LM2597HVN-12/NOPB_技术文档

LM2597, LM2597HV

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SNVS119C –MARCH 1998–REVISED APRIL 2013

®

LM2597/LM2597HV SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down

Voltage Regulator, with Features

Check for Samples: LM2597, LM2597HV

1

FEATURES

DESCRIPTION

The LM2597/LM2597HV series of regulators are

monolithic integrated circuits that provide all the

active functions for a step-down (buck) switching

23

•

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

•

Adjustable Version Output Voltage Range,

1.2V to 37V (57V for HV Version)±4% Max Over

Line and Load Conditions

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 an 8-lead PDIP and an 8-lead surface mount

package.

•

Specified 0.5A Output Current

Available in 8-pin Surface Mount and PDIP-8

Package

•

Input Voltage Range Up to 60V

150 kHz Fixed Frequency Internal Oscillator

Shutdown /Soft-start

This series of switching regulators is similar to the

LM2594 series, with additional supervisory and

performance features added.

Out of Regulation Error Flag

Error Output Delay

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.

Bias Supply Pin (V_BS) for Internal Circuitry

Improves Efficiency at High Input Voltages

•

Low Power Standby Mode, I_QTypically 85 μA

High Efficiency

The LM2597/LM2597HV series operates at

a

Uses Readily Available Standard Inductors

switching frequency of 150 kHz thus allowing smaller

sized filter components than what would be needed

with lower frequency switching regulators. Because of

its high efficiency, the copper traces on the printed

circuit board are normally the only heat sinking

needed.

Thermal Shutdown and Current Limit

Protection

APPLICATIONS

•

Simple High-efficiency Step-down (Buck)

Regulator

A standard series of inductors (both through hole and

surface mount types) are available from several

different manufacturers optimized for use with the

LM2597/LM2597HV series. This feature greatly

simplifies the design of switch-mode power supplies.

•

Efficient Pre-regulator for Linear Regulators

On-card Switching Regulators

Positive to Negative Converter

Other features include a specified ±4% tolerance on

output voltage under all conditions of input voltage

and output load conditions, and ±15% on the

oscillator frequency. External shutdown is included,

featuring typically 85 μA standby current. Self

protection features include a two stage current limit

for the output switch and an over temperature

shutdown for complete protection under fault

conditions.

The LM2597HV is for use in applications requiring

and input voltage up to 60V.

†Patent Number 5,382,918.

1

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

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

2

3

SIMPLE SWITCHER, Switchers Made Simple are registered trademarks of Texas Instruments.

All other trademarks are the property of their respective owners.

PRODUCTION DATA information is current as of publication date.

Products conform to specifications per the terms of the Texas

Instruments standard warranty. Production processing does not

necessarily include testing of all parameters.

LM2597, LM2597HV

SNVS119C –MARCH 1998–REVISED APRIL 2013

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Typical Application

(Fixed Output Voltage Versions)

Figure 1.

Connection Diagrams

Figure 2. 8–Lead PDIP (P) Top View

Figure 3. 8–Lead Surface Mount (D) Top View

See Package Number P0008E

See Package Number D0008A

2

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

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

(1)(2)

Absolute Maximum Ratings

(3)

Maximum Supply Voltage (V_IN

)

LM2597

45V

60V

LM2597HV

(4)

SD /SS Pin Input Voltage

6V

(4)

Delay Pin Voltage

1.5V

Flag Pin Voltage

−0.3 ≤ V ≤45V

−0.3 ≤ V ≤30V

−0.3 ≤ V ≤+25V

−1V

Bias Supply Voltage (V_BS

)

Feedback Pin Voltage

Output Voltage to Ground (Steady State)

Power Dissipation

Internally limited

−65°C to +150°C

Storage Temperature Range

ESD Susceptibility

(5)

Human Body Model

2 kV

+215°C

+220°C

+260°C

+150°C

Lead Temperature

D8 Package

Vapor Phase (60 sec.)

Infrared (15 sec.)

P Package (Soldering, 10 sec.)

Maximum Junction Temperature

(1) Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for

which the device is intended to be functional, but do not ensure specific performance limits. For specifications and test conditions, see

the Electrical Characteristics.

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

specifications.

(3) V_IN= 40V for the LM2597 and 60V for the LM2597HV.

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

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

Operating Conditions

Temperature Range

−40°C ≤ T_J+125°C

4.5V to 40V

Supply Voltage

LM2597

LM2597HV

4.5V to 60V

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LM2597/LM2597HV-3.3

Electrical Characteristics

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

Temperature Range.V_INmax=40V for the LM2597 and 60V for the LM2597HV

Symbol

Parameter

Conditions

LM2597/LM2597HV-3.3

Units

(Limits)

(1)

(2)

Typ

Limit

SYSTEM PARAMETERS Test Circuit Figure 31⁽³⁾⁽⁴⁾

V_OUT

Output Voltage

Efficiency

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

3.3

V

3.168/3.135

3.432/3.465

V(min)

V(max)

%

η

V_IN= 12V, I_LOAD= 0.5A

80

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

(2) All limits specified at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits

are 100% production tested. All limits at temperature extremes are specified via correlation using standard Statistical Quality Control

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

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

When the LM2597/LM2597HV is used as shown in the Figure 31 test circuit, system performance will be as shown in system

parameters section of Electrical Characteristics.

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

LM2597/LM2597HV-5.0

Electrical Characteristics

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

Temperature Range.V_INmax=40V for the LM2597 and 60V for the LM2597HV

Symbol

Parameter

Conditions

LM2597/LM2597HV-5.0

Units

(Limits)

(1)

(2)

Typ

Limit

SYSTEM PARAMETERS Test Circuit Figure 31⁽³⁾⁽⁴⁾

V_OUT

Output Voltage

Efficiency

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

V_IN= 12V, I_LOAD= 0.5A

82

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

(2) All limits specified at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits

are 100% production tested. All limits at temperature extremes are specified via correlation using standard Statistical Quality Control

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

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

When the LM2597/LM2597HV is used as shown in the Figure 31 test circuit, system performance will be as shown in system

parameters section of Electrical Characteristics.

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

4

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LM2597/LM2597HV-12

Electrical Characteristics

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

Temperature Range.V_INmax=40V for the LM2597 and 60V for the LM2597HV

Symbol

Parameter

Conditions

LM2597/LM2597HV-12

Units

(Limits)

(1)

(2)

Typ

Limit

SYSTEM PARAMETERS Test Circuit Figure 31⁽³⁾⁽⁴⁾

V_OUT

Output Voltage

Efficiency

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

12

V

11.52/11.40

12.48/12.60

V(min)

V(max)

%

η

V_IN= 25V, I_LOAD= 0.5A

88

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

(2) All limits specified at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits

are 100% production tested. All limits at temperature extremes are specified via correlation using standard Statistical Quality Control

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

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

When the LM2597/LM2597HV is used as shown in the Figure 31 test circuit, system performance will be as shown in system

parameters section of Electrical Characteristics.

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

LM2597/LM2597HV-ADJ

Electrical Characteristics

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

Temperature Range.V_INmax=40V for the LM2597 and 60V for the LM2597HV

Symbol

Parameter

Conditions

LM2597/LM2597HV-ADJ

Units

(Limits)

(1)

(2)

Typ

Limit

SYSTEM PARAMETERS Test Circuit Figure 31⁽³⁾⁽⁴⁾

V_FB

Feedback Voltage

Efficiency

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

1.230

80

V

V_OUTprogrammed for 3V. Circuit of Figure 31.

1.193/1.180

1.267/1.280

V(min)

V(max)

%

η

V_IN= 12V, V_OUT= 3V, I_LOAD= 0.5A

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

(2) All limits specified at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits

are 100% production tested. All limits at temperature extremes are specified via correlation using standard Statistical Quality Control

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

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

When the LM2597/LM2597HV is used as shown in the Figure 31 test circuit, system performance will be as shown in system

parameters section of Electrical Characteristics.

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

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All Output Voltage Versions

Electrical Characteristics

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

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

12V version. I_LOAD= 100 mA.

Symbol

Parameter

Conditions

LM2597/LM2597HV-XX

Units

(Limits)

(1)

(2)

Typ

Limit

DEVICE PARAMETERS

I_b

Feedback Bias Current

Adjustable Version Only, V_FB= 1.235V

10

50/100

nA

kHz

(3)

f_O

Oscillator Frequency

See

150

0.9

127/110

173/173

kHz(min)

kHz(max)

V

(4)(5)

V_SAT

DC

Saturation Voltage

I_OUT= 0.5A

1.1/1.2

V(max)

%

(5)

Max Duty Cycle (ON)

Min Duty Cycle (OFF)

See

100

0

(6)

See

(4)(5)

I_CL

Current Limit

Peak Current

0.8

A

A(min)

A(max)

μA(max)

mA

0.65/0.58

1.3/1.4

50

(4)(6)(7)

I_L

Output Leakage Current

Output = 0V

Output = −1V

2

5

15

10

mA(max)

mA

(6)

I_Q

Operating Quiescent

Current

SD /SS Pin Open, V_BSPin Open

mA(max)

μA

I_STBY

Standby Quiescent

Current

SD /SS pin = 0V ⁽⁶⁾LM2597

85

200/250

250/300

μA(max)

°C/W

LM2597HV

140

95

(8)

θ_JA

Thermal Resistance

P Package, Junction to Ambient

(8)

D Package, Junction to Ambient

150

SHUTDOWN/SOFT-START CONTROL Test Circuit of Figure 31

V_SD

Shutdown Threshold

Voltage

1.3

V

Low, (Shutdown Mode)

0.6

2

V(max)

V(min)

V

High, (Soft-start Mode)

V_SS

I_SD

I_SS

Soft-start Voltage

Shutdown Current

Soft-start Current

V_OUT= 20% of Nominal Output Voltage

V_OUT= 100% of Nominal Output Voltage

V_SHUTDOWN= 0.5V

2

3

5

μA

10

5

μA(max)

μA

V_Soft-start= 2.5V

1.6

μA(max)

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

(2) All limits specified at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits

are 100% production tested. All limits at temperature extremes are specified via correlation using standard Statistical Quality Control

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

(3) The switching frequency is reduced when the second stage current limit is activated. The amount of reduction is determined by the

severity of current overload.

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

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

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

the output transistor switch OFF.

(7) V_IN= 40V for the LM2597 and 60V for the LM2597HV.

(8) 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.

6

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All Output Voltage Versions

Electrical Characteristics

(continued)

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

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

12V version. I_LOAD= 100 mA.

Symbol

Parameter

Conditions

LM2597/LM2597HV-XX

Units

(Limits)

(1)

(2)

Typ

Limit

FLAG/DELAY CONTROL Test Circuit of Figure 31

Regulator Dropout

Detector

Threshold Voltage

Low (Flag ON)

96

%

%(min)

%(max)

V

92

98

VF_SAT

IF_L

Flag Output Saturation

Voltage

I_SINK= 3 mA

V_DELAY= 0.5V

V_FLAG= 40V

0.3

0.7/1.0

V(max)

μA

Flag Output Leakage

Current

Delay Pin Threshold

Voltage

1.25

V

Low (Flag ON)

1.21

1.29

V(min)

V(max)

μA

High (Flag OFF) and V_OUTRegulated

V_DELAY= 0.5V

Delay Pin Source

Current

3

6

μA(max)

mV

Delay Pin Saturation

Low (Flag ON)

55

350/400

mV(max)

BIAS SUPPLY

I_BSBias Supply Pin Current

(9)

V_BS= 2V

120

4

μA

μA(max)

mA

400

(9)

V_BS= 4.4V

10

2

mA(max)

mA

(9)

I_Q

Operating Quiescent Current V_BS= 4.4V , V_inpin current

1

(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.

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Typical Performance Characteristics

Normalized

Output Voltage

Line Regulation

Figure 4.

Figure 5.

Switch Saturation

Voltage

Efficiency

Figure 6.

Figure 7.

Switch Current Limit

Dropout Voltage

Figure 8.

Figure 9.

8

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Typical Performance Characteristics (continued)

Standby

Quiescent Current

Figure 10.

Figure 11.

Minimum Operating

Supply Voltage

Feedback Pin

Bias Current

Figure 12.

Figure 13.

Flag Saturation

Voltage

Switching Frequency

Figure 14.

Figure 15.

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Typical Performance Characteristics (continued)

Shutdown /Soft-start

Current

Soft-start

Figure 16.

Figure 17.

V_INand V_BSCurrent vs

V_BSand Temperature

Delay Pin Current

Figure 18.

Figure 19.

Shutdown /Soft-start

Threshold Voltage

Soft-start Response

Figure 20.

Figure 21.

10

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Typical Performance Characteristics (continued)

Continuous Mode Switching Waveforms

V_IN= 20V, V_OUT= 5V, I_LOAD= 400 mA

L = 100 μH, C_OUT= 120 μF, C_OUTESR = 140 mΩ

Discontinuous Mode Switching Waveforms

V_IN= 20V, V_OUT= 5V, I_LOAD= 200 mA

L = 33 μH, C_OUT= 220 μF, C_OUTESR = 60 mΩ

A: Output Pin Voltage, 10V/div.

B: Inductor Current 0.2A/div.

A: Output Pin Voltage, 10V/div.

B: Inductor Current 0.2A/div.

C: Output Ripple Voltage, 20 mV/div.

Figure 22. Horizontal Time Base: 2 μs/div.

Figure 23. Horizontal Time Base: 2 μs/div.

Load Transient Response for Continuous Mode

V_IN= 20V, V_OUT= 5V, I_LOAD= 200 mA to 500 mA

L = 100 μH, C_OUT= 120 μF, C_OUTESR = 140 mΩ

Load Transient Response for Discontinuous Mode

V_IN= 20V, V_OUT= 5V, I_LOAD= 100 mA to 200 mA

L = 33 μH, C_OUT= 220 μF, C_OUTESR = 60 mΩ

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

B: 100 mA to 200 mA Load Pulse

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

B: 200 mA to 500 mA Load Pulse

Figure 24. Horizontal Time Base: 50 μs/div.

Figure 25. Horizontal Time Base: 200 μs/div.

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

V_IN(max) = Maximum DC Input Voltage

I_LOAD(max) = Maximum Load Current

V_OUT= 5V

V_IN(max) = 12V

I_LOAD(max) = 0.4A

1. Inductor Selection (L1)

A. Select the correct inductor value selection guide from Figure 26, A. Use the inductor selection guide for the 5V version shown in

Figure 27, or Figure 28. (Output voltages of 3.3V, 5V, or 12V Figure 27.

respectively.) For all other voltages, see the design procedure for the

adjustable version.

B. From the inductor value selection guide shown in Figure 27, the

inductance region intersected by the 12V horizontal line and the 0.4A

B. From the inductor value selection guide, identify the inductance vertical line is 100 μH, and the inductor code is L20.

region intersected by the Maximum Input Voltage line and the

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

Maximum Load Current line. Each region is identified by an

Table 3, go to the L20 line and choose an inductor part number from

inductance value and an inductor code (LXX).

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

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

numbers listed in Table 3.

2. Output Capacitor Selection (C_OUT

)

2. Output Capacitor Selection (C_OUT)

A. In the majority of applications, low ESR (Equivalent Series A. See OUTPUT CAPACITOR in Application Information section.

Resistance) electrolytic capacitors between 82 μF and 220 μF and

low ESR solid tantalum capacitors between 15 μF and 100 μF

B. From the quick design component selection table shown in

Table 1, locate the 5V output voltage section. In the load current

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

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

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

needed in your application, for this example, use the 0.5A line. In the

use capacitors larger than 220 μF.

maximum input voltage column, select the line that covers the input

For additional information, see OUTPUT CAPACITOR in voltage needed in your application, in this example, use the 15V line.

Application Information.

Continuing on this line are recommended inductors and capacitors

that will provide the best overall performance.

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

design component selection table shown in Table 1. This table The capacitor list contains both through hole electrolytic and surface

contains different input voltages, output voltages, and load currents, mount tantalum capacitors from four different capacitor

and lists various inductors and output capacitors that will provide the manufacturers. It is recommended that both the manufacturers and

best design solutions.

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

C. The capacitor voltage rating for electrolytic capacitors should be In this example aluminum electrolytic capacitors from several

at least 1.5 times greater than the output voltage, and often much different manufacturers are available with the range of ESR numbers

higher voltage ratings are needed to satisfy the low ESR needed.

requirements for low output ripple voltage.

120 μF 25V Panasonic HFQ Series

D. For computer aided design software, see Switchers Made

120 μF 25V Nichicon PL Series

Simple^®version 4.1 or later).

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 mΩ of ESR (see the curve in Figure 36 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.

12

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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 A. Refer to Table 6. In this example, a 1A, 20V, 1N5817 Schottky

than the maximum load current. Also, if the power supply design diode will provide the best performance, and will not be overstressed

must withstand a continuous output short, the diode should have a even for a shorted output.

current rating equal to the maximum current limit of the LM2597. 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 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 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.

4. Input Capacitor (C_IN

)

4. Input Capacitor (C_IN)

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

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

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

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

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

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

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

Figure 35 shows typical RMS current ratings for several different

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

aluminum electrolytic capacitor values.

with a 400 mA load, a capacitor with a RMS current rating of at least

This capacitor should be located close to the IC using short leads 200 mA is needed. The curves shown in Figure 35 can be used to

and the voltage rating should be approximately 1.5 times the select an appropriate input capacitor. From the curves, locate the

maximum input voltage.

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.

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

Use caution when using ceramic capacitors for input bypassing,

because it may cause severe ringing at the V_INpin.

(Panasonic HFQ series or Nichicon PL series or equivalent) would

be adequate. Other types or other manufacturers capacitors can be

used provided the RMS ripple current ratings are adequate.

For additional information, see section on INPUT CAPACITOR in

Application Information section.

For surface mount designs, solid tantalum capacitors are

recommended. The TPS series available from AVX, and the 593D

series from Sprague are both surge current tested.

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

Conditions

Inductor

Output Capacitor

Surface Mount

Through Hole

Panasonic Nichicon

HFQ Series

Voltage

Output

(V)

Current

Load

(A)

Voltage

Max Input

(V)

AVX TPS

Series

(μF/V)

Sprague

595D Series

(μF/V)

Inductance

Inductor

(#)

PL Series

(μF/V)

(μH)

(μF/V)

5

7

33

47

L14

L13

L21

L20

L4

220/16

120/25

120/35

120/25

120/16

220/16

120/25

120/35

120/25

120/16

100/16

100/6.3

0.5

0.2

10

40

6

68

3.3

100

68

10

40

150

220

L10

L9

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Table 1. LM2597/LM2597HV Fixed Voltage Quick Design Component Selection Table (continued)

Conditions

Inductor

Output Capacitor

Surface Mount

Through Hole

Panasonic Nichicon

HFQ Series

Voltage

Output

(V)

Current

Load

(A)

Voltage

Max Input

(V)

AVX TPS

Series

(μF/V)

Sprague

595D Series

(μF/V)

Inductance

Inductor

(#)

PL Series

(μF/V)

(μH)

(μF/V)

8

47

L13

L21

L20

L19

L10

L9

180/16

120/25

82/16

120/16

82/25

180/16

120/25

82/16

120/16

82/25

100/16

33/25

15/25

10

15

40

9

68

0.5

0.2

0.5

0.2

100

150

220

330

68

5

20

40

15

18

30

40

15

20

40

L8

L21

L19

L27

L26

L11

L9

150

220

330

100

220

330

12

L17

LM2597/LM2597HV Series Buck Regulator Design Procedure (Adjustable Output)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

Given:

V_OUT= Regulated Output Voltage

V_OUT= 20V

V_IN(max) = Maximum Input Voltage

V_IN(max) = 28V

I_LOAD(max) = 0.5A

I_LOAD(max) = Maximum Load Current

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

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

Figure 31)

Use the following formula to select the appropriate resistor values.

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

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

values minimize noise pickup in the sensitive feedback pin. (For the

lowest temperature coefficient and the best stability with time, use

1% metal film resistors.)

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

R₂= 15.4 kΩ.

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PROCEDURE (Adjustable Output Voltage Version)

2. Inductor Selection (L1)

EXAMPLE (Adjustable Output Voltage Version)

2. Inductor Selection (L1)

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

from the following formula:

where

B. E • T = 35.2 (V • μs)

•

V_SAT= internal switch saturation voltage =

0.9V

C. I_LOAD(max) = 0.5A

D. From the inductor value selection guide shown in Figure 29, the

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

•

V_D= diode forward voltage drop = 0.5V

B. Use the E • T value from the previous formula and match it with the 0.5A vertical line is 150 μH, and the inductor code is L19.

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

Guide shown in Figure 29.

E. From Table 3, locate line L19, and select an inductor part number

from the list of manufacturers part numbers.

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

numbers listed in Table 3.

3. Output Capacitor Selection (C_OUT

)

3. Output Capacitor SeIection (C_OUT)

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

tantalum capacitors between 82 μF and 220 μF provide the best section.

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

B. From the quick design table shown in Table 2, locate the output

capacitor leads and short copper traces. Do not use capacitors

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

larger than 220 μF. For additional information, see OUTPUT

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

24V line. Under OUTPUT CAPACITOR, select a capacitor from the

CAPACITOR in Application Information section.

B. To simplify the capacitor selection procedure, refer to the quick list of through hole electrolytic or surface mount tantalum types from

design table shown in Table 2. This table contains different output four different capacitor manufacturers. It is recommended that both

voltages, and lists various output capacitors that will provide the best the manufacturers and the manufacturers series that are listed in the

design solutions.

table be used.

C. The capacitor voltage rating should be at least 1.5 times greater In this example, through hole aluminum electrolytic capacitors from

than the output voltage, and often much higher voltage ratings are several different manufacturers are available.

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. Refer to the capacitor

manufacturers data sheet for this information.

4. Feedforward Capacitor (C_FF) (See Figure 31)

4. Feedforward Capacitor (C_FF)

For output voltages greater than approximately 10V, an additional Table 2 contains feed forward capacitor values for various output

capacitor is required. The compensation capacitor is typically voltages. In this example, a 1 nF capacitor is needed.

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.

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

(Because of the unstable characteristics of ceramic capacitors made

with Z5U material, they are not recommended.)

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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 A. Refer to Table 6. Schottky diodes provide the best performance,

than the maximum load current. Also, if the power supply design and in this example a 1A, 40V, 1N5819 Schottky diode would be a

must withstand a continuous output short, the diode should have a good choice. The 1A diode rating is more than adequate and will not

current rating equal to the maximum current limit of the LM2597. The be overstressed even for a shorted output.

most stressful condition for this diode is an overload or shorted

output condition.

B. The reverse voltage rating of the diode should be at least 1.25

times the maximum input voltage.

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

located close to the 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. Rectifiers such as the 1N4001 series are much too slow and

should not be used.

6. Input Capacitor (C_IN

)

6. Input Capacitor (C_IN)

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

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

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

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

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

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

Figure 35 shows typical RMS current ratings for several different V_IN) is a conservative guideline, and can be modified somewhat if

aluminum electrolytic capacitor values.

desired.

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

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

maximum input voltage.

with a 400 mA load, a capacitor with a RMS current 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 35 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.

Use caution when using ceramic capacitors for input bypassing,

because it may cause severe ringing at the V_INpin.

For

additional

information,

see

INPUT

CAPACITOR

For

a through hole design, a 47 μF/50V electrolytic capacitor

inApplication Information section.

(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, Texas

Instruments is making available computer design software to be

used with the Simple Switcher line of switching regulators.

Table 2. Output Capacitor and Feedforward Capacitor Selection Table

Output

Voltage

(V)

Through Hole Output Capacitor

Surface Mount Output Capacitor

Panasonic

Nichicon PL

Feedforward

AVX TPS

Sprague

Feedforward

HFQ Series

Series

(μF/V)

Series

(μF/V)

595D Series

Capacitor

(μF/V)

1.2

4

220/25

180/25

82/25

82/50

220/25

180/25

82/25

0

220/10

100/10

100/16

68/20

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

120/50

10/35

820 pF

10/35

15/35

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LM2597/LM2597HV Series Buck Regulator Design Procedure

INDUCTOR VALUE SELECTION GUIDES

(For Continuous Mode Operation)

Figure 26. LM2597/LM2597HV-3.3

Figure 27. LM2597/LM2597HV-5.0

Figure 28. LM2597/LM2597HV-12

Figure 29. LM2597/LM2597HV-ADJ

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Coilcraft

(For Continuous Mode Operation)

Table 3. Inductor Manufacturers' Part Numbers

Inductance Current

Schott

Renco

Pulse Engineering

(μH)

(A)

Through

Hole

Surface

Mount

Through

Hole

Surface

Mount

Through

Hole

Surface

Mount

Surface

Mount

L1

220

150

100

68

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

67143940 67144310 RL-1284-68

67148310 67148420 RL-1284-47

67148320 67148430 RL-1284-33

67148330 67148440 RL-1284-22

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

67148340 67148450 RL-1284-33

67148350 67148460 RL-1284-22

67148360 67148470 RL-1284-15

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

RL1500-220

RL1500-150

RL1500-100

RL1500-68

RL1500-47

RL1500-33

RL1500-22

RL1500-330

RL1500-220

RL1500-150

RL1500-100

RL1500-68

RL1500-47

RL1500-33

RL1500-22

RL1500-15

RL1500-330

RL1500-220

RL1500-150

RL1500-100

RL1500-68

—

PE-53801

PE-53802

PE-53803

PE-53804

PE-53805

PE-53806

PE-53807

PE-53808

PE-53809

PE-53810

PE-53811

PE-53812

PE-53813

PE-53814

PE-53815

PE-53816

PE-53817

PE-53818

PE-53819

PE-53820

PE-53821

PE-53826

PE-53827

PE-53801-S

PE-53802-S

PE-53803-S

PE-53804-S

PE-53805-S

PE-53806-S

PE-53807-S

PE-53808-S

PE-53809-S

PE-53810-S

PE-53811-S

PE-53812-S

PE-53813-S

PE-53814-S

PE-53815-S

PE-53816-S

PE-53817-S

PE-53818-S

PE-53819-S

PE-53820-S

PE-53821-S

PE-53826-S

PE-53827-S

DO1608-224

L2

DO1608-154

DO1608-104

DO1608-68

DO1608-473

DO1608-333

DO1608-223

DO3308-334

DO3308-224

DO3308-154

DO3308-104

DO1608-683

DO3308-473

DO1608-333

DO1608-223

DO1608-153

DO3316-334

DO3316-224

DO3316-154

DO3316-104

DDO3316-683

—

L3

L4

L5

47

L6

33

L7

22

L8

330

220

150

100

68

L9

L10

L11

L12

L13

L14

L15

L16

L17

L18

L19

L20

L21

L26

L27

47

33

22

15

330

220

150

100

68

330

220

—

Table 4. Inductor Manufacturers' Phone Numbers

Coilcraft Inc.

Phone

FAX

(800) 322-2645

(708) 639-1469

Coilcraft Inc., Europe

Phone

FAX

+44 1236 730 595

+44 1236 730 627

(619) 674-8100

(619) 674-8262

+353 93 24 107

+353 93 24 459

(800) 645-5828

(516) 586-5562

(612) 475-1173

(612) 475-1786

Pulse Engineering Inc.

Pulse Engineering Inc., Europe

Renco Electronics Inc.

Schott Corp.

Phone

FAX

Phone

FAX

Phone

FAX

Phone

FAX

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Table 5. Capacitor 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

Phone

FAX

AVX Corp.

Phone

FAX

Sprague/Vishay

Phone

FAX

Table 6. Diode Selection Table

1A Diodes

Surface Mount

Ultra Fast

Recovery

Through Hole

Ultra Fast

VR

Schottky

1N5817

Recovery

All of these diodes are rated to

at least 60V.

All of these diodes are rated to

at least 60V.

20V

30V

SR102

1N5818

SR103

11DQ03

1N5819

SR104

MBRS130

MBRS140

10BQ040

MURS120

10BF10

HER101

MUR120

11DF1

40V

10MQ040

MBRS160

10BQ050

10MQ060

MBRS1100

10MQ090

SGL41-60

SS16

11DQ04

SR105

MBR150

11DQ05

MBR160

SB160

50V

or

more

11DQ10

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Block Diagram

Typical Circuit and Layout Guidelines

Component Values shown are for V_IN= 15V, V_OUT= 5V, I_LOAD= 500 mA.

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

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

D1

L1

— 1A, 30V Schottky Rectifier, 1N5818

— 100 μH, L20

Typical Values

C_SS— 0.1 μF

C_DELAY— 0.1 μF

R_{Pull Up}— 4.7k

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

Figure 30. Fixed Output Voltage Versions

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

Component Values shown are for V_IN= 20V,

V_OUT= 10V, I_LOAD= 500 mA.

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

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

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

L1 — 150 μH, L19

R₁— 1 kΩ, 1%

R₂— 7.15k, 1%

C_FF— 3.3 nF, See Application Information

Typical Values

C_SS— 0.1 μF

C_DELAY— 0.1 μF

R_{PULL UP}— 4.7k

*For output voltages between 4V and 20V

Figure 31. Adjustable Output Voltage Versions

As in any switching regulator, layout is very important. Rapidly switching currents associated with wiring

inductance can generate voltage transients which can cause problems. For minimal inductance and ground

loops, the wires indicated by heavy lines should be wide printed circuit traces and should be kept as short

as possible. For best results, external components should be located as close to the switcher lC as possible

using ground plane construction or single point grounding.

If open core inductors are used, special care must be taken as to the location and positioning of this type of

inductor. Allowing the inductor flux to intersect sensitive feedback, lC groundpath and C_OUTwiring can cause

problems.

When using the adjustable version, special care must be taken as to the location of the feedback resistors and

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

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

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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

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 sensitive circuitry, the PC board copper area

connected to this pin should be kept to a minimum.

Feedback (Pin 4)Senses the regulated output voltage to complete the feedback loop.

Shutdown /Soft-start (Pin 5)This dual function pin provides the following features: (a) Allows the switching

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

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

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

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

reaches 95% of the nominal output voltage and a delay time determined by the Delay pin capacitor. This

signal can be used as a reset to a microprocessor on power-up.

Delay (Pin 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.

Bias Supply (Pin 3)This feature allows the regulators internal circuitry to be powered from the regulated output

voltage or an external supply, instead of the input voltage. This results in increased efficiency under some

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

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 (approximately 5 μA begins charging this capacitor). As the capacitor voltage rises, the regulator goes

through four operating regions (See the bottom curve in Figure 32).

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

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

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

and 1.8V, the internal 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.

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

Soft-start condition. The switch (Pin 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 controlled ramp up. See the center curve in Figure 32. The input supply

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

This is a very useful feature in some switcher topologies that require large startup currents (such as the

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

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NOTE

The lower curve shown in Figure 32 shows the Soft-start region from 0% to 100%. This is

not the duty cycle percentage, but the output voltage percentage. Also, the Soft-start

voltage range has a negative temperature coefficient associated with it. See the Soft-start

curve in Electrical Characteristics.

4. Normal operation. Above 2.8V, the circuit operates as a standard Pulse Width Modulated switching

regulator. The capacitor will continue to charge up until it reaches the internal clamp voltage of approximately

7V. If this pin is driven from a voltage source, the current must be limited to about 1 mA.

If the part is operated with an input voltage at or below the internal soft-start clamp voltage of approximately

7V, the voltage on the SD/SS pin tracks the input voltage and can be disturbed by a step in the voltage. To

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

externally between the 3V maximum soft-start threshold and the 4.5V minimum input voltage. Figure 34 is an

example of an external 3.7V (approx.) clamp that prevents a line-step related glitch but does not interfere

with the soft-start behavior of the device.

Figure 32. Soft-start, Delay, Error, Output

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Figure 33. Timing Diagram for 5V Output

V_IN

LM2597

5

Q1

SD/SS

C_SS

Z1

3V

Figure 34. External 3.7V Soft-Start Clamp

DELAY CAPACITOR

C_DELAY—Provides delay for the error flag output. See the upper curve in Figure 32, and also refer to timing

diagrams in Figure 33. A capacitor on this pin provides a time delay between the time the regulated output

voltage (when it is increasing in value) reaches 95% of the nominal output voltage, and the time the error flag

output goes high. A 3 μA constant current from the delay pin charges the delay capacitor resulting in a voltage

ramp. When this voltage reaches a threshold of approximately 1.3V, the open collector error flag output (or

power OK) goes high. This signal can be used to indicate that the regulated output has reached the correct

voltage and has stabilized.

If, for any reason, the regulated output voltage drops by 5% or more, the error output flag (Pin 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 dropping would only

be approximately 50 μs.

R_Pull

—The error flag output, (or power OK) is the collector of a NPN transistor, with the emitter internally

Up

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

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INPUT CAPACITOR

C_IN—A low ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground pin. It

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

appearing at the input, and provides the instantaneous current needed each time the switch turns on.

The important parameters for the Input capacitor are the voltage rating and the RMS current rating. Because of

the relatively high RMS currents flowing in a buck regulator's input capacitor, this capacitor should be chosen for

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

rating are directly related to the RMS current rating.

The RMS current rating of a capacitor could be viewed as a capacitor's power rating. The RMS current flowing

through the capacitors internal ESR produces power which causes the internal temperature of the capacitor to

rise. The RMS current rating of a capacitor is determined by the amount of current required to raise the internal

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

this heat to the surrounding air will determine the amount of current the capacitor can safely sustain. Capacitors

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

capacitor value, a higher voltage electrolytic capacitor will be physically larger than a lower voltage capacitor, and

thus be able to dissipate more heat to the surrounding air, and therefore will have a higher RMS current rating.

Figure 35. RMS Current Ratings for Low

ESR Electrolytic Capacitors (Typical)

Figure 36. Capacitor ESR vs Capacitor Voltage

Rating (Typical Low ESR Electrolytic Capacitor)

The consequences of operating an electrolytic capacitor above the RMS current rating is a shortened operating

life. The higher temperature speeds up the evaporation of the capacitor's electrolyte, resulting in eventual failure.

Selecting an input capacitor requires consulting the manufacturers data sheet for maximum allowable RMS ripple

current. For a maximum ambient temperature of 40°C, a general guideline would be to select a capacitor with a

ripple current rating of approximately 50% of the DC load current. For ambient temperatures up to 70°C, a

current rating of 75% of the DC load current would be a good choice for a conservative design. The capacitor

voltage rating must be at least 1.25 times greater than the maximum input voltage, and often a much higher

voltage capacitor is needed to satisfy the RMS current requirements.

A graph shown in Figure 35 shows the relationship between an electrolytic capacitor value, its voltage rating, and

the RMS current it is rated for. These curves were obtained from the Nichicon “PL” series of low ESR, high

reliability electrolytic capacitors designed for switching regulator applications. Other capacitor manufacturers offer

similar types of capacitors, but always check the capacitor data sheet.

“Standard” electrolytic capacitors typically have much higher ESR numbers, lower RMS current ratings and

typically have a shorter operating lifetime.

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Because of their small size and excellent performance, surface mount solid tantalum capacitors are often used

for input bypassing, but several precautions must be observed. A small percentage of solid tantalum capacitors

can short if the inrush current rating is exceeded. This can happen at turn on when the input voltage is suddenly

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

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

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

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

current rating must be sized to the load current.

OUTPUT CAPACITOR

C_OUT—An output capacitor is required to filter the output and provide regulator loop stability. Low impedance or

low ESR Electrolytic or solid tantalum capacitors designed for switching regulator applications must be used.

When selecting an output capacitor, the important capacitor parameters are; the 100 kHz Equivalent Series

Resistance (ESR), the RMS ripple current rating, voltage rating, and capacitance value. For the output capacitor,

the ESR value is the most important parameter.

The output capacitor requires an ESR value that has an upper and lower limit. For low output ripple voltage, a

low ESR value is needed. This value is determined by the maximum allowable output ripple voltage, typically 1%

to 2% of the output voltage. But if the selected capacitor's ESR is extremely low, there is a possibility of an

unstable feedback loop, resulting in an oscillation at the output. Using the capacitors listed in the tables, or

similar types, will provide design solutions under all conditions.

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

RIPPLE AND TRANSIENTS for a post ripple filter.

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 36). Often, capacitors with much

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

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

capacitor values and several different voltage ratings. See the quick design component selection tables in

Table 1 and Table 2 for typical capacitor values, voltage ratings, and manufacturers capacitor types.

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

temperatures and typically rises 3X @ −25°C and as much as 10X at −40°C. See curve shown in Figure 37.

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 characteristic may cause instability or

EMI problems. Ultra-fast recovery diodes typically have reverse recovery times of 50 ns or less. Rectifiers such

as the 1N4001 series are much too slow and should not be used.

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Figure 37. Capacitor ESR Change vs Temperature

INDUCTOR SELECTION

All switching regulators have two basic modes of operation; continuous and discontinuous. The difference

between the two types relates to the inductor current, whether it is flowing continuously, or if it drops to zero for a

period of time in the normal switching cycle. Each mode has distinctively different operating characteristics,

which can affect the regulators performance and requirements. Most switcher designs will operate in the

discontinuous mode when the load current is low.

The LM2597 (or any of the Simple Switcher family) can be used for both continuous or discontinuous modes of

operation.

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

peak switch, inductor and diode currents, and can have lower output ripple voltage. But it does require larger

inductor values to keep the inductor current flowing continuously, especially at low output load currents and/or

high input voltages.

To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Figure 26

through Figure 29). 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 38.)

Figure 38. (ΔI_IND) Peak-to-Peak Inductor

Ripple Current (as a Percentage

of the Load Current) vs Load Current

By allowing the percentage of inductor ripple current to increase for low load currents, the inductor value and size

can be kept relatively low.

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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 causing problems with both the

switching regulator operation and nearby sensitive circuitry, and can give incorrect scope readings because of

induced voltages in the scope probe. Also see 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 Engineering.

Exceeding an inductor's maximum current rating may cause the inductor to overheat because of the copper wire

losses, or the core may saturate. If the inductor begins to saturate, the inductance decreases rapidly and the

inductor begins to look mainly resistive (the DC resistance of the winding). This can cause the switch current to

rise very rapidly and force the switch into a cycle-by-cycle current limit, thus reducing the DC output load current.

This can also result in overheating of the inductor and/or the 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 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 sinusoidal ringing present. (See photo titled;

Discontinuous Mode Switching Waveforms) This ringing is normal for discontinuous operation, and is not caused

by feedback loop instabilities. In discontinuous operation, there is a period of time where neither the switch or the

diode are conducting, and the inductor current has dropped to zero. During this time, a small amount of energy

can circulate between the inductor and the switch/diode parasitic capacitance causing this characteristic ringing.

Normally this ringing is not a problem, unless the amplitude becomes great enough to exceed the input voltage,

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

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

inductors have very little core loss and therefore produce the most ringing. The higher core loss of powdered iron

inductors produce less ringing. If desired, a series RC could be placed in parallel with the inductor to dampen the

ringing. The computer aided design software Switchers Made Simple (version 4.1) will provide all component

values for continuous and discontinuous modes of operation.

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Figure 39. Post Ripple Filter Waveform

OUTPUT VOLTAGE RIPPLE AND TRANSIENTS

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

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

waveform.

The output ripple voltage is a function of the inductor sawtooth ripple current and the ESR of the output

capacitor. A typical output ripple voltage can range from approximately 0.5% to 3% of the output voltage. To

obtain low ripple voltage, the ESR of the output capacitor must be low, however, caution must be exercised when

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

very low output ripple voltage is needed (less than 15 mV), a post ripple filter is recommended. (See Figure 31.)

The inductance required is typically between 1 μH and 5 μH, with low DC resistance, to maintain good load

regulation. A low ESR output filter capacitor is also required to assure good dynamic load response and ripple

reduction. The ESR of this capacitor may be as low as desired, because it is out of the regulator feedback loop.

The photo shown in Figure 39 shows a typical output ripple voltage, with and without a post ripple filter.

When observing output ripple with a scope, it is essential that a short, low inductance scope probe ground

connection be used. Most scope probe manufacturers provide a special probe terminator which is soldered onto

the regulator board, preferable at the output capacitor. This provides a very short scope ground thus eliminating

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

cleaner and more accurate picture of the ripple voltage waveform.

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

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

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

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

to the amplitude of these spikes.

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Figure 40. Peak-to-Peak Inductor

Ripple Current vs Load Current

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

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

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

decreases, the entire sawtooth current waveform also rises and falls. The average value (or the center) of this

current waveform is equal to the DC load current.

If the load current drops to a low enough level, the bottom of the sawtooth current waveform will reach zero, and

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

designs (irregardless how large the inductor value is) will be forced to run discontinuous if the output is lightly

loaded. This is a perfectly acceptable mode of operation.

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

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

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

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

Figure 26 through Figure 29 are used to select an inductor value, the peak-to-peak inductor ripple current can

immediately be determined. The curve shown in Figure 40 shows the range of (ΔI_IND) that can be expected for

different load currents. The curve also shows how the peak-to-peak inductor ripple current (ΔI_IND) changes as

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

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

INDUCTOR SELECTION).

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

to select the inductor value

Consider the following example:

V_OUT= 5V, maximum load current of 300 mA

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

The selection guide in Figure 27 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 (ΔI_IND) to flow that will be a

percentage of the maximum load current. Referring to Figure 40, follow the 0.3A line approximately midway into

the inductance region, and read the peak-to-peak inductor ripple current (ΔI_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 40, it can be seen that for a load current of 0.3A, the

peak-to-peak inductor ripple current (ΔI_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).

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

switching regulator circuit.

1. Peak Inductor or peak switch current

(1)

2. Minimum load current before the circuit becomes discontinuous

(2)

3. Output Ripple Voltage

= (ΔI_IND)×(ESR of C_OUT

)

= 0.150A×0.240Ω=36 mV p-p

4. Placeholder to force break

(3)

OPEN CORE INDUCTORS

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

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

the other end. These magnetic lines of flux will induce a voltage into any wire or PC board copper trace that

comes within the inductor's magnetic field. The strength of the magnetic field, the orientation and location of the

PC copper trace to the magnetic field, and the distance between the copper trace and the inductor, determine

the amount of voltage generated in the copper trace. Another way of looking at this inductive coupling is to

consider the PC board copper trace as one turn of a transformer (secondary) with the inductor winding as the

primary. Many millivolts can be generated in a copper trace located near an open core inductor which can cause

stability problems or high output ripple voltage problems.

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

respect to other PC traces may be the problem. To determine if this is the problem, temporarily raise the inductor

away from the board by several inches and then check circuit operation. If the circuit now operates correctly,

then the magnetic flux from the open core inductor is causing the problem. Substituting a closed core inductor

such as a torroid or E-core will correct the problem, or re-arranging the PC layout may be necessary. Magnetic

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

should be minimized.

Sometimes, locating a trace directly beneath a bobbin in- ductor will provide good results, provided it is exactly in

the center of the inductor (because the induced voltages cancel themselves out), but if it is off center one

direction or the other, then problems could arise. If flux problems are present, even the direction of the inductor

winding can make a difference in some circuits.

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

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

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

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Figure 41. Junction Temperature Rise, PDIP-8

Circuit Data for Temperature Rise Curve (PDIP-8)

Through hole electrolytic

Capacitors

Inductor

Diode

Through hole, Schott, 100 μH

Through hole, 1A 40V, Schottky

PC board

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

Figure 42. Junction Temperature Rise, SOIC-8

Circuit Data for Temperature Rise Curve (Surface Mount)

Surface mount tantalum, molded “D” size

Capacitors

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″)

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THERMAL CONSIDERATIONS

The LM2597/LM2597HV is available in two packages, an 8-pin through hole PDIP (P) and an 8-pin surface

mount SOIC-8 (D). 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.

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 output (switch) pin, which should not have

large areas of copper). Large areas of copper provide the best transfer of heat (lower thermal resistance) to the

surrounding air, and even double-sided or multilayer boards provide a better heat path to the surrounding air.

Unless power levels are small, sockets are not recommended 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 factors 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 components such as the catch diode will add heat to the PC board and the

heat can vary as the input voltage changes. For the inductor, depending on the physical size, type of core

material and the DC resistance, it could either act as a heat sink taking heat away from the board, or it could add

heat to the board.

The curves shown in Figure 41 and Figure 42 show the LM2597 junction temperature rise above ambient

temperature with a 500 mA load for various input and output voltages. 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 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.

BIAS SUPPLY FEATURE

The bias supply (V_BS) pin allows the LM2597's internal circuitry to be powered from a power source, other than

V_IN, typically the output voltage. This feature can increase efficiency and lower junction temperatures under some

operating 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 43 and Figure 44. 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.

The bias supply pin requires a minimum of approximately 3.5V at room temperature (4V @ −40°C), and can be

as high 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.

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.

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Figure 43. Effects of Bias Supply Feature on 5V

Regulator Efficiency

Figure 44. Effects of Bias Supply Feature on 12V

Regulator Efficiency

SHUTDOWN /SOFT-START

The circuit shown in Figure 47 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 45 and Figure 46 show the effects of Soft-start on the output voltage,

the input current, with, and without a Soft-start capacitor. Figure 45 also shows the error flag output going high

when the output voltage reaches 95% of the nominal output voltage. The reduced input current required at

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.

Figure 45. Output Voltage, Input Current, Error

Flag Signal, at Start-Up, WITH Soft-start

Figure 46. 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 capacitor can be eliminated.

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Figure 47. Typical Circuit Using Shutdown /Soft-start and Error Flag Features

Figure 48. Inverting −5V Regulator With Shutdown and Soft-start

lNVERTING REGULATOR

The circuit in Figure 48 converts a positive input voltage to a negative output voltage with a common ground. The

circuit operates by bootstrapping the regulators ground pin to the negative output voltage, then grounding the

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

This example uses the LM2597-5 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 49 provides a guide as to the amount

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

The maximum voltage appearing across the regulator is the absolute sum of the input and output voltage, and

this must be limited to a maximum of 40V. In this example, when converting +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).

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Figure 49. Maximum Load Current for Inverting Regulator Circuit

An additional diode is required in this regulator configuration. Diode D1 is used to isolate input voltage ripple or

noise from coupling through the C_INcapacitor to the output, under light or no load conditions. Also, this diode

isolation changes the topology to closely resemble a buck configuration thus providing good closed loop stability.

A Schottky diode is recommended for low input voltages, (because of its lower voltage drop) but for higher input

voltages, a 1N4001 diode could be used.

Because of differences in the operation of the inverting regulator, the standard design procedure is not used to

select the inductor value. In the majority of designs, a 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 48 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 LM2597 current limit (approximately 0.8A) are needed for 1 ms or more,

until the output reaches its nominal output voltage. The actual time depends on the output voltage and the size of

the output capacitor. Input power sources that are current limited or sources that can not deliver these currents

without getting loaded down, may not work correctly. Because of the relatively high startup currents required by

the inverting topology, the Soft-start feature shown in Figure 48 is recommended.

Also shown in Figure 48 are several shutdown methods for the inverting configuration. With the inverting

configuration, some level shifting is required, because the ground pin of the regulator is no longer at ground, but

is now at the negative output voltage. The shutdown methods shown accept ground referenced shutdown

signals.

UNDERVOLTAGE LOCKOUT

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

Figure 50 contains a undervoltage lockout circuit for a buck configuration, while Figure 51 and Figure 52 are for

the inverting types (only the circuitry pertaining to the undervoltage lockout is shown). Figure 50 uses a zener

diode to establish the threshold voltage when the switcher begins operating. When the input voltage is less than

the zener voltage, resistors R1 and R2 hold the Shutdown /Soft-start pin low, keeping the regulator in the

shutdown mode. As the input voltage exceeds the zener voltage, the zener conducts, pulling the Shutdown /Soft-

start pin high, allowing the regulator to begin switching. The threshold voltage for the undervoltage lockout

feature is approximately 1.5V greater than the zener voltage.

36

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SNVS119C –MARCH 1998–REVISED APRIL 2013

Figure 50. Undervoltage Lockout for a Buck Regulator

Figure 51 and Figure 52 apply the same feature to an inverting circuit. Figure 51 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 52 has a turn ON voltage which is different than the turn OFF voltage. The amount of hysteresis is

approximately equal to the value of the output voltage. Since the SD /SS pin has an internal 7V zener clamp, R2

is needed to limit the current into this pin to approximately 1 mA when Q1 is on.

Figure 51. Undervoltage Lockout Without

Hysteresis for an Inverting Regulator

Figure 52. Undervoltage Lockout With

Hysteresis for an Inverting Regulator

NEGATIVE VOLTAGE CHARGE PUMP

Occasionally a low current negative voltage is needed for biasing parts of a circuit. A simple method of

generating a negative voltage using a charge pump technique and the switching waveform present at the OUT

pin, is shown in Figure 53. 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 requirement however, that there

be a minimum load of several hundred mA on the regulated positive output for the charge pump to work

correctly. Also, resistor R1 is required to limit the charging current of C1 to some value less than the LM2597

current limit (typically 800 mA).

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.

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Figure 53. Charge Pump for Generating a

Low Current, Negative Output Voltage

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

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

D1 — 1A, 40V Surface Mount Schottky Rectifier

L1 — Surface Mount Inductor, Coilcraft DO33

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

C_D— Delay Capacitor (surface mount ceramic chip capacitor)

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

Figure 54. Typical Surface Mount PC Board Layout, Fixed Output (2X size)

38

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

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

D1 — 1A, 40V Surface Mount Schottky Rectifier

L1 — Surface Mount Inductor, Coilcraft DO33

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

C_D— Delay Capacitor (surface mount ceramic chip capacitor)

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 55. Typical Surface Mount PC Board Layout, Adjustable Output (2X size)

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REVISION HISTORY

Changes from Revision B (April 2013) to Revision C

Page

•

Changed layout of National Data Sheet to TI format .......................................................................................................... 39

40

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PACKAGE MATERIALS INFORMATION

www.ti.com

11-Oct-2013

TAPE AND REEL INFORMATION

*All dimensions are nominal

Device

Package Package Pins

Type Drawing

SPQ

Reel

A0

B0

K0

P1

W

Pin1

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

(mm) W1 (mm)

LM2597HVMX-12/NOPB

SOIC

D

8

2500

330.0

12.4

6.5

5.4

2.0

8.0

12.0

Q1

LM2597HVMX-3.3/NOPB SOIC

LM2597HVMX-5.0/NOPB SOIC

LM2597HVMX-ADJ/NOPB SOIC

LM2597MX-12/NOPB

LM2597MX-3.3

SOIC

LM2597MX-3.3/NOPB

LM2597MX-5.0/NOPB

LM2597MX-ADJ/NOPB

Pack Materials-Page 1

PACKAGE MATERIALS INFORMATION

www.ti.com

11-Oct-2013

*All dimensions are nominal

Device

Package Type Package Drawing Pins

SPQ

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

LM2597HVMX-12/NOPB

LM2597HVMX-3.3/NOPB

LM2597HVMX-5.0/NOPB

LM2597HVMX-ADJ/NOPB

LM2597MX-12/NOPB

LM2597MX-3.3

SOIC

D

8

2500

367.0

35.0

LM2597MX-3.3/NOPB

LM2597MX-5.0/NOPB

LM2597MX-ADJ/NOPB

Pack Materials-Page 2

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型号：	LM2597HVN-12/NOPB
厂家：	TEXAS INSTRUMENTS
描述：	LM2597/LM2597HV SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down Voltage Regulator LM2597 / LM2597HV SIMPLE SWITCHER系列电源转换器150千赫0.5A降压稳压器转换器稳压器
文件：	总48页 (文件大小：2271K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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