LM2594M-ADJ - Texas Instruments High-Performance Analog

LM2594,LM2594HV

LM2594/LM2594HV SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down

Voltage Regulator

Literature Number: SNVS118B

LM2594/LM2594HV

SIMPLE SWITCHER®Power Converter 150 kHz 0.5A

Step-Down Voltage Regulator

General Description

The LM2594/LM2594HV series of regulators are monolithic

integrated circuits that provide all the active functions for a

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

0.5A 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, and are packaged in

a 8-lead DIP and a 8-lead surface mount package.

Requiring a minimum number of external components, these

regulators are simple to use and feature internal frequency

compensation†, a fixed-frequency oscillator, and improved

line and load regulation specifications.

The LM2594/LM2594HV series operates at a switching fre-

quency of 150 kHz thus allowing smaller sized filter compo-

nents than what would be needed with lower frequency

switching regulators. Because of its high efficiency, the cop-

per traces on the printed circuit board are normally the only

heat sinking needed.

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 LM2594/LM2594HV se-

ries. This feature greatly simplifies the design of switch-

mode power supplies.

Other features include a guaranteed ±4% tolerance on out-

put voltage under all conditions of input voltage and output

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

ternal shutdown is included, featuring typically 85 µA

standby current. Self protection features include a two stage

frequency reducing current limit for the output switch and an

over temperature shutdown for complete protection under

fault conditions.

The LM2594HV is for applications requiring an input voltage

up to 60V.

Features

n3.3V, 5V, 12V, and adjustable output versions

nAdjustable version output voltage range, 1.2V to 37V

(57V for the HV version)±4% max over line and load

conditions

nAvailable in 8-pin surface mount and DIP-8 package

nGuaranteed 0.5A output current

nInput voltage range up to 60V

nRequires only 4 external components

n150 kHz fixed frequency internal oscillator

nTTL Shutdown capability

nLow power standby mode, I

typically 85 µA

nHigh Efficiency

nUses readily available standard inductors

nThermal shutdown and current limit protection

Applications

nSimple high-efficiency step-down (buck) regulator

nEfficient pre-regulator for linear regulators

nOn-card switching regulators

nPositive to Negative convertor

Typical Application (Fixed Output Voltage Versions)

01243901

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

December 1999

LM2594/LM2594HV SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down Voltage

Regulator

Connection Diagrams and Order Information

8-Lead DIP (N) 8-Lead Surface Mount (M)

01243902

Top View

Order Number

LM2594N-3.3, LM2594N-5.0,

LM2594N-12 or LM2594N-ADJ

LM2594HVN-3.3, LM2594HVN-5.0,

LM2594HVN-12 or LM2594HVN-ADJ

See NS Package Number N08E

01243903

Top View

Order Number LM2594M-3.3,

LM2594M-5.0, LM2594M-12 or

LM2594M-ADJ

LM2594HVM-3.3, LM2594HVM-5.0,

LM2594HVM-12 or LM2594HVM-ADJ

See NS Package Number M08A

*No internal connection, but should be soldered to pc board for best heat transfer.

‡Patent Number 5,382,918.

LM2594/LM2594HV

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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.

Maximum Supply Voltage

LM2594 45V

LM2594HV 60V

ON /OFF Pin Input Voltage −0.3 ≤V≤+25V

Feedback Pin Voltage −0.3 ≤V≤+25V

Output Voltage to Ground

(Steady State) −1V

Power Dissipation Internally limited

Storage Temperature Range −65˚C to +150˚C

ESD Susceptibility

Human Body Model (Note 2) 2 kV

Lead Temperature

M8 Package

Vapor Phase (60 sec.) +215˚C

Infrared (15 sec.) +220˚C

N Package (Soldering, 10 sec.) +260˚C

Maximum Junction Temperature +150˚C

Operating Conditions

Temperature Range −40˚C ≤T

+125˚C

Supply Voltage

LM2594 4.5V to 40V

LM2594HV 4.5V to 60V

LM2594/LM2594HV-3.3

Electrical Characteristics

Specifications with standard type face are for T

= 25˚C, and those with boldface type apply over full Operating Tempera-

ture Range.V

INmax

= 40V for the LM2594 and 60V for the LM2594HV.

Symbol Parameter Conditions LM2594/LM2594HV-3.3 Units

(Limits)

Typ Limit

(Note 3) (Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1

OUT

Output Voltage 4.75V ≤V

≤V

INmax

, 0.1A ≤I

LOAD

≤0.5A 3.3 V

3.168/3.135 V(min)

3.432/3.465 V(max)

ηEfficiency V

= 12V, I

LOAD

= 0.5A 80 %

LM2594/LM2594HV-5.0

Electrical Characteristics

Specifications with standard type face are for T

= 25˚C, and those with boldface type apply over full Operating Tempera-

ture Range

Symbol Parameter Conditions LM2594/LM2594HV-5.0 Units

(Limits)

Typ Limit

(Note 3) (Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1

OUT

Output Voltage 7V ≤V

≤V

INmax

, 0.1A ≤I

LOAD

≤0.5A 5.0 V

4.800/4.750 V(min)

5.200/5.250 V(max)

ηEfficiency V

= 12V, I

LOAD

= 0.5A 82 %

LM2594/LM2594HV-12

Electrical Characteristics

Specifications with standard type face are for T

= 25˚C, and those with boldface type apply over full Operating Tempera-

ture Range

Symbol Parameter Conditions LM2594/LM2594HV-12 Units

(Limits)

Typ Limit

(Note 3) (Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1

OUT

Output Voltage 15V ≤V

≤V

INmax

, 0.1A ≤I

LOAD

≤0.5A 12.0 V

11.52/11.40 V(min)

12.48/12.60 V(max)

LM2594/LM2594HV

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LM2594/LM2594HV-12

Electrical Characteristics (Continued)

Specifications with standard type face are for T

= 25˚C, and those with boldface type apply over full Operating Tempera-

ture Range

Symbol Parameter Conditions LM2594/LM2594HV-12 Units

(Limits)

Typ Limit

(Note 3) (Note 4)

ηEfficiency V

= 25V, I

LOAD

= 0.5A 88 %

LM2594/LM2594HV-ADJ

Electrical Characteristics

Specifications with standard type face are for T

= 25˚C, and those with boldface type apply over full Operating Tempera-

ture Range

Symbol Parameter Conditions LM2594/LM2594HV-ADJ Units

(Limits)

Typ Limit

(Note 3) (Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1

Feedback Voltage 4.5V ≤V

≤V

INmax

, 0.1A ≤I

LOAD

≤0.5A 1.230 V

OUT

programmed for 3V. Circuit of Figure 1 1.193/1.180 V(min)

1.267/1.280 V(max)

ηEfficiency V

= 12V, I

LOAD

= 0.5A 80 %

All Output Voltage Versions

Electrical Characteristics

Specifications with standard type face are for T

= 25˚C, and those with boldface type apply over full Operating Tempera-

ture Range . Unless otherwise specified, V

= 12V for the 3.3V, 5V, and Adjustable version and V

= 24V for the 12V ver-

sion. I

LOAD

= 100 mA

Symbol Parameter Conditions LM2594/LM2594HV-XX Units

(Limits)

Typ Limit

(Note 3) (Note 4)

DEVICE PARAMETERS

Feedback Bias Current Adjustable Version Only, VFB = 1.3V 10 50/100 nA

Oscillator Frequency (Note 6) 150 kHz

127/110 kHz(min)

173/173 kHz(max)

SAT

Saturation Voltage I

OUT

= 0.5A (Note 7) (Note 8) 0.9 V

1.1/1.2 V(max)

DC Max Duty Cycle (ON) (Note 8) 100 %

Min Duty Cycle (OFF) (Note 9) 0

Current Limit Peak Current, (Note 7) (Note 8) 0.8 A

0.65/0.58 A(min)

1.3/1.4 A(max)

Output Leakage Current (Note 7) (Note 9) (Note 10) Output = 0V 50 µA(max)

Output = −1V 2 mA

15 mA(max)

Quiescent Current (Note 9) 5 mA

10 mA(max)

STBY

Standby Quiescent ON/OFF pin = 5V (OFF) (Note 10) 85 µA

Current LM2594 200/250 µA(max)

LM2594HV 140 250/300 µA(max)

Thermal Resistance N Package, Junction to Ambient (Note 11) 95 ˚C/W

M Package, Junction to Ambient (Note 11) 150

LM2594/LM2594HV

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All Output Voltage Versions

Electrical Characteristics (Continued)

Specifications with standard type face are for T

= 25˚C, and those with boldface type apply over full Operating Tempera-

ture Range . Unless otherwise specified, V

= 12V for the 3.3V, 5V, and Adjustable version and V

= 24V for the 12V ver-

sion. I

LOAD

= 100 mA

Symbol Parameter Conditions LM2594/LM2594HV-XX Units

(Limits)

Typ Limit

(Note 3) (Note 4)

DEVICE PARAMETERS

ON/OFF CONTROL Test Circuit Figure 1

ON /OFF Pin Logic Input 1.3 V

Threshold Voltage Low (Regulator ON) 0.6 V(max)

High (Regulator OFF) 2.0 V(min)

ON /OFF Pin V

LOGIC

= 2.5V (Regulator OFF) 5 µA

Input Current 15 µA(max)

LOGIC

= 0.5V (Regulator ON) 0.02 µA

5 µA(max)

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

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

Note 2: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.

Note 3: Typical numbers are at 25˚C and represent the most likely norm.

Note 4: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%

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

calculate Average Outgoing Quality Level (AOQL).

Note 5: External components such as the catch diode, inductor, input and output capacitors, and voltage programming resistors can affect switching regulator

system performance. When the LM2594/LM2594HV is used as shown in the Figure 1 test circuit, system performance will be as shown in system parameters section

of Electrical Characteristics.

Note 6: The switching frequency is reduced when the second stage current limit is activated. The amount of reduction is determined by the severity of current

overload.

Note 7: No diode, inductor or capacitor connected to output pin.

Note 8: Feedback pin removed from output and connected to 0V to force the output transistor switch ON.

Note 9: Feedback pin removed from output and connected to 12V for the 3.3V, 5V, and the ADJ. version, and 15V for the 12V version, to force the output transistor

switch OFF.

Note 10: VIN = 40V for the LM2594 and 60V for the LM2594HV.

Note 11: Junction to ambient thermal resistance with approximately 1 square inch of printed circuit board copper surrounding the leads. Additional copper area will

lower thermal resistance further. See application hints in this data sheet and the thermal model in Switchers Made Simple®software.

Typical Performance Characteristics

Normalized

Output Voltage Line Regulation

01243904 01243905

LM2594/LM2594HV

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Typical Performance Characteristics (Continued)

Efficiency

Switch Saturation

Voltage

01243906 01243907

Switch Current Limit Dropout Voltage

01243908 01243909

Quiescent Current

Standby

Quiescent Current

01243910 01243911

LM2594/LM2594HV

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Typical Performance Characteristics (Continued)

Minimum Operating

Supply Voltage

ON /OFF Threshold

Voltage

01243912 01243913

ON /OFF Pin

Current (Sinking) Switching Frequency

01243914 01243915

Feedback Pin

Bias Current

01243916

LM2594/LM2594HV

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Typical Performance Characteristics

Continuous Mode Switching Waveforms

= 20V, V

OUT

= 5V, I

LOAD

= 400 mA

L = 100 µH, C

OUT

= 120 µF, C

OUT

ESR = 140 mΩ

Discontinuous Mode Switching Waveforms

= 20V, V

OUT

= 5V, I

LOAD

= 200 mA

L=33µH,C

OUT

= 220 µF, C

OUT

ESR=60mΩ

01243917

A: Output Pin Voltage, 10V/div.

B: Inductor Current 0.2A/div.

C: Output Ripple Voltage, 20 mV/div.

Horizontal Time Base: 2 µs/div.

01243918

A: Output Pin Voltage, 10V/div.

B: Inductor Current 0.2A/div.

C: Output Ripple Voltage, 20 mV/div.

Horizontal Time Base: 2 µs/div.

Load Transient Response for Continuous Mode

= 20V, V

OUT

= 5V, I

LOAD

= 200 mA to 500 mA

L = 100 µH, C

OUT

= 120 µF, C

OUT

ESR = 140 mΩ

Load Transient Response for Discontinuous Mode

= 20V, V

OUT

= 5V, I

LOAD

= 100 mA to 200 mA

L=33µH,C

OUT

= 220 µF, C

OUT

ESR=60mΩ

01243919

A: Output Voltage, 50 mV/div. (AC)

B: 200 mA to 500 mA Load Pulse

Horizontal Time Base: 50 µs/div.

01243920

A: Output Voltage, 50 mV/div. (AC)

B: 100 mA to 200 mA Load Pulse

Horizontal Time Base: 200 µs/div.

LM2594/LM2594HV

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Typical Circuit and Layout

Guidelines

Fixed Output Voltage Versions

01243922

CIN — 68 µF, 35V, Aluminum Electrolytic Nichicon “PL Series”

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

D1 — 1A, 40V Schottky Rectifier, 1N5819

L1 — 100 µH, L20

Select components with higher voltage ratings for designs using the LM2594HV with an input voltage between 40V and 60V.

Adjustable Output Voltage Versions

01243923

CIN — 68 µF, 35V, Aluminum Electrolytic Nichicon “PL Series”

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

D1 — 1A, 40V Schottky Rectifier, 1N5819

L1 — 100 µH, L20

R1—1kΩ,1%

CFF — See Application Information Section

FIGURE 1. Typical Circuits and Layout Guides

LM2594/LM2594HV

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Typical Circuit and Layout

Guidelines (Continued)

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

idly switching currents associated with wiring inductance can

generate voltage transients which can cause problems. For

minimal inductance and ground loops, the wires indicated by

heavy lines should be wide printed circuit traces and

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

external components should be located as close to the

switcher lC as possible using ground plane construction or

single point grounding.

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

OUT

wiring 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 section for more

information.)

LM2594/LM2594HV Series Buck Regulator Design Procedure (Fixed

Output)

PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)

Given:

OUT

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

(max) = Maximum DC Input Voltage

LOAD

(max) = Maximum Load Current

Given:

OUT

=5V

(max) = 12V

LOAD

(max) = 0.4A

1. Inductor Selection (L1)

A. Select the correct inductor value selection guide from

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

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

procedure for the adjustable version.

B. From the inductor value selection guide, identify the

inductance region intersected by the Maximum Input Voltage

line and the Maximum Load Current line. Each region is

identified by an inductance value and an inductor code

(LXX).

C. Select an appropriate inductor from the four

manufacturer’s part numbers listed in Figure 8.

1. Inductor Selection (L1)

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 0.4A vertical line is 100 µH, and the inductor

code is L20.

C. The inductance value required is 100 µH. From the table

in Figure 8, go to the L20 line and choose an inductor part

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

instance, both through hole and surface mount inductors are

available.)

LM2594/LM2594HV

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LM2594/LM2594HV Series Buck Regulator Design Procedure (Fixed

Output) (Continued)

PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)

2. Output Capacitor Selection (C

OUT

)

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

Series Resistance) electrolytic capacitors between 82 µF

and 220 µF and low ESR solid tantalum capacitors between

15 µF and 100 µF provide the best results. This capacitor

should be located close to the IC using short capacitor leads

and short copper traces. Do not use capacitors larger than

220 µF.

For additional information, see section on output

capacitors 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

capacitors that will provide the best design solutions.

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.

D. For computer aided design software, see Switchers Made

Simple version 4.1 or later.

2. Output Capacitor Selection (C

OUT

)

A. See section on output capacitors in application

information 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 0.5A line. In the maximum input voltage column,

select the line that covers the input voltage needed in your

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

this line are recommended inductors and capacitors that will

provide the best overall performance.

The capacitor list contains both through hole electrolytic and

surface mount tantalum capacitors from four different

capacitor manufacturers. It is recommended that both the

manufacturers and the manufacturer’s series that are listed

in the table be used.

In this example aluminum electrolytic capacitors from

several different manufacturers are available with the range

of ESR numbers needed.

120 µF 25V Panasonic HFQ Series

120 µF 25V Nichicon PL Series

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

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

switching grade, 120 µF 10V aluminum electrolytic capacitor

would exhibit approximately 400 mΩof ESR (see the curve

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

ESR would result in relatively high output ripple voltage. To

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

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

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

voltage by approximately half.

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 LM2594. 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 LM2594 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 1N4001 series are much too slow and should not be

used.

3. Catch Diode Selection (D1)

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

1A, 20V, 1N5817 Schottky diode will provide the best

performance, and will not be overstressed even for a

shorted output.

LM2594/LM2594HV

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LM2594/LM2594HV Series Buck Regulator Design Procedure (Fixed

Output) (Continued)

PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)

4. Input Capacitor (C

)

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 at least

⁄

the DC load current. The capacitor

manufacturers data sheet must be checked to assure that

this current rating is not exceeded. The curve shown in

Figure 13 shows typical RMS current ratings for several

different aluminum electrolytic capacitor values.

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

recommended that they be surge current tested by the

manufacturer.

Use caution when using ceramic capacitors for input

bypassing, because it may cause severe ringing at the V

pin.

For additional information, see section on input

capacitors in Application Information section.

4. Input Capacitor (C

)

The important parameters for the Input capacitor are the

input voltage rating and the RMS current rating. With a

nominal input voltage of 12V, an aluminum electrolytic

capacitor with a voltage rating greater than 18V (1.5 x V

)

would be needed. The next higher capacitor voltage rating is

25V.

The RMS current rating requirement for the input capacitor

in a buck regulator is approximately

⁄

the DC load current.

In this example, with a 400 mA load, a capacitor with a

RMS current rating of at least 200 mA is needed. The

curves shown in Figure 13 can be used to select an

appropriate input capacitor. From the curves, locate the 25V

line and note which capacitor values have RMS current

ratings greater than 200 mA. Either a 47 µF or 68 µF, 25V

capacitor could be used.

For a through hole design, a 68 µF/25V electrolytic capacitor

(Panasonic HFQ series or Nichicon PL series or equivalent)

would be adequate. Other types or other manufacturers

capacitors can be used provided the RMS ripple current

ratings are adequate.

For surface mount designs, solid tantalum capacitors are

recommended. The TPS series available from AVX, and the

593D series from Sprague are both surge current tested.

LM2594/LM2594HV

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LM2594/LM2594HV Series Buck Regulator Design Procedure (Adjustable

Output)

PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)

Given:

OUT

= Regulated Output Voltage

(max) = Maximum Input Voltage

LOAD

(max) = Maximum Load Current

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

Given:

OUT

= 20V

(max) = 28V

LOAD

(max) = 0.5A

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

1. Programming Output Voltage (Selecting R

and R

,as

shown in Figure 1.

Use the following formula to select the appropriate resistor

values.

Select a value for R

between 240Ωand 1.5 kΩ. The lower

resistor values minimize noise pickup in the sensitive feed-

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

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

1. Programming Output Voltage (Selecting R

and R

,as

shown in Figure 1 )

Select R

to be 1 kΩ, 1%. Solve for R

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

= 15.4 kΩ.

Conditions Inductor Output Capacitor

Through Hole Surface Mount

Output Load Max Input Inductance Inductor Panasonic Nichicon AVX TPS Sprague

Voltage Current Voltage (µH) (#) HFQ Series PL Series Series 595D Series

(V) (A) (V) (µF/V) (µF/V) (µF/V) (µF/V)

3.3 0.5 5 33 L14 220/16 220/16 100/16 100/6.3

7 47 L13 120/25 120/25 100/16 100/6.3

10 68 L21 120/25 120/25 100/16 100/6.3

40 100 L20 120/35 120/35 100/16 100/6.3

6 68 L4 120/25 120/25 100/16 100/6.3

0.2 10 150 L10 120/16 120/16 100/16 100/6.3

40 220 L9 120/16 120/16 100/16 100/6.3

50.5 8 47 L13 180/16 180/16 100/16 33/25

10 68 L21 180/16 180/16 100/16 33/25

15 100 L20 120/25 120/25 100/16 33/25

40 150 L19 120/25 120/25 100/16 33/25

9 150 L10 82/16 82/16 100/16 33/25

0.2 20 220 L9 120/16 120/16 100/16 33/25

40 330 L8 120/16 120/16 100/16 33/25

12 0.5 15 68 L21 82/25 82/25 100/16 15/25

18 150 L19 82/25 82/25 100/16 15/25

30 220 L27 82/25 82/25 100/16 15/25

40 330 L26 82/25 82/25 100/16 15/25

15 100 L11 82/25 82/25 100/16 15/25

0.2 20 220 L9 82/25 82/25 100/16 15/25

40 330 L17 82/25 82/25 100/16 15/25

FIGURE 2. LM2594/LM2594HV Fixed Voltage Quick Design Component Selection Table

LM2594/LM2594HV

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LM2594/LM2594HV 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) , from the following formula:

where V

SAT

= internal switch saturation voltage = 0.9V

and V

= diode forward voltage drop = 0.5V

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

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

Selection Guide shown in Figure 7.

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

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

and the Maximum Load Current value. Each region is identi-

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

E. Select an appropriate inductor from the four manufacturer’s

part numbers listed in Figure 8.

2. Inductor Selection (L1)

A. Calculate the inductor Volt •microsecond constant

(E •T) ,

B. E•T = 35.2 (V •µs)

C. I

LOAD

(max) = 0.5A

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

the inductance region intersected by the 35 (V •µs) horizontal

line and the 0.5A vertical line is 150 µH, and the inductor code

is L19.

E. From the table in Figure 8, locate line L19, and select an

inductor part number from the list of manufacturers part num-

bers.

3. Output Capacitor Selection (C

OUT)

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

solid tantalum capacitors between 82 µF and 220 µF provide

the best results. This capacitor should be located close to

the IC using short capacitor leads and short copper traces.

Do not use capacitors larger than 220 µF. For additional

information, see section on output capacitors in

application information section.

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

quick design table shown in Figure 3. This table contains

different output voltages, and lists various output capacitors

that will provide the best design solutions.

C. The capacitor voltage rating 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 needed for low output ripple voltage.

3. Output Capacitor SeIection (C

OUT

)

A. See section on C

OUT

in 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

electrolytic or surface mount tantalum types from four

different capacitor manufacturers. It is recommended that

both the manufacturers and the manufacturers series that

are listed in the table be used.

In this example, through hole aluminum electrolytic

capacitors from several different manufacturers are

available.

82 µF 50V Panasonic HFQ Series

120 µF 50V 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.

LM2594/LM2594HV

www.national.com 14

LM2594/LM2594HV Series Buck Regulator Design Procedure (Adjustable

Output) (Continued)

PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)

4. Feedforward Capacitor (C

)(See Figure 1 )

For output voltages greater than approximately 10V, an

additional capacitor is required. The compensation capacitor

is typically between 50 pF and 10 nF, and is wired in

parallel with the output voltage setting resistor, R

.It

provides additional stability for high output voltages, low

input-output voltages, and/or very low ESR output

capacitors, such as solid tantalum capacitors.

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.)

4. Feedforward Capacitor (C

)

The table shown in Figure 3 contains feed forward capacitor

values for various output voltages. In this example,a1nF

capacitor is needed.

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 LM2594. 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 LM2594 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.

5. Catch Diode Selection (D1)

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

provide the best performance, and in this example a 1A,

40V, 1N5819 Schottky diode would be a good choice. The

1A diode rating is more than adequate and will not be

overstressed even for a shorted output.

LM2594/LM2594HV

www.national.com15

LM2594/LM2594HV Series Buck Regulator Design Procedure (Adjustable

Output) (Continued)

PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)

6. Input Capacitor (C

)

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 at least

⁄

the DC load current. The capacitor

manufacturers data sheet must be checked to assure that

this current rating is not exceeded. The curve shown in

Figure 13 shows typical RMS current ratings for several

different aluminum electrolytic capacitor values.

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.

Use caution when using ceramic capacitors for input

bypassing, because it may cause severe ringing at the V

pin.

For additional information, see section on input

capacitors in application information section.

6. Input Capacitor (C

)

The important parameters for the Input capacitor are the

input voltage rating and the RMS current rating. With a

nominal input voltage of 28V, an aluminum electrolytic

aluminum electrolytic capacitor with a voltage rating greater

than 42V (1.5 x V

) would be needed. Since the the next

higher capacitor voltage rating is 50V, a 50V capacitor

should be used. The capacitor voltage rating of (1.5 x V

)is

a conservative guideline, and can be modified somewhat if

desired.

The RMS current rating requirement for the input capacitor

of a buck regulator is approximately

⁄

the DC load current.

In this example, with a 400 mA load, a capacitor with a RMS

current rating of at least 200 mA is needed.

The curves shown in Figure 13 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 200 mA. A 47 µF/50V low ESR

electrolytic capacitor capacitor is needed.

For a through hole design, a 47 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 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.

To further simplify the buck regulator design procedure,

National Semiconductor is making available computer

design software to be used with the Simple Switcher line ot

switching regulators. Switchers Made Simple (version 4.1

or later) is available from National’s web site,

www.national.com.

Output

Voltage

(V)

Through Hole Output Capacitor Surface Mount Output Capacitor

Panasonic Nichicon PL Feedforward AVX TPS Sprague Feedforward

HFQ Series Series Capacitor Series 595D Series Capacitor

(µF/V) (µF/V) (µF/V) (µF/V)

1.2 220/25 220/25 0 220/10 220/10 0

4180/25 180/25 4.7 nF 100/10 120/10 4.7 nF

682/25 82/25 4.7 nF 100/10 120/10 4.7 nF

982/25 82/25 3.3 nF 100/16 100/16 3.3 nF

12 82/25 82/25 2.2 nF 100/16 100/16 2.2 nF

15 82/25 82/25 1.5 nF 68/20 100/20 1.5 nF

24 82/50 120/50 1 nF 10/35 15/35 220 pF

28 82/50 120/50 820 pF 10/35 15/35 220 pF

FIGURE 3. Output Capacitor and Feedforward Capacitor Selection Table

LM2594/LM2594HV

www.national.com 16

LM2594/LM2594HV Series Buck Regulator Design Procedure

INDUCTOR VALUE SELECTION GUIDES

(For Continuous Mode Operation)

01243924

FIGURE 4. LM2594/LM2594HV-3.3

01243925

FIGURE 5. LM2594/LM2594HV-5.0

01243926

FIGURE 6. LM2594/LM2594HV-12

01243927

FIGURE 7. LM2594/LM2594HV-ADJ

LM2594/LM2594HV

www.national.com17

LM2594/LM2594HV Series Buck Regulator Design Procedure (Continued)

Inductance

(µH)

Current

(A)

Schott Renco Pulse Engineering Coilcraft

Through Surface Through Surface Through Surface Surface

Hole Mount Hole Mount Hole Mount Mount

L1 220 0.18 67143910 67144280 RL-5470-3 RL1500-220 PE-53801 PE-53801-S DO1608-224

L2 150 0.21 67143920 67144290 RL-5470-4 RL1500-150 PE-53802 PE-53802-S DO1608-154

L3 100 0.26 67143930 67144300 RL-5470-5 RL1500-100 PE-53803 PE-53803-S DO1608-104

L4 68 0.32 67143940 67144310 RL-1284-68 RL1500-68 PE-53804 PE-53804-S DO1608-68

L5 47 0.37 67148310 67148420 RL-1284-47 RL1500-47 PE-53805 PE-53805-S DO1608-473

L6 33 0.44 67148320 67148430 RL-1284-33 RL1500-33 PE-53806 PE-53806-S DO1608-333

L7 22 0.60 67148330 67148440 RL-1284-22 RL1500-22 PE-53807 PE-53807-S DO1608-223

L8 330 0.26 67143950 67144320 RL-5470-2 RL1500-330 PE-53808 PE-53808-S DO3308-334

L9 220 0.32 67143960 67144330 RL-5470-3 RL1500-220 PE-53809 PE-53809-S DO3308-224

L10 150 0.39 67143970 67144340 RL-5470-4 RL1500-150 PE-53810 PE-53810-S DO3308-154

L11 100 0.48 67143980 67144350 RL-5470-5 RL1500-100 PE-53811 PE-53811-S DO3308-104

L12 68 0.58 67143990 67144360 RL-5470-6 RL1500-68 PE-53812 PE-53812-S DO1608-683

L13 47 0.70 67144000 67144380 RL-5470-7 RL1500-47 PE-53813 PE-53813-S DO3308-473

L14 33 0.83 67148340 67148450 RL-1284-33 RL1500-33 PE-53814 PE-53814-S DO1608-333

L15 22 0.99 67148350 67148460 RL-1284-22 RL1500-22 PE-53815 PE-53815-S DO1608-223

L16 15 1.24 67148360 67148470 RL-1284-15 RL1500-15 PE-53816 PE-53816-S DO1608-153

L17 330 0.42 67144030 67144410 RL-5471-1 RL1500-330 PE-53817 PE-53817-S DO3316-334

L18 220 0.55 67144040 67144420 RL-5471-2 RL1500-220 PE-53818 PE-53818-S DO3316-224

L19 150 0.66 67144050 67144430 RL-5471-3 RL1500-150 PE-53819 PE-53819-S DO3316-154

L20 100 0.82 67144060 67144440 RL-5471-4 RL1500-100 PE-53820 PE-53820-S DO3316-104

L21 68 0.99 67144070 67144450 RL-5471-5 RL1500-68 PE-53821 PE-53821-S DDO3316-683

L26 330 0.80 67144100 67144480 RL-5471-1 — PE-53826 PE-53826-S —

L27 220 1.00 67144110 67144490 RL-5471-2 — PE-53827 PE-53827-S —

FIGURE 8. Inductor Manufacturers Part Numbers

Coilcraft Inc. Phone (800) 322-2645

FAX (708) 639-1469

Coilcraft Inc., Europe Phone +44 1236 730 595

FAX +44 1236 730 627

Pulse Engineering Inc. Phone (619) 674-8100

FAX (619) 674-8262

Pulse Engineering Inc., Phone +353 93 24 107

Europe FAX +353 93 24 459

Renco Electronics Inc. Phone (800) 645-5828

FAX (516) 586-5562

Schott Corp. Phone (612) 475-1173

FAX (612) 475-1786

FIGURE 9. Inductor Manufacturers Phone Numbers

Nichicon Corp. Phone (708) 843-7500

FAX (708) 843-2798

Panasonic Phone (714) 373-7857

FAX (714) 373-7102

AVX Corp. Phone (803) 448-9411

FAX (803) 448-1943

Sprague/Vishay Phone (207) 324-7223

FAX (207) 324-4140

FIGURE 10. Capacitor Manufacturers Phone Numbers

LM2594/LM2594HV

www.national.com 18

LM2594/LM2594HV Series Buck Regulator Design Procedure (Continued)

Block Diagram

Application Information

PIN FUNCTIONS

— This is the positive input supply for the IC switching

regulator. A suitable input bypass capacitor must be present

at this pin to minimize voltage transients and to supply the

switching currents needed by the regulator.

Ground — Circuit ground.

Output — Internal switch. The voltage at this pin switches

between (+V

−V

SAT

) and approximately −0.5V, with a duty

cycle of V

OUT

. To minimize coupling to sensitive circuitry,

the PC board copper area connected to this pin should be

kept to a minimum.

VR 1A Diodes

Surface Mount Through Hole

Schottky Ultra Fast Schottky Ultra Fast

Recovery Recovery

20V All of these 1N5817 All of these

diodes are SR102 diodes are

MBRS130 rated to at 1N5818 rated to at

30V least 60V. SR103 least 60V.

11DQ03

MBRS140 MURS120 1N5819 MUR120

40V 10BQ040 10BF10 SR104 HER101

10MQ040 11DQ04 11DF1

50V

MBRS160 SR105

10BQ050 MBR150

10MQ060 11DQ05

MBRS1100 MBR160

10MQ090 SB160

SGL41-60 11DQ10

SS16

FIGURE 11. Diode Selection Table

01243921

FIGURE 12.

LM2594/LM2594HV

www.national.com19

Application Information (Continued)

Feedback — Senses the regulated output voltage to com-

plete the feedback loop.

ON /OFF — Allows the switching regulator circuit to be shut

down using logic level signals thus dropping the total input

supply current to approximately 80 µA. Pulling this pin below

a threshold voltage of approximately 1.3V turns the regulator

on, and pulling this pin above 1.3V (up to a maximum of 25V)

shuts the regulator down. If this shutdown feature is not

needed, the ON /OFF pin can be wired to the ground pin or

it can be left open, in either case the regulator will be in the

ON condition.

EXTERNAL COMPONENTS

— A low ESR aluminum or tantalum bypass capacitor is

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

located near the regulator using short leads. This capacitor

prevents large voltage transients from appearing at the in-

put, and provides the instantaneous current needed each

time the switch turns on.

The important parameters for the Input capacitor are the

voltage rating and the RMS current rating. Because of the

relatively high RMS currents flowing in a buck regulator’s

input capacitor, this capacitor should be chosen for its RMS

current rating rather than its capacitance or voltage ratings,

although the capacitance value and voltage rating are di-

rectly related to the RMS current rating.

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

capacitor’s power rating. The RMS current flowing through

the capacitors internal ESR produces power which causes

the internal temperature of the capacitor to rise. The RMS

current rating of a capacitor is determined by the amount of

current required to raise the internal temperature approxi-

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

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

ing air will determine the amount of current the capacitor can

safely sustain. Capacitors that are physically large and have

a large surface area will typically have higher RMS current

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

trolytic capacitor will be physically larger than a lower voltage

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

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

rent rating.

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 manu-

facturers data sheet for maximum allowable RMS ripple

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

general guideline would be to select a capacitor with a ripple

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

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

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

conservative design. The capacitor voltage rating must be at

least 1.25 times greater than the maximum input voltage,

and often a much higher voltage capacitor is needed to

satisfy the RMS current requirements.

A graph shown in Figure 13 shows the relationship between

an electrolytic capacitor value, its voltage rating, and the

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

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

lytic capacitors designed for switching regulator applications.

Other capacitor manufacturers offer similar types of capaci-

tors, but always check the capacitor data sheet.

“Standard” electrolytic capacitors typically have much higher

ESR numbers, lower RMS current ratings and typically have

a shorter operating lifetime.

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

OUT

— An output capacitor is required to filter the output

and provide regulator loop stability. Low impedance or low

ESR Electrolytic or solid tantalum capacitors designed for

switching regulator applications must be used. When select-

ing an output capacitor, the important capacitor parameters

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

RMS ripple current rating, voltage rating, and capacitance

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

important parameter.

The output capacitor requires an ESR value that has an

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

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

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

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

extremely low, there is a possibility of an unstable feedback

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

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

design solutions under all conditions.

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

quired, refer to the section on Output Voltage Ripple and

Transients for a post ripple filter.

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

01243928

FIGURE 13. RMS Current Ratings for Low ESR

Electrolytic Capacitors (Typical)

LM2594/LM2594HV

www.national.com 20

Application Information (Continued)

(see Figure 14 ). Often, capacitors with much higher voltage

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

quired for low output ripple voltage.

The output capacitor for many different switcher designs

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

tor values and several different voltage ratings. See the

quick design component selection tables in Figure 2 and

Figure 3 for typical capacitor values, voltage ratings, and

manufacturers capacitor types.

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 15 .

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 LM2594 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 charac-

teristic may cause instability or EMI problems. Ultra-fast

recovery diodes typically have reverse recovery times of 50

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

too slow and should not be used.

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 LM2594 (or any of the Simple Switcher family) can be

used for both continuous or discontinuous modes of opera-

tion.

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

tinuous mode. It offers greater output power, lower peak

switch, inductor and diode currents, and can have lower

output ripple voltage. But it does require larger inductor

values to keep the inductor current flowing continuously,

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

ages.

To simplify the inductor selection process, an inductor selec-

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

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

in the continuous mode, and selects an inductor that will

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 16.)

01243929

FIGURE 14. Capacitor ESR vs Capacitor Voltage Rating

(Typical Low ESR Electrolytic Capacitor)

01243930

FIGURE 15. Capacitor ESR Change vs Temperature

LM2594/LM2594HV

www.national.com21

Application Information (Continued)

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

wrapped on a ferrite bobbin. This type of construction makes

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

completely contained within the core, it generates more

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

induce voltages into nearby printed circuit traces, thus caus-

ing problems with both the switching regulator operation and

nearby sensitive circuitry, and can give incorrect scope read-

ings because of induced voltages in the scope probe. Also

see section on Open Core Inductors.

The inductors listed in the selection chart include ferrite

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

and Coilcraft, and powdered iron toroid for Pulse Engineer-

ing.

Exceeding an inductor’s maximum current rating may cause

the inductor to overheat because of the copper wire losses,

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

the inductance decreases rapidly and the inductor begins to

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

can cause the switch current to rise very rapidly and force

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

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

ing of the inductor and/or the LM2594. Different inductor

types have different saturation characteristics, and this

should be kept in mind when selecting an inductor.

The inductor manufacturers 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

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

less than the switch current limit.

Discontinuous operation can have voltage waveforms that

are considerable different than a continuous design. The

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

soidal ringing present. (See photo titled; Discontinuous

Mode Switching Waveforms) This ringing is normal for dis-

continuous 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 prob-

lem, 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 differ-

ent amounts of this characteristic ringing. Ferrite core induc-

tors have very little core loss and therefore produce the most

ringing. The higher core loss of powdered iron inductors

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

in parallel with the inductor to dampen the ringing. The

computer aided design software Switchers Made Simple

(version 4.1) will provide all component values for continu-

ous and discontinuous modes of operation.

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 saw-

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

typical output ripple voltage can range from approximately

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

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

ever, caution must be exercised when using extremely low

ESR capacitors because they can affect the loop stability,

01243931

FIGURE 16. (∆I

IND

) Peak-to-Peak

Inductor Ripple Current

(as a Percentage of the Load Current) vs Load Current

01243932

FIGURE 17. Post Ripple Filter Waveform

LM2594/LM2594HV

www.national.com 22

Application Information (Continued)

resulting in oscillation problems. If very low output ripple

voltage is needed (less than 15 mV), a post ripple filter is

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

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

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

pacitor is also required to assure good dynamic load re-

sponse and ripple reduction. The ESR of this capacitor may

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

feedback loop. The photo shown in Figure 17 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 and the diode, and the parasitic inductance

of the output filter capacitor, and its associated wiring. To

minimize these voltage spikes, the output capacitor should

be designed for switching regulator applications, and the

lead lengths must be kept very short. Wiring inductance,

stray capacitance, as well as the scope probe used to evalu-

ate these transients, all contribute to the amplitude of these

spikes.

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 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 discon-

tinuous mode of operation. Most switcher designs (irregard-

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

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

fectly acceptable mode of operation.

In a switching regulator design, knowing the value of the

peak-to-peak inductor ripple current (∆I

IND

) can be useful for

determining a number of other circuit parameters. Param-

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

mum load current before the circuit becomes discontinuous,

output ripple voltage and output capacitor ESR can all be

calculated from the peak-to-peak ∆I

IND

. When the inductor

nomographs shown in Figure 4 through Figure 7 are used to

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

current can immediately be determined. The curve shown in

Figure 18 shows the range of (∆I

IND

) that can be expected

for different load currents. The curve also shows how the

peak-to-peak inductor ripple current (∆I

IND

) changes as you

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

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

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

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

These curves are only correct for continuous mode opera-

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

select the inductor value

Consider the following example:

OUT

= 5V, maximum load current of 300 mA

= 15V, nominal, varying between 11V and 20V.

The selection guide in Figure 5 shows that the vertical line

for a 0.3A load current, and the horizontal line for the 15V

input voltage intersect approximately midway between the

upper and lower borders of the 150 µH inductance region. A

150 µH inductor will allow a peak-to-peak inductor current

(∆I

IND

) to flow that will be a percentage of the maximum load

current. Referring to Figure 18, follow the 0.3A 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 150 mA p-p).

As the input voltage increases to 20V, it approaches the

upper border of the inductance region, and the inductor

ripple current increases. Referring to the curve in Figure 18,

it can be seen that for a load current of 0.3A, the peak-to-

peak inductor ripple current (∆I

IND

) is 150 mA with 15V in,

and can range from 175 mA at the upper border (20V in) to

120 mA at the lower border (11V in).

Once the ∆I

IND

value is known, the following formulas can be

used to calculate additional information about the switching

regulator circuit.

01243933

FIGURE 18. Peak-to-Peak Inductor

Ripple Current vs Load Current

LM2594/LM2594HV

www.national.com23

Application Information (Continued)

1. Peak Inductor or peak switch current

2. Minimum load current before the circuit becomes dis-

continuous

3. Output Ripple Voltage

=(∆I

IND

)x(ESR of C

OUT

)

= 0.150Ax0.240Ω=36 mV p-p

4. ESR of C

OUT

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 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.

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

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

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

mine if this is the problem, temporarily raise the inductor

away from the board by several inches and then check

circuit operation. If the circuit now operates correctly, then

the magnetic flux from the open core inductor is causing the

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

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

PC layout may be necessary. Magnetic flux cutting the IC

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

tive traces of the output capacitor should be minimized.

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” induc-

tors 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 LM2594/LM2594HV is available in two packages, an

8-pin through hole DIP (N) and an 8-pin surface mount SO-8

(M). Both packages are molded plastic with a copper lead

frame. When the package is soldered to the PC board, the

copper and the board are the heat sink for the LM2594 and

the other heat producing components.

For best thermal performance, wide copper traces should be

used and all ground and unused pins should be soldered to

generous amounts of printed circuit board copper, such as a

ground plane (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 re-

sistance) to the surrounding air, and even double-sided or

multilayer boards provide a better heat path to the surround-

ing air. Unless power levels are small, sockets are not rec-

ommended because of the added thermal resistance it adds

and the resultant higher junction temperatures.

Package thermal resistance and junction temperature rise

numbers are all approximate, and there are many factors

that will affect the junction temperature. Some of these fac-

tors include board size, shape, thickness, position, location,

and even board temperature. Other factors are, trace width,

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. Furthermore, some of these com-

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

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

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

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

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

the board.

LM2594/LM2594HV

www.national.com 24

Application Information (Continued)

The curves shown in Figure 19 and Figure 20 show the

LM2594 junction temperature rise above ambient tempera-

ture with a 500 mA load for various input and output volt-

ages. This data was taken with the circuit operating as a

buck switcher with all components mounted on a PC board

to simulate the junction temperature under actual operating

conditions. This curve is typical, and can be used for a quick

check on the maximum junction temperature for various

conditions, but keep in mind that there are many factors that

can affect the junction temperature.

DELAYED STARTUP

The circuit in Figure 21 uses the the ON /OFF pin to provide

a time delay between the time the input voltage is applied

and the time the output voltage comes up (only the circuitry

pertaining to the delayed start up is shown). As the input

voltage rises, the charging of capacitor C1 pulls the ON /OFF

pin high, keeping the regulator off. Once the input voltage

reaches its final value and the capacitor stops charging, and

resistor R

pulls the ON /OFF pin low, thus allowing the

circuit to start switching. Resistor R

is included to limit the

maximum voltage applied to the ON /OFF pin (maximum of

25V), reduces power supply noise sensitivity, and also limits

the capacitor, C1, discharge current. When high input ripple

voltage exists, avoid long delay time, because this ripple can

be coupled into the ON /OFF pin and cause problems.

This delayed startup feature is useful in situations where the

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

deliver. It allows the input voltage to rise to a higher voltage

before the regulator starts operating. Buck regulators require

less input current at higher input voltages.

UNDERVOLTAGE LOCKOUT

Some applications require the regulator to remain off until

the input voltage reaches a predetermined voltage. An und-

ervoltage lockout feature applied to a buck regulator is

shown in Figure 22, while Figure 23 and Figure 24 applies

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

23 features a constant threshold voltage for turn on and turn

01243935

Circuit Data for Temperature Rise Curve (DIP-8)

Capacitors Through hole electrolytic

Inductor Through hole, Schott, 100 µH

Diode Through hole, 1A 40V, Schottky

PC board 4 square inches single sided 2 oz. copper

(0.0028")

FIGURE 19. Junction Temperature Rise, DIP-8

01243934

Circuit Data for Temperature Rise Curve

(Surface Mount)

Capacitors Surface mount tantalum, molded “D” size

Inductor Surface mount, Coilcraft DO33, 100 µH

Diode Surface mount, 1A 40V, Schottky

PC board 4 square inches single sided 2 oz. copper

(0.0028")

FIGURE 20. Junction Temperature Rise, SO-8

01243936

FIGURE 21. Delayed Startup

01243937

FIGURE 22. Undervoltage Lockout

for Buck Regulator

LM2594/LM2594HV

www.national.com25

Application Information (Continued)

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

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

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

hysteresis is approximately equal to the value of the output

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

additional 47 kΩresistor is needed from the ON /OFF pin to

the ground pin to stay within the 25V maximum limit of the

ON /OFF pin.

INVERTING REGULATOR

The circuit in Figure 25 converts a positive input voltage to a

negative output voltage with a common ground. The circuit

operates by bootstrapping the regulators ground pin to the

negative output voltage, then grounding the feedback pin,

the regulator senses the inverted output voltage and regu-

lates it.

01243938

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

FIGURE 23. Undervoltage Lockout for Inverting Regulator

01243939

This circuit has hysteresis

Regulator starts switching at VIN = 13V

Regulator stops switching at VIN =8V

FIGURE 24. Undervoltage Lockout with Hysteresis for Inverting Regulator

LM2594/LM2594HV

www.national.com 26

Application Information (Continued)

This example uses the LM2594-5 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 26 provides a

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

different input and output voltage conditions.

The maximum voltage appearing across the regulator is the

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

be limited to a maximum of 40V. For example, when convert-

ing +20V to −12V, the regulator would see 32V between the

input pin and ground pin. The LM2594 has a maximum input

voltage spec of 40V (60V for the LM2594HV).

Additional diodes are required in this regulator configuration.

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

coupling through the C

capacitor to the output, under light

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

topology to closley resemble a buck configuration thus pro-

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

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

drop) but for higher input voltages, a fast recovery diode

could be used.

Without diode D3, when the input voltage is first applied, the

charging current of C

can pull the output positive by sev-

eral volts for a short period of time. Adding D3 prevents the

output from going positive by more than a diode voltage.

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 100

µH, 1A inductor is the best choice. Capacitor selection can

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

shown in Figure 25 will provide good results in the majority of

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 LM2594 current limit

(approx 0.8A) are needed for at least 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 rela-

tively high startup currents required by the inverting topology,

the delayed startup feature (C1, R

and R

) shown in Figure

25 is recommended. By delaying the regulator startup, the

input capacitor is allowed to charge up to a higher voltage

before the switcher begins operating. A portion of the high

input current needed for startup is now supplied by the input

capacitor (C

). For severe start up conditions, the input

capacitor can be made much larger than normal.

01243940

CIN — 68 µF/25V Tant. Sprague 595D

120 µF/35V Elec. Panasonic HFQ

COUT — 22 µF/20V Tant. Sprague 595D

39 µF/16V Elec. Panasonic HFQ

FIGURE 25. Inverting −5V Regulator with Delayed Startup

01243941

FIGURE 26. Inverting Regulator Typical Load Current

LM2594/LM2594HV

www.national.com27

Application Information (Continued)

INVERTING REGULATOR SHUTDOWN METHODS

To use the ON /OFF pin in a standard buck configuration is

simple, pull it below 1.3V (@25˚C, referenced to ground) to

turn regulator ON, pull it above 1.3V to shut the regulator

OFF. With the inverting configuration, some level shifting is

required, because the ground pin of the regulator is no

longer at ground, but is now setting at the negative output

voltage level. Two different shutdown methods for inverting

regulators are shown in Figure 27 and Figure 28.

01243942

FIGURE 27. Inverting Regulator Ground Referenced Shutdown

01243943

FIGURE 28. Inverting Regulator Ground Referenced Shutdown using Opto Device

LM2594/LM2594HV

www.national.com 28

Application Information (Continued)

TYPICAL SURFACE MOUNT PC BOARD LAYOUT, FIXED OUTPUT (2X SIZE)

01243944

— 10 µF, 35V, Solid Tantalum AVX, “TPS series”

OUT

— 100 µF, 10V Solid Tantalum AVX, “TPS series”

D1 — 1A, 40V Schottky Rectifier, surface mount

L1 — 100 µH, L20, Coilcraft DO33

TYPICAL SURFACE MOUNT PC BOARD LAYOUT, ADJUSTABLE OUTPUT (2X SIZE)

01243945

— 10 µF, 35V, Solid Tantalum AVX, “TPS series”

OUT

— 100 µF, 10V Solid Tantalum AVX, “TPS series”

D1 — 1A, 40V Schottky Rectifier, surface mount

L1 — 100 µH, L20, Coilcraft DO33

R1 — 1 kΩ,1%

— Use formula in Design Procedure

— See Figure 3.

FIGURE 29. PC Board Layout

LM2594/LM2594HV

www.national.com29

Physical Dimensions inches (millimeters)

unless otherwise noted

8-Lead (0.150" Wide) Molded Small Outline Package,

Order Number LM2594M-3.3, LM2594M-5.0,

LM2594M-12 or LM2594M-ADJ JEDEC

NS Package Number M08A

LM2594/LM2594HV

www.national.com 30

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

8-Lead (0.300" Wide) Molded Dual-In-Line Package,

Order Number LM2594N-3.3, LM2594N-5.0, LM2594N-12 or LM2594N-ADJ

NS Package Number N08E

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.

For the most current product information visit us at www.national.com.

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.

BANNED SUBSTANCE COMPLIANCE

National Semiconductor certifies that the products and packing materials meet the provisions of the Customer Products Stewardship

Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned

Substances’’ as defined in CSP-9-111S2.

National Semiconductor

Americas Customer

Support Center

Email: new.feedback@nsc.com

Tel: 1-800-272-9959

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

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www.national.com

LM2594/LM2594HV SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down Voltage

Regulator

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