LM2597-3.3_技术文档

March 1998

LM2597/LM2597HV

SIMPLE SWITCHER^®Power Converter 150 kHz 0.5A

Step-Down Voltage Regulator, with Features

The LM2597HV is for use in applications requiring and input

voltage up to 60V.

General Description

The LM2597/LM2597HV 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 de-

vices are available in fixed output voltages of 3.3V, 5V, 12V,

and an adjustable output version, and are packaged in an

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

Features

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

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

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

conditions

±

n Guaranteed 0.5A output current

n Available in 8-pin surface mount and DIP-8 package

n Input voltage range up to 60V

n 150 kHz fixed frequency internal oscillator

n Shutdown /Soft-start

n Out of regulation error flag

n Error output delay

n Bias Supply Pin (V_BS) for internal circuitry improves

efficiency at high input voltages

n Low power standby mode, I_Qtypically 85 µA

n High Efficiency

This series of switching regulators is similar to the LM2594

series, with additional supervisory and performance features

added.

Requiring a minimum number of external components, these

regulators are simple to use and include internal frequency

†

compensation , improved line and load specifications,

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

delay and error flag output.

The LM2597/LM2597HV 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.

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 LM2597/LM2597HV se-

ries. This feature greatly simplifies the design of

switch-mode power supplies.

Applications

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

n Efficient pre-regulator for linear regulators

n On-card switching regulators

±

Other features include a guaranteed 4% tolerance on out-

n Positive to Negative converter

put voltage under all conditions of input voltage and output

±

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

ternal shutdown is included, featuring typically 85 µA

standby current. Self protection features include a two stage

current limit for the output switch and an over temperature

shutdown for complete protection under fault conditions.

Typical Application (Fixed Output Voltage Versions)

DS012440-1

†

Patent Number 5,382,918.

®

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

DS012440

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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

M8 Package

Vapor Phase (60 sec.)

Infrared (15 sec.)

+215˚C

+220˚C

+260˚C

+150˚C

Maximum Supply Voltage (V_IN

)

LM2597

45V

60V

N Package (Soldering, 10 sec.)

Maximum Junction Temperature

LM2597HV

SD /SS Pin Input Voltage (Note 2)

Delay Pin Voltage (Note 2)

Flag Pin Voltage

6V

1.5V

Operating Conditions

Temperature Range

Supply Voltage

LM2597

−0.3 ≤ V ≤45V

−0.3 ≤ V ≤30V

−0.3 ≤ V ≤+25V

Bias Supply Voltage (V_BS

Feedback Pin Voltage

)

−40˚C ≤ T_J+125˚C

Output Voltage to Ground

(Steady State)

4.5V to 40V

4.5V to 60V

−1V

Internally limited

−65˚C to +150˚C

LM2597HV

Power Dissipation

Storage Temperature Range

LM2597/LM2597HV-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.V_INmax40V for the LM2597 and 60V for the LM2597HV

Symbol Parameter Conditions

LM2597/LM2597HV-3.3

Units

(Limits)

Typ

Limit

(Note 4)

(Note 5)

SYSTEM PARAMETERS (Note 6) Test Circuit Figure 12

V_OUT

Output Voltage

4.75V ≤ V_IN≤ V_INmax, 0.1A ≤ I_LOAD≤ 0.5A

3.3

80

V

3.168/3.135

3.432/3.465

V(min)

V(max)

%

= =

V_IN12V, I_LOAD0.5A

η

Efficiency

LM2597/LM2597HV-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.V_INmax40V for the LM2597 and 60V for the LM2597HV

Symbol Parameter Conditions

LM2597/LM2597HV-5.0

Units

(Limits)

Typ

Limit

(Note 4)

(Note 5)

SYSTEM PARAMETERS (Note 6) Test Circuit Figure 12

V_OUT

Output Voltage

7V ≤ V_IN≤ V_INmax, 0.1A ≤ I_LOAD≤ 0.5A

5

V

4.800/4.750

5.200/5.250

V(min)

V(max)

%

=

η

Efficiency

V_IN12V, I_LOAD0.5A

82

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2

LM2597/LM2597HV-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.V_INmax40V for the LM2597 and 60V for the LM2597HV

Symbol Parameter Conditions

LM2597/LM2597HV-12

Units

(Limits)

Typ

Limit

(Note 4)

(Note 5)

SYSTEM PARAMETERS (Note 6) Test Circuit Figure 12

V_OUT

Output Voltage

15V ≤ V_IN≤ V_INmax, 0.1A ≤ I_LOAD≤ 0.5A

12

88

V

11.52/11.40

12.48/12.60

V(min)

V(max)

%

= =

V_IN25V, I_LOAD0.5A

η

Efficiency

LM2597/LM2597HV-ADJ

Electrical Characteristics

=

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

=

ture Range.V_INmax40V for the LM2597 and 60V for the LM2597HV

Symbol Parameter Conditions

LM2597/LM2597HV-ADJ

Units

(Limits)

Typ

Limit

(Note 4)

(Note 5)

SYSTEM PARAMETERS (Note 6) Test Circuit Figure 12

V_FB

Feedback Voltage

4.5V ≤ V_IN≤ V_INmax, 0.1A ≤ I_LOAD≤ 0.5A

1.230

80

V

V_OUTprogrammed for 3V. Circuit of Figure 12.

1.193/1.180

1.267/1.280

V(min)

V(max)

%

= = =

V_IN12V, V_OUT3V, I_LOAD0.5A

η

Efficiency

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_LOAD100 mA.

Symbol

Parameter

Conditions

LM2597/LM2597HV-XX

Units

(Limits)

Typ

Limit

(Note 4)

(Note 5)

DEVICE PARAMETERS

=

I_b

Feedback Bias Current

Adjustable Version Only, V_FB1.235V

10

50/100

nA

kHz

f_O

Oscillator Frequency

(Note 7)

150

127/110

173/173

kHz(min)

kHz(max)

V

=

V_SAT

DC

Saturation Voltage

I_OUT0.5A (Notes 8 and 9)

0.9

1.1/1.2

V(max)

%

Max Duty Cycle (ON)

Min Duty Cycle (OFF)

Current Limit

(Note 9)

100

0

(Note 10)

I_CL

Peak Current, (Notes 8 and 9)

0.8

A

0.65/0.58

1.3/1.4

50

A(min)

A(max)

µA(max)

mA

=

Output 0V

I_L

Output Leakage Current

(Notes 8, 10 and 11)

=

Output −1V

2

5

15

10

mA(max)

mA

I_Q

Operating Quiescent

Current

SD /SS Pin Open, V_BSPin 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_LOAD100 mA.

Symbol

Parameter

Conditions

LM2597/LM2597HV-XX

Units

(Limits)

Typ

Limit

(Note 4)

(Note 5)

DEVICE PARAMETERS

=

I_STBY

Standby Quiescent

SD /SS pin 0V

(Note 10)LM2597

85

µA

Current

200/250

250/300

µA(max)

˚C/W

LM2597HV

140

95

θ_JA

Thermal Resistance

N Package, Junction to Ambient (Note 12)

M Package, Junction to Ambient (Note 12)

150

SHUTDOWN/SOFT-START CONTROL Test Circuit of Figure 12

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 12

Regulator Dropout

Detector

Low (Flag ON)

%

%(min)

%(max)

V

92

98

Threshold Voltage

Flag Output Saturation

Voltage

=

VF_SAT

IF_L

I_SINK3 mA

0.3

=

V_DELAY0.5V

0.7/1.0

V(max)

µA

=

Flag Output Leakage

Current

V_FLAG40V

0.3

Delay Pin Threshold

Voltage

1.25

V

Low (Flag ON)

1.21

1.29

V(min)

V(max)

µA

High (Flag OFF) and V_OUTRegulated

=

Delay Pin Source

Current

V_DELAY0.5V

3

6

µA(max)

mV

Delay Pin Saturation

Low (Flag ON)

55

350/400

mV(max)

BIAS SUPPLY

I_BSBias Supply Pin Current

=

V_BS2V

(Note 10)

120

4

µA

µA(max)

mA

400

=

V_BS4.4V (Note 10)

10

2

mA(max)

mA

=

I_Q

Operating Quiescent

Current

V_BS4.4V , V_inpin current(Note 10)

1

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% produc-

tion 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).

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4

All Output Voltage Versions

Electrical Characteristics ^(Continued)

Note 6: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2597/

LM2597HV is used as shown in the Figure 12 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.

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.

=

40V for the LM2597 and 60V for the LM2597HV.

Note 11:

V

IN

Note 12: 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

Line Regulation

Efficiency

Output Voltage

DS012440-3

DS012440-4

DS012440-2

Switch Saturation

Voltage

Switch Current Limit

Dropout Voltage

DS012440-6

DS012440-7

DS012440-5

Quiescent Current

Standby

Quiescent Current

Minimum Operating

Supply Voltage

DS012440-8

DS012440-9

DS012440-10

5

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Typical Performance Characteristics (Continued)

Feedback Pin

Bias Current

Flag Saturation

Voltage

Switching Frequency

DS012440-13

DS012440-11

DS012440-12

Soft-start

Shutdown /Soft-start

Current

Delay Pin Current

DS012440-14

DS012440-16

DS012440-15

V_INand V_BSCurrent vs

V_BSand Temperature

Soft-start Response

Shutdown /Soft-start

Threshold Voltage

DS012440-18

DS012440-25

DS012440-17

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6

Typical Performance Characteristics (Continued)

Continuous Mode Switching Waveforms

Discontinuous Mode Switching Waveforms

=

V_IN20V, V_OUT5V, I_LOAD400 mA

=

V_IN20V, V_OUT5V, I_LOAD200 mA

=

= =

100 µH, C_OUT120 µF, C_OUTESR 140 mΩ

L

=

= =

33 µH, C_OUT220 µF, C_OUTESR 60 mΩ

L

DS012440-19

DS012440-20

A: Output Pin Voltage, 10V/div.

B: Inductor Current 0.2A/div.

C: Output Ripple Voltage, 20 mV/div.

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

Load Transient Response for Discontinuous Mode

=

V_IN20V, V_OUT5V, I_LOAD200 mA to 500 mA

=

V_IN20V, V_OUT5V, I_LOAD100 mA to 200 mA

=

= =

100 µH, C_OUT120 µF, C_OUTESR 140 mΩ

L

=

= =

33 µH, C_OUT220 µF, C_OUTESR 60 mΩ

L

DS012440-21

DS012440-22

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

B: 200 mA to 500 mA Load Pulse

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

B: 100 mA to 200 mA Load Pulse

Horizontal Time Base: 50 µs/div.

Horizontal Time Base: 200 µs/div.

Connection Diagrams and Ordering Information

8–Lead DIP (N)

8–Lead Surface Mount (M)

DS012440-23

DS012440-24

Top View

Order Number LM2597N-3.3,

LM2597N-5.0, LM2597N-12 or

LM2597N-ADJ

Order Number LM2597M-3.3,

LM2597M-5.0, LM2597M-12 or

LM2597M-ADJ

LM2597HVN-3.3, LM2597HVN-5.0,

LM2597HVN-12 or LM2597HVN-ADJ

See NS Package Number N08E

LM2597HVM-3.3, LM2597HVM-5.0,

LM2597HVM-12 or LM2597HVM-ADJ

See NS Package Number M08A

7

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LM2597/LM2597HV 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) Maximum Load Current

I_LOAD(max) 0.4A

1. Inductor Selection (L1)

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

ure 3, Figure 4, or Figure 5. (Output voltages of 3.3V, 5V, or

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

cedure for the adjustable version.

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

in Figure 4.

B. From the inductor value selection guide shown in Figure 4,

the inductance region intersected by the 12V horizontal line

and the 0.4A vertical line is 100 µH, and the inductor code is

L20.

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 100 µH. From the table in

Figure 7, go to the L20 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 manufacturer’s

part numbers listed in Figure 7.

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 220 µF

and low ESR solid tantalum capacitors between 15 µF and

100 µF provide the best results. This capacitor should be lo-

cated close to the IC using short capacitor leads and short

copper traces. Do not use capacitors larger than 220 µF.

A. See section on output capacitors in application infor-

mation section.

B. From the quick design component selection table shown in

Figure 1, locate the 5V output voltage section. In the load cur-

rent 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.

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 1.

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 4.1 or later).

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, switch-

ing grade, 120 µF 10V aluminum electrolytic capacitor would

exhibit approximately 400 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 voltage rating (lower ESR) should be selected. A 16V

or 25V capacitor will reduce the ripple voltage by approxi-

mately half.

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8

LM2597/LM2597HV 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 cur-

rent limit of the LM2597. The most stressful condition for this

diode is an overload or shorted output condition.

A. Refer to the table shown in Figure 10. In this example, a

1A, 20V, 1N5817 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 LM2597 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 re-

sults. Ultra-fast recovery diodes typically have reverse recov-

ery times of 50 ns or less. Rectifiers such as the 1N4001 se-

ries 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

The important parameters for the Input capacitor are the input

voltage rating and the RMS current rating. With a nominal in-

put 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.

current rating of the input capacitor should be selected to be

1

at least

⁄2 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 15 shows typical

RMS current ratings for several different aluminum electrolytic

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 400 mA load, a capacitor with a RMS cur-

rent rating of at least 200 mA is needed. The curves shown in

Figure 15 can be used to select an appropriate input capaci-

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

pacitor values have RMS current ratings greater than 200 mA.

Either a 47 µF or 68 µF, 25V capacitor could be used.

This capacitor should be located close to the IC using short

leads and the voltage rating should be approximately 1.5

times the maximum input voltage.

If solid tantalum input capacitors are used, it is recommended

that they be surge current tested by the manufacturer.

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 ca-

pacitors can be used provided the RMS ripple current ratings

are adequate.

Use caution when using ceramic capacitors for input bypass-

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

For additional information, see section on input capaci-

tors in Application Information section.

For surface mount designs, solid tantalum capacitors are rec-

ommended. The TPS series available from AVX, and the

593D series from Sprague are both surge current tested.

9

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LM2597/LM2597HV Series Buck Regulator Design Procedure (Fixed

Output) ^(Continued)

Conditions

Inductor

Output Capacitor

Through Hole Surface Mount

Panasonic

Output

Voltage

(V)

Load

Current

(A)

Max Input

Inductance

Inductor

(#)

Nichicon

PL Series

(µF/V)

220/16

120/25

120/35

120/25

120/16

180/16

120/25

82/16

AVX TPS

Series

(µF/V)

100/16

Sprague

595D Series

(µF/V)

100/6.3

33/25

Voltage

(V)

5

(µH)

HFQ Series

(µF/V)

220/16

120/25

120/35

120/25

120/16

180/16

120/25

82/16

3.3

0.5

33

47

L14

L13

L21

L20

L4

7

10

40

6

68

100

68

0.2

0.5

10

40

8

150

220

47

L10

L9

5

L13

L21

L20

L19

L10

L9

10

15

40

9

68

33/25

100

150

220

330

68

33/25

0.2

0.5

20

40

15

18

30

40

15

20

40

120/16

82/25

120/16

82/25

33/25

L8

33/25

12

L21

L19

L27

L26

L11

L9

15/25

150

220

330

100

220

330

82/25

15/25

82/25

15/25

82/25

15/25

82/25

15/25

0.2

82/25

15/25

L17

82/25

15/25

FIGURE 1. LM2597/LM2597HV Fixed Voltage Quick Design Component Selection Table

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10

LM2597/LM2597HV 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

=

I_LOAD(max) 0.5A

I_LOAD(max) Maximum Load Current

=

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 12)

Use the following formula to select the appropriate resistor

values.

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

=

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 re-

sistor values minimize noise pickup in the sensitive feedback

pin. (For the lowest temperature coefficient and the best sta-

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

=

R₂15.4 kΩ.

2. Inductor Selection (L1)

A. Calculate the inductor Volt microsecond constant E • T

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

(V • µs), from the following formula:

=

where V_SATinternal switch saturation voltage 0.9V

=

and V_Ddiode forward voltage drop 0.5V

=

B. E • T 35.2 (V • µs)

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 6.

=

C. I_LOAD(max) 0.5A

D. From the inductor value selection guide shown in Figure 6,

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.

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

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

value and the Maximum Load Current value. Each region is

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

E. From the table in Figure 7, locate line L19, and select an in-

ductor part number from the list of manufacturers part num-

bers.

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

part numbers listed in Figure 7.

11

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LM2597/LM2597HV Series Buck Regulator Design Procedure (Adjustable

Output) ^(Continued)

PROCEDURE (Adjustable Output Voltage Version)

3. Output Capacitor Selection (C_OUT

EXAMPLE (Adjustable Output Voltage Version)

3. Output Capacitor SeIection (C_OUT

)

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

tantalum capacitors between 82 µF and 220 µF provide the

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

using short capacitor leads and short copper traces. Do not

use capacitors larger than 220 µF. For additional informa-

tion, see section on output capacitors in application in-

formation section.

A. See section on C_OUTin Application Information section.

B. From the quick design table shown in Figure 2, 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 sec-

tion, 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 manufactur-

ers 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 2. 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 capacitors

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.

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. Re-

fer to the capacitor manufacturers data sheet for this informa-

tion.

4. Feedforward Capacitor (C_FF) (See Figure 12)

4. Feedforward Capacitor (C_FF)

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

tional capacitor is required. The compensation capacitor is

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

the output voltage setting resistor, R₂. It provides additional

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

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

capacitors.

The table shown in Figure 2 contains feed forward capacitor

values for various output voltages. In this example, a 1 nF ca-

pacitor is needed.

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.)

www.national.com

12

LM2597/LM2597HV 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 cur-

rent limit of the LM2597. The most stressful condition for this

diode is an overload or shorted output condition.

A. Refer to the table shown in Figure 10. Schottky diodes pro-

vide 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.

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 LM2597 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. Recti-

fiers 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

The important parameters for the Input capacitor are the input

voltage rating and the RMS current rating. With a nominal in-

put voltage of 28V, an aluminum electrolytic aluminum electro-

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

current rating of the input capacitor should be selected to be

1

at least

⁄

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 electrolytic

capacitor values.

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

pacitor voltage rating of (1.5 x V_IN) is a conservative guideline,

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 400 mA load, a capacitor with a RMS cur-

rent rating of at least 200 mA is needed.

If solid tantalum input capacitors are used, it is recomended

that they be surge current tested by the manufacturer.

The curves shown in Figure 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 ratings

greater than 200 mA. A 47 µF/50V low ESR electrolytic ca-

pacitor capacitor is needed.

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 capacitor

in application information section.

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 ca-

pacitors can be used provided the RMS ripple current ratings

are adequate.

For surface mount designs, solid tantalum capacitors are rec-

ommended. 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 switching

regulators. Switchers Made Simple^®(version 4.1 or later) is

available at National’s web site, www.national.com.

13

www.national.com

LM2597/LM2597HV 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)

220/25

180/25

82/25

Feedforward

AVX TPS

Sprague

595D Series

(µF/V)

Feedforward

HFQ Series

(µF/V)

220/25

180/25

82/25

Capacitor

Series

(µF/V)

220/10

100/10

100/16

68/20

Capacitor

1.2

4

0

220/10

120/10

100/16

100/20

15/35

0

4.7 nF

3.3 nF

2.2 nF

1.5 nF

1 nF

4.7 nF

3.3 nF

2.2 nF

1.5 nF

220 pF

6

9

82/25

1 2

1 5

2 4

2 8

82/25

82/50

120/50

10/35

82/50

820 pF

10/35

15/35

FIGURE 2. Output Capacitor and Feedforward Capacitor Selection Table

www.national.com

14

LM2597/LM2597HV Series Buck Regulator Design Procedure

INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)

DS012440-57

DS012440-30

FIGURE 3. LM2597/LM2597HV-3.3

FIGURE 4. LM2597/LM2597HV-5.0

DS012440-58

DS012440-32

FIGURE 5. LM2597/LM2597HV-12

FIGURE 6. LM2597/LM2597HV-ADJ

15

www.national.com

LM2597/LM2597HV Series Buck Regulator Design Procedure (Continued)

Induc-

tance

(µH)

Cur-

rent

(A)

Schott

Renco

Through

Hole

Pulse Engineering

Coilcraft

Surface

Mount

Through Surface

Surface

Mount

Through

Hole

Surface

Mount

Hole

Mount

L1

220

0.18

0.21

0.26

0.32

0.37

0.44

0.60

0.26

0.32

0.39

0.48

0.58

0.70

0.83

0.99

1.24

0.42

0.55

0.66

0.82

0.99

0.80

1.00

67143910 67144280 RL-5470-3

67143920 67144290 RL-5470-4

67143930 67144300 RL-5470-5

RL1500-220 PE-53801 PE-53801-S DO1608-224

RL1500-150 PE-53802 PE-53802-S DO1608-154

RL1500-100 PE-53803 PE-53803-S DO1608-104

L2

150

100

68

L3

L4

67143940 67144310 RL-1284-68 RL1500-68

67148310 67148420 RL-1284-47 RL1500-47

67148320 67148430 RL-1284-33 RL1500-33

67148330 67148440 RL-1284-22 RL1500-22

PE-53804 PE-53804-S DO1608-68

PE-53805 PE-53805-S DO1608-473

PE-53806 PE-53806-S DO1608-333

PE-53807 PE-53807-S DO1608-223

L5

47

L6

33

L7

22

L8

330

220

150

100

68

67143950 67144320 RL-5470-2

67143960 67144330 RL-5470-3

67143970 67144340 RL-5470-4

67143980 67144350 RL-5470-5

67143990 67144360 RL-5470-6

67144000 67144380 RL-5470-7

RL1500-330 PE-53808 PE-53808-S DO3308-334

RL1500-220 PE-53809 PE-53809-S DO3308-224

RL1500-150 PE-53810 PE-53810-S DO3308-154

RL1500-100 PE-53811 PE-53811-S DO3308-104

L9

L10

L11

L12

L13

L14

L15

L16

L17

L18

L19

L20

L21

L26

L27

RL1500-68

RL1500-47

PE-53812 PE-53812-S DO1608-683

PE-53813 PE-53813-S DO3308-473

PE-53814 PE-53814-S DO1608-333

PE-53815 PE-53815-S DO1608-223

PE-53816 PE-53816-S DO1608-153

47

33

67148340 67148450 RL-1284-33 RL1500-33

67148350 67148460 RL-1284-22 RL1500-22

67148360 67148470 RL-1284-15 RL1500-15

22

15

330

220

150

100

68

67144030 67144410 RL-5471-1

67144040 67144420 RL-5471-2

67144050 67144430 RL-5471-3

67144060 67144440 RL-5471-4

67144070 67144450 RL-5471-5

67144100 67144480 RL-5471-1

67144110 67144490 RL-5471-2

RL1500-330 PE-53817 PE-53817-S DO3316-334

RL1500-220 PE-53818 PE-53818-S DO3316-224

RL1500-150 PE-53819 PE-53819-S DO3316-154

RL1500-100 PE-53820 PE-53820-S DO3316-104

RL1500-68

PE-53821 PE-53821-S DDO3316-683

330

220

—

PE-53826 PE-53826-S

PE-53827 PE-53827-S

—

FIGURE 7. Inductor Manufacturers Part Numbers

Coilcraft Inc.

Phone (800) 322-2645

FAX (708) 639-1469

Phone +44 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

FIGURE 8. Inductor Manufacturers Phone Numbers

Nichicon Corp.

Panasonic

Phone

FAX

(708) 843-7500

(708) 843-2798

(714) 373-7857

(714) 373-7102

(803) 448-9411

(803) 448-1943

(207) 324-7223

(207) 324-4140

Coilcraft Inc., Europe

Pulse Engineering Inc.

Phone

FAX

AVX Corp.

Phone

FAX

Pulse Engineering Inc.,

Europe

Sprague/Vishay

Phone

FAX

Renco Electronics Inc.

FIGURE 9. Capacitor Manufacturers Phone Numbers

Schott Corp.

www.national.com

16

LM2597/LM2597HV Series Buck Regulator Design Procedure (Continued)

VR

1A Diodes

Surface Mount

Ultra Fast

Recovery

Through Hole

Ultra Fast

Schottky

Recovery

All of these diodes are rated to

at least 60V.

20V

30V

40V

All of these diodes are rated to

at least 60V.

1N5817

SR102

MBRS130

1N5818

SR103

11DQ03

1N5819

SR104

MBRS140

10BQ040

10MQ040

MBRS160

10BQ050

10MQ060

MBRS1100

10MQ090

SGL41-60

SS16

MURS120

10BF10

MUR120

HER101

11DF1

11DQ04

SR105

50V

or

more

MBR150

11DQ05

MBR160

SB160

11DQ10

FIGURE 10. Diode Selection Table

Block Diagram

DS012440-26

FIGURE 11.

17

www.national.com

Typical Circuit and Layout Guidelines

Fixed Output Voltage Versions

DS012440-27

=

5V, I 500 mA.

LOAD

Component Values shown are for V

IN

15V, V

OUT

C

D1

L1

— 47 µF, 50V, Aluminum Electrolytic Nichicon “PL Series”

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

— 1A, 30V Schottky Rectifier, 1N5818

IN

OUT

— 100 µH, L20

Typical Values

C

R

— 0.1 µF

SS

— 0.1 µF

— 4.7k

DELAY

Pull Up

*Use Bias Supply pin for 5V and 12V Versions

Adjustable Output Voltage Versions

DS012440-56

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

1

=

Component Values shown are for V

IN

20V,

=

500 mA.

V

10V, I

OUT

LOAD

C

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

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

IN

OUT

D1 — 1A, 30V Schottky Rectifier, 1N5818

L1 — 150 µH, L19

R

C

— 1 kΩ, 1%

— 7.15k, 1%

— 3.3 nF, See Application Information Section

1

2

FF

Typical Values

C

R

— 0.1 µF

SS

— 0.1 µF

DELAY

— 4.7k

PULL UP

*For output voltages between 4V and 20V

FIGURE 12. Typical Circuits and Layout Guides

www.national.com

18

Bias Supply (Pin 3)—This feature allows the regulators in-

ternal circuitry to be powered from the regulated output volt-

age or an external supply, instead of the input voltage. This

results in increased efficiency under some operating condi-

tions, such as low output current and/or high input voltage.

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, ex-

ternal components should be located as close to the

switcher lC as possible using ground plane construction or

single point grounding.

Special Note If any of the above four features (Shutdown

/Soft-start, Error Flag, Delay, or Bias Supply) are not used,

the respective pins should be left open.

EXTERNAL COMPONENTS

SOFT-START CAPACITOR

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

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

is first applied to the regulator, or when the Shutdown

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

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.)

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.

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

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

the internal regulatory circuitry is operating, the quiescent

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

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

Soft-start capacitor charging current decreases from 5 µA

down to approximately 1.6 µA. This decreases the slope of

capacitor voltage ramp.

Application Information

PIN FUNCTIONS

+V_IN(Pin 7)—This is the positive input supply for the IC

switching regulator. A suitable input bypass capacitor must

be present at this pin to minimize voltage transients and to

supply the switching currents needed by the regulator.

Ground (Pin 6)—Circuit ground.

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

Output (Pin 8)—Internal switch. The voltage at this pin

switches between (+V_IN− V_SAT) and approximately −0.5V,

with a duty cycle of V_OUT/V_IN. To minimize coupling to sensi-

tive circuitry, the PC board copper area connected to this pin

should be kept to a minimum.

@

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

condition. The switch (Pin 8) 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

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.

Feedback (Pin 4)—Senses the regulated output voltage to

complete the feedback loop.

Shutdown /Soft-start (Pin 5)—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.

Error Flag (Pin 1)—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.

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.

Delay (Pin 2)—At power-up, this pin can be used to provide

a time delay between the time the regulated output voltage

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

error flag output goes high.

19

www.national.com

Application Information (Continued)

DS012440-33

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

www.national.com

20

Application Information (Continued)

DS012440-34

FIGURE 14. Timing Diagram for 5V Output

DELAY CAPACITOR

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.

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-

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 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.

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 1) 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.

INPUT CAPACITOR

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.

21

www.national.com

bypassing, but several precautions must be observed. A

small percentage of solid tantalum capacitors can short if the

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

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

higher input voltages produce higher inrush currents. Sev-

eral capacitor manufacturers do a 100% surge current test-

ing on their products to minimize this potential problem. If

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

limit this current by adding either some resistance or induc-

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

age capacitor. As with aluminum electrolytic capacitors, the

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

Application Information (Continued)

OUTPUT CAPACITOR

C_OUT—An output capacitor is required to filter the output

and provide regulator loop stability. Low impedance or low

ESR Electrolytic or solid tantalum capacitors designed for

switching regulator applications must be used. When select-

ing an output capacitor, the important capacitor parameters

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

RMS ripple current rating, voltage rating, and capacitance

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

important parameter.

DS012440-28

FIGURE 15. RMS Current Ratings for Low

ESR Electrolytic Capacitors (Typical)

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

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.

DS012440-29

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.

FIGURE 16. Capacitor ESR vs Capacitor Voltage Rating

(Typical Low ESR Electrolytic Capacitor)

The consequences of operating an electrolytic capacitor

above the RMS current rating is a shortened operating life.

The higher temperature speeds up the evaporation of the ca-

pacitor’s electrolyte, resulting in eventual failure.

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 1 and Figure 2

for typical capacitor values, voltage ratings, and manufactur-

ers capacitor types.

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

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.

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.

Solid tantalum capacitors have a much better ESR spec for

cold temperatures and are recommended for temperatures

below −25˚C.

A graph shown in Figure 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 LM2594 using

short leads and short printed circuit traces.

“Standard” electrolytic capacitors typically have much higher

ESR numbers, lower RMS current ratings and typically have

a shorter operating lifetime.

Because of their very fast switching speed and low forward

voltage drop, Schottky diodes provide the best performance,

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

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

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

Because of their small size and excellent performance, sur-

face mount solid tantalum capacitors are often used for input

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22

Application Information (Continued)

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

DS012440-31

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.

DS012440-37

FIGURE 17. Capacitor ESR Change vs Temperature

INDUCTOR SELECTION

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.

All switching regulators have two basic modes of operation;

continuous and discontinuous. The difference between the

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

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

normal switching cycle. Each mode has distinctively different

operating characteristics, which can affect the regulators

performance and requirements. Most switcher designs will

operate in the discontinuous mode when the load current is

low.

Inductors are available in different styles such as pot core,

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

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

sive, the bobbin, rod or stick core, consists of wire wrapped

on a ferrite bobbin. This type of construction makes for a in-

expensive 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.

The LM2597 (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 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 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.

To simplify the inductor selection process, an inductor selec-

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

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

in the continuous mode, and selects an inductor that will al-

low 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 18.)

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 LM2597. 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

23

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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 feed-

back loop. The photo shown in Figure 19 shows a typical

output ripple voltage, with and without a post ripple filter.

Application Information (Continued)

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 (200

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

than the switch current limit.

When observing output ripple with a scope, it is essential

that a short, low inductance scope probe ground connection

be used. Most scope probe manufacturers provide a special

probe terminator which is soldered onto the regulator board,

preferable at the output capacitor. This provides a very short

scope ground thus eliminating the problems associated with

the 3 inch ground lead normally provided with the probe, and

provides a much cleaner and more accurate picture of the

ripple voltage waveform.

Discontinuous operation can have voltage waveforms that

are considerable different than a continuous design. The out-

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

dal ringing present. (See photo titled; Discontinuous Mode

Switching Waveforms) This ringing is normal for discontinu-

ous operation, and is not caused by feedback loop instabili-

ties. 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 char-

acteristic ringing. Normally this ringing is not a problem, un-

less the amplitude becomes great enough to exceed the in-

put voltage, and even then, there is very little energy present

to cause damage.

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.

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-

puter aided design software Switchers Made Simple (ver-

sion 4.1) will provide all component values for continuous

and discontinuous modes of operation.

DS012440-40

FIGURE 20. Peak-to-Peak Inductor

Ripple Current vs Load Current

When a switching regulator is operating in the continuous

mode, the inductor current waveform ranges from a triangu-

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

voltage). For

a given input and output voltage, the

DS012440-39

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.

FIGURE 19. Post Ripple Filter Waveform

OUTPUT VOLTAGE RIPPLE AND TRANSIENTS

The output voltage of a switching power supply operating in

the continuous mode will contain a sawtooth ripple voltage at

the switcher frequency, and may also contain short voltage

spikes at the peaks of the sawtooth waveform.

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.

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 15 mV), a post ripple filter is recom-

mended. (See Figure 12.) The inductance required is typi-

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

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

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 3 through Figure 6 are used to

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24

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.

Application Information (Continued)

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 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 se-

lect the inductor value

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.

Consider the following example:

=

V_OUT5V, maximum load current of 300 mA

=

V_IN15V, nominal, varying between 11V and 20V.

The selection guide in Figure 4 shows that the vertical line

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

put voltage intersect approximately midway between the up-

per 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 20, 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).

Sometimes, locating a trace directly beneath a bobbin in-

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

center of the inductor (because the induced voltages cancel

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

other, then problems could arise. If flux problems are

present, even the direction of the inductor winding can make

a difference in some circuits.

As the input voltage increases to 20V, 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 0.3A, the peak-to-peak in-

ductor 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).

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.

Once the ∆I_INDvalue is known, the following formulas can be

used to calculate additional information about the switching

regulator circuit.

1. Peak Inductor or peak switch current

2. Minimum load current before the circuit becomes dis-

continuous

3. Output Ripple Voltage

=

(∆I_IND)x(ESR of C_OUT

)

=

0.150Ax0.240Ω 36 mV p-p

4.

DS012440-41

Circuit Data for Temperature Rise Curve (DIP-8)

Capacitors Through hole electrolytic

Inductor

Diode

Through hole, Schott, 100 µH

Through hole, 1A 40V, Schottky

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.

PC board 4 square inches single sided 2 oz. copper

(0.0028")

FIGURE 21. Junction Temperature Rise, DIP-8

25

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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.

Application Information (Continued)

The curves shown in Figure 21 and Figure 22 show the

LM2597 junction temperature rise above ambient tempera-

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

ages. The Bias Supply pin was not used (left open) for these

curves. Connecting the Bias Supply pin to the output voltage

would reduce the junction temperature by approximately 5˚C

to 15˚C, depending on the input and output voltages, and the

load current. 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 op-

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

a quick check on the maximum junction temperature for vari-

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

that can affect the junction temperature.

BIAS SUPPLY FEATURE

DS012440-42

The bias supply (V_BS) pin allows the LM2597’s internal cir-

cuitry to be powered from a power source, other than V_IN

,

typically the output voltage. This feature can increase effi-

ciency and lower junction temperatures under some operat-

ing conditions. The greatest increase in efficiency occur with

light load currents, high input voltage and low output voltage

(4V to 12V). See efficiency curves shown in Figure 23 and

Figure 24. The curves with solid lines are with the V_BSpin

connected to the regulated output voltage, while the curves

with dashed lines are with the V_BSpin open.

Circuit Data for Temperature Rise Curve (Surface

Mount)

Capacitors Surface mount tantalum, molded “D” size

Inductor

Diode

Surface mount, Coilcraft DO33, 100 µH

Surface mount, 1A 40V, Schottky

PC board 4 square inches single sided 2 oz. copper

(0.0028")

The bias supply pin requires a minimum of approximately

@

3.5V at room temperature (4V −40˚C), and can be as high

FIGURE 22. Junction Temperature Rise, SO-8

THERMAL CONSIDERATIONS

as 30V, but there is little advantage of using the bias supply

feature with voltages greater than 15V or 20V. The current

required for the V_INpin is typically 4 mA.

The LM2597/LM2597HV 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 LM2597 and

the other heat producing components.

To use the bias supply feature with output voltages between

4V and 15V, wire the bias pin to the regulated output. Since

the V_BSpin requires a minimum of 4V to operate, the 3.3V

part cannot be used this way. When the V_BSpin is left open,

the intemal regulator circuitry is powered from the input

voltage.

For best thermal performance, wide copper traces should be

used. Pins should be soldered to generous amounts of

printed circuit board copper, (one exception to this is the out-

put (switch) pin, which should not have large areas of cop-

per). Large areas of copper provide the best transfer of heat

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

double-sided or multilayer boards provide a better heat path

to the surrounding air. Unless power levels are small, sock-

ets are not recommended because of the added thermal re-

sistance it adds and the resultant higher junction tempera-

tures.

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

DS012440-43

FIGURE 23. Effects of Bias Supply Feature on 5V

Regulator Efficiency

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26

Application Information (Continued)

DS012440-44

FIGURE 25. Output Voltage, Input Current, Error Flag

Signal, at Start-Up, WITH Soft-start

DS012440-45

FIGURE 24. Effects of Bias Supply Feature on 12V

Regulator Efficiency

SHUTDOWN /SOFT-START

The circuit shown in Figure 27 is a standard buck regulator

with 24V in, 12V out, 100 mA load, and using a 0.068 µF

Soft-start capacitor. The photo in Figure 25 and Figure 26

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

current, with, and without a Soft-start capacitor. Figure 25

also shows the error flag output going high when the output

voltage reaches 95% of the nominal output voltage. The re-

duced input current required at startup is very evident when

comparing the two photos. The Soft-start feature reduces the

startup current from 700 mA down to 160 mA, and delays

and slows down the output voltage rise time.

DS012440-46

FIGURE 26. 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.

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.

DS012440-47

FIGURE 27. Typical Circuit Using Shutdown /Soft-start and Error Flag Features

27

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Application Information (Continued)

DS012440-48

FIGURE 28. Inverting −5V Regulator With Shutdown and Soft-start

lNVERTING REGULATOR

The circuit in Figure 28 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.

This example uses the LM2597-5 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 29 provides a

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

different input and output voltage conditions.

DS012440-49

FIGURE 29. Maximum Load Current for Inverting

Regulator Circuit

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-

verting +20V to −5V, the regulator would see 25V between

the input pin and ground pin. The LM2597 has a maximum

input voltage rating of 40V (60V for the LM2597HV).

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 1N4001 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 100 µH, 1 Amp

inductor is the best choice. Capacitor selection can also be

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

in Figure 28 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 LM2597 current limit (ap-

proximately 0.8A) are needed for 1 ms or more, until the out-

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

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28

Application Information (Continued)

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 28 is recommended.

Also shown in Figure 28 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.

DS012440-52

FIGURE 31. Undervoltage Lockout Without

Hysteresis for an Inverting Regulator

UNDERVOLTAGE LOCKOUT

Some applications require the regulator to remain off until

the input voltage reaches a predetermined voltage. Figure

30 contains a undervoltage lockout circuit for a buck configu-

ration, while Figure 31 and Figure 32 are for the inverting

types (only the circuitry pertaining to the undervoltage lock-

out is shown). Figure 30 uses a zener diode to establish the

threshold voltage when the switcher begins operating. When

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

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

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

ceeds the zener voltage, the zener conducts, pulling the

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

switching. The threshold voltage for the undervoltage lockout

feature is approximately 1.5V greater than the zener voltage.

DS012440-53

FIGURE 32. Undervoltage Lockout With

Hysteresis for an Inverting Regulator

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 33. This unregulated negative voltage is approximately

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

can supply up to a 100 mA of output current. There is a re-

quirement however, that there be a minimum load of several

hundred mA on the regulated positive output for the charge

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

the charging current of C1 to some value less than the

LM2597 current limit (typically 800 mA).

DS012440-50

FIGURE 30. Undervoltage Lockout for a Buck

Regulator

Figure 31 and Figure 32 apply the same feature to an invert-

ing circuit. Figure 31 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 32 has

a turn ON voltage which is different than the turn OFF volt-

age. The amount of hysteresis is approximately equal to the

value of the output voltage. Since the SD /SS pin has an in-

ternal 7V zener clamp, R2 is needed to limit the current into

this pin to approximately 1 mA when Q1 is on.

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.

29

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Application Information (Continued)

DS012440-51

FIGURE 33. Charge Pump for Generating a

Low Current, Negative Output Voltage

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30

Application Information (Continued)

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

DS012440-54

C

— 10 µF, 35V, Solid Tantalum, AVX, “TPS Series” (surface mount, “D” size)

— 100 µF, 10V Solid Tantalum, AVX, “TPS Series” (surface mount, “D” size)

IN

OUT

D1 — 1A, 40V Surface Mount Schottky Rectifier

L1 — Surface Mount Inductor, Coilcraft DO33

C

— Soft-start Capacitor (surface mount ceramic chip capacitor)

— Delay Capacitor (surface mount ceramic chip capacitor)

SS

D

R3 — Error Flag Pullup Resistor (surface mount chip resistor)

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

DS012440-55

C

— 10 µF, 35V, Solid Tantalum, AVX, “TPS Series” (surface mount, “D” size)

— 68 µF, 20V Solid Tantalum, AVX, “TPS Series” (surface mount, “D” size)

IN

OUT

D1 — 1A, 40V Surface Mount Schottky Rectifier

L1 — Surface Mount Inductor, Coilcraft DO33

C

— Soft-start Capacitor (surface mount ceramic chip capacitor)

— Delay Capacitor (surface mount ceramic chip capacitor)

SS

D

CFF — Feedforward Capacitor (surface mount ceramic chip capacitor)

R1 — Output Voltage Program Resistor (surface mount chip resistor)

R2 — Output Voltage Program Resistor (surface mount chip resistor)

R3 — Error Flag Pullup Resistor (surface mount chip resistor)

FIGURE 34. 2X Printed Circuit Board Layout

31

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32

Physical Dimensions inches (millimeters) unless otherwise noted

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

Order Number LM2597M-3.3, LM2597M-5.0,

LM2597M-12 or LM2597M-ADJ

LM2597HVM-3.3, LM2597HVM-5.0,

LM2597HVM-12 or LM2597HVM-ADJ

NS Package Number M08A

33

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

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

Order Number LM2597N-3.3, LM2597N-5.0, LM2597N-12 or LM2597N-ADJ

LM2597HVN-3.3, LM2597HVN-5.0, LM2597HVN-12 or LM2597HVN-ADJ

NS Package Number N08E

LIFE SUPPORT POLICY

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

VICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMI-

CONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or sys-

tems which, (a) are intended for surgical implant into

the body, or (b) support or sustain life, and whose fail-

ure 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 in any component of a life support

device or system whose failure to perform can be rea-

sonably 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: 81-3-5620-6175

Fax: 81-3-5620-6179

Fax: +49 (0) 1 80-530 85 86

Email: europe.support@nsc.com

Deutsch Tel: +49 (0) 1 80-530 85 85

English Tel: +49 (0) 1 80-532 78 32

Français Tel: +49 (0) 1 80-532 93 58

Italiano Tel: +49 (0) 1 80-534 16 80

Email: sea.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：	LM2597-3.3
厂家：	ETC
描述：	SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down Voltage Regulator with Features(897.00 k) SIMPLE SWITCHER系列电源转换器150千赫0.5A降压稳压器与特点（ 897.00 K）\n 转换器稳压器
文件：	总34页 (文件大小：898K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LM2597-3.3 [ETC]

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