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LM2598SX-12
Datasheet (NSC)
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Datasheet LM2598T-5.0, LM2598T-3.3, LM2598SX-ADJ, LM2598SX-5.0, LM2598SX-3.3 Datasheet (NSC)

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Page 1
LM2598 SIMPLE SWITCHER
®
Power Converter 150 kHz
1A Step-Down Voltage Regulator, with Features
General Description
The LM2598 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 avail­able in fixed output voltages of 3.3V, 5V, 12V, and an adjust­able output version.
This series of switching regulators is similar to the LM2595 series, with additionalsupervisoryand performance features added.
†
, improved line and load specifications, fixed-frequency oscillator, Shutdown /Soft-start, error flag delay and error flag output.
The LM2598 series operates at a switching frequency of 150 kHz thus allowing smaller sized filter components than what would be needed with lower frequency switching regulators. Available in a standard 7-lead TO-220 package with several different lead bend options, and a 7-lead TO-263 surface mount package. Typically, for output voltages less than 12V, and ambient temperatures less than 50˚C, no heat sink is re­quired.
A standard series of inductors (both through hole and sur­face mount types) are available from several different manu­facturers optimized for use with the LM2598 series. This fea­ture greatly simplifies the design of switch-mode power supplies.
Other features include a guaranteed
±
4%tolerance on out­put voltage under all conditions of input voltage and output load conditions, and
±
15%on the oscillator frequency. Ex-
ternal shutdown is included, featuring typically 85 µA standby current. Self protection features include a two stage current limit for the output switch and an over temperature shutdown for complete protection under fault conditions.
Features
n 3.3V, 5V, 12V, and adjustable output versions n Adjustable version output voltage range, 1.2V to 37V
±
4%max over line and load conditions
n Guaranteed 1A output current n Available in 7-pin TO-220 and TO-263 (surface mount)
package
n Input voltage range up to 40V n Excellent line and load regulation specifications n 150 kHz fixed frequency internal oscillator n Shutdown /Soft-start n Out of regulation error flag n Error output delay n Low power standby mode, I
Q
typically 85 µA
n High Efficiency n Uses readily available standard inductors n Thermal shutdown and current limit protection
Applications
n Simple high-efficiency step-down (buck) regulator n Efficient pre-regulator for linear regulators n On-card switching regulators n Positive to Negative converter
Typical Application (Fixed Output Voltage Versions)
†
Patent Number 5,382,918.
SIMPLE SWITCHER
®
and
Switchers Made Simple
®
are registered trademarks of National Semiconductor Corporation.
DS012593-1
March 1998
LM2598 SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator, with
Features
© 1999 National Semiconductor Corporation DS012593 www.national.com
Page 2
Connection Diagrams and Order Information
Bent and Staggered Leads, Through Hole Package
7-Lead TO-220 (T)
DS012593-50
Order Number LM2598T-3.3, LM2598T-5.0,
LM2598T-12 or LM2598T-ADJ
See NS Package Number TA07B
Surface Mount Package
7-Lead TO-263 (S)
DS012593-22
Order Number LM2598S-3.3, LM2598S-5.0,
LM2598S-12 or LM2598S-ADJ
See NS Package Number TS7B
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Page 3
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 (V
IN
) 45V
SD/SS Pin Input Voltage (Note 2)
6V Delay Pin Voltage (Note 2) 1.5V Flag Pin Voltage −0.3 ≤ V ≤ +45V Feedback Pin Voltage −0.3 ≤ V ≤ +25V Output Voltage to Ground
(Steady State) −1V Power Dissipation Internally limited Storage Temperature Range −65˚C to +150˚C
ESD Susceptibility
Human Body Model (Note 3) 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 −25˚C ≤ TJ≤ +125˚C Supply Voltage 4.5V to 40V
LM2598-3.3 Electrical Characteristics
Specifications with standard type face are for T
J
=
25˚C, and those with boldface type apply over full Operating Tempera-
ture Range.
Symbol Parameter Conditions LM2598-3.3 Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit
Figure 1
V
OUT
Output Voltage 4.75V ≤ VIN≤ 40V, 0.1A ≤ I
LOAD
≤ 1A 3.3 V
3.168/3.135 V(min)
3.432/3.465 V(max)
η Efficiency V
IN
=
12V, I
LOAD
=
1A 78
%
LM2598-5.0 Electrical Characteristics
Specifications with standard type face are for T
J
=
25˚C, and those with boldface type apply over full Operating Tempera-
ture Range.
Symbol Parameter Conditions LM2598-5.0 Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit
Figure 1
V
OUT
Output Voltage 7V ≤ VIN≤ 40V, 0.1A ≤ I
LOAD
≤ 1A 5 V
4.800/4.750 V(min)
5.200/5.250 V(max)
η Efficiency V
IN
=
12V, I
LOAD
=
1A 82
%
LM2598-12 Electrical Characteristics
Specifications with standard type face are for T
J
=
25˚C, and those with boldface type apply over full Operating Tempera-
ture Range.
Symbol Parameter Conditions LM2598-12 Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit
Figure 1
V
OUT
Output Voltage 15V ≤ VIN≤ 40V, 0.1A ≤ I
LOAD
≤ 1A 12 V
11.52/11.40 V(min)
12.48/12.60 V(max)
η Efficiency V
IN
=
25V, I
LOAD
=
1A 90
%
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Page 4
LM2598-ADJ Electrical Characteristics
Specifications with standard type face are for T
J
=
25˚C, and those with boldface type apply over full Operating Tempera-
ture Range.
Symbol Parameter Conditions LM2598-ADJ Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit
Figure 1
V
FB
Feedback Voltage 4.5V ≤ VIN≤ 40V, 0.1A ≤ I
LOAD
≤ 1A 1.230 V
V
OUT
programmed for 3V. Circuit of
Figure 12
. 1.193/1.180 V(min)
1.267/1.280 V(max)
η Efficiency V
IN
=
12V, V
OUT
=
3V, I
LOAD
=
1A 78
%
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
=
200 mA
Symbol Parameter Conditions LM2598-XX Units
(Limits)
Typ Limit
(Note 4) (Note 5)
DEVICE PARAMETERS
I
b
Feedback Bias Current Adjustable Version Only, V
FB
=
1.3V 10 nA 50/100 nA(max)
f
O
Oscillator Frequency (Note 7) 150 kHz
127/110 kHz(min) 173/173 kHz(max)
V
SAT
Saturation Voltage I
OUT
=
1A (Note 8) (Note 9) 1 V
1.2/1.3 V(max)
DC Max Duty Cycle (ON) (Note 9) 100
%
Min Duty Cycle (OFF) (Note 10) 0
I
CL
Current Limit Peak Current, (Note 8) (Note 9) 1.5 A
1.2/1.15 A(min)
2.4/2.6 A(max)
I
L
Output Leakage Current Output=0V (Note 9) (Note 10) (Note 11) 50 µA(max)
Output=−1V 2 mA
15 mA(max)
I
Q
Operating Quiescent SD /SS Pin Open, (Note 10) 5mA Current 10 mA(max)
I
STBY
Standby Quiescent SD /SS pin=0V, (Note 11) 85 µA Current 200/250 µA(max)
θ
JC
Thermal Resistance TO220 or TO263 Package, Junction to Case 2 ˚C/W
θ
JA
TO220 Package, Junction to Ambient (Note 12) 50 ˚C/W
θ
JA
TO263 Package, Junction to Ambient (Note 13) 50 ˚C/W
θ
JA
TO263 Package, Junction to Ambient (Note 14) 30 ˚C/W
θ
JA
TO263 Package, Junction to Ambient (Note 15) 20 ˚C/W
SHUTDOWN/SOFT-START CONTROL Test Circuit of
Figure 1
V
SD
Shutdown Threshold 1.3 V Voltage Low, (Shutdown Mode) 0.6 V(max)
High, (Soft-start Mode) 2 V(min)
V
SS
Soft-start Voltage V
OUT
=
20%of Nominal Output Voltage 2 V
V
OUT
=
100%of Nominal Output Voltage 3
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Page 5
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
=
200 mA
Symbol Parameter Conditions LM2598-XX Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SHUTDOWN/SOFT-START CONTROL Test Circuit of
Figure 1
I
SD
Shutdown Current V
SHUTDOWN
=
0.5V
5µA
10 µA(max)
I
SS
Soft-start Current V
Soft-start
=
2.5V 1.6 µA 5 µA(max)
FLAG/DELAY CONTROL Test Circuit of
Figure 1
Regulator Dropout Detector Low (Flag ON) 96
%
Threshold Voltage 92
%
(min)
98
%
(max)
VF
SAT
Flag Output Saturation I
SINK
=
3 mA 0.3 V
Voltage V
DELAY
=
0.5V 0.7/1.0 V(max)
IF
L
Flag Output Leakage Current
V
FLAG
=
40V 0.3 µA
Delay Pin Threshold 1.25 V Voltage Low (Flag ON) 1.21 V(min)
High (Flag OFF) and V
OUT
Regulated 1.29 V(max)
Delay Pin Source Current V
DELAY
=
0.5V 3 µA 6 µA(max)
Delay Pin Saturation Low (Flag ON) 55 mV
350/400 mV(max)
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is in­tended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: Voltage internally clamped. If clamp voltage is exceeded, limit current to a maximum of 1 mA. Note 3: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin. Note 4: Typical numbers are at 25˚C and represent the most likely norm. Note 5: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%pro-
duction tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to cal­culate Average Outgoing Quality Level (AOQL).
Note 6: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2598 is used as shown in the
Figure 1
test circuit, system performance will be as shown in system parameters section of Electrical Characteristics.
Note 7: The switching frequency is reduced when the second stage current limit is activated. The amount of reduction is determined by the severity of current over­load.
Note 8: No diode, inductor or capacitor connected to output pin. Note 9: Feedback pin removed from output and connected to 0V to force the output transistor switch ON. Note 10: Feedback pin removed from output and connected to 12V for the 3.3V, 5V,andtheADJ. version, and 15V for the 12V version, to force the output transistor
switch OFF. Note 11: V
IN
=
40V.
Note 12: 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
2
.
Note 13: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 0.5 in
2
of (1 oz.) copper area.
Note 14: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 2.5 in
2
of (1 oz.) copper area.
Note 15: Junction to ambient thermal resistance with the TO-263 package tab soldered to a double sided printed circuit board with 3 in
2
of (1 oz.) copper area on
the LM2598S side of the board, and approximately 16 in
2
of copper on the other side of the p-c board. See application hints in this data sheet and the thermal model
in
Switchers Made Simple
®
version 4.2 software.
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Page 6
Typical Performance Characteristics (Circuit of
Figure 1
)
Normalized Output Voltage
DS012593-2
Line Regulation
DS012593-3
Efficiency
DS012593-14
Switch Saturation Voltage
DS012593-15
Switch Current Limit
DS012593-16
Dropout Voltage
DS012593-17
Operating Quiescent Current
DS012593-4
Shutdown Quiescent Current
DS012593-5
Minimum Operating Supply Voltage
DS012593-6
Feedback Pin Bias Current
DS012593-49
Flag Saturation Voltage
DS012593-7
Switching Frequency
DS012593-8
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Page 7
Typical Performance Characteristics (Circuit of
Figure 1
) (Continued)
Typical Performance Characteristics (Circuit of
Figure 1
)
Soft-start
DS012593-9
Shutdown/Soft-start Current
DS012593-10
Delay Pin Current
DS012593-11
Soft-start Response
DS012593-12
Shutdown/Soft-start Threshold Voltage
DS012593-13
Continuous Mode Switching Waveforms V
IN
=
20V, V
OUT
=
5V, I
LOAD
=
1A
L=68 µH, C
OUT
=
120 µF, C
OUT
ESR=100 mΩ
DS012593-18
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.
Discontinuous Mode Switching Waveforms V
IN
=
20V, V
OUT
=
5V, I
LOAD
=
600 mA
L=22 µH, C
OUT
=
220 µF, C
OUT
ESR=50 mΩ
DS012593-19
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.
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Page 8
Typical Performance Characteristics (Circuit of
Figure 1
) (Continued)
Load Transient Response for Continuous Mode V
IN
=
20V, V
OUT
=
5V, I
LOAD
=
250 mA to 750 mA
L=68 µH, C
OUT
=
120 µF, C
OUT
ESR=100 mΩ
DS012593-20
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
IN
=
20V, V
OUT
=
5V, I
LOAD
=
250 mA to 750 mA
L=22 µH, C
OUT
=
220 µF, C
OUT
ESR=50 mΩ
DS012593-21
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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Page 9
Test Circuit and Layout Guidelines
Fixed Output Voltage Versions
DS012593-23
Component Values shown are for V
IN
=
15V, V
OUT
=
5V, I
LOAD
=
1A. 120 µF, 50V, Aluminum Electrolytic Nichicon “PL Series” 120 µF, 35V Aluminum Electrolytic, Nichicon “PL Series” 3A, 40V Schottky Rectifier, 1N5822 68 µH, L30
Typical Values
*CSS: — 0.1 µF C
DELAY
: — 0.1 µF
R
Pull Up
: — 4.7k
Adjustable Output Voltage Versions
DS012593-24
where V
REF
=
1.23V
Select R1to be approximately 1kΩ,usea1%resistor for best stability. Component Values shown are for V
IN
=
20V,
V
OUT
=
10V, I
LOAD
=
1A.
C
IN
— 120 µF, 35V,Aluminum Electrolytic Nichicon “PL Series”
C
OUT
— 120 µF, 35V Aluminum Electrolytic, Nichicon “PL Series” D1 — 3A, 40V Schottky Rectifier, 1N5822 L1 — 100 µH, L29 R
1
—1kΩ,1
%
R
2
— 7.15k, 1
%
C
FF
— 3.3 nF, See Application Information Section
R
FF
—3kΩ, See Application Information Section
Typical Values
CSS— 0.1 µF C
DELAY
— 0.1 µF
R
PULL UP
— 4.7k
FIGURE 1. Standard Test Circuits and Layout Guides
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Page 10
Test Circuit and Layout Guidelines
(Continued)
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, 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 induc­tor.Allowing the inductor flux to intersect sensitive feedback, lC groundpath and C
OUT
wiring can cause problems.
When using the adjustable version, special care must be taken as to the location of the feedback resistors and the as­sociated wiring. Physically locate both resistors near the IC, and route the wiring away from the inductor, especially an open core type of inductor. (See application section for more information.)
LM2598 Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
Given:
Given:
V
OUT
=
Regulated Output Voltage (3.3V, 5V or 12V)
V
OUT
=
5V
V
IN
(max)=Maximum DC Input Voltage
V
IN
(max)=12V
I
LOAD
(max)=Maximum Load Current I
LOAD
(max)=1A
1. Inductor Selection (L1) 1. Inductor Selection (L1) A. Select the correct inductor value selection guide from Fig-
ures
Figure 4,Figure 5
,or
Figure 6
(Output voltages of 3.3V, 5V, or 12V respectively.) For all other voltages, see the de­sign procedure for the adjustable version.
A. Use the inductor selection guide for the 5V version shown in
Figure 5
.
B. From the inductor value selection guide, identify the induc­tance region intersected by the Maximum Input Voltage line and the Maximum Load Current line. Each region is identified by an inductance value and an inductor code (LXX).
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. Select an appropriate inductor from the four manufactur­er’s part numbers listed in
Figure 8
.
C. The inductance value required is 68 µH. From the table in
Figure 8
, go to the L30 line and choose an inductor part num­ber from any of the four manufacturers shown. (In most in­stance, both through hole and surface mount inductors are available.)
2. Output Capacitor Selection (C
OUT
) 2. Output Capacitor Selection (C
OUT
)
A. In the majority of applications, low ESR (Equivalent Series
Resistance) electrolytic capacitors between 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 capaci­tors in application information section.
A. See section on output capacitors in application infor­mation section.
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Page 11
LM2598 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
B. To simplify the capacitor selection procedure, refer to the
quick design component selection table shown in
Figure 2
. This table contains different input voltages, output voltages, and load currents, and lists various inductors and output ca­pacitors that will provide the best design solutions.
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 capaci­tor manufacturers. It is recommended that both the manufac­turers and the manufacturer’s series that are listed in the table be used.
In this example aluminum electrolytic capacitors from several different manufacturers are available with the range of ESR numbers needed.
220 µF 25V Panasonic HFQ Series 220 µF 25V Nichicon PL Series
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 .
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 mΩ of ESR (see the curve in
Figure 16
for the ESR vs voltage rating). This amount of ESR would result in relatively high output ripple voltage. To reduce the ripple to 1%of the output voltage, or less, a ca­pacitor with a higher voltage rating (lower ESR) should be se­lected. A 16V or 25V capacitor will reduce the ripple voltage by approximately half.
D. For computer aided design software, see
Switchers Made
Simple
™
(version 4.2 or later).
3. Catch Diode Selection (D1) 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 LM2598. The most stressful condition for this diode is an overload or shorted output condition.
A. Refer to the table shown in
Figure 11
. In this example, a 3A, 20V,1N5820 Schottky diode will provide the best perfor­mance, and will not be overstressed even for a shorted out­put.
B. The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage. C. This diode must be fast (short reverse recovery time) and
must be located close to the LM2598 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 1N5400 series are much too slow and should not be used.
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Page 12
LM2598 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
4. Input Capacitor (C
IN
)
Alow 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
1
⁄2the DC load current. The capacitor manufacturers data sheet must be checked to assure that this current rating is not exceeded. The curve shown in
Figure 15
shows typical RMS current ratings for several different aluminum electro­lytic capacitor values.
This capacitor should be located close to the IC using short leads and the voltage rating should be approximately 1.5 times the maximum input voltage.
If solid tantalum input capacitors are used, it is recomended that they be surge current tested by the manufacturer.
Use caution when using ceramic capacitors for input bypass­ing, because it may cause severe ringing at the V
IN
pin.
For additional information, see section on input capaci­tors in Application Information section.
4. Input Capacitor (C
IN
)
The important parameters for the Input capacitor are the in­put voltage rating and the RMS current rating. With a nominal input voltage of 12V, an aluminum electrolytic capacitor with a voltage rating greater than 18V (1.5 x V
IN
) 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
1
⁄2the DC load current. In this example, with a 1A load, a capacitor with a RMS current rating of at least 500 mAis needed. The curves shown in
Fig-
ure 15
can be used to select an appropriate input capacitor. From the curves, locate the 25V line and note which capaci­tor values have RMS current ratings greater than 500 mA. Ei­ther a 180 µF or 220 µF, 25V capacitor could be used.
For a through hole design, a 220 µF/25V electrolytic capaci­tor (Panasonic HFQ series or Nichicon PL series or equiva­lent) would be adequate. other types or other manufacturers capacitors can be used provided the RMS ripple current rat­ings are adequate.
For surface mount designs, solid tantalum capacitors are recommended. The TPS series available from AVX, and the 593D series from Sprague are both surge current tested.
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) (µF/V) (µF/V) (µF/V)
3.3 1 5 22 L24 330/16 330/16 220/10 330/10
7 33 L23 270/25 270/25 220/10 270/10 10 47 L31 220/25 220/35 220/10 220/10 40 68 L30 180/35 220/35 220/10 180/10
6 47 L13 220/25 220/16 220/16 220/10
0.5 10 68 L21 150/35 150/25 100/16 150/16 40 100 L20 150/35 82/35 100/16 100/20
5 1 8 33 L28 330/16 330/16 220/10 270/10
10 47 L31 220/25 220/25 220/10 220/10 15 68 L30 180/35 180/35 220/10 150/16 40 100 L29 180/35 120/35 100/16 120/16
9 68 L21 180/16 180/16 220/10 150/16
0.5 20 150 L19 120/25 120/25 100/16 100/20 40 150 L19 100/25 100/25 68/20 68/25
12 1 15 47 L31 220/25 220/25 68/20 120/20
18 68 L30 180/35 120/25 68/20 120/20 30 150 L36 82/25 82/25 68/20 100/20 40 220 L35 82/25 82/25 68/20 68/25 15 68 L21 180/25 180/25 68/20 120/20
0.5 20 150 L19 82/25 82/25 68/20 100/20 40 330 L26 56/25 56/25 68/20 68/25
FIGURE 2. LM2598 Fixed Voltage Quick Design Component Selection Table
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LM2598 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version) Given:
V
OUT
=
Regulated Output Voltage
V
IN
(max)=Maximum Input Voltage
I
LOAD
(max)=Maximum Load Current
F=Switching Frequency
(Fixed at a nominal 150 kHz).
Given:
V
OUT
=
20V
V
IN
(max)=28V
I
LOAD
(max)=1A
F=Switching Frequency
(Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R1and R2,as shown in
Figure 1
)
Use the following formula to select the appropriate resistor values.
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.)
1. Programming Output Voltage (Selecting R1and R2,as shown in
Figure 1
)
Select R
1
to be 1 kΩ,1%. Solve for R2.
R
2
=
1k (16.26 − 1)=15.26k, closest 1%value is 15.4 kΩ.
R
2
=
15.4 kΩ.
2. Inductor Selection (L1) A. Calculate the inductor Volt
•
microsecond constant E•T
(V
•
µs), from the following formula:
where V
SAT
=
internal switch saturation voltage=1V
and V
D
=
diode forward voltage drop=0.5V
2. Inductor Selection (L1) A. Calculate the inductor Volt
•
microsecond constant
(E
•
T),
B. Use the E•T value from the previous formula and match it with the E
•
T number on the vertical axis of the Inductor
Value Selection Guide shown in
Figure 7
.
B. E
•
T=34.8 (V•µs)
C. on the horizontal axis, select the maximum load current. C. I
LOAD
(max)=1A
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).
D. From the inductor value selection guide shown in
Figure 7
,
the inductance region intersected by the 35 (V
•
µs) horizon­tal line and the 1A vertical line is 100 µH, and the inductor code is L29.
E. Select an appropriate inductor from the four manufactur­er’s part numbers listed in
Figure 8
.
E. From the table in
Figure 8
, locate line L29, and select an inductor part number from the list of manufacturers part num­bers.
3. Output Capacitor Selection (C
OUT
) 3. Output Capacitor SeIection (C
OUT
)
A. In the majority of applications, low ESR electrolytic or solid
tantalum capacitors between 82 µF and 220 µF provide the best results. This capacitor should be located close to the IC using short capacitor leads and short copper traces. Do not use capacitors larger than 220 µF. For additional informa-
tion, see section on output capacitors in application in­formation section.
A. See section on C
OUT
in Application Information section.
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LM2598 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
B. To simplify the capacitor selection procedure, refer to the
quick design table shown in
Figure 3
. This table contains dif­ferent output voltages, and lists various output capacitors that will provide the best design solutions.
B. From the quick design table shown in
Figure 3
, locate the output voltage column. From that column, locate the output voltage closest to the output voltage in your application. In this example, select the 24V line. Under the output capacitor section, select a capacitor from the list of through hole elec­trolytic or surface mount tantalum types from four different capacitor manufacturers. It is recommended that both the manufacturers and the manufacturers series that are listed in the table be used.
In this example, through hole aluminum electrolytic capaci­tors from several different manufacturers are available.
82 µF 35V Panasonic HFQ Series 82 µF 35V Nichicon PL Series
C. The capacitor voltage rating should be at least 1.5 times greater than the output voltage, and often much higher volt­age ratings are needed to satisfy the low ESR requirements needed for low output ripple voltage.
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 35V rating was chosen although a 50V rat­ing 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. Re­fer to the capacitor manufacturers data sheet for this informa­tion.
4. Feedforward Capacitor (C
FF
) (See
Figure 1
)
For output voltages greater than approximately 10V, an addi­tional capacitor is required. The compensation capacitor is typically between 50 pF and 10 nF, and is wired in parallel with the output voltage setting resistor, R
2
. It provides addi­tional stability for high output voltages, low input-output volt­ages, and/or very low ESR output capacitors, such as solid tantalum capacitors.
This capacitor type can be ceramic, plastic, silver mica, etc. (Because of the unstable characteristics of ceramic capaci­tors made with Z5U material, they are not recommended.)
4. Feedforward Capacitor (C
FF
)
The table shown in
Figure 3
contains feed forward capacitor values for various output voltages. In this example,a1nF capacitor is needed.
5. Catch Diode Selection (D1) A. The catch diode current rating must be at least 1.3 times
greater than the maximum load current. Also, if the power supply design must withstand a continuous output short, the diode should have a current rating equal to the maximum current limit of the LM2598. 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 LM2598 using short leads and short printed circuit traces. Because of their fast switching speed and low forward voltage drop, Schottky diodes provide the best performance and efficiency, and should be the first choice, especially in low output voltage applications. Ultra-fast recovery, or High-Efficiency rectifiers are also a good choice, but some types with an abrupt turn-off charac­teristic may cause instability or EMl problems. Ultra-fast re­covery diodes typically have reverse recovery times of 50 ns or less. Rectifiers such as the 1N4001 series are much too slow and should not be used.
5. Catch Diode Selection (D1) A. Refer to the table shown in
Figure 11
. Schottky diodes provide the best performance, and in this example a 3A, 40V, 1N5822 Schottky diode would be a good choice. The 3A di­ode rating is more than adequate and will not be over­stressed even for a shorted output.
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LM2598 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
6. Input Capacitor (C
IN
)
Alow 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
1
⁄2the DC load current. The capacitor manufacturers data sheet must be checked to assure that this current rating is not exceeded. The curve shown in
Figure 15
shows typical RMS current ratings for several different aluminum electro­lytic capacitor values.
This capacitor should be located close to the IC using short leads and the voltage rating should be approximately 1.5 times the maximum input voltage.
If solid tantalum input capacitors are used, it is recomended that they be surge current tested by the manufacturer.
Use caution when using a high dielectric constant ceramic capacitor for input bypassing, because it may cause severe ringing at the V
IN
pin.
6. Input Capacitor (C
IN
)
The important parameters for the Input capacitor are the in­put voltage rating and the RMS current rating. With a nominal input voltage of 28V,an aluminum electrolytic aluminum elec­trolytic capacitor with a voltage rating greater than 42V (1.5 x V
IN
) 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
IN
) 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
1
⁄2the 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 15
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 500 mA. Either a 100 µF or 120 µF,50V ca­pacitor could be used.
For a through hole design, a 120 µF/50V electrolytic capaci­tor (Panasonic HFQ series or Nichicon PL series or equiva­lent) would be adequate. Other types or other manufacturers capacitors can be used provided the RMS ripple current rat­ings are adequate.
For surface mount designs, solid tantalum capacitors can be used, but caution must be exercised with regard to the ca­pacitor surge current rating (see Application Information or input capacitors in this data sheet). The TPS series available from AVX, and the 593D series from Sprague are both surge current tested.
For additional information, see section on input capaci­tor in application information section.
To further simplify the buck regulator design procedure, Na­tional Semiconductor is making available computer design software to be used with the Simple Switcher line ot switch­ing regulators. Switchers Made Simple (version 4.2 or later) is available on a 3
1
⁄2" diskette for IBM compatible computers.
Output
Voltage
(V)
Through Hole Electrolytic Output Capacitor Surface Mount Tantalum Output Capacitor
Panasonic Nichicon PL Feedforward AVX TPS Sprague Feedforward
HFQ Series Series Capacitor Series 595D Series Capacitor
(µF/V) (µF/V) (µF/V) (µF/V)
1.2 330/50 330/50 0 330/6.3 330/6.3 0 4 220/25 220/25 4.7 nF 220/10 220/10 4.7 nF 6 220/25 220/25 3.3 nF 220/10 220/10 3.3 nF 9 180/25 180/25 1.5 nF 100/16 180/16 1.5 nF
12 120/25 120/25 1.5 nF 68/20 120/20 1.5 nF 15 120/25 120/25 1.5 nF 68/20 100/20 1.5 nF 24 82/35 82/35 1 nF 33/25 33/35 220 pF 28 82/50 82/50 1 nF 10/35 33/35 220 pF
FIGURE 3. Output Capacitor and Feedforward Capacitor Selection Table
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LM2598 Series Buck Regulator Design Procedure
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)
DS012593-25
FIGURE 4. LM2598-3.3
DS012593-26
FIGURE 5. LM2598-5.0
DS012593-27
FIGURE 6. LM2598-12
DS012593-28
FIGURE 7. LM2598-ADJ
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Page 17
LM2598 Series Buck Regulator Design Procedure (Continued)
Inductance
(µH)
Current
(A)
Schott Renco Pulse Engineering Coilcraft
Through Surface Through Surface Through Surface Surface
Hole Mount Hole Mount Hole Mount Mount
L4 68 0.32 67143940 67144310 RL-1284-68-43 RL1500-68 PE-53804 PE-53804-S DO1608-68 L5 47 0.37 67148310 67148420 RL-1284-47-43 RL1500-47 PE-53805 PE-53805-S DO1608-473 L6 33 0.44 67148320 67148430 RL-1284-33-43 RL1500-33 PE-53806 PE-53806-S DO1608-333 L9 220 0.32 67143960 67144330 RL-5470-3 RL1500-220 PE-53809 PE-53809-S DO3308-224 L10 150 0.39 67143970 67144340 RL-5470-4 RL1500-150 PE-53810 PE-53810-S DO3308-154 L11 100 0.48 67143980 67144350 RL-5470-5 RL1500-100 PE-53811 PE-53811-S DO3308-104 L12 68 0.58 67143990 67144360 RL-5470-6 RL1500-68 PE-53812 PE-53812-S DO3308-683 L13 47 0.70 67144000 67144380 RL-5470-7 RL1500-47 PE-53813 PE-53813-S DO3308-473 L14 33 0.83 67148340 67148450 RL-1284-33-43 RL1500-33 PE-53814 PE-53814-S DO3308-333 L15 22 0.99 67148350 67148460 RL-1284-22-43 RL1500-22 PE-53815 PE-53815-S DO3308-223 L16 15 1.24 67148360 67148470 RL-1284-15-43 RL1500-15 PE-53816 PE-53816-S DO3308-153 L17 330 0.42 67144030 67144410 RL-5471-1 RL1500-330 PE-53817 PE-53817-S DO3316-334 L18 220 0.55 67144040 67144420 RL-5471-2 RL1500-220 PE-53818 PE-53818-S DO3316-224 L19 150 0.66 67144050 67144430 RL-5471-3 RL1500-150 PE-53819 PE-53819-S DO3316-154 L20 100 0.82 67144060 67144440 RL-5471-4 RL1500-100 PE-53820 PE-53820-S DO3316-104 L21 68 0.99 67144070 67144450 RL-5471-5 RL1500-68 PE-53821 PE-53821-S DO3316-683 L22 47 1.17 67144080 67144460 RL-5471-6 — PE-53822 PE-53822-S DO3316-473 L23 33 1.40 67144090 67144470 RL-5471-7 — PE-53823 PE-53823-S DO3316-333 L24 22 1.70 67148370 67144480 RL-1283-22-43 — PE-53824 PE-53824-S DO3316-223 L26 330 0.80 67144100 67144480 RL-5471-1 — PE-53826 PE-53826-S DO5022P-334 L27 220 1.00 67144110 67144490 RL-5471-2 — PE-53827 PE-53827-S DO5022P-224 L28 150 1.20 67144120 67144500 RL-5471-3 — PE-53828 PE-53828-S DO5022P-154 L29 100 1.47 67144130 67144510 RL-5471-4 — PE-53829 PE-53829-S DO5022P-104 L30 68 1.78 67144140 67144520 RL-5471-5 — PE-53830 PE-53830-S DO5022P-683 L35 47 2.15 67144170 — RL-5473-1 — PE-53935 PE-53935-S —
FIGURE 8. Inductor Manufacturers Part Numbers
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Page 18
LM2598 Series Buck Regulator Design Procedure (Continued)
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
Schott Corp. Phone (612) 475-1173
FAX (612) 475-1786
FIGURE 9. Inductor Manufacturers Phone Numbers
Nichicon Corp. Phone (708) 843-7500
FAX (708) 843-2798
Panasonic Phone (714) 373-7857
FAX (714) 373-7102
AVX Corp. Phone (803) 448-9411
FAX (803) 448-1943
Sprague/Vishay Phone (207) 324-4140
FAX (207) 324-7223
FIGURE 10. Capacitor Manufacturers Phone Numbers
VR 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 Recovery Recovery Recovery
20V SK12 All of these 1N5817 All of these All of these IN5820 All of these
diodes are SR102 diodes are SK32 diodes are SR302 diodes are rated to at rated to at rated to at MBR320 rated to at
30V SK13 least 50V. 1N5818 least 50V. least 50V. 1N5821 least 50V.
MBRS130 SR103 SK33 MBR330
11DQ03 31DQ03
40V SK14 1N5822
MBRS140 1N5819 SK34 SR304
10BQ040 SR104 MBRS340 MBR340 10MQ040 MURS120 11DQ04 MUR120 30WQ04 MURS320 31DQ04 MUR320
50V
or
more
MBRS160 10BF10 SR105 SK35 30WF10 SR305 30WF10
10BQ050 MBR150 MBRS360 MBR350 10MQ060 11DQ05 30WQ05 31DQ05
FIGURE 11. Diode Selection Table
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Page 19
Block Diagram
Application Information
PIN FUNCTIONS +V
IN
(Pin 2)— 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 4) — Circuit ground. Output (Pin 1)—Internal switch. The voltage at this pin
switches between approximately (+V
IN−VSAT
) and approxi-
mately −0.5V, with a duty cycle of V
OUT/VIN
. To minimize coupling to sensitive circuitry, the PC board copper area con­nected to this pin should be kept to a minimum.
Feedback (Pin 6)— Senses the regulated output voltage to complete the feedback loop.
Shutdown /Soft-start (Pin 7)—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 85 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 3)— Open collector output that provides a low signal (flag transistor ON) when the regulated output voltage drops more than 5%from the nominal output 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 5) — At power-up, this pin can be used to provide a time delay between the time the regulated output voltage reaches 95%of the nominal output voltage, and the time the error flag output goes high.
Special Note If any of the above three features (Shutdown /Soft-start, Error Flag, or Delay) are not used, the respective pins should be left open.
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 13
).
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 regulator 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 1) 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 13
. The input
supply current requirement also starts out at a low level for
DS012593-29
FIGURE 12.
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Page 20
Application Information (Continued)
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.
Note: The lower curve shown in
Figure 13
shows the Soft-start region from 0%to 100%. This is not the duty cycle percentage, but the output volt­age percentage. Also, the Soft-start voltage range has a negative tem­perature coefficient associated with it. See the Soft-start curve in the electrical characteristics section.
4. Normal operation.
Above 2.8V, the circuit operates as a standard Pulse Width Modulated switching regulator. The capacitor will continue to charge up until it reaches the inter­nal clamp voltage of approximately 7V. If this pin is driven from a voltage source, the current must be limited to about 1 mA.
DS012593-30
FIGURE 13. Soft-start, Delay, Error, Output
DS012593-31
FIGURE 14. Timing Diagram for 5V Output
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Page 21
Application Information (Continued)
DELAY CAPACITOR C
DELAY
—Provides delay for the error flag output. See the
upper curve in
Figure 13
, and also refer to timing diagrams in
Figure 14
. A capacitor on this pin provides a time delay 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 3) immediately goes low (internal transistor turns on). The delay capacitor provides very little delay if the regulated output is dropping out of regulation. The delay time for an output that is decreasing is approximately a 1000 times less than the delay for the rising output. For a 0.1 µF delay capacitor, the delay time would be approximately 50 ms when the output is rising and passes through the 95%threshold, but the delay for the output drop­ping would only be approximately 50 µs.
R
Pull Up
—The error flag output, (or power OK) is the collec­tor of a NPN transistor, with the emitter internally grounded. To use the error flag, a pullup resistor to a positive voltage is needed. The error flag transistor is rated up to a maximum of 45V and can sink approximately 3 mA. If the error flag is not used, it can be left open.
FEEDFORWARD CAPACITOR
(Adjustable Output Voltage Version)
C
FF
— A Feedforward Capacitor CFF, shown across R2 in
Figure 1
is used when the output voltage is greater than 10V or then 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
FF
selection, see the de-
sign procedure section. If the output ripple is large (
>
5%of the nominal output volt­age), this ripple can be coupled to the feedback pin through the feedforward capacitor and cause the error comparator to trigger the error flag. In this situation, adding a resistor, R
FF
, in series with the feedforward capacitor, approximately 3 times R1, will attenuate the ripple voltage at the feedback pin.
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.
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.
DS012593-32
FIGURE 15. RMS Current Ratings for Low
ESR Electrolytic Capacitors (Typical)
DS012593-33
FIGURE 16. Capacitor ESR vs Capacitor Voltage Rating
(Typical Low ESR Electrolytic Capacitor)
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Application Information (Continued)
A graph shown in
Figure 15
shows the relationship between an electrolytic capacitor value, its voltage rating, and the RMS current it is rated for. These curves were obtained from the Nichicon “PL” series of low ESR, high reliability 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.
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.
An aluminum electrolytic capacitor’s ESR value is related to the capacitance value and its voltage rating. In most cases, higher voltage electrolytic capacitors have lower ESR values (see
Figure 16
). Often, capacitors with much higher voltage ratings may be needed to provide the low ESR values 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
Figure 2
and
Figure 3
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 17
.
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 LM2598 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 1N5400 series are much too slow and should not be used.
INDUCTOR SELECTION
The LM2598 (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 3
through
Figure 6
). 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 18
.)
DS012593-34
FIGURE 17. Capacitor ESR Change vs Temperature
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Page 23
Application Information (Continued)
By allowing the percentage of inductor ripple current to in­crease for low load currents, the inductor value and size can be kept relatively low.
When operating in the continuous mode, the inductor current waveform ranges from a triangular to a sawtooth type of waveform (depending on the input voltage), with the average value of this current waveform equal to the DC output load current.
Inductors are available in different styles such as pot core, toroid, E-core, bobbin core, etc., as well as different core ma­terials, such as ferrites and powdered iron. The least expen­sive, the bobbin, rod or stick core, consists of wire wound on a ferrite bobbin. This type of construction makes for an 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 section on Open Core Inductors.
When multiple switching regulators are located on the same PC board, open core magnetics can cause interference be­tween two or more of the regulator circuits, especially at high currents.A torroid or E-core inductor (closed magnetic struc­ture) 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 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 LM2598. 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.
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 TypicalPerfomance Characteristics 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.
Switchers Made Simple
(ver­sion 4.2) 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 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 20 mV), a post ripple filter is recom-
DS012593-35
FIGURE 18. (∆I
IND
) Peak-to-Peak Inductor
Ripple Current (as a Percentage of the
Load Current) vs Load Current
DS012593-36
FIGURE 19. Post Ripple Filter Waveform
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Page 24
Application Information (Continued)
mended. (See
Figure 1
.) The inductance required is typically between 1 µH and 5 µH, with low DC resistance, to maintain good load regulation.Alow 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 19
shows a typical output ripple volt-
age, with and without a post ripple filter. When observing output ripple with a scope, it is essential
that a short, low inductance scope probe ground connection be used. Most scope probe manufacturers provide a special probe terminator which is soldered onto the regulator board, preferable at the output capacitor. This provides a very short scope ground thus eliminating the problems associated with the 3 inch ground lead normally provided with the probe, and provides a much cleaner and more accurate picture of the ripple voltage waveform.
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.
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 (∆I
IND
) can be useful for determining a number of other circuit parameters. Param­eters such as, peak inductor or peak switch current, mini-
mum load current before the circuit becomes discontinuous, output ripple voltage and output capacitor ESR can all be calculated from the peak-to-peak ∆I
IND
. When the inductor
nomographs shown in
Figure 4
through
Figure 7
are used to select an inductor value, the peak-to-peak inductor ripple current can immediately be determined. The curve shown in
Figure 20
shows the range of (∆I
IND
) that can be expected for different load currents. The curve also shows how the peak-to-peak inductor ripple current (∆I
IND
) changes as you go from the lower border to the upper border (for a given load current) within an inductance region. The upper border rep­resents a higher input voltage, while the lower border repre­sents a lower input voltage (see Inductor Selection Guides).
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 800 mA
V
IN
=
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 in­put voltage intersect approximately midway between the up­per and lower borders of the 68 µH inductance region. A 68 µH inductor will allow a peak-to-peak inductor current (∆I
IND
) to flow that will be a percentage of the maximum load cur­rent. Referring to
Figure 20
, follow the 0.8A line approxi­mately midway into the inductance region, and read the peak-to-peak inductor ripple current (∆I
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 20
,itcan be seen that for a load current of 0.8A, the peak-to-peak in­ductor ripple current (∆I
IND
) is 300 mA with 12V in, and can range from 340 mAat the upper border (14V in) to 225 mAat the lower border (10V in).
Once the ∆I
IND
value is known, the following formulas can be used to calculate additional information about the switching regulator circuit.
1. Peak Inductor or peak switch current
2. Minimum load current before the circuit becomes dis-
continuous
3. Output Ripple Voltage=(∆I
IND
) x (ESR of C
OUT
)
=
0.3A x 0.16Ω=48 mV p-p
4. ESR of C
OUT
OPEN CORE INDUCTORS
Another possible source of increased output ripple voltage or unstable operation is from an open core inductor. Ferrite bobbin or stick inductors have magnetic lines of flux flowing through the air from one end of the bobbin to the other end.
DS012593-37
FIGURE 20. Peak-to-Peak Inductor
Ripple Current vs Load Current
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Page 25
Application Information (Continued)
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 in­ductor 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.
THERMAL CONSIDERATIONS
The LM2598 is available in two packages, a 7-pin TO-220 (T) and a 7-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
21
show the LM2598T junction temperature rises above am­bient temperature for different input and output voltages. The data for these curves was taken with the LM2598T (TO-220 package) operating as a switching regulator in an ambient temperature of 25˚C (still air). These temperature rise num­bers are all approximate and there are many factors that can affect these temperatures. Higher ambient temperatures re­quire 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
2
, and ideally should
have 2 or more square inches of 2 oz. (0.0028) in) copper.
DS012593-38
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 21. Junction Temperature Rise, TO-220
DS012593-39
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 22. Junction Temperature Rise, TO-263
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Page 26
Application Information (Continued)
Additional copper area improves the thermal characteristics, but with copper areas greater than approximately 3 in
2
, only small improvements in heat dissipation are realized. If fur­ther thermal improvements are needed, double sided or mul­tilayer PC-board with large copper areas are recommended.
The curves shown in
Figure 22
show the LM2598S (TO-263 package) junction temperature rise above ambient tempera­ture 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 junc­tion 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 surround­ing air is still or moving. 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.
SHUTDOWN /SOFT-START
The circuit shown in
Figure 25
is a standard buck regulator with 24V in, 12V out, 280 mA load, and using a 0.068 µF Soft-start capacitor. The photo in
Figure 23
and
Figure 24
Figure 23
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 1A down to 240 mA, and delays and slows down the output voltage rise time.
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.
DS012593-40
FIGURE 23. Output Voltage, Input Current, Error Flag
Signal, at Start-Up, WITH Soft-start
DS012593-41
FIGURE 24. Output Voltage, Input Current, at Start-Up,
WITHOUT Soft-start
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Page 27
Application Information (Continued)
lNVERTING REGULATOR
The circuit in
Figure 26
converts a positive input voltage to a negative output voltage with a common ground. The circuit operates by bootstrapping the 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 LM2598-5 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 27
provides a guide as to the amount of output load current possible for the different input and output voltage conditions.
The maximum voltage appearing across the regulator is the absolute sum of the input and output voltage, and this must be limited to a maximum of 40V. In this example, when con­verting +20V to −5V, the regulator would see 25V between the input pin and ground pin. The LM2598 has a maximum input voltage rating of 40V.
DS012593-42
FIGURE 25. Typical Circuit Using Shutdown /Soft-start and Error Flag Features
DS012593-43
FIGURE 26. Inverting −5V Regulator With Shutdown and Soft-start
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Page 28
Application Information (Continued)
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
IN
capacitor 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 1N5400 diode could be used.
Figure 26
will provide good results in the majority of invert-
ing designs. This type of inverting regulator can require relatively large
amounts of input current when starting up, even with light loads. Input currents as high as the LM2598 current limit (ap­proximately 1.5A) are needed for 2 ms or more, until the out­put reaches its nominal output voltage. The actual time de­pends on the output voltage and the size of the output capacitor. Input power sources that are current limited or sources that can not deliver these currents without getting loaded down, may not work correctly. Because of the rela­tively high startup currents required by the inverting topology, the Soft-start feature shown in
Figure 26
is recommended.
Also shown in
Figure 26
are several shutdown methods for the inverting configuration. With the inverting configuration, some level shifting is required, because the ground pin of the regulator is no longer at ground, but is now at the negative output voltage. The shutdown methods shown accept ground referenced shutdown signals.
UNDERVOLTAGE LOCKOUT
Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage.
Figure
28
contains a undervoltage lockout circuit for a buck configu-
ration, while
Figure 29
and
Figure 30
are for the inverting types (only the circuitry pertaining to the undervoltage lock­out is shown).
Figure 28
uses a zener diode to establish the threshold voltage when the switcher begins operating. When the input voltage is less than the zener voltage, resistors R1 and R2 hold the Shutdown /Soft-start pin low, keeping the regulator in the shutdown mode. As the input voltage 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.
Figure 29
and
Figure 30
apply the same feature to an invert-
ing circuit.
Figure 29
features a constant threshold voltage for turn on and turn off (zener voltage plus approximately one volt). 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. If hysteresis is needed, the circuit in
Figure 30
has a turn ON voltage which is different than the turn OFF voltage. The amount of hysteresis is ap­proximately equal to the value of the output voltage.
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
Fig-
ure 31
. This unregulated negative voltage is approximately equal to the positive input voltage (minus a few volts), and can supply up to a 200 mA of output current. There is a re­quirement however, that there be a minimum load of several
DS012593-44
FIGURE 27. Maximum Load Current for Inverting
Regulator Circuit
DS012593-45
FIGURE 28. Undervoltage Lockout for a Buck
Regulator
DS012593-47
FIGURE 29. Undervoltage Lockout Without
Hysteresis for an Inverting Regulator
DS012593-46
FIGURE 30. Undervoltage Lockout With
Hysteresis for an Inverting Regulator
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Page 29
Application Information (Continued)
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 LM2598 current limit (typically 1.5A).
This method of generating a negative output voltage without an additional inductor can be used with other members of the Simple Switcher Family, using either the buck or boost topology.
DS012593-48
FIGURE 31. Charge Pump for Generating a
Low Current, Negative Output Voltage
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Page 30
Application Information (Continued)
TYPICAL THROUGH HOLE PC BOARD LAYOUT, FIXED OUTPUT (1X SIZE), DOUBLE SIDED, THROUGH HOLE
PLATED
DS012593-51
CIN— 150 µF/50V Aluminum Electrolytic, Panasonic “HFQ series” R
PULL-UP
—10 kΩ
C
OUT
— 120 µF/25V Aluminum Electrolytic, Panasonic “HFQ series” C
DELAY
— 0.1 µF
D1—3A, 40V Schottky Rectifier, 1N5822 C
SD/SS
— 0.1 µF
L1—68 µH, L30, Renco, Through hole
FIGURE 32. Fixed Output PC Board Layout
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Page 31
Application Information (Continued)
TYPICAL THROUGH HOLE PC BOARD LAYOUT, ADJUSTABLE OUTPUT (1X SIZE), DOUBLE SIDED,
THROUGH HOLE PLATED
DS012593-52
CIN— 150 µF/50V, Aluminum Electrolytic, Panasonic “HFQ series” CFF— See
Figure 4
.
C
OUT
— 120 µF/25V Aluminum Electrolytic, Panasonic “HFQ series” RFF— See Application Information Section (CFFSection)
D1—3A, 40V Schottky Rectifier, 1N5822 R
PULL-UP
—10 kΩ
L1—68 µH, L30, Renco, Through hole C
DELAY
— 0.1 µF
R1—1 kΩ,1
%
C
SD/SS
— 0.1 µF
R2—Use formula in Design Procedure
FIGURE 33. Adjustable Output PC Board Layout
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Page 32
Physical Dimensions inches (millimeters) unless otherwise noted
7-Lead TO-220 (T)
Order Number LM2598T-3.3, LM2598T-5.0,
LM2598T-12 or LM2598T-ADJ
NS Package Number TA07B
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Page 33
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.
National Semiconductor Corporation
Americas Tel: 1-800-272-9959 Fax: 1-800-737-7018 Email: support@nsc.com
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Fax: +49 (0) 1 80-530 85 86
Email: europe.support@nsc.com Deutsch Tel: +49 (0) 1 80-530 85 85 English Tel: +49 (0) 1 80-532 78 32 Français Tel: +49 (0) 1 80-532 93 58 Italiano Tel: +49 (0) 1 80-534 16 80
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Tel: 81-3-5639-7560 Fax: 81-3-5639-7507
www.national.com
7-Lead TO-263 Surface Mount Package (S)
Order Number LM2598S-3.3, LM2598S-5.0, LM2598S-12 or LM2598S-ADJ
NS Package Number TS7B
LM2598 SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator, with
Features
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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