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LM2598
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National Semiconductor LM2598 Technical data

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LM2598 SIMPLE SWITCHER
®
Power Converter 150 kHz
1A Step-Down Voltage Regulator, with Features
January 2001
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 additionalsupervisory and performance features added.
Requiring a minimum number of external components, these regulators are simple to use and include internal frequency compensation fixed-frequency oscillator, Shutdown /Soft-start, error flag delay and error flag output.
Other features include a guaranteed put voltage under all conditions of input voltage and output load conditions, and
†
, improved line and load specifications,
±
4% tolerance on out-
±
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 n High Efficiency n Uses readily available standard inductors n Thermal shutdown and current limit protection
typically 85 µA
Q
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)
DS012593-1
†
Patent Number 5,382,918.
SIMPLE SWITCHER
© 2001 National Semiconductor Corporation DS012593 www.national.com
®
and
Switchers Made Simple
®
are registered trademarks of National Semiconductor Corporation.
Connection Diagrams and Order Information
LM2598
Bent and Staggered Leads, Through Hole Package
7-Lead TO-220 (T)
Surface Mount Package
7-Lead TO-263 (S)
DS012593-50
Order Number LM2598T-3.3, LM2598T-5.0,
LM2598T-12 or LM2598T-ADJ
See NS Package Number TA07B
DS012593-22
Order Number LM2598S-3.3, LM2598S-5.0,
LM2598S-12 or LM2598S-ADJ
See NS Package Number TS7B
www.national.com 2
LM2598
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 SD/SS Pin Input Voltage (Note 2) 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
) 45V
IN
6V
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 TJ= 25˚C, and those with boldface type apply over full Operating Tempera­ture Range.
Symbol Parameter Conditions LM2598-3.3 Units
Typ Limit
(Note 4) (Note 5) SYSTEM PARAMETERS (Note 6) Test Circuit V
OUT
η Efficiency V
Output Voltage 4.75V ≤ VIN≤ 40V, 0.1A ≤ I
IN
= 12V, I
Figure 1
≤ 1A 3.3 V
LOAD
3.168/3.135 V(min)
3.432/3.465 V(max)
=1A 78 %
LOAD
(Limits)
LM2598-5.0 Electrical Characteristics
Specifications with standard type face are for TJ= 25˚C, and those with boldface type apply over full Operating Tempera­ture Range.
Symbol Parameter Conditions LM2598-5.0 Units
Typ Limit
(Note 4) (Note 5) SYSTEM PARAMETERS (Note 6) Test Circuit V
OUT
η Efficiency V
Output Voltage 7V ≤ VIN≤ 40V, 0.1A ≤ I
IN
= 12V, I
Figure 1
≤ 1A 5 V
LOAD
4.800/4.750 V(min)
5.200/5.250 V(max)
=1A 82 %
LOAD
(Limits)
LM2598-12 Electrical Characteristics
Specifications with standard type face are for TJ= 25˚C, and those with boldface type apply over full Operating Tempera­ture Range.
Symbol Parameter Conditions LM2598-12 Units
Typ Limit
(Note 4) (Note 5) SYSTEM PARAMETERS (Note 6) Test Circuit V
OUT
η Efficiency V
Output Voltage 15V ≤ VIN≤ 40V, 0.1A ≤ I
IN
= 25V, I
Figure 1
≤ 1A 12 V
LOAD
11.52/11.40 V(min)
12.48/12.60 V(max)
=1A 90 %
LOAD
(Limits)
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LM2598-ADJ Electrical Characteristics
LM2598
Specifications with standard type face are for TJ= 25˚C, and those with boldface type apply over full Operating Tempera­ture Range.
Symbol Parameter Conditions LM2598-ADJ Units
Typ Limit
(Note 4) (Note 5) SYSTEM PARAMETERS (Note 6) Test Circuit V
FB
Feedback Voltage 4.5V ≤ VIN≤ 40V, 0.1A ≤ I
V
OUT
Figure 1
≤ 1A 1.230 V
LOAD
programmed for 3V. Circuit of
Figure 12
. 1.193/1.180 V(min)
1.267/1.280 V(max)
η Efficiency V
= 12V, V
IN
OUT
= 3V, I
=1A 78 %
LOAD
All Output Voltage Versions Electrical Characteristics
Specifications with standard type face are for TJ= 25˚C, and those with boldface type apply over full Operating Tempera­ture Range. Unless otherwise specified, V
sion. I
LOAD
= 200 mA
Symbol Parameter Conditions LM2598-XX Units
DEVICE PARAMETERS
I
b
f
O
V
SAT
Feedback Bias Current Adjustable Version Only, VFB= 1.3V 10 nA
Oscillator Frequency (Note 7) 150 kHz
Saturation Voltage I
DC Max Duty Cycle (ON) (Note 9) 100 %
Min Duty Cycle (OFF) (Note 10) 0
I
CL
I
L
I
Q
Current Limit Peak Current, (Note 8) (Note 9) 1.5 A
Output Leakage Current Output = 0V (Note 9) (Note 10) (Note 11) 50 µA(max)
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
θ
JA
θ
JA
θ
JA
θ
JA
Thermal Resistance TO220 or TO263 Package, Junction to Case 2 ˚C/W
SHUTDOWN/SOFT-START CONTROL Test Circuit of V
SD
Shutdown Threshold 1.3 V Voltage Low, (Shutdown Mode) 0.6 V(max)
V
SS
Soft-start Voltage V
= 12V for the 3.3V, 5V, and Adjustable version and VIN= 24V for the 12V ver-
IN
Typ Limit
(Note 4) (Note 5)
50/100 nA(max)
127/110 kHz(min) 173/173 kHz(max)
= 1A (Note 8) (Note 9) 1 V
OUT
1.2/1.3 V(max)
1.2/1.15 A(min)
2.4/2.6 A(max)
Output = −1V 2 mA
15 mA(max)
TO220 Package, Junction to Ambient (Note 12) 50 ˚C/W TO263 Package, Junction to Ambient (Note 13) 50 ˚C/W TO263 Package, Junction to Ambient (Note 14) 30 ˚C/W TO263 Package, Junction to Ambient (Note 15) 20 ˚C/W
Figure 1
High, (Soft-start Mode) 2 V(min)
= 20% of Nominal Output Voltage 2 V
OUT
V
= 100% of Nominal Output Voltage 3
OUT
(Limits)
(Limits)
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All Output Voltage Versions Electrical Characteristics
Specifications with standard type face are for TJ= 25˚C, and those with boldface type apply over full Operating Tempera­ture Range. Unless otherwise specified, V
sion. I
Symbol Parameter Conditions LM2598-XX Units
SHUTDOWN/SOFT-START CONTROL Test Circuit of
I
SD
I
SS
FLAG/DELAY CONTROL Test Circuit of
VF
SAT
IF
L
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%
production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
Note 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
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 to12Vfor the 3.3V, 5V, and theADJ. version, and 15V for the 12V version, to force theoutput transistor
switch OFF.
Note 11: V Note 12: Junction to ambient thermal resistance (no external heat sink) forthe TO-220 package mounted vertically,with the leads soldered to a printed circuit board
with (1 oz.) copper area of approximately 1 in
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 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 Note 15: Junction to ambient thermal resistance with the TO-263 package tab soldered to a double sided printed circuit board with 3 in
the LM2598S side of the board, and approximately 16 in
Switchers Made Simple
in
= 200 mA
LOAD
Shutdown Current V
Soft-start Current V
Regulator Dropout Detector Low (Flag ON) 96 % Threshold Voltage 92 %(min)
Flag Output Saturation I Voltage V Flag Output Leakage
Current Delay Pin Threshold 1.25 V Voltage Low (Flag ON) 1.21 V(min)
Delay Pin Source Current V
Delay Pin Saturation Low (Flag ON) 55 mV
Figure 1
test circuit, system performance will be as shown in system parameters section of Electrical Characteristics.
= 40V.
IN
®
version 4.2 software.
(Continued)
= 12V for the 3.3V, 5V, and Adjustable version and VIN= 24V for the 12V ver-
IN
SHUTDOWN
= 2.5V 1.6 µA
Soft-start
Figure 1
= 3 mA 0.3 V
SINK
= 0.5V 0.7/1.0 V(max)
DELAY
V
= 40V 0.3 µA
FLAG
High (Flag OFF) and V
= 0.5V 3 µA
DELAY
2
.
2
of copper on the other side of the p-c board. See application hints in this data sheetandthe thermal model
Typ Limit
(Limits)
(Note 4) (Note 5)
Figure 1
= 0.5V 5µA
10 µA(max)
5 µA(max)
98 %(max)
Regulated 1.29 V(max)
OUT
6 µA(max)
350/400 mV(max)
2
of (1 oz.) copper area.
2
of (1 oz.) copper area.
2
of (1 oz.) copper area on
LM2598
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Typical Performance Characteristics (Circuit of
LM2598
Normalized Output Voltage
Line Regulation
Figure 1
)
Efficiency
Switch Saturation Voltage
Operating Quiescent Current
DS012593-2
DS012593-15
Switch Current Limit
Shutdown Quiescent Current
DS012593-3
DS012593-16
DS012593-14
Dropout Voltage
DS012593-17
Minimum Operating Supply Voltage
DS012593-4
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DS012593-5
DS012593-6
LM2598
Typical Performance Characteristics (Circuit of
Feedback Pin Bias Current
DS012593-49
Soft-start
Flag Saturation Voltage
Shutdown/Soft-start Current
Figure 1
DS012593-7
) (Continued)
Switching Frequency
DS012593-8
Delay Pin Current
Soft-start Response
DS012593-9
DS012593-10
DS012593-11
Shutdown/Soft-start Threshold Voltage
DS012593-12
DS012593-13
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Typical Performance Characteristics (Circuit of
LM2598
Continuous Mode Switching Waveforms V
= 20V, V
IN
L = 68 µH, C
OUT
OUT
=5V,I
LOAD
= 120 µF, C
=1A
ESR = 100 mΩ
OUT
Figure 1
)
Discontinuous Mode Switching Waveforms
= 20V, V
V
IN
L = 22 µH, C
OUT
OUT
=5V,I
LOAD
= 220 µF, C
= 600 mA
ESR=50mΩ
OUT
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
IN
L = 68 µH, C
A: Output Voltage, 100 mV/div. (AC) B: 250 mA to 750 mA Load Pulse
OUT
OUT
=5V,I
LOAD
= 120 µF, C
= 250 mA to 750 mA
ESR = 100 mΩ
OUT
Horizontal Time Base: 100 µs/div.
DS012593-18
DS012593-20
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.
Load Transient Response for Discontinuous Mode
= 20V, V
V
IN
L = 22 µH, C
A: Output Voltage, 100 mV/div. (AC) B: 250 mA to 750 mA Load Pulse
OUT
OUT
=5V,I
LOAD
= 220 µF, C
= 250 mA to 750 mA
ESR=50mΩ
OUT
DS012593-21
Horizontal Time Base: 200 µs/div.
www.national.com 8
Test Circuit and Layout Guidelines
Fixed Output Voltage Versions
Component Values shown are for VIN= 15V, V 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
: — 0.1 µF
C
DELAY
: — 4.7k
R
Pull Up
OUT
=5V,I
LOAD
= 1A.
Adjustable Output Voltage Versions
LM2598
DS012593-23
where V
REF
= 1.23V
Select R1to be approximately 1kΩ, use a 1% resistor for best stability. Component Values shown are for V
= 10V, I
V
OUT
— 120 µF, 35V,Aluminum Electrolytic Nichicon “PL Series”
C
IN
— 120 µF, 35V Aluminum Electrolytic, Nichicon “PL Series”
C
OUT
D1 —3A, 40V Schottky Rectifier, 1N5822
LOAD
= 1A.
IN
= 20V,
L1 —100 µH, L29
—1 kΩ,1%
R
1
—7.15k, 1%
R
2
—3.3 nF, See Application Information Section
C
FF
—3 kΩ, See Application Information Section
R
FF
Typical Values
CSS—0.1 µF
—0.1 µF
C
DELAY
—4.7k
R
PULL UP
FIGURE 1. Standard Test Circuits and Layout Guides
DS012593-24
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Test Circuit and Layout Guidelines
(Continued)
LM2598
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
external components should be located as close to the
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
wiring can cause problems.
OUT
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.) switcher lC as possible using ground plane construction or single point grounding.
LM2598 Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
Given:
V
= Regulated Output Voltage (3.3V, 5V or 12V)
OUT
(max) = Maximum DC Input Voltage
V
IN
I
(max) = Maximum Load Current I
LOAD
1. Inductor Selection (L1) 1. Inductor Selection (L1) A. Select the correct inductor value selection guide from
Figures
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.
B. From the inductor value selection guide, identify the in­ductance region intersected by the Maximum Input Voltage line and the Maximum Load Current line. Each region is identified by an inductance value and an inductor code (LXX).
C. Select an appropriate inductor from the four manufactur­er’s part numbers listed in
2. Output Capacitor Selection (C
Figure 8
.
) 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.
Given:
V
=5V
OUT
V
(max) = 12V
IN
(max) = 1A
LOAD
A. Use the inductor selection guide for the 5V version shown in
Figure 5
.
B. From the inductor value selection guide shown in 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 number from any of the four manufacturers shown. (In most instance, both through hole and surface mount inductors are available.)
A. See section on output capacitors in application infor­mation section.
OUT
Figure 5
,
)
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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 This table contains different input voltages, output voltages, and load currents, and lists various inductors and output capacitors that will provide the best design solutions.
C. The capacitor voltage rating for electrolytic capacitors should be at least 1.5 times greater than the output voltage, and often much higher voltage ratings are needed to satisfy the low ESR requirements for low output ripple voltage .
D. For computer aided design software, see
™
Simple
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.
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.
(version 4.2 or later).
Figure 2
Switchers Made
B. From the quick design component selection table shown in
.
Figure 2
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.
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 17
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.
A. Refer to the table shown in 3A, 20V, 1N5820 Schottky diode will provide the best perfor­mance, and will not be overstressed even for a shorted output.
, locate the 5V output voltage section. In the load
220 µF 25V Panasonic HFQ Series 220 µF 25V Nichicon PL Series
for the ESR vs voltage rating). This amount of
Figure 11
. In this example, a
LM2598
www.national.com11
LM2598 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
LM2598
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
1
at least
⁄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 16
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
For additional information, see section on input capaci­tors in Application Information section.
shows typical
pin.
IN
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 mA is needed. The curves shown in
Figure 16
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
www.national.com 12
LM2598 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version) Given:
V
= Regulated Output Voltage
OUT
V
(max) = Maximum Input Voltage
IN
I
(max) = Maximum Load Current
LOAD
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.
Given:
V
= 20V
OUT
V
(max) = 28V
IN
I
(max) = 1A
LOAD
F = Switching Frequency
(Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R1and R2,as shown in
Select R
Figure 1
1
)
to be 1 kΩ, 1%. Solve for R2.
LM2598
Select a value for R1between 240Ω and 1.5 kΩ. The lower resistor values minimize noise pickup in the sensitive feed-
R2= 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ. R
2
back pin. (For the lowest temperature coefficient and the best stability with time, use 1% metal film resistors.)
2. Inductor Selection (L1) A. Calculate the inductor Volt
(V
µs), from the following formula:
•
where V and V
= internal switch saturation voltage = 1V
SAT
= diode forward voltage drop = 0.5V
D
microsecond constant E•T
•
B. Use the E•T value from the previous formula and match it with the E Value Selection Guide shown in
T number on the vertical axis of the Inductor
•
Figure 7
.
2. Inductor Selection (L1) A. Calculate the inductor Volt
(E
B. E
C. on the horizontal axis, select the maximum load current. C. I D. Identify the inductance region intersected by the E
•
value and the Maximum Load Current value. Each region is identified by an inductance value and an inductor code (LXX).
E. Select an appropriate inductor from the four manufactur­er’s part numbers listed in
3. Output Capacitor Selection (C
Figure 8
.
) 3. Output Capacitor SeIection (C
OUT
A. In the majority of applications, low ESR electrolytic or solid
T
D. From the inductor value selection guide shown in the inductance region intersected by the 35 (V tal line and the 1A vertical line is 100 µH, and the inductor code is L29.
E. From the table in inductor part number from the list of manufacturers part numbers.
A. See section on C 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.
= 15.4 kΩ.
T),
•
T = 34.8 (V•µs)
•
(max) = 1A
LOAD
microsecond constant
•
•
Figure 8
OUT
, locate line L29, and select an
)
OUT
in Application Information section.
Figure 7
µs) horizon-
,
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LM2598 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
LM2598
PROCEDURE (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.
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.
4. Feedforward Capacitor (C
) (See
FF
Figure 1
)
For output voltages greater than approximately 10V, an ad­ditional capacitor is required. The compensation capacitor is typically between 50 pF and 10 nF, and is wired in parallel with the output voltage setting resistor, R
. It provides addi-
2
tional stability for high output voltages, low input-output volt­ages, and/or very low ESR output capacitors, such as solid tantalum capacitors.
EXAMPLE (Adjustable Output Voltage Version) B. From the quick design table shown in
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. 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
The table shown in
Figure 3
)
FF
contains feed forward capacitor values for various output voltages. In this example,a1nF capacitor is needed.
Figure 3
, locate the
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.)
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.
www.national.com 14
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.
LM2598 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
LM2598
PROCEDURE (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
1
at least
⁄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 16
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
pin.
IN
For additional information, see section on input capaci­tor in application information section.
EXAMPLE (Adjustable Output Voltage Version)
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
) would be needed. Since the the next higher
IN
capacitor voltage rating is 50V, a 50V capacitor should be used. The capacitor voltage rating of (1.5 x V
) is a conser-
IN
vative guideline, 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 16
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 (seeApplication 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.
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
LM2598
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)
FIGURE 4. LM2598-3.3
FIGURE 5. LM2598-5.0
DS012593-25
DS012593-26
DS012593-27
FIGURE 6. LM2598-12
DS012593-28
FIGURE 7. LM2598-ADJ
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LM2598 Series Buck Regulator Design Procedure (Continued)
LM2598
Inductance
(µH)
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 —
Current
(A)
Schott Renco Pulse Engineering Coilcraft
Through Surface Through Surface Through Surface Surface
Hole Mount Hole Mount Hole Mount Mount
FIGURE 8. Inductor Manufacturers Part Numbers
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LM2598 Series Buck Regulator Design Procedure (Continued)
LM2598
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
MBRS160 10BF10 SR105 SK35 30WF10 SR305 30WF10
or
more
10BQ050 MBR150 MBRS360 MBR350
10MQ060 11DQ05 30WQ05 31DQ05
FIGURE 11. Diode Selection Table
www.national.com 18
Block Diagram
LM2598
FIGURE 12.
Application Information
PIN FUNCTIONS +V
(Pin 2)—This is the positive input supply for the IC
IN
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
mately −0.5V, with a duty cycle of V 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 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.
OUT/VIN
reaches 95% of
OUT
) and approxi­. To minimize
DS012593-29
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
—A capacitor on this pin provides the regulator with a
SS
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
1. Regulator in Shutdown.
When the SD /SS pin voltage is
Figure 13
).
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
www.national.com19
Application Information (Continued)
the narrow pulses and ramp up in a controlled manner. This
LM2598
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
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.
standard Pulse Width Modulated switching regulator. The capacitor will continue to charge up until it reaches the internal clamp voltage of approximately 7V. If this pin is driven from a voltage source, the current must be limited to about 1 mA.
If the part is operated with an input voltage at or below the internal soft-start clamp voltage of approximately 7V, the voltage on the SD/SS pin tracks the input voltage and can be disturbed by a step in the voltage. To maintain proper func­tion under these conditions, it is strongly recommended that the SD/SS pin be clamped externally between the 3V maxi­mum soft-start threshold and the 4.5V minimum input volt­age.
Figure 15
is an example of an external 3.7V (approx.) clamp that prevents a line-step related glitch but does not interfere with the soft-start behavior of the device.
Figure 13
shows the Soft-start region from
Above 2.8V, the circuit operates as a
FIGURE 13. Soft-start, Delay, Error, Output
FIGURE 14. Timing Diagram for 5V Output
DS012593-30
DS012593-31
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Application Information (Continued)
FIGURE 15. External 3.7V Soft-Start Clamp
DELAY CAPACITOR C
upper curve in
Figure 14
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
lector 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
Figure 1
or then C compensation to the feedback loop and increases the phase margin for better loop stability. For C design procedure section.
If the output ripple is large ( 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 proximately 3 times R1, will attenuate the ripple voltage at the feedback pin.
—Provides delay for the error flag output. See the
DELAY
Figure 13
, and also refer to timing diagrams in
. A capacitor on this pin provides a time delay
—The error flag output, (or power OK) is the col-
Pull Up
— A Feedforward Capacitor CFF, shown across R2 in
FF
is used when the output voltage is greater than 10V
has a very low ESR. This capacitor adds lead
OUT
selection, see the
FF
>
5% of the nominal output
, in series with the feedforward capacitor, ap-
FF
DS012593-65
INPUT CAPACITOR
—A low ESR aluminum or tantalum bypass capacitor is
C
IN
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.
LM2598
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Application Information (Continued)
LM2598
DS012593-32
FIGURE 16. RMS Current Ratings for Low
ESR Electrolytic Capacitors (Typical)
DS012593-33
FIGURE 17. Capacitor ESR vs Capacitor Voltage Rating
(Typical Low ESR Electrolytic Capacitor)
The consequences of operating an electrolytic capacitor above the RMS current rating is a shortened operating life. The higher temperature speeds up the evaporation of the capacitor’s electrolyte, resulting in eventual failure.
Selecting an input capacitor requires consulting the manu­facturers data sheet for maximum allowable RMS ripple current. For a maximum ambient temperature of 40˚C, a general guideline would be to select a capacitor with a ripple current rating of approximately 50% of the DC load current. For ambient temperatures up to 70˚C, a current rating of 75% of the DC load current would be a good choice for a conservative design. The capacitor voltage rating must be at least 1.25 times greater than the maximum input voltage, and often a much higher voltage capacitor is needed to satisfy the RMS current requirements.
A graph shown in 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.
Figure 16
shows the relationship between
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
—An output capacitor is required to filter the output
OUT
and provide regulator loop stability. Low impedance or low ESR Electrolytic or solid tantalum capacitors designed for switching regulator applications must be used. When select­ing an output capacitor, the important capacitor parameters are; the 100 kHz Equivalent Series Resistance (ESR), the RMS ripple current rating, voltage rating, and capacitance value. For the output capacitor, the ESR value is the most important parameter.
The output capacitor requires an ESR value that has an upper and lower limit. For low output ripple voltage, a low ESR value is needed. This value is determined by the maxi­mum allowable output ripple voltage, typically 1% to 2% of the output voltage. But if the selected capacitor’s ESR is extremely low, there is a possibility of an unstable feedback loop, resulting in an oscillation at the output. Using the capacitors listed in the tables, or similar types, will provide design solutions under all conditions.
If very low output ripple voltage (less than 15 mV) is re­quired, refer to the section on Output Voltage Ripple and Transients for a post ripple filter.
An aluminum electrolytic capacitor’s ESR value is related to the capacitance value and its voltage rating. In most cases, higher voltage electrolytic capacitors have lower ESR values (see
Figure 17
). 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 3
for typical capacitor values, voltage ratings, and
Figure 2
and
manufacturers capacitor types. Electrolytic capacitors are not recommended for tempera-
tures below −25˚C. The ESR rises dramatically at cold tem­peratures and typically rises 3X 10X at −40˚C. See curve shown in
@
−25˚C and as much as
Figure 18
.
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 charac-
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Application Information (Continued)
DS012593-34
FIGURE 18. Capacitor ESR Change vs Temperature
INDUCTOR SELECTION
All switching regulators have two basic modes of operation; continuous and discontinuous. The difference between the two types relates to the inductor current, whether it is flowing continuously, or if it drops to zero for a period of time in the normal switching cycle. Each mode has distinctively different operating characteristics, which can affect the regulators performance and requirements. Most switcher designs will operate in the discontinuous mode when the load current is low.
The 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 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 19
.)
Figure 3
through
DS012593-35
FIGURE 19. (∆I
) Peak-to-Peak Inductor
IND
Ripple Current (as a Percentage of the
Load Current) vs Load Current
By allowing the percentage of inductor ripple current to increase for low load currents, the inductor value and size can be kept relatively low.
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.
LM2598
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Application Information (Continued)
DISCONTINUOUS MODE OPERATION
LM2598
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 Typical Perfomance 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.
Different inductor types and/or core materials produce differ­ent amounts of this characteristic ringing. Ferrite core induc­tors have very little core loss and therefore produce the most ringing. The higher core loss of powdered iron inductors produce less ringing. If desired, a series RC could be placed in parallel with the inductor to dampen the ringing. The computer aided design software (version 4.2) will provide all component values for continu­ous and discontinuous modes of operation.
Switchers Made Simple
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 recommended. (See
Figure 1
.) The inductance required is typically between 1 µH and 5 µH, with low DC resistance, to maintain good load regulation. A low ESR output filter 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 20
shows a typical output ripple voltage, with and without a post ripple filter.
When observing output ripple with a scope, it is essential that a short, low inductance scope probe ground connection be used. Most scope probe manufacturers provide a special probe terminator which is soldered onto the regulator board, preferable at the output capacitor. This provides a very short scope ground thus eliminating the problems associated with the 3 inch ground lead normally provided with the probe, and provides a much cleaner and more accurate picture of the ripple voltage waveform.
The voltage spikes are caused by the fast switching action of the output switch, the diode, and the parasitic inductance of the output filter capacitor, and its associated wiring. To mini­mize these voltage spikes, the output capacitor should be designed for switching regulator applications, and the lead lengths must be kept very short. Wiring inductance, stray capacitance, as well as the scope probe used to evaluate these transients, all contribute to the amplitude of these spikes.
DS012593-36
FIGURE 20. Post Ripple Filter Waveform
OUTPUT VOLTAGE RIPPLE AND TRANSIENTS
The output voltage of a switching power supply operating in the continuous mode will contain a sawtooth ripple voltage at the switcher frequency, and may also contain short voltage spikes at the peaks of the sawtooth waveform.
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
www.national.com 24
DS012593-37
FIGURE 21. Peak-to-Peak Inductor
Ripple Current vs Load Current
When a switching regulator is operating in the continuous mode, the inductor current waveform ranges from a triangu­lar to a sawtooth type of waveform (depending on the input voltage). For a given input and output voltage, the peak-to-peak amplitude of this inductor current waveform remains constant. As the load current increases or de­creases, the entire sawtooth current waveform also rises and falls. The average value (or the center) of this current waveform is equal to the DC load current.
If the load current drops to a low enough level, the bottom of the sawtooth current waveform will reach zero, and the switcher will smoothly change from a continuous to a discon­tinuous mode of operation. Most switcher designs (irregard­less how large the inductor value is) will be forced to run discontinuous if the output is lightly loaded. This is a per­fectly acceptable mode of operation.
Application Information (Continued)
In a switching regulator design, knowing the value of the peak-to-peak inductor ripple current (∆I determining a number of other circuit parameters. Param­eters such as, peak inductor or peak switch current, mini­mum load current before the circuit becomes discontinuous, output ripple voltage and output capacitor ESR can all be calculated from the peak-to-peak ∆I nomographs shown in
Figure 4
through select an inductor value, the peak-to-peak inductor ripple current can immediately be determined. The curve shown in
Figure 21
shows the range of (∆I
IND
for different load currents. The curve also shows how the peak-to-peak inductor ripple current (∆I 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
= 5V, maximum load current of 800 mA
OUT
V
= 12V, nominal, varying between 10V and 14V.
IN
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
) to flow that will be a percentage of the maximum load
IND
current. Referring to
Figure 21
, follow the 0.8A line approxi­mately midway into the inductance region, and read the peak-to-peak inductor ripple current (∆I axis (approximately 300 mA p-p).
As the input voltage increases to 14V, it approaches the upper border of the inductance region, and the inductor ripple current increases. Referring to the curve in it can be seen that for a load current of 0.8A, the peak-to-peak inductor ripple current (∆I 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 ∆I
value is known, the following formulas can be
IND
used to calculate additional information about the switching regulator circuit.
1. Peak Inductor or peak switch current
) can be useful for
IND
. When the inductor
IND
Figure 7
are used to
) that can be expected
) changes as you
IND
) on the left hand
IND
Figure 21
) is 300 mA with
IND
LM2598
OPEN CORE INDUCTORS
Another possible source of increased output ripple voltage or unstable operation is from an open core inductor. Ferrite bobbin or stick inductors have magnetic lines of flux flowing through the air from one end of the bobbin to the other end. These magnetic lines of flux will induce a voltage into any wire or PC board copper trace that comes within the induc­tor’s magnetic field. The strength of the magnetic field, the orientation and location of the PC copper trace to the mag­netic field, and the distance between the copper trace and the inductor, determine the amount of voltage generated in the copper trace. Another way of looking at this inductive coupling is to consider the PC board copper trace as one turn of a transformer (secondary) with the inductor winding as the primary. Many millivolts can be generated in a copper trace located near an open core inductor which can cause stability problems or high output ripple voltage problems.
If unstable operation is seen, and an open core inductor is used, it’s possible that the location of the inductor with respect to other PC traces may be the problem. To deter­mine if this is the problem, temporarily raise the inductor away from the board by several inches and then check circuit operation. If the circuit now operates correctly, then the magnetic flux from the open core inductor is causing the problem. Substituting a closed core inductor such as a tor­roid or E-core will correct the problem, or re-arranging the PC layout may be necessary. Magnetic flux cutting the IC device ground trace, feedback trace, or the positive or nega­tive traces of the output capacitor should be minimized.
Sometimes, locating a trace directly beneath a bobbin 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.
2. Minimum load current before the circuit becomes dis-
continuous
3. Output Ripple Voltage = (∆I
) x (ESR of C
IND
OUT
)
= 0.3A x 0.16Ω=48mVp-p
4. ESR of C
OUT
DS012593-38
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Application Information (Continued)
LM2598
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 22. 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 23. Junction Temperature Rise, TO-263
THERMAL CONSIDERATIONS
22
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
Figure
2
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, 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 23
show the LM2598S (TO-263 package) junction temperature rise above ambient tempera­ture with a 1Aload 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 26
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 24
and
Figure 25
show the effects of Soft-start on the output voltage, the input current, with, and without a Soft-start capacitor.
Figure 24
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.
If a very slow output voltage ramp is desired, the Soft-start capacitor can be made much larger. Many seconds or even minutes are possible.
If only the shutdown feature is needed, the Soft-start capaci­tor can be eliminated.
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Application Information (Continued)
LM2598
DS012593-40
FIGURE 24. Output Voltage, Input Current, Error Flag
Signal, at Start-Up, WITH Soft-start
FIGURE 26. Typical Circuit Using Shutdown /Soft-start and Error Flag Features
DS012593-41
FIGURE 25. Output Voltage, Input Current, at Start-Up,
WITHOUT Soft-start
DS012593-42
FIGURE 27. Inverting −5V Regulator With Shutdown and Soft-start
DS012593-43
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Application Information (Continued)
lNVERTING REGULATOR
LM2598
The circuit in 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 guide as to the amount of output load current possible for the different input and output voltage conditions.
Figure 27
converts a positive input voltage to a
Figure 28
provides a
loaded down, may not work correctly. Because of the rela­tively high startup currents required by the inverting topology, the Soft-start feature shown in
Also shown in
Figure 27
are several shutdown methods for
Figure 27
is recommended.
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.
29
contains a undervoltage lockout circuit for a buck configu-
ration, while
Figure 30
and
Figure 31
are for the inverting
Figure
types (only the circuitry pertaining to the undervoltage lock­out is shown).
Figure 29
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.
DS012593-44
FIGURE 28. Maximum Load Current for Inverting
Regulator Circuit
capacitor to the output, under light
IN
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.
Because of differences in the operation of the inverting regulator, the standard design procedure is not used to select the inductor value. In the majority of designs, a 68 µH,
Figure 27
will provide good results in the majority of
inverting designs. This type of inverting regulator can require relatively large
amounts of input current when starting up, even with light loads. Input currents as high as the 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
DS012593-45
FIGURE 29. Undervoltage Lockout for a Buck
Regulator
Figure 30
inverting circuit.
and
Figure 31
Figure 30
apply the same feature to an
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 31
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.
DS012593-47
FIGURE 30. Undervoltage Lockout Without
Hysteresis for an Inverting Regulator
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Application Information (Continued)
DS012593-46
FIGURE 31. Undervoltage Lockout With
Hysteresis for an Inverting Regulator
NEGATIVE VOLTAGE CHARGE PUMP
Occasionally a low current negative voltage is needed for biasing parts of a circuit. A simple method of generating a negative voltage using a charge pump technique and the switching waveform present at the OUT pin, is shown in
Figure 32
mately 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 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.
. This unregulated negative voltage is approxi-
LM2598
FIGURE 32. Charge Pump for Generating a
Low Current, Negative Output Voltage
DS012593-48
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Application Information (Continued)
LM2598
TYPICAL THROUGH HOLE PC BOARD LAYOUT, FIXED OUTPUT (1X SIZE), DOUBLE SIDED, THROUGH HOLE
PLATED
CIN—150 µF/50V Aluminum Electrolytic, Panasonic “HFQ series” R C
—120 µF/25V Aluminum Electrolytic, Panasonic “HFQ series” C
OUT
D1— 3A, 40V Schottky Rectifier, 1N5822 C L1— 68 µH, L30, Renco, Through hole
FIGURE 33. Fixed Output PC Board Layout
PULL-UP DELAY SD/SS
DS012593-51
—10 kΩ
—0.1 µF
—0.1 µF
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Application Information (Continued)
TYPICAL THROUGH HOLE PC BOARD LAYOUT, ADJUSTABLE OUTPUT (1X SIZE), DOUBLE SIDED,
THROUGH HOLE PLATED
LM2598
CIN—150 µF/50V, Aluminum Electrolytic, Panasonic “HFQ series” CFF—See
—120 µF/25V Aluminum Electrolytic, Panasonic “HFQ series” RFF—See Application Information Section (CFFSection)
C
OUT
D1— 3A, 40V Schottky Rectifier, 1N5822 R L1— 68 µH, L30, Renco, Through hole C R1—1 kΩ,1% C R2— Use formula in Design Procedure
PULL-UP DELAY SD/SS
Figure 4
—10 kΩ
—0.1 µF
—0.1 µF
.
FIGURE 34. Adjustable Output PC Board Layout
DS012593-52
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Physical Dimensions inches (millimeters) unless otherwise noted
LM2598
7-Lead TO-220 (T)
Order Number LM2598T-3.3, LM2598T-5.0,
LM2598T-12 or LM2598T-ADJ
NS Package Number TA07B
www.national.com 32
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
LM2598 SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator, with
Features
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
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