2599S-3.3

LM2599S-3.3

LM2599S-3.3/NOPB

LM2599S-5.0

LM2599S-5.0

LM2599S-5.0/NOPB

LM2599S-ADJ

LM2599S-ADJ

LM2599S-ADJ/NOPB

LM2599SX-12/NOPB