LM2595
SIMPLE SWITCHER

Power Converter 150 kHz

1A Step-Down Voltage Regulator

General Description

The LM2595 series of regulators are monolithic integrated
circuits that provide all the active functions for a step-down
(buck) switching regulator, capable of driving a 1A load with
excellent line and load regulation. These devices are avail-
able in fixed output voltages of 3.3V, 5V, 12V, and an adjust-
able output version.

Requiring a minimum number of external components, these
regulators are simple to use and include internal frequency
compensation

, and a fixed-frequency oscillator.

The LM2595 series operates at a switching frequency of
150 kHz thus allowing smaller sized filter components than
what would be needed with lower frequency switching regu-
lators. Available in a standard 5-lead TO-220 package with
several different lead bend options, and a 5-lead TO-263
surface mount package. Typically, for output voltages less
than 12V, and ambient temperatures less than 50°C, no heat
sink is required.

A standard series of inductors are available from several dif-
ferent manufacturers optimized for use with the LM2595 se-
ries.

This

feature

greatly

simplifies

the

design

switch-mode power supplies.

Other features include a guaranteed

4% tolerance on out-

put voltage under specified input voltage and output load
conditions, and

15% on the oscillator frequency. External

shutdown is included, featuring typically 85 µA stand-by cur-
rent. Self protection features include a two stage frequency
reducing current limit for the output switch and an over tem-
perature shutdown for complete protection under fault condi-
tions.

Features

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

Adjustable version output voltage range, 1.2V to 37V

4% max over line and load conditions

Available in TO-220 and TO-263 (surface mount)
packages

Guaranteed 1A output load current

Input voltage range up to 40V

Requires only 4 external components

Excellent line and load regulation specifications

150 kHz fixed frequency internal oscillator

TTL shutdown capability

Low power standby mode, I

typically 85 µA

High efficiency

Uses readily available standard inductors

Thermal shutdown and current limit protection

Applications

Simple high-efficiency step-down (buck) regulator

Efficient pre-regulator for linear regulators

On-card switching regulators

Positive to negative converter

Note:

Patent Number 5,382,918.

Typical Application

(Fixed Output Voltage Versions)

SIMPLE SWITCHER

and

Switchers Made Simple

are registered trademarks of National Semiconductor Corporation.

DS012565-1

May 1999

LM2595

SIMPLE

SWITCHER

Power

Converter

150

kHz

Step-Down

oltage

Regulator

DS012565

www.national.com

Absolute Maximum Ratings

(Note 1)

If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.

Maximum Supply Voltage

45V

ON /OFF Pin Input Voltage

-0.3

+25V

Feedback Pin Voltage

-0.3

+25V

Output Voltage to Ground

(Steady State)

-1V

Power Dissipation

Internally limited

Storage Temperature Range

-65°C to +150°C

ESD Susceptibility

Human Body Model (Note 2)

2 kV

Lead Temperature

S Package

Vapor Phase (60 sec.)

+215°C

Infrared (10 sec.)

+245°C

T Package (Soldering, 10 sec.)

+260°C

Maximum Junction Temperature

+150°C

Operating Conditions

Temperature Range

-40°C

+125°C

Supply Voltage

4.5V to 40V

LM2595-3.3
Electrical Characteristics

Specifications with standard type face are for T

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

ture Range.

Symbol

Parameter

Conditions

LM2595-3.3

Units

(Limits)

Typ

Limit

(Note 3)

(Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit

Figure 1

OUT

Output Voltage

4.75V

40V, 0.1A

LOAD

3.3

3.168/3.135

V(min)

3.432/3.465

V(max)

Efficiency

= 12V, I

LOAD

= 1A

LM2595-5.0
Electrical Characteristics

Specifications with standard type face are for T

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

ture Range.

Symbol

Parameter

Conditions

LM2595-5.0

Units

(Limits)

Typ

Limit

(Note 3)

(Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit

Figure 1

OUT

Output Voltage

40V, 0.1A

LOAD

5.0

4.800/4.750

V(min)

5.200/5.250

V(max)

Efficiency

= 12V, I

LOAD

= 1A

LM2595-12
Electrical Characteristics

Specifications with standard type face are for T

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

ture Range.

Symbol

Parameter

Conditions

LM2595-12

Units

(Limits)

Typ

Limit

(Note 3)

(Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit

Figure 1

OUT

Output Voltage

15V

40V, 0.1A

LOAD

12.0

11.52/11.40

V(min)

12.48/12.60

V(max)

Efficiency

= 25V, I

LOAD

= 1A

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LM2595-ADJ
Electrical Characteristics

Specifications with standard type face are for T

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

ture Range.

Symbol

Parameter

Conditions

LM2595-ADJ

Units

(Limits)

Typ

Limit

(Note 3)

(Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit

Figure 1

Feedback Voltage

4.5V

40V, 0.1A

LOAD

1.230

OUT

programmed for 3V. Circuit of

Figure 1

1.193/1.180

V(min)

1.267/1.280

V(max)

Efficiency

= 12V, V

OUT

= 3V, I

LOAD

= 1A

All Output Voltage Versions
Electrical Characteristics

Specifications with standard type face are for T

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

ture Range. Unless otherwise specified, V

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

= 24V for the 12V ver-

sion. I

LOAD

= 200 mA.

Symbol

Parameter

Conditions

LM2595-XX

Units

(Limits)

Typ

Limit

(Note 3)

(Note 4)

DEVICE PARAMETERS

Feedback Bias Current

Adjustable Version Only,V

= 1.3V

50/100

nA (max)

Oscillator Frequency

(Note 6)

150

kHz

127/110

kHz(min)

173/173

kHz(max)

SAT

Saturation Voltage

OUT

= 1A (Notes 7, 8)

1.2/1.3

V(max)

Max Duty Cycle (ON)

(Note 8)

100

Min Duty Cycle (OFF)

(Note 9)

Current Limit

Peak Current (Notes 7, 8)

1.5

1.2/1.15

A(min)

2.4/2.6

A(max)

Output Leakage Current

Output = 0V

(Notes 7, 9) and (Note 10)

µA(max)

Output = -1V

mA(max)

Quiescent Current

(Note 9)

mA(max)

STBY

Standby Quiescent

ON/OFF pin = 5V (OFF)

(Note 10)

µA

Current

200/250

µA(max)

Thermal Resistance

TO-220 or TO-263 Package, Junction to Case

°C/W

TO-220 Package, Junction to Ambient (Note 11)

°C/W

TO-263 Package, Junction to Ambient (Note 12)

°C/W

TO-263 Package, Junction to Ambient (Note 13)

°C/W

TO-263 Package, Junction to Ambient (Note 14)

°C/W

ON/OFF CONTROL Test Circuit

Figure 1

ON /OFF Pin Logic Input

1.3

Threshold Voltage

Low (Regulator ON)

0.6

V(max)

High (Regulator OFF)

2.0

V(min)

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All Output Voltage Versions
Electrical Characteristics

(Continued)

Specifications with standard type face are for T

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

ture Range. Unless otherwise specified, V

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

= 24V for the 12V ver-

sion. I

LOAD

= 200 mA.

Symbol

Parameter

Conditions

LM2595-XX

Units

(Limits)

Typ

Limit

(Note 3)

(Note 4)

ON/OFF CONTROL Test Circuit

Figure 1

ON/OFF Pin
Input Current

LOGIC

= 2.5V (Regulator OFF)

µA

µA(max)

LOGIC

= 0.5V (Regulator ON)

0.02

µA

µA(max)

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is in-
tended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.

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

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

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

Note 5: External components such as the catch diode, inductor, input and output capacitors, and voltage programming resistors can affect switching regulator sys-
tem performance. When the LM2595 is used as shown in the

Figure 1 test circuit, system performance will be as shown in system parameters section of Electrical

Characteristics.

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

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

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

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

Note 10: V

= 40V.

Note 11: Junction to ambient thermal resistance (no external heat sink) for the TO-220 package mounted vertically, with the leads soldered to a printed circuit board
with (1 oz.) copper area of approximately 1 in

Note 12: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single printed circuit board with 0.5 in

of (1 oz.) copper area.

Note 13: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 2.5 in

of (1 oz.) copper area.

Note 14: Junction to ambient thermal resistance with the TO-263 package tab soldered to a double sided printed circuit board with 3 in

of (1 oz.) copper area on

the LM2595S side of the board, and approximately 16 in

of copper on the other side of the p-c board. See Application Information in this data sheet and the thermal

model in

Switchers Made Simple

version 4.3 software.

Typical Performance Characteristics

(Circuit of

Figure 1)

Normalized
Output Voltage

DS012565-11

Line Regulation

DS012565-12

Efficiency

DS012565-13

www.national.com

Typical Performance Characteristics

(Circuit of

Figure 1) (Continued)

Feedback Pin
Bias Current

DS012565-10

Continuous Mode Switching Waveforms
V

= 20V, V

OUT

= 5V, I

LOAD

= 1A

L = 68 µH, C

OUT

= 120 µF, C

OUT

ESR = 100 m

DS012565-17

A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.5A/div.
C: Output Ripple Voltage, 50 mV/div.

Horizontal Time Base: 2 µs/div.

Discontinuous Mode Switching Waveforms
V

= 20V, V

OUT

= 5V, I

LOAD

= 600 mA

L = 22 µH, C

OUT

= 220 µF, C

OUT

ESR = 50 m

DS012565-18

A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.5A/div.
C: Output Ripple Voltage, 50 mV/div.

Horizontal Time Base: 2 µs/div.

Load Transient Response for Continuous Mode
V

= 20V, V

OUT

= 5V, I

LOAD

= 250 mA to 750 mA

L = 68 µH, C

OUT

= 120 µF, C

OUT

ESR = 100 m

DS012565-19

A: Output Voltage, 100 mV/div. (AC)
B: 250 mA to 750 mA Load Pulse

Horizontal Time Base: 100 µs/div.

Load Transient Response for Discontinuous Mode
V

= 20V, V

OUT

= 5V, I

LOAD

= 250 mA to 750 mA

L = 22 µH, C

OUT

= 220 µF, C

OUT

ESR = 50 m

DS012565-20

A: Output Voltage, 100 mV/div. (AC)
B: 250 mA to 750 mA Load Pulse

Horizontal Time Base: 200 µs/div.

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Test Circuit and Layout Guidelines

As in any switching regulator, layout is very important. Rap-
idly switching currents associated with wiring inductance can
generate voltage transients which can cause problems. For
minimal inductance and ground loops, the wires indicated by
heavy lines should be wide printed circuit traces and
should be kept as short as possible. For best results, ex-
ternal components should be located as close to the
switcher lC as possible using ground plane construction or
single point grounding.

If open core inductors are used, special care must be
taken as to the location and positioning of this type of induc-
tor. Allowing the inductor flux to intersect sensitive feedback,
lC groundpath and C

OUT

wiring can cause problems.

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

Fixed Output Voltage Versions

DS012565-22

-- 120 µF, 50V, Aluminum Electrolytic Nichicon "PL Series"

OUT

-- 120 µF, 25V Aluminum Electrolytic, Nichicon "PL Series"

D1 -- 3A, 40V Schottky Rectifier, 1N5822
L1 -- 100 µH, L29

Adjustable Output Voltage Versions

DS012565-23

-- 120 µF, 50V, Aluminum Electrolytic Nichicon "PL Series"

OUT

-- 120 µF, 25V Aluminum Electrolytic, Nichicon "PL Series"

D1 -- 3A, 40V Schottky Rectifier, 1N5822
L1 -- 100 µH, L29
R

-- 1 k

, 1%

-- See Application Information Section

FIGURE 1. Standard Test Circuits and Layout Guides

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LM2595 Series Buck Regulator Design Procedure (Fixed Output)

PROCEDURE (Fixed Output Voltage Version)

EXAMPLE (Fixed Output Voltage Version)

Given:

OUT

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

(max) = Maximum DC Input Voltage

LOAD

(max) = Maximum Load Current

Given:

OUT

= 5V

(max) = 12V

LOAD

(max) = 1A

1. Inductor Selection (L1)

A. Select the correct inductor value selection guide from

Fig-

ure 4 , Figure 5, or Figure 6. (Output voltages of 3.3V, 5V, or
12V respectively.) For all other voltages, see the design pro-
cedure for the adjustable version.

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. Select an appropriate inductor from the four manufactur-
er's part numbers listed in

Figure 8.

1. Inductor Selection (L1)

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

Figure 5.

B. From the inductor value selection guide shown in

Figure 5,

the inductance region intersected by the 12V horizontal line
and the 1A vertical line is 68 µH, and the inductor code is
L30.

C. The inductance value required is 68 µH. From the table in
Figure 8, go to the L30 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.)

2. Output Capacitor Selection (C

OUT

)

A. In the majority of applications, low ESR (Equivalent Series
Resistance) electrolytic capacitors between 47 µF and 330
µF and low ESR solid tantalum capacitors between 56 µF
and 270 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 330 µF.

For additional information, see section on output capaci-
tors in application information section.

B. To simplify the capacitor selection procedure, refer to the
quick design component selection table shown in

Figure 2.

This table contains different input voltages, output voltages,
and load currents, and lists various inductors and output ca-
pacitors that will provide the best design solutions.

C. The capacitor voltage rating for electrolytic capacitors
should be at least 1.5 times greater than the output voltage,
and often much higher voltage ratings are needed to satisfy
the low ESR requirements for low output ripple voltage.

D. For computer aided design software, see

Switchers

Made Simple

version 4.2 or later.

2. Output Capacitor Selection (C

OUT

)

A. See section on output capacitors in application infor-
mation section.

B. From the quick design component selection table shown
in

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

current column, choose the load current line that is closest to
the current needed in your application, for this example, use
the 1A line. In the maximum input voltage column, select the
line that covers the input voltage needed in your application,
in this example, use the 15V line. Continuing on this line are
recommended inductors and capacitors that will provide the
best overall performance.

The capacitor list contains both through hole electrolytic and
surface mount tantalum capacitors from four different capaci-
tor manufacturers. It is recommended that both the manufac-
turers and the manufacturer's series that are listed in the
table be used.

In this example aluminum electrolytic capacitors from several
different manufacturers are available with the range of ESR
numbers needed.

220 µF

25V

Panasonic HFQ Series

220 µF

25V

Nichicon PL Series

C. For a 5V output, a capacitor voltage rating at least 7.5V or
more is needed. But, in this example, even a low ESR,
switching grade, 220 µF 10V aluminum electrolytic capacitor
would exhibit approximately 225 m

of ESR (see the curve

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

ESR would result in relatively high output ripple voltage. To
reduce the ripple to 1% of the output voltage, or less, a ca-
pacitor with a higher voltage rating (lower ESR) should be se-
lected. A 16V or 25V capacitor will reduce the ripple voltage
by approximately half.

Procedure continued on next page.

Example continued on next page.

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LM2595 Series Buck Regulator Design Procedure (Fixed Output)

(Continued)

PROCEDURE (Fixed Output Voltage Version)

EXAMPLE (Fixed Output Voltage Version)

3. Catch Diode Selection (D1)

A. The catch diode current rating must be at least 1.3 times
greater than the maximum load current. Also, if the power
supply design must withstand a continuous output short, the
diode should have a current rating equal to the maximum
current limit of the LM2595. 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 LM2595 using short leads and
short printed circuit traces. Because of their fast switching
speed and low forward voltage drop, Schottky diodes provide
the best performance and efficiency, and should be the first
choice, especially in low output voltage applications.
Ultra-fast recovery, or High-Efficiency rectifiers also provide
good results. Ultra-fast recovery diodes typically have re-
verse recovery times of 50 ns or less. Rectifiers such as the
1N5400 series are much too slow and should not be used.

3. Catch Diode Selection (D1)

A. Refer to the table shown in

Figure 11 In this example, a

3A, 20V, 1N5820 Schottky diode will provide the best perfor-
mance, and will not be overstressed even for a shorted out-
put.

4. Input Capacitor (C

)

A low ESR aluminum or tantalum bypass capacitor is needed
between the input pin and ground pin to prevent large volt-
age transients from appearing at the input. This capacitor
should be located close to the IC using short leads. In addi-
tion, the RMS current rating of the input capacitor should be
selected to be at least

the DC load current. The capacitor

manufacturers data sheet must be checked to assure that
this current rating is not exceeded. The curve shown in

Fig-

ure 13 shows typical RMS current ratings for several different
aluminum electrolytic capacitor values.

For an aluminum electrolytic, the capacitor voltage rating
should be approximately 1.5 times the maximum input volt-
age. Caution must be exercised if solid tantalum capacitors
are used (see Application Information on input capacitor).
The tantalum capacitor voltage rating should be 2 times the
maximum input voltage and it is recommended that they be
surge current tested by the manufacturer.

Use caution when using ceramic capacitors for input bypass-
ing, because it may cause severe ringing at the V

pin.

For additional information, see section on input capaci-
tors in Application Information section.

4. Input Capacitor (C

)

The important parameters for the Input capacitor are the in-
put voltage rating and the RMS current rating. With a nominal
input voltage of 12V, an aluminum electrolytic capacitor with
a voltage rating greater than 18V (1.5 x V

) would be

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

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

the DC load current. In

this example, with a 1A load, a capacitor with a RMS current
rating of at least 500 mA is needed. The curves shown in

Fig-

ure 13 can be used to select an appropriate input capacitor.
From the curves, locate the 25V line and note which capaci-
tor values have RMS current ratings greater than 500 mA. Ei-
ther a 180 µF or 220 µF, 25V capacitor could be used.

For a through hole design, a 220 µF/25V electrolytic capaci-
tor (Panasonic HFQ series or Nichicon PL series or equiva-
lent) would be adequate. other types or other manufacturers
capacitors can be used provided the RMS ripple current rat-
ings are adequate.

For surface mount designs, solid tantalum capacitors can be
used, but caution must be exercised with regard to the ca-
pacitor surge current rating (see Application Information on
input capacitors in this data sheet). The TPS series available
from AVX, and the 593D series from Sprague are both surge
current tested.

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LM2595 Series Buck Regulator Design Procedure (Fixed Output)

(Continued)

LM2595 Series Buck Regulator Design Procedure (Adjustable Output)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

Given:

OUT

= Regulated Output Voltage

(max) = Maximum Input Voltage

LOAD

(max) = Maximum Load Current

F = Switching Frequency

(Fixed at a nominal 150 kHz).

Given:

OUT

= 20V

(max) = 28V

LOAD

(max) = 1A

F = Switching Frequency

(Fixed at a nominal 150 kHz).

1. Programming Output Voltage (Selecting R

and R

, as

shown in

Figure 1)

Use the following formula to select the appropriate resistor
values.

Select a value for R

between 240

and 1.5 k

. The lower

resistor values minimize noise pickup in the sensitive feed-
back pin. (For the lowest temperature coefficient and the best
stability with time, use 1% metal film resistors.)

1. Programming Output Voltage (Selecting R

and R

, as

shown in

Figure 1)

Select R

to be 1 k

, 1%. Solve for R

= 1k (16.26 - 1) = 15.26k, closest 1% value is 15.4 k

= 15.4 k

Procedure continued on next page.

Example continued on next page.

Conditions

Inductor

Output Capacitor

Through Hole Electrolytic

Surface Mount Tantalum

Output

Load

Max Input

Inductance

Inductor

Panasonic

Nichicon

AVX TPS

Sprague

Voltage

Current

Voltage

(µH)

(#)

HFQ Series

PL Series

Series

595D Series

(V)

(A)

(V)

(µF/V)

3.3

L24

330/16

220/10

330/10

L23

270/25

220/10

270/10

L31

220/25

220/35

220/10

L30

180/35

220/35

220/10

180/10

L13

220/25

220/16

220/10

0.5

L21

150/35

150/25

100/16

150/16

100

L20

150/35

82/35

100/16

100/20

L28

330/16

220/10

270/10

L31

220/25

220/10

L30

180/35

220/10

150/16

100

L29

180/35

120/35

100/16

120/16

L21

180/16

220/10

150/16

0.5

150

L19

120/25

1200/25

100/16

100/20

150

L19

100/25

68/20

68/25

L31

220/25

68/20

120/20

L30

180/35

120/25

68/20

120/20

150

L36

82/25

68/20

100/20

220

L35

82/25

68/20

68/25

L21

180/25

68/20

120/20

0.5

150

L19

82/25

68/20

100/20

330

L26

56/25

68/20

68/25

FIGURE 2. LM2595 Fixed Voltage Quick Design Component Selection Table

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LM2595 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

2. Inductor Selection (L1)

A. Calculate the inductor Volt

· microsecond constant E · T

· µs), from the following formula:

where V

SAT

= internal switch saturation voltage = 1V

and V

= diode forward voltage drop = 0.5V

B. Use the E

· T value from the previous formula and match

it with the E

· T number on the vertical axis of the Inductor

Value Selection Guide shown in

Figure 7.

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

D. Identify the inductance region intersected by the E

· T

value and the Maximum Load Current value. Each region is
identified by an inductance value and an inductor code
(LXX).

E. Select an appropriate inductor from the four manufactur-
er's part numbers listed in

Figure 8.

2. Inductor Selection (L1)

A. Calculate the inductor Volt

· microsecond constant

· T),

B. E

· T = 34.8 (V · µs)

C. I

LOAD

(max) = 1A

D. From the inductor value selection guide shown in

Figure 7,

the inductance region intersected by the 35 (V

· µs) horizon-

tal line and the 1A vertical line is 100 µH, and the inductor
code is L29.

E. From the table in

Figure 8, locate line L29, and select an

inductor part number from the list of manufacturers part num-
bers.

3. Output Capacitor Selection (C

OUT

)

A. In the majority of applications, low ESR electrolytic or solid
tantalum capacitors between 47 µF and 330 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 330 µF. For additional informa-
tion, see section on output capacitors in application in-
formation section.

B. To simplify the capacitor selection procedure, refer to the
quick design table shown in

Figure 3. This table contains dif-

ferent output voltages, and lists various output capacitors
that will provide the best design solutions.

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.

3. Output Capacitor SeIection (C

OUT

)

A. See section on C

OUT

in Application Information section.

B. From the quick design table shown in

Figure 3, locate the

output voltage column. From that column, locate the output
voltage closest to the output voltage in your application. In
this example, select the 24V line. Under the output capacitor
section, select a capacitor from the list of through hole elec-
trolytic or surface mount tantalum types from four different
capacitor manufacturers. It is recommended that both the
manufacturers and the manufacturers series that are listed in
the table be used.

In this example, through hole aluminum electrolytic capaci-
tors from several different manufacturers are available.

82 µF, 35V

Panasonic HFQ Series

82 µF, 35V

Nichicon PL Series

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

more is needed. In this example, either a 35V or 50V capaci-
tor would work. A 35V rating was chosen, although a 50V rat-
ing could also be used if a lower output ripple voltage is
needed.

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

) (See

Figure 1)

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

ditional capacitor is required. The compensation capacitor is
typically between 50 pF and 10 nF, and is wired in parallel
with the output voltage setting resistor, R

. It provides addi-

tional stability for high output voltages, low input-output volt-
ages, 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 capaci-
tors made with Z5U material, they are not recommended.)

4. Feedforward Capacitor (C

)

The table shown in

Figure 3 contains feed forward capacitor

values for various output voltages. In this example, a 1 nF
capacitor is needed.

www.national.com

LM2595 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

Procedure continued on next page.

Example continued on next page.

5. Catch Diode Selection (D1)

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 LM2595 using short leads and
short printed circuit traces. Because of their fast switching
speed and low forward voltage drop, Schottky diodes provide
the best performance and efficiency, and should be the first
choice, especially in low output voltage applications.
Ultra-fast recovery, or High-Efficiency rectifiers are also a
good choice, but some types with an abrupt turn-off charac-
teristic may cause instability or EMl problems. Ultra-fast re-
covery diodes typically have reverse recovery times of 50 ns
or less. Rectifiers such as the 1N4001 series are much too
slow and should not be used.

5. Catch Diode Selection (D1)

A. Refer to the table shown in

Figure 11 Schottky diodes pro-

vide the best performance, and in this example a 3A, 40V,
1N5822 Schottky diode would be a good choice. The 3A di-
ode rating is more than adequate and will not be over-
stressed even for a shorted output.

6. Input Capacitor (C

)

A low ESR aluminum or tantalum bypass capacitor is needed
between the input pin and ground to prevent large voltage
transients from appearing at the input. In addition, the RMS
current rating of the input capacitor should be selected to be
at least

the DC load current. The capacitor manufacturers

data sheet must be checked to assure that this current rating
is not exceeded. The curve shown in

Figure 13 shows typical

RMS current ratings for several different aluminum electro-
lytic capacitor values.

This capacitor should be located close to the IC using short
leads and the voltage rating should be approximately 1.5
times the maximum input voltage.

If solid tantalum input capacitors are used, it is recomended
that they be surge current tested by the manufacturer.

se caution when using a high dielectric constant ceramic ca-
pacitor for input bypassing, because it may cause severe
ringing at the V

pin.

For additional information, see section on input capaci-
tors in application information section.

6. Input Capacitor (C

)

The important parameters for the Input capacitor are the in-
put voltage rating and the RMS current rating. With a nominal
input voltage of 28V, an aluminum electrolytic aluminum elec-
trolytic capacitor with a voltage rating greater than 42V (1.5 x
V

) would be needed. Since the the next higher capacitor

voltage rating is 50V, a 50V capacitor should be used. The
capacitor voltage rating of (1.5 x V

) is a conservative guide-

line, and can be modified somewhat if desired.

The RMS current rating requirement for the input capacitor of
a buck regulator is approximately

the DC load current. In

this example, with a 1A load, a capacitor with a RMS current
rating of at least 500 mA is needed.

The curves shown in

Figure 13 can be used to select an ap-

propriate input capacitor. From the curves, locate the 50V
line and note which capacitor values have RMS current rat-
ings greater than 500 mA. Either a 100 µF or 120 µF, 50V ca-
pacitor could be used.

For a through hole design, a 120 µF/50V electrolytic capaci-
tor (Panasonic HFQ series or Nichicon PL series or equiva-
lent) would be adequate. Other types or other manufacturers
capacitors can be used provided the RMS ripple current rat-
ings are adequate.

For surface mount designs, solid tantalum capacitors can be
used, but caution must be exercised with regard to the ca-
pacitor surge current rting (see Application Information or in-
put capacitors in this data sheet). The TPS series available
from AVX, and the 593D series from Sprague are both surge
current tested.

To further simplify the buck regulator design procedure, Na-
tional Semiconductor is making available computer design
software to be used with the Simple Switcher line ot switch-
ing regulators. Switchers Made Simple (version 4.2 or later)
is available on a 3

" diskette for IBM compatible computers.

www.national.com

LM2595 Series Buck Regulator Design Procedure

(Continued)

Inductance

(µH)

Current

(A)

Renco

Pulse Engineering

Coilcraft

Through Hole

Surface

Mount

Through

Hole

Surface

Mount

Surface Mount

0.32

RL-1284-68-43

RL1500-68

PE-53804

PE-53804-S

DO1608-68

0.37

RL-1284-47-43

RL1500-47

PE-53805

PE-53805-S

DO1608-473

0.44

RL-1284-33-43

RL1500-33

PE-53806

PE-53806-S

DO1608-333

220

0.32

RL-5470-3

RL1500-220

PE-53809

PE-53809-S

DO3308-224

L10

150

0.39

RL-5470-4

RL1500-150

PE-53810

PE-53810-S

DO3308-154

L11

100

0.48

RL-5470-5

RL1500-100

PE-53811

PE-53811-S

DO3308-104

L12

0.58

RL-5470-6

RL1500-68

PE-53812

PE-53812-S

DO3308-683

L13

0.70

RL-5470-7

RL1500-47

PE-53813

PE-53813-S

DO3308-473

L14

0.83

RL-1284-33-43

RL1500-33

PE-53814

PE-53814-S

DO3308-333

L15

0.99

RL-1284-22-43

RL1500-22

PE-53815

PE-53815-S

DO3308-223

L16

1.24

RL-1284-15-43

RL1500-15

PE-53816

PE-53816-S

DO3308-153

L17

330

0.42

RL-5471-1

RL1500-330

PE-53817

PE-53817-S

DO3316-334

L18

220

0.55

RL-5471-2

RL1500-220

PE-53818

PE-53818-S

DO3316-224

L19

150

0.66

RL-5471-3

RL1500-150

PE-53819

PE-53819-S

DO3316-154

L20

100

0.82

RL-5471-4

RL1500-100

PE-53820

PE-53820-S

DO3316-104

L21

0.99

RL-5471-5

RL1500-68

PE-53821

PE-53821-S

DO3316-683

L22

1.17

RL-5471-6

PE-53822

PE-53822-S

DO3316-473

L23

1.40

RL-5471-7

PE-53823

PE-53823-S

DO3316-333

L24

1.70

RL-1283-22-43

PE-53824

PE-53824-S

DO3316-223

L26

330

0.80

RL-5471-1

PE-53826

PE-53826-S

DO5022P-334

L27

220

1.00

RL-5471-2

PE-53827

PE-53827-S

DO5022P-224

L28

150

1.20

RL-5471-3

PE-53828

PE-53828-S

DO5022P-154

L29

100

1.47

RL-5471-4

PE-53829

PE-53829-S

DO5022P-104

L30

1.78

RL-5471-5

PE-53830

PE-53830-S

DO5022P-683

L35

2.15

RL-5473-1

PE-53935

PE-53935-S

FIGURE 8. Inductor Manufacturers Part Numbers

Coilcraft Inc.

Phone

(800) 322-2645

FAX

(708) 639-1469

Coilcraft Inc., Europe

Phone

+11 1236 730 595

FAX

+44 1236 730 627

Pulse Engineering Inc.

Phone

(619) 674-8100

FAX

(619) 674-8262

Pulse Engineering Inc.,

Phone

+353 93 24 107

Europe

FAX

+353 93 24 459

Renco Electronics Inc.

Phone

(800) 645-5828

FAX

(516) 586-5562

FIGURE 9. Inductor Manufacturers Phone Numbers

www.national.com

LM2595 Series Buck Regulator Design Procedure

(Continued)

Nichicon Corp.

Phone

(708) 843-7500

FAX

(708) 843-2798

Panasonic

Phone

(714) 373-7857

FAX

(714) 373-7102

AVX Corp.

Phone

(803) 448-9411

FAX

(803) 448-1943

Sprague/Vishay

Phone

(207) 324-4140

FAX

(207) 324-7223

FIGURE 10. Capacitor Manufacturers Phone Numbers

1A Diodes

3A Diodes

Surface Mount

Through Hole

Surface Mount

Through Hole

Schottky

Ultra Fast

Schottky

Ultra Fast

Schottky

Ultra Fast

Schottky

Ultra Fast

Recovery

20V

SK12

All of

these

diodes

are

rated to

at least

50V.

1N5817

All of

these

diodes

are

rated to

at least

50V.

All of

these

diodes

are

rated to

at least

50V.

1N5820

All of

these

diodes

are

rated to

at least

50V.

SR102

SK32

SR302

MBR320

30V

SK13

1N5818

1N5821

MBRS130

SR103

SK33

MBR330

11DQ03

31DQ03

SK14

1N5822

40V

MBRS140

1N5819

SK34

SR304

10BQ040

SR104

MBRS340

MBR340

10MQ040

MURS120

11DQ04

MUR120

30WQ04

MURS320

31DQ04

MUR320

50V

MBRS160

10BF10

SR105

SK35

30WF10

SR305

30WF10

10BQ050

MBR150

MBR360

MBR350

10MQ060

11DQ05

30WQ05

31DQ05

FIGURE 11. Diode Selection Table

www.national.com

Block Diagram

Application Information

PIN FUNCTIONS

-- This is the positive input supply for the IC switching

regulator. A suitable input bypass capacitor must be present
at this pin to minimize voltage transients and to supply the
switching currents needed by the regulator.

Ground -- Circuit ground.

Output -- Internal switch. The voltage at this pin switches
between (+V

- V

SAT

) and approximately -0.5V, with a duty

cycle of approximately V

OUT

. To minimize coupling to

sensitive circuitry, the PC board copper area connected to
this pin should be kept to a minimum.

Feedback -- Senses the regulated output voltage to com-
plete the feedback loop.

ON/OFF -- Allows the switching regulator circuit to be shut
down using logic level signals thus dropping the total input
supply current to approximately 85 µA. Pulling this pin below
a threshold voltage of approximately 1.3V turns the regulator
on, and pulling this pin above 1.3V (up to a maximum of 25V)
shuts the regulator down. If this shutdown feature is not
needed, the ON/OFF pin can be wired to the ground pin or it
can be left open, in either case the regulator will be in the ON
condition.

EXTERNAL COMPONENTS
INPUT CAPACITOR

-- 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.

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.

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.

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.

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.

A graph shown in

Figure 13 shows the relationship between

an electrolytic capacitor value, its voltage rating, and the
RMS current it is rated for. These curves were obtained from
the Nichicon "PL" series of low ESR, high reliability electro-

DS012565-21

FIGURE 12.

www.national.com

Application Information

(Continued)

lytic capacitors designed for switching regulator applications.
Other capacitor manufacturers offer similar types of capaci-
tors, but always check the capacitor data sheet.

"Standard" electrolytic capacitors typically have much higher
ESR numbers, lower RMS current ratings and typically have
a shorter operating lifetime.

Because of their small size and excellent performance, sur-
face mount solid tantalum capacitors are often used for input
bypassing, but several precautions must be observed. A
small percentage of solid tantalum capacitors can short if the
inrush current rating is exceeded. This can happen at turn on
when the input voltage is suddenly applied, and of course,
higher input voltages produce higher inrush currents. Sev-
eral capacitor manufacturers do a 100% surge current test-
ing on their products to minimize this potential problem. If
high turn on currents are expected, it may be necessary to
limit this current by adding either some resistance or induc-
tance before the tantalum capacitor, or select a higher volt-
age capacitor. As with aluminum electrolytic capacitors, the
RMS ripple current rating must be sized to the load current.

FEEDFORWARD CAPACITOR

(Adjustable Output Voltage Version)

-A Feedforward Capacitor C

, shown across R2 in

Fig-

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

OUT

has a very low ESR. This capacitor adds lead

compensation to the feedback loop and increases the phase
margin for better loop stability. For C

selection, see the de-

sign procedure section.

OUTPUT CAPACITOR

OUT

-- An output capacitor is required to filter the output

and provide regulator loop stability. Low impedance or low
ESR Electrolytic or solid tantalum capacitors designed for
switching regulator applications must be used. When select-
ing an output capacitor, the important capacitor parameters
are; the 100 kHz Equivalent Series Resistance (ESR), the
RMS ripple current rating, voltage rating, and capacitance
value. For the output capacitor, the ESR value is the most
important parameter.

The output capacitor requires an ESR value that has an 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.

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 14). Often, capacitors with much higher voltage

ratings may be needed to provide the low ESR values re-
quired for low output ripple voltage.

The output capacitor for many different switcher designs of-
ten can be satisfied with only three or four different capacitor
values and several different voltage ratings. See the quick
design component selection tables in

Figure 2 and Figure 3

for typical capacitor values, voltage ratings, and manufactur-
ers capacitor types.

Electrolytic capacitors are not recommended for tempera-
tures below -25°C. The ESR rises dramatically at cold tem-
peratures and typically rises 3X

-25°C and as much as

10X at -40°C. See curve shown in

Figure 15.

Solid tantalum capacitors have a much better ESR spec for
cold temperatures and are recommended for temperatures
below -25°C.

CATCH DIODE

Buck regulators require a diode to provide a return path for
the inductor current when the switch turns off. This must be
a fast diode and must be located close to the LM2595 using
short leads and short printed circuit traces.

Because of their very fast switching speed and low forward
voltage drop, Schottky diodes provide the best performance,
especially in low output voltage applications (5V and lower).
Ultra-fast recovery, or High-Efficiency rectifiers are also a
good choice, but some types with an abrupt turnoff charac-
teristic may cause instability or EMI problems. Ultra-fast re-
covery diodes typically have reverse recovery times of 50 ns
or less. Rectifiers such as the 1N5400 series are much too
slow and should not be used.

DS012565-28

FIGURE 13. RMS Current Ratings for Low ESR

Electrolytic Capacitors (Typical)

DS012565-29

FIGURE 14. Capacitor ESR vs Capacitor Voltage Rating

(Typical Low ESR Electrolytic Capacitor)

www.national.com

Application Information

(Continued)

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 LM2595 (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.

To simplify the inductor selection process, an inductor selec-
tion guide (nomograph) was designed (see

Figure 4 through

Figure 7). This guide assumes that the regulator is operating
in the continuous mode, and selects an inductor that will 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 16.)

By allowing the percentage of inductor ripple current to in-
crease for low load currents, the inductor value and size can
be kept relatively low.

When operating in the continuous mode, the inductor current
waveform ranges from a triangular to a sawtooth type of
waveform (depending on the input voltage), with the average
value of this current waveform equal to the DC output load
current.

Inductors are available in different styles such as pot core,
toroid, E-core, bobbin core, etc., as well as different core ma-
terials, such as ferrites and powdered iron. The least expen-
sive, the bobbin, rod or stick core, consists of wire wound on
a ferrite bobbin. This type of construction makes for an inex-
pensive inductor, but since the magnetic flux is not com-
pletely contained within the core, it generates more
Electro-Magnetic Interference (EMl). This magnetic flux can
induce voltages into nearby printed circuit traces, thus caus-
ing problems with both the switching regulator operation and
nearby sensitive circuitry, and can give incorrect scope read-
ings because of induced voltages in the scope probe. Also
see section on Open Core Inductors.

When multiple switching regulators are located on the same
PC board, open core magnetics can cause interference be-
tween two or more of the regulator circuits, especially at high
currents. A toroid or E-core inductor (closed magnetic struc-
ture) should be used in these situations.

The inductors listed in the selection chart include ferrite
E-core construction for Schott, ferrite bobbin core for Renco
and Coilcraft, and powdered iron toroid for Pulse Engineer-
ing.

Exceeding an inductor's maximum current rating may cause
the inductor to overheat because of the copper wire losses,
or the core may saturate. If the inductor begins to saturate,
the inductance decreases rapidly and the inductor begins to
look mainly resistive (the DC resistance of the winding). This
can cause the switch current to rise very rapidly and force
the switch into a cycle-by-cycle current limit, thus reducing
the DC output load current. This can also result in overheat-
ing of the inductor and/or the LM2595. Different inductor
types have different saturation characteristics, and this
should be kept in mind when selecting an inductor.

The inductor manufacturer's data sheets include current and
energy limits to avoid inductor saturation.

DS012565-30

FIGURE 15. Capacitor ESR Change vs Temperature

DS012565-31

FIGURE 16. (

IND

) Peak-to-Peak

Inductor Ripple Current (as a Percentage

of the Load Current) vs Load Current

www.national.com

Application Information

(Continued)

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 de-
sign. The peak switch and inductor currents will be higher in
a discontinuous design, but at these low load currents
(400 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 out-
put pin (switch) waveform can have some damped sinusoi-
dal ringing present. (See Typical Performance Characteris-
tics photo titled Discontinuous Mode Switching Waveforms)
This ringing is normal for discontinuous operation, and is not
caused by feedback loop instabilities. In discontinuous op-
eration, there is a period of time where neither the switch or
the diode are conducting, and the inductor current has
dropped to zero. During this time, a small amount of energy
can circulate between the inductor and the switch/diode
parasitic capacitance causing this characteristic ringing. Nor-
mally this ringing is not a problem, unless the amplitude be-
comes great enough to exceed the input voltage, and even
then, there is very little energy present to cause damage.

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 4.3) will provide all component values for continuous
and discontinuous modes of operation.

OUTPUT VOLTAGE RIPPLE AND TRANSIENTS

The output voltage of a switching power supply operating in
the continuous mode will contain a sawtooth ripple voltage at
the switcher frequency, and may also contain short voltage
spikes at the peaks of the sawtooth waveform.

The output ripple voltage is a function of the inductor saw-
tooth ripple current and the ESR of the output capacitor. A
typical output ripple voltage can range from approximately
0.5% to 3% of the output voltage. To obtain low ripple volt-
age, the ESR of the output capacitor must be low, however,
caution must be exercised when using extremely low ESR
capacitors because they can affect the loop stability, result-

ing in oscillation problems. If very low output ripple voltage is
needed (less than 20 mV), a post ripple filter is recom-
mended. (See

Figure 1.) The inductance required is typically

between 1 µH and 5 µH, with low DC resistance, to maintain
good load regulation. A low ESR output filter capacitor is also
required to assure good dynamic load response and ripple
reduction. The ESR of this capacitor may be as low as de-
sired, because it is out of the regulator feedback loop. The
photo shown in

Figure 17 shows a typical output ripple volt-

age, with and without a post ripple filter.

When observing output ripple with a scope, it is essential
that a short, low inductance scope probe ground connection
be used. Most scope probe manufacturers provide a special
probe terminator which is soldered onto the regulator board,
preferable at the output capacitor. This provides a very short
scope ground thus eliminating the problems associated with
the 3 inch ground lead normally provided with the probe, and
provides a much cleaner and more accurate picture of the
ripple voltage waveform.

The voltage spikes are caused by the fast switching action of
the output switch and the diode, and the parasitic inductance
of the output filter capacitor, and its associated wiring. To
minimize these voltage spikes, the output capacitor should
be designed for switching regulator applications, and the
lead lengths must be kept very short. Wiring inductance,
stray capacitance, as well as the scope probe used to evalu-
ate these transients, all contribute to the amplitude of these
spikes.

When a switching regulator is operating in the continuous
mode, the inductor current waveform ranges from a triangu-
lar to a sawtooth type of waveform (depending on the input
voltage). For a given input and output voltage, the
peak-to-peak amplitude of this inductor current waveform 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.

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.

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

IND

) can be useful for

determining a number of other circuit parameters. Param-

DS012565-32

FIGURE 17. Post Ripple Filter Waveform

DS012565-33

FIGURE 18. Peak-to-Peak Inductor

Ripple Current vs Load Current

www.national.com

Application Information

(Continued)

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

IND

. When the inductor

nomographs shown in

Figure 4 through Figure 7 are used to

select an inductor value, the peak-to-peak inductor ripple
current can immediately be determined. The curve shown in
Figure 18 shows the range of (

IND

) that can be expected

for different load currents. The curve also shows how the
peak-to-peak inductor ripple current (

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

Consider the following example:

OUT

= 5V, maximum load current of 800 mA

= 12V, nominal, varying between 10V and 14V.

The selection guide in

Figure 5 shows that the vertical line

for a 0.8A load current, and the horizontal line for the 12V in-
put voltage intersect approximately midway between the up-
per and lower borders of the 68 µH inductance region. A 68
µH inductor will allow a peak-to-peak inductor current (

IND

)

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

Figure 18, follow the 0.8A line approxi-

mately midway into the inductance region, and read the
peak-to-peak inductor ripple current (

IND

) on the left hand

axis (approximately 300 mA p-p).

As the input voltage increases to 14V, it approaches the up-
per border of the inductance region, and the inductor ripple
current increases. Referring to the curve in

Figure 18, it can

be seen that for a load current of 0.8A, the peak-to-peak in-
ductor ripple current (

IND

) is 300 mA with 12V in, and can

range from 340 mA at the upper border (14V in) to 225 mA at
the lower border (10V in).

Once the

IND

value is known, the following formulas can be

used to calculate additional information about the switching
regulator circuit.

Peak Inductor or peak switch current

Minimum load current before the circuit becomes dis-
continuous

Output Ripple Voltage = (

IND

)x(ESR of C

OUT

)

= 0.30Ax0.16

=48 mV p-p

ESR of C

OUT

OPEN CORE INDUCTORS

Another possible source of increased output ripple voltage or
unstable operation is from an open core inductor. Ferrite
bobbin or stick inductors have magnetic lines of flux flowing
through the air from one end of the bobbin to the other end.
These magnetic lines of flux will induce a voltage into any
wire or PC board copper trace that comes within the induc-
tor's magnetic field. The strength of the magnetic field, the
orientation and location of the PC copper trace to the mag-
netic field, and the distance between the copper trace and
the inductor, determine the amount of voltage generated in
the copper trace. Another way of looking at this inductive
coupling is to consider the PC board copper trace as one
turn of a transformer (secondary) with the inductor winding
as the primary. Many millivolts can be generated in a copper
trace located near an open core inductor which can cause
stability problems or high output ripple voltage problems.

If unstable operation is seen, and an open core inductor is
used, it's possible that the location of the inductor with re-
spect to other PC traces may be the problem. To determine
if this is the problem, temporarily raise the inductor away
from the board by several inches and then check circuit op-
eration. If the circuit now operates correctly, then the mag-
netic flux from the open core inductor is causing the problem.
Substituting a closed core inductor such as a torroid or
E-core will correct the problem, or re-arranging the PC layout
may be necessary. Magnetic flux cutting the IC device
ground trace, feedback trace, or the positive or negative
traces of the output capacitor should be minimized.

Sometimes, locating a trace directly beneath a bobbin induc-
tor will provide good results, provided it is exactly in the cen-
ter of the inductor (because the induced voltages cancel
themselves out), but if it is off center one direction or the
other, then problems could arise. If flux problems are
present, even the direction of the inductor winding can make
a difference in some circuits.

This discussion on open core inductors is not to frighten the
user, but to alert the user on what kind of problems to watch
out for when using them. Open core bobbin or "stick" induc-
tors are an inexpensive, simple way of making a compact ef-
ficient inductor, and they are used by the millions in many dif-
ferent applications.

THERMAL CONSIDERATIONS

The LM2595 is available in two packages, a 5-pin TO-220
(T) and a 5-pin surface mount TO-263 (S).

The TO-220 package can be used without a heat sink for
ambient temperatures up to approximately 50°C (depending
on the output voltage and load current). The curves in

Figure

19 show the LM2595T junction temperature rises above am-
bient temperature for different input and output voltages. The
data tor these curves was taken with the LM2595T (TO-220
package) operating as a switching regutator in an ambient
temperature of 25°C (still air). These temperature rise num-
bers are all approximate and there are many factors that can
affect these temperatures. Higher ambient temperatures re-
quire some heat sinking, either to the PC board or a small
external heat sink.

The TO-263 surface mount package tab is designed to be
soldered to the copper on a printed circuit board. The copper
and the board are the heat sink for this package and the
other heat producing components, such as the catch diode
and inductor. The PC board copper area that the package is
soldered to should be at least 0.4 in

, and ideally should

have 2 or more square inches of 2 oz. (0.0028 in) copper.
Additional copper area improves the thermal characteristics,

www.national.com

Application Information

(Continued)

but with copper areas greater than approximately 3 in

, only

small improvements in heat dissipation are realized. If fur-
ther thermal improvements are needed, double sided or mul-
tilayer PC-board with large copper areas are recommended.

The curves shown in

Figure 20 show the LM2595S (TO-263

package) junction temperature rise above ambient tempera-
ture with a 1A load for various input and output voltages. This
data was taken with the circuit operating as a buck switching
regulator with all components mounted on a PC board to
simulate the junction temperature under actual operating
conditions. This curve can be used for a quick check for the
approximate junction temperature for various conditions, but
be aware that there are many factors that can affect the junc-
tion temperature.

For the best thermal performance, wide copper traces and
generous amounts of printed circuit board copper should be
used in the board layout. (One exception to this is the output
(switch) pin, which should not have large areas of copper.)
Large areas of copper provide the best transfer of heat
(lower thermal resistance) to the surrounding air, and moving
air lowers the thermal resistance even further.

Package thermal resistance and junction temperature rise
numbers are all approximate, and there are many factors
that will affect these numbers. Some of these factors include
board size, shape, thickness, position, location, and even
board temperature. Other factors are, trace width, total
printed circuit copper area, copper thickness, single- or
double-sided, multilayer board and the amount of solder on
the board. The effectiveness of the PC board to dissipate
heat also depends on the size, quantity and spacing of other
components on the board, as well as whether the surround-
ing air is still or moving. Furthermore, some of these compo-
nents such as the catch diode will add heat to the PC board
and the heat can vary as the input voltage changes. For the
inductor, depending on the physical size, type of core mate-
rial and the DC resistance, it could either act as a heat sink
taking heat away from the board, or it could add heat to the
board.

DS012565-34

Circuit Data for Temperature Rise Curve

TO-220 Package (T)

Capacitors

Through hole electrolytic

Inductor

Through hole, Schott, 68 µH

Diode

Through hole, 3A 40V, Schottky

PC board

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

FIGURE 19. Junction Temperature Rise, TO-220

DS012565-35

Circuit Data for Temperature Rise Curve

TO-263 Package (S)

Capacitors

Surface mount tantalum, molded "D" size

Inductor

Surface mount, Schott, 68 µH

Diode

Surface mount, 3A 40V, Schottky

PC board

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

FIGURE 20. Junction Temperature Rise, TO-263

www.national.com

Application Information

(Continued)

DELAYED STARTUP

The circuit in

Figure 21 uses the the ON /OFF pin to provide

a time delay between the time the input voltage is applied
and the time the output voltage comes up (only the circuitry
pertaining to the delayed start up is shown). As the input volt-
age rises, the charging of capacitor C1 pulls the ON /OFF pin
high, keeping the regulator off. Once the input voltage
reaches its final value and the capacitor stops charging, and
resistor R

pulls the ON /OFF pin low, thus allowing the cir-

cuit to start switching. Resistor R

is included to limit the

maximum voltage applied to the ON /OFF pin (maximum of
25V), reduces power supply noise sensitivity, and also limits
the capacitor, C1, discharge current. When high input ripple
voltage exists, avoid long delay time, because this ripple can
be coupled into the ON /OFF pin and cause problems.

This delayed startup feature is useful in situations where the
input power source is limited in the amount of current it can
deliver. It allows the input voltage to rise to a higher voltage
before the regulator starts operating. Buck regulators require
less input current at higher input voltages.

UNDERVOLTAGE LOCKOUT

Some applications require the regulator to remain off until
the input voltage reaches a predetermined voltage. An und-
ervoltage lockout feature applied to a buck regulator is
shown in

Figure 22, while Figure 23 and Figure 24 applies

the same feature to an inverting circuit. The circuit in

Figure

23 features a constant threshold voltage for turn on and turn
off (zener voltage plus approximately one volt). If hysteresis

is needed, the circuit in

Figure 24 has a turn ON voltage

which is different than the turn OFF voltage. The amount of
hysteresis is approximately equal to the value of the output
voltage. If zener voltages greater than 25V are used, an ad-
ditional 47 k

resistor is needed from the ON /OFF pin to the

ground pin to stay within the 25V maximum limit of the ON
/OFF pin.

INVERTING REGULATOR

The circuit in

Figure 25 converts a positive input voltage to a

negative output voltage with a common ground. The circuit
operates by bootstrapping the regulator's ground pin to the
negative output voltage, then grounding the feedback pin,
the regulator senses the inverted output voltage and regu-
lates it.

This example uses the LM2595-5.0 to generate a -5V out-
put, but other output voltages are possible by selecting other
output voltage versions, including the adjustable version.

Since this regulator topology can produce an output voltage
that is either greater than or less than the input voltage, the
maximum output current greatly depends on both the input
and output voltage. The curve shown in

Figure 26 provides a

guide as to the amount of output load current possible for the
different input and output voltage conditions.

The maximum voltage appearing across the regulator is the
absolute sum of the input and output voltage, and this must
be limited to a maximum of 40V. For example, when convert-
ing +20V to -12V, the regulator would see 32V between the
input pin and ground pin. The LM2595 has a maximum input
voltage spec of 40V.

Additional diodes are required in this regulator configuration.
Diode D1 is used to isolate input voltage ripple or noise from
coupling through the C

capacitor to the output, under light

or no load conditions. Also, this diode isolation changes the
topology to closley resemble a buck configuration thus pro-
viding good closed loop stability. A Schottky diode is recom-
mended for low input voltages, (because of its lower voltage
drop) but for higher input voltages, a fast recovery diode
could be used.

Without diode D3, when the input voltage is first applied, the
charging current of C

can pull the output positive by sev-

eral volts for a short period of time. Adding D3 prevents the
output from going positive by more than a diode voltage.

DS012565-36

FIGURE 21. Delayed Startup

DS012565-37

FIGURE 22. Undervoltage Lockout

for Buck Regulator

DS012565-46

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

FIGURE 23. Undervoltage Lockout

for Inverting Regulator

www.national.com

Application Information

(Continued)

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 68 µH, 1.5A in-
ductor is the best choice. Capacitor selection can also be
narrowed down to just a few values. Using the values shown
in

Figure 25 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 LM2595 current limit (ap-
prox 1.5A) are needed for at least 2 ms or more, until the out-
put reaches its nominal output voltage. The actual time de-
pends on the output voltage and the size of the output
capacitor. Input power sources that are current limited or
sources that can not deliver these currents without getting
loaded down, may not work correctly. Because of the rela-
tively high startup currents required by the inverting topology,
the delayed startup feature (C1, R

and R

) shown in

Figure

25 is recommended. By delaying the regulator startup, the
input capacitor is allowed to charge up to a higher voltage
before the switcher begins operating. A portion of the high in-
put current needed for startup is now supplied by the input
capacitor (C

). For severe start up conditions, the input ca-

pacitor can be made much larger than normal.

INVERTING REGULATOR SHUTDOWN METHODS

To use the ON /OFF pin in a standard buck configuration is
simple, pull it below 1.3V (

25°C, referenced to ground) to

turn regulator ON, pull it above 1.3V to shut the regulator
OFF. With the inverting configuration, some level shifting is
required, because the ground pin of the regulator is no
longer at ground, but is now setting at the negative output
voltage level. Two different shutdown methods for inverting
regulators are shown in

Figure 27 and Figure 28.

DS012565-39

This circuit has hysteresis

Regulator starts switching at V

= 13V

Regulator stops switching at V

= 8V

FIGURE 24. Undervoltage Lockout with Hysteresis for Inverting Regulator

DS012565-40

-- 220 µF/25V Tant. Sprague 595D

120 µF/50V Elec. Panasonic HFQ
C

OUT

-- 22 µF/20V Tant. Sprague 595D

120 µF/25V Elec. Panasonic HFQ

FIGURE 25. Inverting -5V Regulator with Delayed Startup

DS012565-41

FIGURE 26. Inverting Regulator Typical Load Current

www.national.com

Physical Dimensions

inches (millimeters) unless otherwise noted (Continued)

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NATIONAL'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or

systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.

2. A critical component is any component of a life

support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.

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Corporation
Americas
Tel: 1-800-272-9959
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Email: support@nsc.com

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Tel: 81-3-5639-7560
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www.national.com

16-Lead Ceramic Dual-in-Line Package (J)

Order Number LM2595J-3.3-QML (5962-9687901QEA),

LM2595J-5.0-QML (5962-9650301QEA),

LM2595J-12-QML (5962-9650201QEA),

or LM2595J-ADJ-QML (5962-9650401QEA)

NS Package Number J16A

LM2595

SIMPLE

SWITCHER

Power

Converter

150

kHz

Step-Down

oltage

Regulator

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.

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