ETC LM2598-5.0

LM2598
SIMPLE SWITCHERÉ 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 additional supervisory and performance features
added.
Requiring a minimum number of external components,
these regulators are simple to use and include internal frequency compensation ² , 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 g 4% tolerance on output voltage under all conditions of input voltage and output
load conditions, and g 15% on the oscillator frequency. External shutdown is included, featuring typically 85 mA stand-
by 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
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
3.3V, 5V, 12V, and adjustable output versions
Adjustable version output voltage range, 1.2V to 37V
g 4% max over line and load conditions
Guaranteed 1A output current
Available in 7-pin TO-220 and TO-263 (surface mount)
package
Input voltage range up to 40V
Excellent line and load regulation specifications
150 kHz fixed frequency internal oscillator
Shutdown/Soft-start
Out of regulation error flag
Error output delay
Low power standby mode, IQ typically 85 mA
High Efficiency
Uses readily available standard inductors
Thermal shutdown and current limit protection
Applications
Y
Y
Y
Y
Simple high-efficiency step-down (buck) regulator
Efficient pre-regulator for linear regulators
On-card switching regulators
Positive to Negative converter
Typical Application (Fixed Output Voltage Versions)
TL/H/12593 – 1
² Patent Number 5,382,918.
SIMPLE SWITCHERÉ and Switchers Made Simple É are registered trademarks of National Semiconductor Corporation.
C1996 National Semiconductor Corporation
TL/H/12593
RRD-B30M66/Printed in U. S. A.
LM2598 SIMPLE SWITCHER Power Converter
150 kHz 1A Step-Down Voltage Regulator, with Features
May 1996
Absolute Maximum Ratings (Note 1)
Lead Temperature
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.
S Package
Vapor Phase (60 sec.)
Infrared (10 sec.)
Maximum Supply Voltage (VIN)
45V
SD/SS Pin Input Voltage (Note 2)
6V
Delay Pin Voltage (Note 2)
1.5V
b 0.3 s V s 45V
Flag Pin Voltage
b 0.3 s V s a 25V
Feedback Pin Voltage
b 1V
Output Voltage to Ground (Steady State)
Power Dissipation
Internally limited
b 65§ C to a 150§ C
Storage Temperature Range
ESD Susceptibility
Human Body Model (Note 3)
2 kV
a 215§ C
a 245§ C
T Package (Soldering, 10 sec.)
Maximum Junction Temperature
a 260§ C
a 150§ C
Operating Conditions
Temperature Range
Supply Voltage
b 25§ C s TJ a 125§ C
4.5V to 40V
LM2598-3.3
Electrical Characteristics Specifications with standard type face are for TJ e 25§ C, and those with boldface
type apply over full Operating Temperature Range.
LM2598-3.3
Symbol
Parameter
Conditions
Typ
(Note 4)
Limit
(Note 5)
Units
(Limits)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1
VOUT
Output Voltage
4.75V s VIN s 40V, 0.1A s ILOAD s 1A
3.3
3.168/3.135
3.432/3.465
h
Efficiency
VIN e 12V, ILOAD e 1A
78
V
V(min)
V(max)
%
LM2598-5.0
Electrical Characteristics Specifications with standard type face are for TJ e 25§ C, and those with boldface
type apply over full Operating Temperature Range.
LM2598-5.0
Symbol
Parameter
Conditions
Typ
(Note 4)
Limit
(Note 5)
Units
(Limits)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1
VOUT
Output Voltage
7V s VIN s 40V, 0.1A s ILOAD s 1A
5
4.800/4.750
5.200/5.250
h
Efficiency
VIN e 12V, ILOAD e 1A
82
V
V(min)
V(max)
%
LM2598-12
Electrical Characteristics Specifications with standard type face are for TJ e 25§ C, and those with boldface
type apply over full Operating Temperature Range.
LM2598-12
Symbol
Parameter
Conditions
Typ
(Note 4)
Limit
(Note 5)
Units
(Limits)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1
VOUT
Output Voltage
15V s VIN s 40V, 0.1A s ILOAD s 1A
12
11.52/11.40
12.48/12.60
h
Efficiency
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VIN e 25V, ILOAD e 1A
90
2
V
V(min)
V(max)
%
LM2598-ADJ
Electrical Characteristics Specifications with standard type face are for TJ e 25§ C, and those with boldface
type apply over full Operating Temperature Range.
LM2598-ADJ
Symbol
Parameter
Conditions
Typ
(Note 4)
4.5V s VIN s 40V, 0.1A s ILOAD s 1A
VOUT programmed for 3V. Circuit of Figure 12 .
1.230
Limit
(Note 5)
Units
(Limits)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1
VFB
h
Feedback Voltage
Efficiency
VIN e 12V, VOUT e 3V, ILOAD e 1A
1.193/1.180
1.267/1.280
78
V
V(min)
V(max)
%
All Output Voltage Versions
Electrical Characteristics Specifications with standard type face are for TJ e 25§ C, and those with boldface
type apply over full Operating Temperature Range. Unless otherwise specified, VIN e 12V for the 3.3V, 5V, and
Adjustable version and VIN e 24V for the 12V version. ILOAD e 200 mA
LM2598-XX
Symbol
Parameter
Conditions
Typ
(Note 4)
Limit
(Note 5)
Units
(Limits)
DEVICE PARAMETERS
Ib
fO
VSAT
Feedback Bias Current
Oscillator Frequency
Saturation Voltage
Adjustable Version Only, VFB e 1.3V
(Note 7)
10
50/100
nA
nA(max)
127/110
173/173
kHz
kHz(min)
kHz(max)
1.2/1.3
V
V(max)
150
IOUT e 1A (Notes 8 and 9)
1
DC
Max Duty Cycle (ON)
Min Duty Cycle (OFF)
(Note 9)
(Note 10)
100
0
ICL
Current Limit
Peak Current, (Notes 8 and 9)
1.5
%
1.2/1.15
2.4/2.6
IL
IQ
ISTBY
iJC
iJA
iJA
iJA
iJA
Output Leakage Current
(Notes 8, 10 and 11)
Output e 0V
Output e b1V
Operating Quiescent
Current
SD/SS Pin Open, (Note 10)
Standby Quiescent
Current
SD/SS pin e 0V, (Note 11)
Thermal Resistance
TO220 or TO263 Package, Junction to Case
TO220 Package, Junction to Ambient (Note 12)
TO263 Package, Junction to Ambient (Note 13)
TO263 Package, Junction to Ambient (Note 14)
TO263 Package, Junction to Ambient (Note 15)
3
50
15
mA(max)
mA
mA(max)
10
mA
mA(max)
200/250
mA
mA(max)
2
5
85
2
50
50
30
20
A
A(min)
A(max)
§ C/W
§ C/W
§ C/W
§ C/W
§ C/W
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All Output Voltage Versions (Continued)
Electrical Characteristics Specifications with standard type face are for TJ e 25§ C, and those with boldface
type apply over full Operating Temperature Range. Unless otherwise specified, VIN e 12V for the 3.3V, 5V, and
Adjustable version and VIN e 24V for the 12V version. ILOAD e 200 mA
LM2598-XX
Symbol
Parameter
Conditions
Typ
(Note 4)
Limit
(Note 5)
Units
(Limits)
SHUTDOWN/SOFT-START CONTROL Test Circuit of Figure 1
VSD
Shutdown Threshold
Voltage
1.3
Low, (Shutdown Mode)
High, (Soft-start Mode)
0.6
2
VSS
Soft-start Voltage
VOUT e 20% of Nominal Output Voltage
VOUT e 100% of Nominal Output Voltage
2
3
ISD
Shutdown Current
VSHUTDOWN e 0.5V
5
ISS
Soft-start Current
VSoft-start e 2.5V
V
V(max)
V(min)
V
10
mA
mA(max)
5
mA
mA(max)
92
98
%
%(min)
%(max)
0.7/1.0
V
V(max)
1.6
FLAG/DELAY CONTROL Test Circuit of Figure 1
VFSAT
IFL
Regulator Dropout Detector
Threshold Voltage
Low (Flag ON)
Flag Output Saturation
Voltage
ISINK e 3 mA
VDELAY e 0.5V
Flag Output Leakage Current
VFLAG e 40V
Delay Pin Threshold
Voltage
Delay Pin Source Current
Delay Pin Saturation
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96
0.3
0.3
mA
1.25
Low (Flag ON)
High (Flag OFF) and VOUT Regulated
VDELAY e 0.5V
1.21
1.29
V
V(min)
V(max)
6
mA
mA(max)
350/400
mV
mV(max)
3
Low (Flag ON)
55
4
Electrical Characteristics (Continued)
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 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, and the ADJ. version, and 15V for the 12V version, to force the output
transistor switch OFF.
Note 11: VIN e 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 in2.
Note 13: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 0.5 in2 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 in2 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 in2 of (1 oz.) copper area on
the LM2598S side of the board, and approximately 16 in2 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.
Typical Performance Characteristics (Circuit of Figure 1 )
Normalized
Output Voltage
Line Regulation
TL/H/12593–2
Switch Saturation
Voltage
Efficiency
TL/H/12593 – 3
Switch Current Limit
TL/H/12593–15
TL/H/12593 – 16
5
TL/H/12593 – 14
Dropout Voltage
TL/H/12593 – 17
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Typical Performance Characteristics (Circuit of Figure 1 ) (Continued)
Operating
Quiescent Current
Shutdown
Quiescent Current
TL/H/12593–4
Feedback Pin
Bias Current
TL/H/12593 – 5
Flag Saturation
Voltage
TL/H/12593–49
TL/H/12593 – 7
TL/H/12593–9
Soft-start Response
TL/H/12593 – 10
Shutdown/Soft-start
Threshold Voltage
TL/H/12593–12
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TL/H/12593 – 13
6
TL/H/12593 – 6
Switching Frequency
Shutdown/Soft-start
Current
Soft-start
Minimum Operating
Supply Voltage
TL/H/12593 – 8
Delay Pin Current
TL/H/12593 – 11
Typical Performance Characteristics (Circuit of Figure 1 )
Discontinuous Mode Switching Waveforms
VIN e 20V, VOUT e 5V, ILOAD e 600 mA
L e 22 mH, COUT e 220 mF, COUT ESR e 50 mX
Continuous Mode Switching Waveforms
VIN e 20V, VOUT e 5V, ILOAD e 1A
L e 68 mH, COUT e 120 mF, COUT ESR e 100 mX
TL/H/12593 – 18
TL/H/12593 – 19
A: Output Pin Voltage, 10V/div.
A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.5A/div.
B: Inductor Current 0.5A/div.
C: Output Ripple Voltage, 50 mV/div.
C: Output Ripple Voltage, 50 mV/div.
Horizontal Time Base: 2 ms/div.
Horizontal Time Base: 2 ms/div.
Load Transient Response for Discontinuous Mode
VIN e 20V, VOUT e 5V, ILOAD e 250 mA to 750 mA
L e 22 mH, COUT e 220 mF, COUT ESR e 50 mX
Load Transient Response for Continuous Mode
VIN e 20V, VOUT e 5V, ILOAD e 250 mA to 750 mA
L e 68 mH, COUT e 120 mF, COUT ESR e 100 mX
TL/H/12593 – 21
TL/H/12593 – 20
A: Output Voltage, 100 mV/div. (AC)
A: Output Voltage, 100 mV/div. (AC)
B: 250 mA to 750 mA Load Pulse
B: 250 mA to 750 mA Load Pulse
Horizontal Time Base: 200 ms/div.
Horizontal Time Base: 100 ms/div.
Connection Diagrams and Order Information
Surface Mount Package
7-Lead TO-263 (S)
Bent and Staggered Leads, Through Hole Package
7-Lead TO-220 (T)
TL/H/12593 – 22
TL/H/12593 – 50
Order Number LM2598S-3.3, LM2598S-5.0,
LM2598S-12 or LM2598S-ADJ
See NS Package Number TS7B
Order Number LM2598T-3.3, LM2598T-5.0,
LM2598T-12 or LM2598T-ADJ
See NS Package Number TA07B
7
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Test Circuit and Layout Guidelines
Fixed Output Voltage Versions
CIN
TL/H/12593 – 23
Typical Values
Component Values shown are for VIN e 15V,
VOUT e 5V, ILOAD e 1A.
CSS
Ð 120 mF, 50V, Aluminum Electrolytic
Nichicon ‘‘PL Series’’
Ð 0.1 mF
CDELAY Ð 0.1 mF
RPull Up
COUT Ð 120 mF, 35V Aluminum Electrolytic,
Nichicon ‘‘PL Series’’
D1
Ð 3A, 40V Schottky Rectifier, 1N5822
L1
Ð 68 mH, L30
Ð 4.7k
Adjustable Output Voltage Versions
TL/H/12593 – 24
VOUT e VREF
R2 e R1
#V
#1
VOUT
REF
a
b1
R2
R1
J
J
D1 Ð 3A, 40V Schottky Rectifier, 1N5822
where VREF e 1.23V
Select R1 to be approximately 1 kX,
use a 1% resistor for best stability.
Ð 100 mH, L29
Ð 1 kX, 1%
R2
Ð 7.15k, 1%
CFF Ð 3.3 nF, See Application Information Section
Component Values shown are for VIN e 20V,
VOUT e 10V, ILOAD e 1A.
CIN
L1
R1
RFF Ð 3 kX, See Application Information Section
Ð 120 mF, 35V, Aluminum Electrolytic Nichicon ‘‘PL Series’’
Typical Values
COUT Ð 120 mF, 35V Aluminum Electrolytic, Nichicon ‘‘PL Series’’
CSSÐ0.1 mF
CDELAYÐ0.1 mF
RPULL UPÐ4.7k
FIGURE 1. Standard Test Circuits and Layout Guides
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 COUT 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.)
As in any switching regulator, layout is very important. Rapidly switching currents associated with wiring inductance
can generate voltage transients which can cause problems.
For minimal inductance and ground loops, the wires indicated by heavy lines should be wide printed circuit traces
and should be kept as short as possible. For best results,
external components should be located as close to the
switcher lC as possible using ground plane construction or
single point grounding.
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8
LM2598 Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version)
EXAMPLE (Fixed Output Voltage Version)
Given:
VOUT e Regulated Output Voltage (3.3V, 5V or 12V)
VIN(max) e Maximum DC Input Voltage
ILOAD(max) e Maximum Load Current
1. Inductor Selection (L1)
A. Select the correct inductor value selection guide from
Figures 4, 5, or 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 Figure 8.
2. Output Capacitor Selection (COUT)
A. In the majority of applications, low ESR (Equivalent
Series Resistance) electrolytic capacitors between
47 mF and 330 mF and low ESR solid tantalum capacitors between 56 mF and 270 mF 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 mF.
For additional information, see section on output capacitors in application information section.
B. To simplify the capacitor selection procedure, refer to
the quick design component selection table shown in
Figure 2. This table contains different input voltages, output voltages, and load currents, and lists various inductors and output 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.
Given:
VOUT e 5V
VIN(max) e 12V
ILOAD(max) e 1A
1. Inductor Selection (L1)
A. Use the inductor selection guide for the 5V version
shown in Figure 5.
B. From the inductor value selection guide shown in Figure 5, the inductance region intersected by the 12V horizontal line and the 1A vertical line is 68 mH, and the
inductor code is L30.
C. The inductance value required is 68 mH. 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 (COUT)
A. See section on output capacitors in application
information section.
B. From the quick design component selection table
shown in Figure 2, locate the 5V output voltage section.
In the load current column, choose the load current line
that is closest to the current needed in your application,
for this example, use the 1A line. In the maximum input
voltage column, select the line that covers the input voltage needed in your application, in this example, use the
15V line. Continuing on this line are recommended inductors and capacitors that will provide the best overall
performance.
The capacitor list contains both through hole electrolytic
and surface mount tantalum capacitors from four different 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.
D. For computer aided design software, see Switchers
Made SimpleÉ (version 4.2 or later).
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 HighProcedure continued on next page.
220 mF
25V
Panasonic HFQ Series
220 mF 25V Nichicon PL Series
C. For a 5V output, a capacitor voltage rating at least
7.5V or more is needed. But, in this example, even a low
ESR, switching grade, 220 mF 10V aluminum electrolytic
capacitor would exhibit approximately 225 mX 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.
3. Catch Diode Selection (D1)
A. Refer to the table shown in Figure 11. In this example,
a 3A, 20V, 1N5820 Schottky diode will provide the best
performance, and will not be overstressed even for a
shorted output.
Example continued on next page.
9
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LM2598 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
PROCEDURE (Fixed Output Voltage Version)
EXAMPLE (Fixed Output Voltage Version)
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.
4. Input Capacitor (CIN)
A low ESR aluminum or tantalum bypass capacitor is
needed between the input pin and ground to prevent
large voltage transients from appearing at the input. In
addition, the RMS current rating of the input capacitor
should be selected to be at least (/2 the DC load current.
The capacitor manufacturers data sheet must be
checked to assure that this current rating is not exceeded. The curve shown in Figure 15 shows typical RMS
current ratings for several different aluminum electrolytic
capacitor values.
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 VIN
pin.
For additional information, see section on input capacitors in Application Information section.
4. Input Capacitor (CIN)
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 c
VIN) 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 (/2 the DC load
current. In this example, with a 1A load, a capacitor with
a RMS current rating of at least 500 mA is needed. The
curves shown in Figure 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 mF or
220 mF, 25V capacitor could be used.
For a through hole design, a 220 mF/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.
Output Capacitor
Conditions
Output
Voltage
(V)
Load
Current
(A)
1
3.3
0.5
1
5
0.5
1
12
0.5
Inductor
Max Input
Voltage
(V)
Inductance
(mH)
Inductor
(Ý)
Through Hole Electrolytic
Surface Mount Tantalum
Panasonic
HFQ Series
(mF/V)
AVX TPS
Series
(mF/V)
Nichicon
PL Series
(mF/V)
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
10
68
L21
150/35
150/25
100/16
150/16
40
100
L20
150/35
82/35
100/16
100/20
8
33
L28
330/16
330/16
220/10
270/10
10
47
L31
220/25
220/25
220/10
220/10
150/16
15
68
L30
180/35
180/35
220/10
40
100
L29
180/35
120/35
100/16
120/16
9
68
L21
180/16
180/16
220/10
150/16
20
150
L19
120/25
120/25
100/16
100/20
40
150
L19
100/25
100/25
68/20
68/25
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
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
http://www.national.com
Sprague
595D Series
(mF/V)
10
LM2598 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
Given:
VOUT e Regulated Output Voltage
VIN(max) e Maximum Input Voltage
ILOAD(max) e Maximum Load Current
F e Switching Frequency (Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R1 and R2, as
shown in Figure 1 )
Use the following formula to select the appropriate resistor values.
R2
VOUT e VREF 1 a
where VREF e 1.23V
R1
Given:
VOUT e 20V
VIN(max) e 28V
ILOAD(max) e 1A
F e Switching Frequency (Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R1 and R2, as
shown in Figure 1 )
#
Select R1 to be 1 kX, 1%. Solve for R2.
J
R2 e R1
#V
VOUT
REF
b1
b1
REF
J
e 1k
# 1.23V 1 J
20V
b
R2 e 15.4 kX.
J
2. Inductor Selection (L1)
A. Calculate the inductor Volt # microsecond constant
E # T (V # ms), from the following formula:
E # T e (VIN b VOUT b VSAT) #
VOUT
R2 e 1k (16.26 b 1) e 15.26k, closest 1% value is
15.4 kX.
Select a value for R1 between 240X and 1.5 kX. The
lower resistor values minimize noise pickup in the sensitive feedback pin. (For the lowest temperature coefficient and the best stability with time, use 1% metal film
resistors.)
R2 e R1
#V
2. Inductor Selection (L1)
A. Calculate the inductor Volt # microsecond constant
(E # T),
VOUT a VD
1000
(V # ms)
#
VIN b VSAT a VD 150 kHz
E # T e (28 b 20 b 1) #
where VSAT e internal switch saturation voltage e 1V
and VD e diode forward voltage drop e 0.5V
E # T e (7) #
B. Use the E # T value from the previous formula and
match it with the E # T number on the vertical axis of the
Inductor Value Selection Guide shown in Figure 7.
C. on the horizontal axis, select the maximum load current.
D. Identify the inductance region intersected by the E # T
value and the Maximum Load Current value. Each region
is identified by an inductance value and an inductor code
(LXX).
E. Select an appropriate inductor from the four manufacturer’s part numbers listed in Figure 8.
3. Output Capacitor Selection (COUT)
A. In the majority of applications, low ESR electrolytic or
solid tantalum capacitors between 82 mF and 220 mF
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 mF.
For additional information, see section on output capacitors in application information section.
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.
20 a 0.5
1000
(V # ms)
#
28 b 1 a 0.5 150
20.5
# 6.67 (V # ms) e 34.8 (V # ms)
27.6
B. E # T e 34.8 (V # ms)
C. ILOAD(max) e 1A
D. From the inductor value selection guide shown in Figure 7, the inductance region intersected by the 35 (V #
ms) horizontal line and the 1A vertical line is 100 mH, and
the inductor code is L29.
E. From the table in Figure 8, locate line L29, and select
an inductor part number from the list of manufacturers
part numbers.
3. Output Capacitor SeIection (COUT)
A. See section on COUT in Application Information section.
B. From the quick design table shown in Figure 3, locate
the output voltage column. From that column, locate the
output voltage closest to the output voltage in your application. In this example, select the 24V line. Under the
output capacitor section, select a capacitor from the list
of through hole 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 mF
82 mF
Procedure continued on next page.
11
35V Panasonic HFQ Series
35V Nichicon PL Series
Example continued on next page.
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LM2598 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
4. Feedforward Capacitor (CFF) (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, R2. It
provides additional stability for high output voltages, low
input-output voltages, and/or very low ESR output capacitors, such as solid tantalum capacitors.
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 (CFF)
CFF e
1
31 c 103 c R2
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)
The table shown in Figure 3 contains feed forward capacitor values for various output voltages. In this example, a 1 nF capacitor is needed.
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.
6. Input Capacitor (CIN)
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 c VIN) 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 c VIN) is a conservative guideline, and can be modified somewhat if desired.
The RMS current rating requirement for the input capacitor of a buck regulator is approximately (/2 the DC load
current. In this example, with a 1A load, a capacitor with
a RMS current rating of at least 500 mA is needed.
The curves shown in Figure 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 mF or
120 mF, 50V capacitor could be used.
For a through hole design, a 120 mF/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.
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 SimpleÉ (version 4.2 or
later) is available on a 3(/2× diskette for IBM compatible
computers.
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 HighEfficiency rectifiers are also a good choice, but some
types with an abrupt turn-off characteristic may cause
instability or EMl problems. Ultra-fast recovery diodes
typically have reverse recovery times of 50 ns or less.
Rectifiers such as the 1N4001 series are much too slow
and should not be used.
6. Input Capacitor (CIN)
A low ESR aluminum or tantalum bypass capacitor is
needed between the input pin and ground to prevent
large voltage transients from appearing at the input. In
addition, the RMS current rating of the input capacitor
should be selected to be at least (/2 the DC load current.
The capacitor manufacturers data sheet must be
checked to assure that this current rating is not exceeded. The curve shown in Figure 15 shows typical RMS
current ratings for several different aluminum electrolytic
capacitor values.
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 VIN pin.
For additional information, see section on input capacitor in application information section.
http://www.national.com
12
LM2598 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
Through Hole Electrolytic Output Capacitor
Output
Voltage
(V)
Panasonic
HFQ Series
(mF/V)
Nichicon PL
Series
(mF/V)
1.2
330/50
4
220/25
6
220/25
Surface Mount Tantalum Output Capacitor
Feedforward
Capacitor
AVX TPS
Series
(mF/V)
Sprague
595D Series
(mF/V)
330/50
0
330/6.3
330/6.3
0
220/25
4.7 nF
220/10
220/10
4.7 nF
220/25
3.3 nF
220/10
220/10
3.3 nF
Feedforward
Capacitor
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
LM2598 Series Buck Regulator Design Procedure
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)
TL/H/12593 – 25
TL/H/12593 – 26
FIGURE 4. LM2598-3.3
FIGURE 5. LM2598-5.0
TL/H/12593 – 28
TL/H/12593 – 27
FIGURE 7. LM2598-ADJ
FIGURE 6. LM2598-12
13
http://www.national.com
LM2598 Series Buck Regulator Design Procedure (Continued)
Inductance
(mH)
Current
(A)
L4
68
L5
L6
Schott
Renco
Pulse Engineering
Coilcraft
Through
Hole
Surface
Mount
Through
Hole
Surface
Mount
Through
Hole
Surface
Mount
Surface
Mount
0.32
67143940
67144310
RL-1284-68-43
RL1500-68
PE-53804
PE-53804-S
DO1608-68
47
0.37
67148310
67148420
RL-1284-47-43
RL1500-47
PE-53805
PE-53805-S
DO1608-473
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
Ð
PE-53822
PE-53822-S
DO3316-473
L23
33
1.40
67144090
67144470
RL-5471-7
Ð
PE-53823
PE-53823-S
DO3316-333
L24
22
1.70
67148370
67144480
RL-1283-22-43
Ð
PE-53824
PE-53824-S
DO3316-223
L26
330
0.80
67144100
67144480
RL-5471-1
Ð
PE-53826
PE-53826-S
DO5022P-334
L27
220
1.00
67144110
67144490
RL-5471-2
Ð
PE-53827
PE-53827-S
DO5022P-224
L28
150
1.20
67144120
67144500
RL-5471-3
Ð
PE-53828
PE-53828-S
DO5022P-154
L29
100
1.47
67144130
67144510
RL-5471-4
Ð
PE-53829
PE-53829-S
DO5022P-104
L30
68
1.78
67144140
67144520
RL-5471-5
Ð
PE-53830
PE-53830-S
DO5022P-683
L35
47
2.15
67144170
Ð
RL-5473-1
Ð
PE-53935
PE-53935-S
Ð
FIGURE 8. Inductor Manufacturers Part Numbers
Coilcraft Inc.
Coilcraft Inc., Europe
Pulse Engineering Inc.
Pulse Engineering Inc.,
Europe
Renco Electronics Inc.
Schott Corp.
Phone
(800) 322-2645
FAX
(708) 639-1469
Phone
a 11 1236 730 595
FAX
a 44 1236 730 627
Phone
(619) 674-8100
FAX
(619) 674-8262
Phone
a 353 93 24 107
FAX
a 353 93 24 459
Phone
(800) 645-5828
FAX
(516) 586-5562
Phone
(612) 475-1173
FAX
(612) 475-1786
Nichicon Corp.
Panasonic
AVX Corp.
Sprague/Vishay
(708) 843-7500
(708) 843-2798
Phone
(714) 373-7857
FAX
(714) 373-7102
Phone
(803) 448-9411
FAX
(803) 448-1943
Phone
(207) 324-4140
FAX
(207) 324-7223
FIGURE 10. Capacitor Manufacturers Phone Numbers
FIGURE 9. Inductor Manufacturers Phone Numbers
http://www.national.com
Phone
FAX
14
LM2598 Series Buck Regulator Design Procedure (Continued)
1A Diodes
VR
Surface Mount
Schottky
SK12
20V
Ultra Fast
Recovery
All of these
diodes are
rated to at
least 50V.
SK13
30V
3A Diodes
Through Hole
Schottky
1N5817
SR102
Surface Mount
Ultra Fast
Recovery
Schottky
All of these
diodes are
rated to at
least 50V.
SK32
1N5818
MBRS130
SR103
Ultra Fast
Recovery
All of
these
diodes
are rated
to at least
50V.
SK33
IN5820
SR302
MBR320
1N5821
1N5819
SK34
SR304
10BQ040
SR104
MBRS340
MBR340
MBRS160
All of
these
diodes
are rated
to at least
50V.
1N5822
MBRS140
10MQ040
Ultra Fast
Recovery
31DQ03
SK14
50V
or
more
Schottky
MBR330
11DQ03
40V
Through Hole
MURS120
10BF10
11DQ04
MUR120
30WQ04
MURS320
30WF10
31DQ04
SR105
SK35
SR305
10BQ050
MBR150
MBRS360
MBR350
10MQ060
11DQ05
30WQ05
31DQ05
MUR320
30WF10
FIGURE 11. Diode Selection Table
Block Diagram
TL/H/12593 – 29
FIGURE 12
15
http://www.national.com
Application Information
3. Soft-start Region. When the SD/SS pin voltage is between 1.8V and 2.8V ( @ 25§ C), the regulator is in a Softstart 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 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.
PIN FUNCTIONS
a VIN (Pin 2)ÐThis 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)ÐCircuit ground.
Output (Pin 1)ÐInternal switch. The voltage at this pin
switches between approximately ( a VIN b VSAT) and approximately b0.5V, with a duty cycle of VOUT/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)ÐSenses the regulated output voltage to
complete the feedback loop.
Shutdown/Soft-start (Pin 7)ÐThis 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 mA. (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)ÐOpen 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 VOUT 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)ÐAt 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.
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.
EXTERNAL COMPONENTS
SOFT-START CAPACITOR
CSSÐA 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/Softstart pin is allowed to go high, a constant current (approximately 5 mA 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 mA.
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 mA
down to approximately 1.6 mA. This decreases the slope of
capacitor voltage ramp.
TL/H/12593 – 30
FIGURE 13. Soft-start, Delay, Error, Output
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16
Application Information (Continued)
TL/H/12593 – 31
FIGURE 14. Timing Diagram for 5V Output
If the output ripple is large ( l 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, RFF, in series with the feedforward capacitor, approximately 3 times R1, will attenuate the ripple voltage at
the feedback pin.
DELAY CAPACITOR
CDELAYÐProvides 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 mA
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 mF 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 ms.
RPull UpÐThe 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.
INPUT CAPACITOR
CINÐA 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.
FEEDFORWARD CAPACITOR
(Adjustable Output Voltage Version)
CFFÐA Feedforward Capacitor CFF, shown across R2 in
Figure 1 is used when the output voltage is greater than 10V
or then COUT has a very low ESR. This capacitor adds lead
compensation to the feedback loop and increases the
phase margin for better loop stability. For CFF selection, see
the design procedure section.
17
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Application Information (Continued)
TL/H/12593–32
TL/H/12593 – 33
FIGURE 15. RMS Current Ratings for Low
ESR Electrolytic Capacitors (Typical)
FIGURE 16. Capacitor ESR vs Capacitor Voltage Rating
(Typical Low ESR Electrolytic Capacitor)
The consequences of operating an electrolytic capacitor
above the RMS current rating is a shortened operating life.
The higher temperature speeds up the evaporation of the
capacitor’s electrolyte, resulting in eventual failure.
Selecting an input capacitor requires consulting the manufacturers data sheet for maximum allowable RMS ripple current. For a maximum ambient temperature of 40§ C, a general guideline would be to select a capacitor with a ripple current rating of approximately 50% of the DC load current. For
ambient temperatures up to 70§ C, a current rating of 75% of
the DC load current would be a good choice for a conservative design. The capacitor voltage rating must be at least
1.25 times greater than the maximum input voltage, and often a much higher voltage capacitor is needed to satisfy the
RMS current requirements.
A graph shown in Figure 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.
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OUTPUT CAPACITOR
COUTÐAn output capacitor is required to filter the output
and provide regulator loop stability. Low impedance or low
ESR Electrolytic or solid tantalum capacitors designed for
switching regulator applications must be used. When selecting an output capacitor, the important capacitor parameters
are; the 100 kHz Equivalent Series Resistance (ESR), the
RMS ripple current rating, voltage rating, and capacitance
value. For the output capacitor, the ESR value is the most
important parameter.
The output capacitor requires an ESR value that has an
upper and lower limit. For low output ripple voltage, a low
ESR value is needed. This value is determined by the maximum allowable output ripple voltage, typically 1% to 2% of
the output voltage. But if the selected capacitor’s ESR is
extremely low, there is a possibility of an unstable feedback
loop, resulting in an oscillation at the output. Using the capacitors listed in the tables, or similar types, will provide
design solutions under all conditions.
If very low output ripple voltage (less than 15 mV) is required, refer to the section on Output Voltage Ripple and
Transients for a post ripple filter.
An aluminum electrolytic capacitor’s ESR value is related to
the capacitance value and its voltage rating. In most cases,
higher voltage electrolytic capacitors have lower ESR values (see Figure 16 ). Often, capacitors with much higher
voltage ratings may be needed to provide the low ESR values required for low output ripple voltage.
18
Application Information (Continued)
To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Figures 3
through 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 peakto-peak inductor ripple current percentage is not fixed, but is
allowed to change as different design load currents are selected. (See Figure 18 .)
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 Figures 2 and 3
for typical capacitor values, voltage ratings, and manufacturers capacitor types.
Electrolytic capacitors are not recommended for temperatures below b25§ C. The ESR rises dramatically at cold temperatures and typically rises 3X @ b25§ C and as much as
10X at b40§ 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 b25§ 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. Ultrafast 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.
TL/H/12593 – 35
FIGURE 18. (DIIND) Peak-to-Peak Inductor Ripple Current
(as a Percentage of the Load Current) vs Load Current
By allowing the percentage of inductor ripple current to increase for low load currents, the inductor value and size can
be kept relatively low.
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
TL/H/12593 – 34
FIGURE 17. 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.
19
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Application Information (Continued)
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.
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 recommended. (See Figure 1 .) The inductance required is typically
between 1 mH and 5 mH, 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 19 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.
DISCONTINUOUS MODE OPERATION
The selection guide chooses inductor values suitable for
continuous mode operation, but for low current applications
and/or high input voltages, a discontinuous mode design
may be a better choice. It would use an inductor that would
be physically smaller, and would need only one half to one
third the inductance value needed for a continuous mode
design. The peak switch and inductor currents will be higher
in a discontinuous design, but at these low load currents
(200 mA and below), the maximum switch current will still be
less than the switch current limit.
Discontinuous operation can have voltage waveforms that
are considerable different than a continuous design. The
output pin (switch) waveform can have some damped sinusoidal ringing present. (See photo titled; Discontinuous Mode
Switching Waveforms) This ringing is normal for discontinuous operation, and is not caused by feedback loop instabilities. In discontinuous operation, there is a period of time
where neither the switch or the diode are conducting, and
the inductor current has dropped to zero. During this time, a
small amount of energy can circulate between the inductor
and the switch/diode parasitic capacitance causing this
characteristic ringing. Normally this ringing is not a problem,
unless the amplitude becomes great enough to exceed the
input voltage, and even then, there is very little energy present to cause damage.
Different inductor types and/or core materials produce different amounts of this characteristic ringing. Ferrite core inductors have very little core loss and therefore produce the
most ringing. The higher core loss of powdered iron inductors produce less ringing. If desired, a series RC could be
placed in parallel with the inductor to dampen the ringing.
The computer aided design software Switchers Made Simple (version 4.2) will provide all component values for continuous and discontinuous modes of operation.
TL/H/12593–36
FIGURE 19. Post Ripple Filter Waveform
TL/H/12593 – 37
FIGURE 20. Peak-to-Peak Inductor
Ripple Current vs Load Current
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20
Application Information (Continued)
Once the DIIND value is known, the following formulas can
be used to calculate additional information about the switching regulator circuit.
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-topeak amplitude of this inductor current waveform remains
constant. As the load current increases or decreases, the
entire sawtooth current waveform also rises and falls. The
average value (or the center) of this current waveform is
equal to the DC load current.
If the load current drops to a low enough level, the bottom
of the sawtooth current waveform will reach zero, and the
switcher will smoothly change from a continuous to a discontinuous mode of operation. Most switcher designs (irregardless how large the inductor value is) will be forced to
run discontinuous if the output is lightly loaded. This is a
perfectly acceptable mode of operation.
In a switching regulator design, knowing the value of the
peak-to-peak inductor ripple current (DIIND) can be useful
for determining a number of other circuit parameters. Parameters such as, peak inductor or peak switch current,
minimum load current before the circuit becomes discontinuous, output ripple voltage and output capacitor ESR can all
be calculated from the peak-to-peak DIIND. When the inductor nomographs shown in Figures 4 through 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 (DIIND) that can be expected
for different load currents. The curve also shows how the
peak-to-peak inductor ripple current (DIIND) 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:
VOUT e 5V, maximum load current of 800 mA
1. Peak Inductor or peak switch current
e
#I
LOAD a
DIIND
2
J # 0.8A
e
a
0.3
2
J
e 0.95A
2. Minimum load current before the circuit becomes discontinuous
DI
0.3
e IND e
e 0.15A
2
2
3. Output Ripple Voltage e (DIIND) c (ESR of COUT)
4. ESR of COUT e
e
e 0.3A c 0.16X e 48 mV p-p
Output Ripple Voltage (DVOUT)
DIIND
0.048V
e 0.16X
0.30A
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.
VIN e 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 mH inductance region. A
68 mH inductor will allow a peak-to-peak inductor current
(DIIND) 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 (DIIND) on the left
hand 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 Figure 20, it
can be seen that for a load current of 0.8A, the peak-topeak inductor ripple current (DIIND) is 300 mA with 12V in,
and can range from 340 mA at the upper border (14V in) to
225 mA at the lower border (10V in).
21
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Application Information (Continued)
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 (TO220 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 in2, 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 in2, 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.
TL/H/12593–38
Circuit Data for Temperature Rise Curve TO-220 Package (T)
Capacitors
Through hole electrolytic
Inductor
Through hole, Schott, 68 mH
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
TL/H/12593–39
Circuit Data for Temperature Rise Curve TO-263 Package (S)
Capacitors
Surface mount tantalum, molded ‘‘D’’ size
Inductor
Surface mount, Schott, 68 mH
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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22
Application Information (Continued)
SHUTDOWN/SOFT-START
The circuit shown in Figure 23 is a standard buck regulator
with 24V in, 12V out, 280 mA load, and using a 0.068 mF
Soft-start capacitor. The photo in Figures 24 and 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.
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.
TL/H/12593 – 40
FIGURE 24. Output Voltage, Input Current, Error Flag
Signal, at Start-Up, WITH Soft-start
TL/H/12593 – 41
FIGURE 25. Output Voltage, Input Current, at Start-Up,
WITHOUT Soft-start
TL/H/12593 – 42
FIGURE 23. Typical Circuit Using Shutdown/Soft-start and Error Flag Features
23
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Application Information (Continued)
TL/H/12593 – 43
FIGURE 26. Inverting b5V Regulator With Shutdown and Soft-start
An additional diode is required in this regulator configuration. Diode D1 is used to isolate input voltage ripple or noise
from coupling through the CIN 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.
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 mH,
1.5 Amp inductor is the best choice. Capacitor selection can
also be narrowed down to just a few values. Using the values shown in Figure 26 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
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.
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 b5V 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 a 20V to b5V, the regulator would see 25V between the input pin and ground pin. The LM2598 has a maximum input voltage rating of 40V.
TL/H/12593–44
FIGURE 27. Maximum Load Current for Inverting
Regulator Circuit
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24
Application Information (Continued)
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 configuration, while Figures 29 and 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.
TL/H/12593 – 46
FIGURE 30. 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 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 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.
TL/H/12593 – 45
FIGURE 28. Undervoltage Lockout for a Buck Regulator
Figures 29 and 30 apply the same feature to an inverting
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.
TL/H/12593 – 48
FIGURE 31. Charge Pump for Generating a
Low Current, Negative Output Voltage
TL/H/12593 – 47
FIGURE 29. Undervoltage Lockout Without
Hysteresis for an Inverting Regulator
25
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Application Information (Continued)
TYPICAL THROUGH HOLE PC BOARD LAYOUT, FIXED OUTPUT (1X SIZE), DOUBLE SIDED, THROUGH HOLE PLATED
RPULL-UPÐ10 kX
CINÐ150 mF/50V Aluminum Electrolytic, Panasonic ‘‘HFQ series’’
COUTÐ120 mF/25V Aluminum Electrolytic, Panasonic ‘‘HFQ series’’
CDELAYÐ0.1 mF
D1Ð3A, 40V Schottky Rectifier, 1N5822
CSD/SSÐ0.1 mF
TL/H/12593 – 51
L1Ð68 mH, L30, Renco, Through hole
TYPICAL THROUGH HOLE PC BOARD LAYOUT, ADJUSTABLE OUTPUT (1X SIZE), DOUBLE SIDED,
THROUGH HOLE PLATED
CINÐ150 mF/50V, Aluminum Electrolytic, Panasonic ‘‘HFQ series’’
RPULL-UPÐ10 kX
COUTÐ120 mF/25V Aluminum Electrolytic, Panasonic ‘‘HFQ series’’
CDELAYÐ0.1 mF
D1Ð3A, 40V Schottky Rectifier, 1N5822
CSD/SSÐ0.1 mF
L1Ð68 mH, L30, Renco, Through hole
R1Ð1 kX, 1%
R2ÐUse formula in Design Procedure
CFFÐSee Figure 4 .
RFFÐSee Application Information Section (CFF Section)
FIGURE 32. PC Board Layout
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26
TL/H/12593 – 52
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
27
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LM2598 SIMPLE SWITCHER Power Converter
150 kHz 1A Step-Down Voltage Regulator, with Features
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
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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