NSC LM2597HVN-12

LM2597/LM2597HV
SIMPLE SWITCHER ® Power Converter 150 kHz 0.5A
Step-Down Voltage Regulator, with Features
General Description
The LM2597/LM2597HV series of regulators are monolithic
integrated circuits that provide all the active functions for a
step-down (buck) switching regulator, capable of driving a
0.5A load with excellent line and load regulation. These
devices are available in fixed output voltages of 3.3V, 5V,
12V, and an adjustable output version, and are packaged in
an 8-lead DIP and an 8-lead surface mount package.
This series of switching regulators is similar to the LM2594
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†, improved line and load specifications,
fixed-frequency oscillator, Shutdown /Soft-start, error flag
delay and error flag output.
The LM2597/LM2597HV 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. Because of its high efficiency, the copper traces on the printed circuit board are normally the only
heat sinking needed.
A standard series of inductors (both through hole and surface mount types) are available from several different manufacturers optimized for use with the LM2597/LM2597HV series. This feature greatly simplifies the design of
switch-mode power supplies.
Other features include a guaranteed ± 4% tolerance on output voltage under all conditions of input voltage and output
load conditions, and ± 15% on the oscillator frequency. External shutdown is included, featuring typically 85 µA
standby current. Self protection features include a two stage
current limit for the output switch and an over temperature
shutdown for complete protection under fault conditions.
Typical Application
The LM2597HV is for use in applications requiring and input
voltage up to 60V.
Features
n 3.3V, 5V, 12V, and adjustable output versions
n Adjustable version output voltage range, 1.2V to 37V
(57V for HV version) ± 4% max over line and load
conditions
n Guaranteed 0.5A output current
n Available in 8-pin surface mount and DIP-8 package
n Input voltage range up to 60V
n 150 kHz fixed frequency internal oscillator
n Shutdown /Soft-start
n Out of regulation error flag
n Error output delay
n Bias Supply Pin (VBS) for internal circuitry improves
efficiency at high input voltages
n Low power standby mode, IQ typically 85 µA
n High Efficiency
n Uses readily available standard inductors
n Thermal shutdown and current limit protection
Applications
n
n
n
n
Simple high-efficiency step-down (buck) regulator
Efficient pre-regulator for linear regulators
On-card switching regulators
Positive to Negative converter
(Fixed Output Voltage Versions)
DS012440-1
†Patent Number 5,382,918.
SIMPLE SWITCHER ® and Switchers Made Simple
®
are registered trademarks of National Semiconductor Corporation.
© 2001 National Semiconductor Corporation
DS012440
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LM2597/LM2597HV SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down Voltage
Regulator, with Features
December 2000
LM2597/LM2597HV
Absolute Maximum Ratings (Note 1)
ESD Susceptibility
Human Body Model (Note 3)
Lead Temperature
M8 Package
Vapor Phase (60 sec.)
Infrared (15 sec.)
N Package (Soldering, 10 sec.)
Maximum Junction Temperature
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Maximum Supply Voltage (VIN)
LM2597
LM2597HV
SD /SS Pin Input Voltage (Note 2)
Delay Pin Voltage (Note 2)
Flag Pin Voltage
Bias Supply Voltage (VBS)
Feedback Pin Voltage
Output Voltage to Ground
(Steady State)
Power Dissipation
Storage Temperature Range
45V
60V
6V
1.5V
−0.3 ≤ V ≤45V
−0.3 ≤ V ≤30V
−0.3 ≤ V ≤+25V
2 kV
+215˚C
+220˚C
+260˚C
+150˚C
Operating Conditions
−40˚C ≤ TJ +125˚C
Temperature Range
Supply Voltage
LM2597
LM2597HV
−1V
Internally limited
−65˚C to +150˚C
4.5V to 40V
4.5V to 60V
LM2597/LM2597HV-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.VINmax =40V for the LM2597 and 60V for the LM2597HV
Symbol
Parameter
Conditions
LM2597/LM2597HV-3.3
Typ
Limit
(Note 4)
(Note 5)
Units
(Limits)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 12
VOUT
η
Output Voltage
Efficiency
4.75V ≤ VIN ≤ VINmax, 0.1A ≤ ILOAD ≤ 0.5A
VIN = 12V, ILOAD = 0.5A
3.3
V
3.168/3.135
V(min)
3.432/3.465
V(max)
80
%
LM2597/LM2597HV-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.VINmax =40V for the LM2597 and 60V for the LM2597HV
Symbol
Parameter
Conditions
LM2597/LM2597HV-5.0
Typ
Limit
(Note 4)
(Note 5)
Units
(Limits)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 12
VOUT
η
Output Voltage
Efficiency
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7V ≤ VIN ≤ VINmax, 0.1A ≤ ILOAD ≤ 0.5A
VIN = 12V, ILOAD = 0.5A
5
82
2
V
4.800/4.750
V(min)
5.200/5.250
V(max)
%
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range.VINmax =40V for the LM2597 and 60V for the LM2597HV
Symbol
Parameter
Conditions
LM2597/LM2597HV-12
Typ
Limit
(Note 4)
(Note 5)
Units
(Limits)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 12
VOUT
η
15V ≤ VIN ≤ VINmax, 0.1A ≤ ILOAD ≤ 0.5A
Output Voltage
Efficiency
VIN = 25V, ILOAD = 0.5A
12
V
11.52/11.40
V(min)
12.48/12.60
V(max)
88
%
LM2597/LM2597HV-ADJ
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range.VINmax =40V for the LM2597 and 60V for the LM2597HV
Symbol
Parameter
Conditions
LM2597/LM2597HV-ADJ
Typ
Limit
(Note 4)
(Note 5)
Units
(Limits)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 12
VFB
Feedback Voltage
4.5V ≤ VIN ≤ VINmax, 0.1A ≤ ILOAD ≤ 0.5A
1.230
VOUT programmed for 3V. Circuit of Figure 12.
η
Efficiency
VIN = 12V, VOUT = 3V, ILOAD = 0.5A
V
1.193/1.180
V(min)
1.267/1.280
V(max)
80
%
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, VIN = 12V for the 3.3V, 5V, and Adjustable version and VIN = 24V for the 12V version. ILOAD = 100 mA.
Symbol
Parameter
Conditions
LM2597/LM2597HV-XX
Units
(Limits)
Typ
Limit
(Note 4)
(Note 5)
50/100
nA
127/110
kHz(min)
173/173
kHz(max)
DEVICE PARAMETERS
Ib
Feedback Bias Current
Adjustable Version Only, VFB = 1.235V
10
fO
Oscillator Frequency
(Note 7)
150
VSAT
Saturation Voltage
IOUT = 0.5A (Notes 8 and 9)
0.9
DC
Max Duty Cycle (ON)
(Note 9)
100
Min Duty Cycle (OFF)
(Note 10)
0
Current Limit
Peak Current, (Notes 8 and 9)
kHz
V
1.1/1.2
ICL
IL
Output Leakage Current
(Notes 8, 10 and 11)
IQ
Operating Quiescent
%
0.8
Output = 0V
Output = −1V
A
0.65/0.58
A(min)
1.3/1.4
A(max)
50
µA(max)
15
mA(max)
2
SD /SS Pin Open, VBS Pin Open(Note 10)
3
5
V(max)
mA
mA
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LM2597/LM2597HV
LM2597/LM2597HV-12
Electrical Characteristics
LM2597/LM2597HV
All Output Voltage Versions
Electrical Characteristics (Continued)
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range. Unless otherwise specified, VIN = 12V for the 3.3V, 5V, and Adjustable version and VIN = 24V for the 12V version. ILOAD = 100 mA.
Symbol
Parameter
Conditions
LM2597/LM2597HV-XX
Typ
Limit
(Note 4)
(Note 5)
Units
(Limits)
DEVICE PARAMETERS
Current
ISTBY
Standby Quiescent
SD /SS pin = 0V
(Note 10)LM2597
Current
LM2597HV
θJA
Thermal Resistance
10
mA(max)
200/250
µA(max)
250/300
µA(max)
85
140
N Package, Junction to Ambient (Note 12)
95
M Package, Junction to Ambient (Note 12)
150
µA
˚C/W
SHUTDOWN/SOFT-START CONTROL Test Circuit of Figure 12
VSD
Shutdown Threshold
Voltage
VSS
ISD
ISS
Soft-start Voltage
Shutdown Current
1.3
V
Low, (Shutdown Mode)
0.6
V(max)
High, (Soft-start Mode)
2
V(min)
VOUT = 20% of Nominal Output Voltage
2
VOUT = 100% of Nominal Output Voltage
3
VSHUTDOWN = 0.5V
5
V
µA
10
µA(max)
5
µA(max)
Detector
92
%(min)
Threshold Voltage
98
%(max)
0.7/1.0
V(max)
Soft-start Current
VSoft-start = 2.5V
1.6
µA
FLAG/DELAY CONTROL Test Circuit of Figure 12
Regulator Dropout
VFSAT
IFL
Low (Flag ON)
Flag Output Saturation
ISINK = 3 mA
Voltage
VDELAY = 0.5V
Flag Output Leakage
Current
VFLAG = 40V
96
0.3
VDELAY = 0.5V
V
1.21
V(min)
1.29
V(max)
6
µA(max)
350/400
mV(max)
400
µA(max)
10
mA(max)
2
mA
3
Current
Delay Pin Saturation
µA
1.25
Low (Flag ON)
High (Flag OFF) and VOUT Regulated
Delay Pin Source
V
0.3
Delay Pin Threshold
Voltage
%
Low (Flag ON)
µA
55
mV
BIAS SUPPLY
IBS
Bias Supply Pin Current
VBS = 2V
(Note 10)
VBS = 4.4V (Note 10)
IQ
Operating Quiescent
Current
120
µA
4
VBS = 4.4V , Vin pin current(Note 10)
1
mA
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.
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4
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
LM2597/LM2597HV is used as shown in the Figure 12 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 = 40V for the LM2597 and 60V for the LM2597HV.
Note 12: Junction to ambient thermal resistance with approximately 1 square inch of printed circuit board copper surrounding the leads. Additional copper area will
lower thermal resistance further. See application hints in this data sheet and the thermal model in Switchers Made Simple ™ software.
Typical Performance Characteristics
Normalized
Output Voltage
Efficiency
Line Regulation
DS012440-4
DS012440-3
DS012440-2
Switch Saturation
Voltage
Dropout Voltage
Switch Current Limit
DS012440-6
DS012440-7
DS012440-5
5
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LM2597/LM2597HV
All Output Voltage Versions
Electrical Characteristics (Continued)
LM2597/LM2597HV
Typical Performance Characteristics
Quiescent Current
(Continued)
Standby
Quiescent Current
Minimum Operating
Supply Voltage
DS012440-8
DS012440-9
Feedback Pin
Bias Current
Flag Saturation
Voltage
DS012440-10
Switching Frequency
DS012440-13
DS012440-11
Soft-start
DS012440-12
Shutdown /Soft-start
Current
Delay Pin Current
DS012440-14
DS012440-16
DS012440-15
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6
VIN and VBS Current vs
VBS and Temperature
LM2597/LM2597HV
Typical Performance Characteristics
(Continued)
Shutdown /Soft-start
Threshold Voltage
Soft-start Response
DS012440-18
DS012440-25
DS012440-17
Continuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 400 mA
L = 100 µH, COUT = 120 µF, COUT ESR = 140 mΩ
Discontinuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 200 mA
L = 33 µH, COUT = 220 µF, COUT ESR = 60 mΩ
DS012440-19
DS012440-20
A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.2A/div.
C: Output Ripple Voltage, 20 mV/div.
A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.2A/div.
C: Output Ripple Voltage, 20 mV/div.
Horizontal Time Base: 2 µs/div.
Horizontal Time Base: 2 µs/div.
Load Transient Response for Continuous Mode
VIN = 20V, VOUT = 5V, ILOAD = 200 mA to 500 mA
L = 100 µH, COUT = 120 µF, COUT ESR = 140 mΩ
Load Transient Response for Discontinuous Mode
VIN = 20V, VOUT = 5V, ILOAD = 100 mA to 200 mA
L = 33 µH, COUT = 220 µF, COUT ESR = 60 mΩ
DS012440-21
A: Output Voltage, 50 mV/div. (AC)
B: 200 mA to 500 mA Load Pulse
Horizontal Time Base: 50 µs/div.
DS012440-22
A: Output Voltage, 50 mV/div. (AC)
B: 100 mA to 200 mA Load Pulse
Horizontal Time Base: 200 µs/div.
7
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LM2597/LM2597HV
Connection Diagrams and Ordering Information
8–Lead DIP (N)
8–Lead Surface Mount (M)
DS012440-23
DS012440-24
Top View
Order Number LM2597N-3.3,
LM2597N-5.0, LM2597N-12 or
LM2597N-ADJ
LM2597HVN-3.3, LM2597HVN-5.0,
LM2597HVN-12 or LM2597HVN-ADJ
See NS Package Number N08E
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Top View
Order Number LM2597M-3.3,
LM2597M-5.0, LM2597M-12 or
LM2597M-ADJ
LM2597HVM-3.3, LM2597HVM-5.0,
LM2597HVM-12 or LM2597HVM-ADJ
See NS Package Number M08A
8
PROCEDURE (Fixed Output Voltage Version)
EXAMPLE (Fixed Output Voltage Version)
Given:
VOUT = Regulated Output Voltage (3.3V, 5V or 12V)
VIN(max) = Maximum DC Input Voltage
Given:
VOUT = 5V
VIN(max) = 12V
ILOAD(max) = 0.4A
ILOAD(max) = Maximum Load Current
1. Inductor Selection (L1)
A. Select the correct inductor value selection guide from
Figure 3, Figure 4, or Figure 5. (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 7.
1. Inductor Selection (L1)
A. Use the inductor selection guide for the 5V version shown
in Figure 4.
B. From the inductor value selection guide shown in Figure 4,
the inductance region intersected by the 12V horizontal line
and the 0.4A vertical line is 100 µH, and the inductor code is
L20.
C. The inductance value required is 100 µH. From the table in
Figure 7, go to the L20 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. In the majority of applications, low ESR (Equivalent Series
Resistance) electrolytic capacitors between 82 µF and 220 µF
and low ESR solid tantalum capacitors between 15 µF and
100 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 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 1.
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 Switchers Made
Simple ® version 4.1 or later).
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 1, 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 0.5A 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.
120 µF 25V Panasonic HFQ Series
120 µF 25V Nichicon PL Series
C. For a 5V output, a capacitor voltage rating at least 7.5V or
more is needed. But, in this example, even a low ESR,
switching grade, 120 µF 10V aluminum electrolytic capacitor
would exhibit approximately 400 mΩ of ESR (see the curve in
Figure 17 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.
9
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LM2597/LM2597HV
LM2597/LM2597HV Series Buck Regulator Design Procedure (Fixed
Output)
LM2597/LM2597HV
LM2597/LM2597HV Series Buck Regulator Design Procedure (Fixed
Output) (Continued)
PROCEDURE (Fixed Output Voltage Version)
EXAMPLE (Fixed Output Voltage Version)
3. Catch Diode Selection (D1)
A. The catch diode current rating must be at least 1.3 times
greater than the maximum load current. Also, if the power
supply design must withstand a continuous output short, the
diode should have a current rating equal to the maximum
current limit of the LM2597. The most stressful condition for
this diode is an overload or shorted output condition.
B. The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage.
C. This diode must be fast (short reverse recovery time) and
must be located close to the LM2597 using short leads and
short printed circuit traces. Because of their fast switching
speed and low forward voltage drop, Schottky diodes provide
the best performance and efficiency, and should be the first
choice, especially in low output voltage applications. Ultra-fast
recovery, or High-Efficiency rectifiers also provide good results. Ultra-fast recovery diodes typically have reverse recovery times of 50 ns or less. Rectifiers such as the 1N4001
series are much too slow and should not be used.
3. Catch Diode Selection (D1)
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 1⁄2 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 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 recommended
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 x 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 1⁄2 the DC load current. In
this example, with a 400 mA load, a capacitor with a RMS
current rating of at least 200 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 200
mA. Either a 47 µF or 68 µF, 25V capacitor could be used.
For a through hole design, a 68 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.
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A. Refer to the table shown in Figure 10. In this example, a
1A, 20V, 1N5817 Schottky diode will provide the best performance, and will not be overstressed even for a shorted output.
10
Conditions
Inductor
Output Capacitor
Through Hole
Surface Mount
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
0.5
0.2
5
0.5
0.2
12
0.5
0.2
5
33
L14
220/16
220/16
100/16
100/6.3
7
47
L13
120/25
120/25
100/16
100/6.3
10
68
L21
120/25
120/25
100/16
100/6.3
40
100
L20
120/35
120/35
100/16
100/6.3
6
68
L4
120/25
120/25
100/16
100/6.3
10
150
L10
120/16
120/16
100/16
100/6.3
40
220
L9
120/16
120/16
100/16
100/6.3
8
47
L13
180/16
180/16
100/16
33/25
10
68
L21
180/16
180/16
100/16
33/25
15
100
L20
120/25
120/25
100/16
33/25
40
150
L19
120/25
120/25
100/16
33/25
9
150
L10
82/16
82/16
100/16
33/25
20
220
L9
120/16
120/16
100/16
33/25
40
330
L8
120/16
120/16
100/16
33/25
15
68
L21
82/25
82/25
100/16
15/25
18
150
L19
82/25
82/25
100/16
15/25
30
220
L27
82/25
82/25
100/16
15/25
40
330
L26
82/25
82/25
100/16
15/25
15
100
L11
82/25
82/25
100/16
15/25
20
220
L9
82/25
82/25
100/16
15/25
40
330
L17
82/25
82/25
100/16
15/25
FIGURE 1. LM2597/LM2597HV Fixed Voltage Quick Design Component Selection Table
11
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LM2597/LM2597HV
LM2597/LM2597HV Series Buck Regulator Design Procedure (Fixed
Output) (Continued)
LM2597/LM2597HV
LM2597/LM2597HV Series Buck Regulator Design Procedure (Adjustable
Output)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
Given:
VOUT = Regulated Output Voltage
VIN(max) = Maximum Input Voltage
Given:
ILOAD(max) = Maximum Load Current
F = Switching Frequency (Fixed at a nominal 150 kHz).
ILOAD(max) = 0.5A
F = Switching Frequency (Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R1 and R2, as
shown in Figure 12)
1. Programming Output Voltage (Selecting R1 and R2, as
shown in Figure 12)
Use the following formula to select the appropriate resistor
values.
Select R1 to be 1 kΩ, 1%. Solve for R2.
VOUT = 20V
VIN(max) = 28V
R2 = 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ.
R2 = 15.4 kΩ.
Select a value for R1 between 240Ω and 1.5 kΩ. The lower
resistor values minimize noise pickup in the sensitive feedback pin. (For the lowest temperature coefficient and the best
stability with time, use 1% metal film resistors.)
2. Inductor Selection (L1)
A. Calculate the inductor Volt microsecond constant E • T
(V • µs), from the following formula:
2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant (E • T),
where VSAT = internal switch saturation voltage = 0.9V
and VD = diode forward voltage drop = 0.5V
B. E • T = 35.2 (V • µs)
C. ILOAD(max) = 0.5A
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 6.
D. From the inductor value selection guide shown in Figure 6,
the inductance region intersected by the 35 (V • µs) horizontal
line and the 0.5A vertical line is 150 µH, and the inductor code
is L19.
E. From the table in Figure 7, locate line L19, and select an
inductor part number from the list of manufacturers part numbers.
C. on the horizontal axis, select the maximum load current.
D. Identify the inductance region intersected by the E • T
value and the Maximum Load Current value. Each region is
identified by an inductance value and an inductor code (LXX).
E. Select an appropriate inductor from the four manufacturer’s
part numbers listed in Figure 7.
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12
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
3. Output Capacitor SeIection (COUT)
3. Output Capacitor Selection (COUT)
A. In the majority of applications, low ESR electrolytic or solid
tantalum capacitors between 82 µF and 220 µF provide the
best results. This capacitor should be located close to the IC
using short capacitor leads and short copper traces. Do not
use capacitors larger than 220 µF. For additional 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 2. 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.
A. See section on COUT in Application Information section.
B. From the quick design table shown in Figure 2, locate the
output voltage column. From that column, locate the output
voltage closest to the output voltage in your application. In this
example, select the 24V line. Under the output capacitor
section, select a capacitor from the list of through hole electrolytic or surface mount tantalum types from four different
capacitor manufacturers. It is recommended that both the
manufacturers and the manufacturers series that are listed in
the table be used.
In this example, through hole aluminum electrolytic capacitors
from several different manufacturers are available.
82 µF 50V Panasonic HFQ Series
120 µF 50V Nichicon PL Series
C. For a 20V output, a capacitor rating of at least 30V or more
is needed. In this example, either a 35V or 50V capacitor
would work. A 50V rating was chosen because it has a lower
ESR which provides a lower output ripple voltage.
Other manufacturers or other types of capacitors may also be
used, provided the capacitor specifications (especially the
100 kHz ESR) closely match the types listed in the table.
Refer to the capacitor manufacturers data sheet for this information.
4. Feedforward Capacitor (CFF) (See Figure 12)
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.
4. Feedforward Capacitor (CFF)
The table shown in Figure 2 contains feed forward capacitor
values for various output voltages. In this example, a 1 nF
capacitor is needed.
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.)
13
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LM2597/LM2597HV
LM2597/LM2597HV Series Buck Regulator Design Procedure (Adjustable
Output) (Continued)
LM2597/LM2597HV
LM2597/LM2597HV Series Buck Regulator Design Procedure (Adjustable
Output) (Continued)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
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 LM2597. The most stressful condition for
this diode is an overload or shorted output condition.
B. The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage.
C. This diode must be fast (short reverse recovery time) and
must be located close to the LM2597 using short leads and
short printed circuit traces. Because of their fast switching
speed and low forward voltage drop, Schottky diodes provide
the best performance and efficiency, and should be the first
choice, especially in low output voltage applications. Ultra-fast
recovery, or High-Efficiency rectifiers are also a good choice,
but some types with an abrupt turn-off characteristic may
cause instability or EMl problems. Ultra-fast recovery diodes
typically have reverse recovery times of 50 ns or less. Rectifiers such as the 1N4001 series are much too slow and should
not be used.
5. Catch Diode Selection (D1)
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 1⁄2 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 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 ceramic capacitors 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.
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 x
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 x 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 1⁄2 the DC load current. In
this example, with a 400 mA load, a capacitor with a RMS
current rating of at least 200 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 200 mA. A 47 µF/50V low ESR electrolytic
capacitor capacitor is needed.
For a through hole design, a 47 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 are
recommended. The TPS series available from AVX, and the
593D series from Sprague are both surge current tested.
A. Refer to the table shown in Figure 10. Schottky diodes
provide the best performance, and in this example a 1A, 40V,
1N5819 Schottky diode would be a good choice. The 1A diode
rating is more than adequate and will not be overstressed
even for a shorted output.
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 4.1 or later) is
available at National’s web site, www.national.com.
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14
Output
Voltage
(V)
Through Hole Output Capacitor
Surface Mount 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
220/25
220/25
0
220/10
220/10
0
4
180/25
180/25
4.7 nF
100/10
120/10
4.7 nF
6
82/25
82/25
4.7 nF
100/10
120/10
4.7 nF
9
82/25
82/25
3.3 nF
100/16
100/16
3.3 nF
12
82/25
82/25
2.2 nF
100/16
100/16
2.2 nF
15
82/25
82/25
1.5 nF
68/20
100/20
1.5 nF
24
82/50
120/50
1 nF
10/35
15/35
220 pF
28
82/50
120/50
820 pF
10/35
15/35
220 pF
FIGURE 2. Output Capacitor and Feedforward Capacitor Selection Table
15
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LM2597/LM2597HV
LM2597/LM2597HV Series Buck Regulator Design Procedure (Adjustable
Output) (Continued)
LM2597/LM2597HV
LM2597/LM2597HV Series Buck Regulator Design Procedure
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)
DS012440-57
DS012440-30
FIGURE 3. LM2597/LM2597HV-3.3
FIGURE 4. LM2597/LM2597HV-5.0
DS012440-58
DS012440-32
FIGURE 5. LM2597/LM2597HV-12
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FIGURE 6. LM2597/LM2597HV-ADJ
16
Inductance
(µH)
(Continued)
Current
(A)
Through
Surface
Through
Surface
Through
Surface
Hole
Mount
Hole
Mount
Hole
Mount
Mount
Schott
Renco
Pulse Engineering
Coilcraft
Surface
L1
220
0.18
67143910
67144280 RL-5470-3
RL1500-220 PE-53801
PE-53801-S
DO1608-224
L2
150
0.21
67143920
67144290 RL-5470-4
RL1500-150 PE-53802
PE-53802-S
DO1608-154
L3
100
0.26
67143930
67144300 RL-5470-5
RL1500-100 PE-53803
PE-53803-S
DO1608-104
L4
68
0.32
67143940
67144310 RL-1284-68
RL1500-68
PE-53804
PE-53804-S
DO1608-68
L5
47
0.37
67148310
67148420 RL-1284-47
RL1500-47
PE-53805
PE-53805-S
DO1608-473
L6
33
0.44
67148320
67148430 RL-1284-33
RL1500-33
PE-53806
PE-53806-S
DO1608-333
L7
22
0.60
67148330
67148440 RL-1284-22
RL1500-22
PE-53807
PE-53807-S
DO1608-223
L8
330
0.26
67143950
67144320 RL-5470-2
RL1500-330 PE-53808
PE-53808-S
DO3308-334
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
DO1608-683
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
RL1500-33
PE-53814
PE-53814-S
DO1608-333
L15
22
0.99
67148350
67148460 RL-1284-22
RL1500-22
PE-53815
PE-53815-S
DO1608-223
L16
15
1.24
67148360
67148470 RL-1284-15
RL1500-15
PE-53816
PE-53816-S
DO1608-153
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
DDO3316-683
L26
330
0.80
67144100
67144480 RL-5471-1
—
PE-53826
PE-53826-S
—
L27
220
1.00
67144110
67144490 RL-5471-2
—
PE-53827
PE-53827-S
—
FIGURE 7. Inductor Manufacturers Part Numbers
Coilcraft Inc.
Coilcraft Inc., Europe
Pulse Engineering Inc.
Phone
(800) 322-2645
FAX
(708) 639-1469
Phone
+44 1236 730 595
FAX
+44 1236 730 627
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
Nichicon Corp.
Panasonic
AVX Corp.
Sprague/Vishay
Phone
(708) 843-7500
FAX
(708) 843-2798
Phone
(714) 373-7857
FAX
(714) 373-7102
Phone
(803) 448-9411
FAX
(803) 448-1943
Phone
(207) 324-7223
FAX
(207) 324-4140
FIGURE 9. Capacitor Manufacturers Phone Numbers
FIGURE 8. Inductor Manufacturers Phone Numbers
17
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LM2597/LM2597HV
LM2597/LM2597HV Series Buck Regulator Design Procedure
LM2597/LM2597HV
LM2597/LM2597HV Series Buck Regulator Design Procedure
VR
(Continued)
1A Diodes
Surface Mount
Schottky
Through Hole
Ultra Fast
Schottky
Recovery
20V
Ultra Fast
Recovery
All of these diodes are rated to
1N5817
All of these diodes are rated to
at least 60V.
SR102
at least 60V.
MBRS130
1N5818
30V
SR103
11DQ03
40V
50V
or
more
MBRS140
MURS120
1N5819
MUR120
10BQ040
10BF10
SR104
HER101
10MQ040
11DQ04
MBRS160
SR105
10BQ050
MBR150
10MQ060
11DQ05
MBRS1100
MBR160
10MQ090
SB160
SGL41-60
11DQ10
11DF1
SS16
FIGURE 10. Diode Selection Table
Block Diagram
DS012440-26
FIGURE 11.
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18
LM2597/LM2597HV
Typical Circuit and Layout Guidelines
Fixed Output Voltage Versions
DS012440-27
Component Values shown are for VIN = 15V, VOUT = 5V, ILOAD = 500 mA.
— 47 µF, 50V, Aluminum Electrolytic Nichicon “PL Series”
CIN
COUT — 120 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”
D1
— 1A, 30V Schottky Rectifier, 1N5818
L1
— 100 µH, L20
Typical Values
CSS
— 0.1 µF
CDELAY — 0.1 µF
RPull Up — 4.7k
*Use Bias Supply pin for 5V and 12V Versions
Adjustable Output Voltage Versions
DS012440-56
Select R1 to be approximately 1 kΩ, use a 1% resistor for best stability.
Component Values shown are for VIN = 20V,
VOUT = 10V, ILOAD = 500 mA.
— 68 µF, 35V, Aluminum Electrolytic Nichicon “PL Series”
CIN
COUT — 120 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”
D1
— 1A, 30V Schottky Rectifier, 1N5818
L1
— 150 µH, L19
— 1 kΩ, 1%
R1
— 7.15k, 1%
R2
CFF — 3.3 nF, See Application Information Section
Typical Values
CSS — 0.1 µF
CDELAY — 0.1 µF
RPULL UP — 4.7k
*For output voltages between 4V and 20V
FIGURE 12. Typical Circuits and Layout Guides
19
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LM2597/LM2597HV
Typical Circuit and Layout
Guidelines (Continued)
Special Note If any of the above four features (Shutdown
/Soft-start, Error Flag, Delay, or Bias Supply) are not used,
the respective pins should be left open.
As in any switching regulator, layout is very important. Rapidly switching currents associated with wiring inductance can
generate voltage transients which can cause problems. For
minimal inductance and ground loops, the wires indicated by
heavy lines should be wide printed circuit traces and
should be kept as short as possible. For best results,
external components should be located as close to the
switcher lC as possible using ground plane construction or
single point grounding.
If open core inductors are used, special care must be
taken as to the location and positioning of this type of inductor. Allowing the inductor flux to intersect sensitive feedback,
lC groundpath and 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.)
EXTERNAL COMPONENTS
SOFT-START CAPACITOR
CSS — A capacitor on this pin provides the regulator with a
Soft-start feature (slow start-up). When the DC input voltage
is first applied to the regulator, or when the Shutdown
/Soft-start pin is allowed to go high, a constant current
(approximately 5 µA begins charging this capacitor). As the
capacitor voltage rises, the regulator goes through four operating regions (See the bottom curve in Figure 13).
1. Regulator in Shutdown. When the SD /SS pin voltage is
between 0V and 1.3V, the regulator is in shutdown, the
output voltage is zero, and the IC quiescent current is approximately 85 µA.
2. Regulator ON, but the output voltage is zero. With the
SD /SS pin voltage between approximately 1.3V and 1.8V,
the internal regulatory circuitry is operating, the quiescent
current rises to approximately 5 mA, but the output voltage is
still zero. Also, as the 1.3V threshold is exceeded, the
Soft-start capacitor charging current decreases from 5 µA
down to approximately 1.6 µA. This decreases the slope of
capacitor voltage ramp.
3. Soft-start Region. When the SD /SS pin voltage is between 1.8V and 2.8V (@ 25˚C), the regulator is in a Soft-start
condition. The switch (Pin 8) duty cycle initially starts out
very low, with narrow pulses and gradually get wider as the
capacitor SD /SS pin ramps up towards 2.8V. As the duty
cycle increases, the output voltage also increases at a controlled ramp up. See the center curve in Figure 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.
Application Information
PIN FUNCTIONS
+VIN (Pin 7) — This is the positive input supply for the IC
switching regulator. A suitable input bypass capacitor must
be present at this pin to minimize voltage transients and to
supply the switching currents needed by the regulator.
Ground (Pin 6) — Circuit ground.
Output (Pin 8) — Internal switch. The voltage at this pin
switches between (+VIN − VSAT) and approximately −0.5V,
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 4) — Senses the regulated output voltage to
complete the feedback loop.
Shutdown /Soft-start (Pin 5) — This dual function pin provides the following features: (a) Allows the switching regulator circuit to be shut down using logic level signals thus
dropping the total input supply current to approximately
80 µA. (b) Adding a capacitor to this pin provides a soft-start
feature which minimizes startup current and provides a controlled ramp up of the output voltage.
Error Flag (Pin 1) — Open collector output that provides a
low signal (flag transistor ON) when the regulated output
voltage drops more than 5% from the nominal output voltage. On start up, Error Flag is low until 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 2) — At power-up, this pin can be used to provide
a time delay between the time the regulated output voltage
reaches 95% of the nominal output voltage, and the time the
error flag output goes high.
Bias Supply (Pin 3) — This feature allows the regulators
internal circuitry to be powered from the regulated output
voltage or an external supply, instead of the input voltage.
This results in increased efficiency under some operating
conditions, such as low output current and/or high input
voltage.
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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.
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.
20
LM2597/LM2597HV
Application Information
(Continued)
DS012440-33
FIGURE 13. Soft-start, Delay, Error, Output
21
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LM2597/LM2597HV
Application Information
(Continued)
DS012440-34
FIGURE 14. Timing Diagram for 5V Output
DS012440-75
FIGURE 15. External 3.7V Soft-Start Clamp
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.
RPull Up — The error flag output, (or power OK) is the collector of a NPN transistor, with the emitter internally
grounded. To use the error flag, a pullup resistor to a positive
voltage is needed. The error flag transistor is rated up to a
maximum of 45V and can sink approximately 3 mA. If the
error flag is not used, it can be left open.
DELAY CAPACITOR
CDELAY — Provides delay for the error flag output. See the
upper curve in Figure 13, and also refer to timing diagrams in
Figure 14. A capacitor on this pin provides a time delay
between the time the regulated output voltage (when it is
increasing in value) reaches 95% of the nominal output
voltage, and the time the error flag output goes high. A 3 µA
constant current from the delay pin charges the delay capacitor resulting in a voltage ramp. When this voltage
reaches a threshold of approximately 1.3V, the open collector error flag output (or power OK) goes high. This signal can
be used to indicate that the regulated output has reached the
correct voltage and has stabilized.
If, for any reason, the regulated output voltage drops by 5%
or more, the error output flag (Pin 1) immediately goes low
(internal transistor turns on). The delay capacitor provides
very little delay if the regulated output is dropping out of
regulation. The delay time for an output that is decreasing is
approximately a 1000 times less than the delay for the rising
output. For a 0.1 µF delay capacitor, the delay time would be
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INPUT CAPACITOR
CIN — A low ESR aluminum or tantalum bypass capacitor is
needed between the input pin and ground pin. It must be
located near the regulator using short leads. This capacitor
prevents large voltage transients from appearing at the input, and provides the instantaneous current needed each
time the switch turns on.
The important parameters for the Input capacitor are the
voltage rating and the RMS current rating. Because of the
relatively high RMS currents flowing in a buck regulator’s
input capacitor, this capacitor should be chosen for its RMS
22
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 16 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.
(Continued)
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.
OUTPUT CAPACITOR
COUT — An output capacitor is required to filter the output
and provide regulator loop stability. Low impedance or low
ESR Electrolytic or solid tantalum capacitors designed for
switching regulator applications must be used. When selecting an output capacitor, the important capacitor parameters
are; the 100 kHz Equivalent Series Resistance (ESR), the
RMS ripple current rating, voltage rating, and capacitance
value. For the output capacitor, the ESR value is the most
important parameter.
The output capacitor requires an ESR value that has an
upper and lower limit. For low output ripple voltage, a low
ESR value is needed. This value is determined by the maximum allowable output ripple voltage, typically 1% to 2% of
the output voltage. But if the selected capacitor’s ESR is
extremely low, there is a possibility of an unstable feedback
loop, resulting in an oscillation at the output. Using the
capacitors listed in the tables, or similar types, will provide
design solutions under all conditions.
If very low output ripple voltage (less than 15 mV) is required, refer to the section on Output Voltage Ripple and
Transients for a post ripple filter.
An aluminum electrolytic capacitor’s ESR value is related to
the capacitance value and its voltage rating. In most cases,
Higher voltage electrolytic capacitors have lower ESR values
(see Figure 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
DS012440-28
FIGURE 16. RMS Current Ratings for Low
ESR Electrolytic Capacitors (Typical)
DS012440-29
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
23
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LM2597/LM2597HV
Application Information
LM2597/LM2597HV
Application Information
To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Figure 3 through
Figure 6). This guide assumes that the regulator is operating
in the continuous mode, and selects an inductor that will
allow a peak-to-peak inductor ripple current to be a certain
percentage of the maximum design load current. This
peak-to-peak inductor ripple current percentage is not fixed,
but is allowed to change as different design load currents are
selected. (See Figure 19.)
(Continued)
quick design component selection tables in Figure 1 and
Figure 2 for typical capacitor values, voltage ratings, and
manufacturers capacitor types.
Electrolytic capacitors are not recommended for temperatures below −25˚C. The ESR rises dramatically at cold temperatures and typically rises 3X @ −25˚C and as much as
10X at −40˚C. See curve shown in Figure 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 LM2594 using
short leads and short printed circuit traces.
Because of their very fast switching speed and low forward
voltage drop, Schottky diodes provide the best performance,
especially in low output voltage applications (5V and lower).
Ultra-fast recovery, or High-Efficiency rectifiers are also a
good choice, but some types with an abrupt turnoff characteristic may cause instability or EMI problems. Ultra-fast
recovery diodes typically have reverse recovery times of 50
ns or less. Rectifiers such as the 1N4001 series are much
too slow and should not be used.
DS012440-31
FIGURE 19. (∆IIND) 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
wrapped on a ferrite bobbin. This type of construction makes
for a inexpensive inductor, but since the magnetic flux is not
completely contained within the core, it generates more
Electro-Magnetic Interference (EMl). This magnetic flux can
induce voltages into nearby printed circuit traces, thus causing problems with both the switching regulator operation and
nearby sensitive circuitry, and can give incorrect scope readings because of induced voltages in the scope probe. Also
see section on Open Core Inductors.
The inductors listed in the selection chart include ferrite
E-core construction for Schott, ferrite bobbin core for Renco
and Coilcraft, and powdered iron toroid for Pulse Engineering.
Exceeding an inductor’s maximum current rating may cause
the inductor to overheat because of the copper wire losses,
or the core may saturate. If the inductor begins to saturate,
the inductance decreases rapidly and the inductor begins to
look mainly resistive (the DC resistance of the winding). This
can cause the switch current to rise very rapidly and force
the switch into a cycle-by-cycle current limit, thus reducing
the DC output load current. This can also result in overheating of the inductor and/or the LM2597. Different inductor
types have different saturation characteristics, and this
should be kept in mind when selecting an inductor.
DS012440-37
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 LM2597 (or any of the Simple Switcher family) can be
used for both continuous or discontinuous modes of operation.
In many cases the preferred mode of operation is the continuous mode. It offers greater output power, lower peak
switch, inductor and diode currents, and can have lower
output ripple voltage. But it does require larger inductor
values to keep the inductor current flowing continuously,
especially at low output load currents and/or high input voltages.
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24
voltage, the ESR of the output capacitor must be low, however, caution must be exercised when using extremely low
ESR capacitors because they can affect the loop stability,
resulting in oscillation problems. If very low output ripple
voltage is needed (less than 15 mV), a post ripple filter is
recommended. (See Figure 12.) 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.
(Continued)
The inductor manufacturers data sheets include current and
energy limits to avoid inductor saturation.
DISCONTINUOUS MODE OPERATION
The selection guide chooses inductor values suitable for
continuous mode operation, but for low current applications
and/or high input voltages, a discontinuous mode design
may be a better choice. It would use an inductor that would
be physically smaller, and would need only one half to one
third the inductance value needed for a continuous mode
design. The peak switch and inductor currents will be higher
in a discontinuous design, but at these low load currents
(200 mA and below), the maximum switch current will still be
less than the switch current limit.
Discontinuous operation can have voltage waveforms that
are considerable different than a continuous design. The
output pin (switch) waveform can have some damped sinusoidal ringing present. (See photo titled; Discontinuous
Mode Switching Waveforms) This ringing is normal for discontinuous operation, and is not caused by feedback loop
instabilities. In discontinuous operation, there is a period of
time where neither the switch or the diode are conducting,
and the inductor current has dropped to zero. During this
time, a small amount of energy can circulate between the
inductor and the switch/diode parasitic capacitance causing
this characteristic ringing. Normally this ringing is not a problem, unless the amplitude becomes great enough to exceed
the input voltage, and even then, there is very little energy
present to cause damage.
Different inductor types and/or core materials produce different amounts of this characteristic ringing. Ferrite core inductors have very little core loss and therefore produce the most
ringing. The higher core loss of powdered iron inductors
produce less ringing. If desired, a series RC could be placed
in parallel with the inductor to dampen the ringing. The
computer aided design software Switchers Made Simple
(version 4.1) will provide all component values for continuous and discontinuous modes of operation.
DS012440-40
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.
DS012440-39
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.
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
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 (irregard-
25
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LM2597/LM2597HV
Application Information
LM2597/LM2597HV
Application Information
(Continued)
less 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 (∆IIND) 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 ∆IIND. When the inductor
nomographs shown in Figure 3 through Figure 6 are used to
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 (∆IIND) that can be expected
for different load currents. The curve also shows how the
peak-to-peak inductor ripple current (∆IIND) 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 = 5V, maximum load current of 300 mA
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 = 15V, nominal, varying between 11V and 20V.
The selection guide in Figure 4 shows that the vertical line
for a 0.3A load current, and the horizontal line for the 15V
input voltage intersect approximately midway between the
upper and lower borders of the 150 µH inductance region. A
150 µH inductor will allow a peak-to-peak inductor current
(∆IIND) to flow that will be a percentage of the maximum load
current. Referring to Figure 21, follow the 0.3A line approximately midway into the inductance region, and read the
peak-to-peak inductor ripple current (∆IIND) on the left hand
axis (approximately 150 mA p-p).
As the input voltage increases to 20V, it approaches the
upper border of the inductance region, and the inductor
ripple current increases. Referring to the curve in Figure 21,
it can be seen that for a load current of 0.3A, the
peak-to-peak inductor ripple current (∆IIND) is 150 mA with
15V in, and can range from 175 mA at the upper border (20V
in) to 120 mA at the lower border (11V in).
Once the ∆IIND value is known, the following formulas can be
used to calculate additional information about the switching
regulator circuit.
1. Peak Inductor or peak switch current
2.
Minimum load current before the circuit becomes discontinuous
3.
Output Ripple Voltage
= (∆IIND)x(ESR of COUT)
= 0.150Ax0.240Ω=36 mV p-p
4.
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26
THERMAL CONSIDERATIONS
(Continued)
The LM2597/LM2597HV is available in two packages, an
8-pin through hole DIP (N) and an 8-pin surface mount SO-8
(M). Both packages are molded plastic with a copper lead
frame. When the package is soldered to the PC board, the
copper and the board are the heat sink for the LM2597 and
the other heat producing components.
For best thermal performance, wide copper traces should be
used. Pins should be soldered to generous amounts of
printed circuit board copper, (one exception to this is the
output (switch) pin, which should not have large areas of
copper). Large areas of copper provide the best transfer of
heat (lower thermal resistance) to the surrounding air, and
even double-sided or multilayer boards provide a better heat
path to the surrounding air. Unless power levels are small,
sockets are not recommended because of the added thermal resistance it adds and the resultant higher junction
temperatures.
Package thermal resistance and junction temperature rise
numbers are all approximate, and there are many factors
that will affect the junction temperature. Some of these factors include board size, shape, thickness, position, location,
and even board temperature. Other factors are, trace width,
printed circuit copper area, copper thickness, single- or
double-sided, multilayer board, and the amount of solder on
the board. The effectiveness of the PC board to dissipate
heat also depends on the size, quantity and spacing of other
components on the board. Furthermore, some of these components such as the catch diode will add heat to the PC
board and the heat can vary as the input voltage changes.
For the inductor, depending on the physical size, type of core
material and the DC resistance, it could either act as a heat
sink taking heat away from the board, or it could add heat to
the board.
The curves shown in Figure 22 and Figure 23 show the
LM2597 junction temperature rise above ambient temperature with a 500 mA load for various input and output voltages. The Bias Supply pin was not used (left open) for these
curves. Connecting the Bias Supply pin to the output voltage
would reduce the junction temperature by approximately 5˚C
to 15˚C, depending on the input and output voltages, and the
load current. This data was taken with the circuit operating
as a buck switcher with all components mounted on a PC
board to simulate the junction temperature under actual
operating conditions. This curve is typical, and can be used
for a quick check on the maximum junction temperature for
various conditions, but keep in mind that there are many
factors that can affect the junction temperature.
DS012440-41
Circuit Data for Temperature Rise Curve (DIP-8)
Capacitors Through hole electrolytic
Inductor
Through hole, Schott, 100 µH
Diode
Through hole, 1A 40V, Schottky
PC board
4 square inches single sided 2 oz. copper
(0.0028")
FIGURE 22. Junction Temperature Rise, DIP-8
DS012440-42
BIAS SUPPLY FEATURE
The bias supply (VBS) pin allows the LM2597’s internal
circuitry to be powered from a power source, other than VIN,
typically the output voltage. This feature can increase efficiency and lower junction temperatures under some operating conditions. The greatest increase in efficiency occur with
light load currents, high input voltage and low output voltage
(4V to 12V). See efficiency curves shown in Figure 24 and
Figure 25. The curves with solid lines are with the VBS pin
connected to the regulated output voltage, while the curves
with dashed lines are with the VBS pin open.
The bias supply pin requires a minimum of approximately
3.5V at room temperature (4V @ −40˚C), and can be as high
as 30V, but there is little advantage of using the bias supply
feature with voltages greater than 15V or 20V. The current
required for the VIN pin is typically 4 mA.
Circuit Data for Temperature Rise Curve (Surface
Mount)
Capacitors Surface mount tantalum, molded “D” size
Inductor
Surface mount, Coilcraft DO33, 100 µH
Diode
Surface mount, 1A 40V, Schottky
PC board
4 square inches single sided 2 oz. copper
(0.0028")
FIGURE 23. Junction Temperature Rise, SO-8
27
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LM2597/LM2597HV
Application Information
LM2597/LM2597HV
Application Information
current, with, and without a Soft-start capacitor. Figure 26
also shows the error flag output going high when the output
voltage reaches 95% of the nominal output voltage. The
reduced input current required at startup is very evident
when comparing the two photos. The Soft-start feature reduces the startup current from 700 mA down to 160 mA, and
delays and slows down the output voltage rise time.
(Continued)
To use the bias supply feature with output voltages between
4V and 15V, wire the bias pin to the regulated output. Since
the VBS pin requires a minimum of 4V to operate, the 3.3V
part cannot be used this way. When the VBS pin is left open,
the intemal regulator circuitry is powered from the input
voltage.
DS012440-44
FIGURE 26. Output Voltage, Input Current, Error Flag
Signal, at Start-Up, WITH Soft-start
DS012440-43
FIGURE 24. Effects of Bias Supply Feature on 5V
Regulator Efficiency
DS012440-46
FIGURE 27. Output Voltage, Input Current, at Start-Up,
WITHOUT Soft-start
DS012440-45
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.
FIGURE 25. Effects of Bias Supply Feature on 12V
Regulator Efficiency
SHUTDOWN /SOFT-START
The circuit shown in Figure 28 is a standard buck regulator
with 24V in, 12V out, 100 mA load, and using a 0.068 µF
Soft-start capacitor. The photo in Figure 26 and Figure 27
show the effects of Soft-start on the output voltage, the input
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28
LM2597/LM2597HV
Application Information
(Continued)
DS012440-47
FIGURE 28. Typical Circuit Using Shutdown /Soft-start and Error Flag Features
DS012440-48
FIGURE 29. Inverting −5V Regulator With Shutdown and Soft-start
lNVERTING REGULATOR
The circuit in Figure 29 converts a positive input voltage to a
negative output voltage with a common ground. The circuit
operates by bootstrapping the regulators ground pin to the
negative output voltage, then grounding the feedback pin,
the regulator senses the inverted output voltage and regulates it.
This example uses the LM2597-5 to generate a −5V output,
but other output voltages are possible by selecting other
output voltage versions, including the adjustable version.
Since this regulator topology can produce an output voltage
that is either greater than or less than the input voltage, the
maximum output current greatly depends on both the input
and output voltage. The curve shown in Figure 30 provides a
guide as to the amount of output load current possible for the
different input and output voltage conditions.
The maximum voltage appearing across the regulator is the
absolute sum of the input and output voltage, and this must
be limited to a maximum of 40V. In this example, when
converting +20V to −5V, the regulator would see 25V between the input pin and ground pin. The LM2597 has a
maximum input voltage rating of 40V (60V for the
LM2597HV).
29
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LM2597/LM2597HV
Application Information
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.
(Continued)
DS012440-50
FIGURE 31. Undervoltage Lockout for a Buck
Regulator
DS012440-49
FIGURE 30. Maximum Load Current for Inverting
Regulator Circuit
Figure 32 and Figure 33 apply the same feature to an
inverting circuit. Figure 32 features a constant threshold
voltage for turn on and turn off (zener voltage plus approximately one volt). If hysteresis is needed, the circuit in Figure
33 has a turn ON voltage which is different than the turn OFF
voltage. The amount of hysteresis is approximately equal to
the value of the output voltage. Since the SD /SS pin has an
internal 7V zener clamp, R2 is needed to limit the current into
this pin to approximately 1 mA when Q1 is on.
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 1N4001 diode could be
used.
Because of differences in the operation of the inverting
regulator, the standard design procedure is not used to
select the inductor value. In the majority of designs, a 100
µH, 1 Amp inductor is the best choice. Capacitor selection
can also be narrowed down to just a few values. Using the
values shown in Figure 29 will provide good results in the
majority of inverting designs.
This type of inverting regulator can require relatively large
amounts of input current when starting up, even with light
loads. Input currents as high as the LM2597 current limit
(approximately 0.8A) are needed for 1 ms or more, until the
output reaches its nominal output voltage. The actual time
depends on the output voltage and the size of the output
capacitor. Input power sources that are current limited or
sources that can not deliver these currents without getting
loaded down, may not work correctly. Because of the relatively high startup currents required by the inverting topology,
the Soft-start feature shown in Figure 29 is recommended.
Also shown in Figure 29 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.
DS012440-52
FIGURE 32. Undervoltage Lockout Without
Hysteresis for an Inverting Regulator
UNDERVOLTAGE LOCKOUT
Some applications require the regulator to remain off until
the input voltage reaches a predetermined voltage. Figure
31 contains a undervoltage lockout circuit for a buck configuration, while Figure 32 and Figure 33 are for the inverting
types (only the circuitry pertaining to the undervoltage lockout is shown). Figure 31 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
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DS012440-53
FIGURE 33. 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 34. This unregulated negative voltage is approximately equal to the positive input voltage (minus a few volts),
and can supply up to a 100 mA of output current. There is a
requirement however, that there be a minimum load of sev30
LM2597/LM2597HV
Application Information
(Continued)
eral hundred mA on the regulated positive output for the
charge pump to work correctly. Also, resistor R1 is required
to limit the charging current of C1 to some value less than
the LM2597 current limit (typically 800 mA).
This method of generating a negative output voltage without
an additional inductor can be used with other members of
the Simple Switcher Family, using either the buck or boost
topology.
DS012440-51
FIGURE 34. Charge Pump for Generating a
Low Current, Negative Output Voltage
31
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LM2597/LM2597HV
Application Information
(Continued)
TYPICAL SURFACE MOUNT PC BOARD LAYOUT, FIXED OUTPUT (2X size)
DS012440-54
CIN
— 10 µF, 35V, Solid Tantalum, AVX, “TPS Series” (surface mount, “D” size)
— 100 µF, 10V Solid Tantalum, AVX, “TPS Series” (surface mount, “D” size)
COUT
D1
— 1A, 40V Surface Mount Schottky Rectifier
L1
— Surface Mount Inductor, Coilcraft DO33
— Soft-start Capacitor (surface mount ceramic chip capacitor)
CSS
— Delay Capacitor (surface mount ceramic chip capacitor)
CD
R3
— Error Flag Pullup Resistor (surface mount chip resistor)
TYPICAL SURFACE MOUNT PC BOARD LAYOUT, ADJUSTABLE OUTPUT (2X size)
DS012440-55
CIN
— 10 µF, 35V, Solid Tantalum, AVX, “TPS Series” (surface mount, “D” size)
COUT — 68 µF, 20V Solid Tantalum, AVX, “TPS Series” (surface mount, “D” size)
D1
— 1A, 40V Surface Mount Schottky Rectifier
L1
— Surface Mount Inductor, Coilcraft DO33
— Soft-start Capacitor (surface mount ceramic chip capacitor)
CSS
— Delay Capacitor (surface mount ceramic chip capacitor)
CD
CFF
— Feedforward Capacitor (surface mount ceramic chip capacitor)
R1
— Output Voltage Program Resistor (surface mount chip resistor)
R2
— Output Voltage Program Resistor (surface mount chip resistor)
R3
— Error Flag Pullup Resistor (surface mount chip resistor)
FIGURE 35. 2X Printed Circuit Board Layout
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32
LM2597/LM2597HV
Physical Dimensions
inches (millimeters) unless otherwise noted
8-Lead (0.150" Wide) Molded Small Outline Package,
Order Number LM2597M-3.3, LM2597M-5.0,
LM2597M-12 or LM2597M-ADJ
LM2597HVM-3.3, LM2597HVM-5.0,
LM2597HVM-12 or LM2597HVM-ADJ
NS Package Number M08A
33
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LM2597/LM2597HV SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down Voltage
Regulator, with Features
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
8-Lead (0.300" Wide) Molded Dual-In-Line Package,
Order Number LM2597N-3.3, LM2597N-5.0, LM2597N-12 or LM2597N-ADJ
LM2597HVN-3.3, LM2597HVN-5.0, LM2597HVN-12 or LM2597HVN-ADJ
NS Package Number N08E
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DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
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2. A critical component is any component of a life
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can be reasonably expected to cause the failure of
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