How it Works
Datasheet
Loading...
LM2597HV

Datasheet LM2597HV Datasheet (National Semiconductor)

Download
Page 1
LM2597/LM2597HV SIMPLE SWITCHER
®
Power Converter 150 kHz 0.5A
Step-Down Voltage Regulator, with Features
December 2000
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
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 fixed-frequency oscillator, Shutdown /Soft-start, error flag delay and error flag output.
Other features include a guaranteed put voltage under all conditions of input voltage and output load conditions, and ternal shutdown is included, featuring typically 85 µA standby current. Self protection features include a two stage current limit for the output switch and an over temperature shutdown for complete protection under fault conditions.
†
, improved line and load specifications,
±
4% tolerance on out-
±
15% on the oscillator frequency. Ex-
Features
n 3.3V, 5V, 12V, and adjustable output versions n Adjustable version output voltage range, 1.2V to 37V
(57V for HV version) 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 (V
efficiency at high input voltages
n Low power standby mode, I n High Efficiency n Uses readily available standard inductors n Thermal shutdown and current limit protection
±
4% max over line and load
) for internal circuitry improves
BS
typically 85 µA
Q
Applications
n Simple high-efficiency step-down (buck) regulator n Efficient pre-regulator for linear regulators n On-card switching regulators n Positive to Negative converter
Typical Application (Fixed Output Voltage Versions)
DS012440-1
†
Patent Number 5,382,918.
SIMPLE SWITCHER®and
© 2001 National Semiconductor Corporation DS012440 www.national.com
Switchers Made Simple
®
are registered trademarks of National Semiconductor Corporation.
Page 2
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications.
Maximum Supply Voltage (V
LM2597 45V
LM2597/LM2597HV
LM2597HV 60V
SD /SS Pin Input Voltage (Note 2)
)
IN
6V
ESD Susceptibility
Human Body Model (Note 3) 2 kV
Lead Temperature
M8 Package
Vapor Phase (60 sec.) +215˚C Infrared (15 sec.) +220˚C
N Package (Soldering, 10 sec.) +260˚C
Maximum Junction Temperature +150˚C
Delay Pin Voltage (Note 2) 1.5V Flag Pin Voltage −0.3 ≤ V ≤45V Bias Supply Voltage (V
) −0.3 ≤ V ≤30V
BS
Feedback Pin Voltage −0.3 ≤ V ≤+25V Output Voltage to Ground
(Steady State) −1V
Power Dissipation Internally limited
Operating Conditions
Temperature Range −40˚C ≤ TJ+125˚C Supply Voltage
LM2597 4.5V to 40V
LM2597HV 4.5V to 60V
Storage Temperature Range −65˚C to +150˚C
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.V
Symbol Parameter Conditions LM2597/LM2597HV-3.3 Units
SYSTEM PARAMETERS (Note 6) Test Circuit
V
OUT
Output Voltage 4.75V ≤ VIN≤ V
η Efficiency V
=40V for the LM2597 and 60V for the LM2597HV
INmax
Figure 12
, 0.1A ≤ I
INmax
= 12V, I
IN
= 0.5A 80 %
LOAD
Typ Limit
(Note 4) (Note 5)
≤ 0.5A 3.3 V
LOAD
3.168/3.135 V(min)
3.432/3.465 V(max)
(Limits)
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.V
Symbol Parameter Conditions LM2597/LM2597HV-5.0 Units
SYSTEM PARAMETERS (Note 6) Test Circuit
V
OUT
Output Voltage 7V ≤ VIN≤ V
η Efficiency V
=40V for the LM2597 and 60V for the LM2597HV
INmax
Figure 12
, 0.1A ≤ I
INmax
= 12V, I
IN
= 0.5A 82 %
LOAD
Typ Limit
(Note 4) (Note 5)
≤ 0.5A 5 V
LOAD
4.800/4.750 V(min)
5.200/5.250 V(max)
(Limits)
www.national.com 2
Page 3
LM2597/LM2597HV-12 Electrical Characteristics
Specifications with standard type face are for TJ= 25˚C, and those with boldface type apply over full Operating Tempera­ture Range.V
=40V for the LM2597 and 60V for the LM2597HV
INmax
Symbol Parameter Conditions LM2597/LM2597HV-12 Units
Typ Limit
(Limits)
(Note 4) (Note 5) SYSTEM PARAMETERS (Note 6) Test Circuit V
OUT
Output Voltage 15V ≤ VIN≤ V
Figure 12
, 0.1A ≤ I
INmax
≤ 0.5A 12 V
LOAD
11.52/11.40 V(min)
12.48/12.60 V(max)
η Efficiency V
= 25V, I
IN
= 0.5A 88 %
LOAD
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.V
Symbol Parameter Conditions LM2597/LM2597HV-ADJ Units
SYSTEM PARAMETERS (Note 6) Test Circuit
V
FB
Feedback Voltage 4.5V ≤ VIN≤ V
η Efficiency V
=40V for the LM2597 and 60V for the LM2597HV
INmax
Figure 12
, 0.1A ≤ I
INmax
V
programmed for 3V. Circuit of
OUT
= 12V, V
IN
OUT
= 3V, I
Typ Limit
(Note 4) (Note 5)
≤ 0.5A 1.230 V
LOAD
Figure 12
. 1.193/1.180 V(min)
1.267/1.280 V(max)
= 0.5A 80 %
LOAD
(Limits)
LM2597/LM2597HV
All Output Voltage Versions Electrical Characteristics
Specifications with standard type face are for TJ= 25˚C, and those with boldface type apply over full Operating Tempera­ture Range. Unless otherwise specified, V
sion. I
LOAD
= 100 mA.
Symbol Parameter Conditions LM2597/LM2597HV-XX Units
DEVICE PARAMETERS
I
b
f
O
V
SAT
Feedback Bias Current Adjustable Version Only, VFB= 1.235V 10 50/100 nA Oscillator Frequency (Note 7) 150 kHz
Saturation Voltage I
DC Max Duty Cycle (ON) (Note 9) 100 %
Min Duty Cycle (OFF) (Note 10) 0
I
CL
I
L
I
Q
Current Limit Peak Current, (Notes 8 and 9) 0.8 A
Output Leakage Current (Notes 8, 10 and 11) Output = 0V 50 µA(max)
Operating Quiescent SD /SS Pin Open, VBSPin Open(Note 10) 5mA
= 12V for the 3.3V, 5V, and Adjustable version and VIN= 24V for the 12V ver-
IN
Typ Limit
(Limits)
(Note 4) (Note 5)
127/110 kHz(min) 173/173 kHz(max)
= 0.5A (Notes 8 and 9) 0.9 V
OUT
1.1/1.2 V(max)
0.65/0.58 A(min)
1.3/1.4 A(max)
Output = −1V 2 mA
15 mA(max)
www.national.com3
Page 4
All Output Voltage Versions Electrical Characteristics
Specifications with standard type face are for TJ= 25˚C, and those with boldface type apply over full Operating Tempera­ture Range. Unless otherwise specified, V
sion. I
Symbol Parameter Conditions LM2597/LM2597HV-XX Units
LM2597/LM2597HV
DEVICE PARAMETERS
I
STBY
θ
JA
SHUTDOWN/SOFT-START CONTROL Test Circuit of V
SD
V
SS
I
SD
I
SS
FLAG/DELAY CONTROL Test Circuit of
VF
SAT
IF
L
BIAS SUPPLY
I
BS
I
Q
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.
= 100 mA.
LOAD
Current 10 mA(max) Standby Quiescent SD /SS pin = 0V (Note 10)LM2597 85 µA Current 200/250 µA(max)
Thermal Resistance N Package, Junction to Ambient (Note 12) 95 ˚C/W
Shutdown Threshold 1.3 V Voltage Low, (Shutdown Mode) 0.6 V(max)
Soft-start Voltage V
Shutdown Current V
Soft-start Current V
Regulator Dropout Low (Flag ON) 96 % Detector 92 %(min) Threshold Voltage 98 %(max) Flag Output Saturation I Voltage V Flag Output Leakage
Current Delay Pin Threshold 1.25 V Voltage Low (Flag ON) 1.21 V(min)
Delay Pin Source V Current 6 µA(max) Delay Pin Saturation Low (Flag ON) 55 mV
Bias Supply Pin Current VBS= 2V (Note 10) 120 µA
Operating Quiescent Current
(Continued)
= 12V for the 3.3V, 5V, and Adjustable version and VIN= 24V for the 12V ver-
IN
Typ Limit
(Note 4) (Note 5)
LM2597HV 140 250/300 µA(max)
M Package, Junction to Ambient (Note 12) 150
Figure 12
High, (Soft-start Mode) 2 V(min)
= 20% of Nominal Output Voltage 2 V
OUT
V
= 100% of Nominal Output Voltage 3
OUT SHUTDOWN
= 0.5V 5µA
10 µA(max)
= 2.5V 1.6 µA
Soft-start
5 µA(max)
Figure 12
= 3 mA 0.3 V
SINK
= 0.5V 0.7/1.0 V(max)
DELAY
V
= 40V 0.3 µA
FLAG
High (Flag OFF) and V
= 0.5V 3 µA
DELAY
Regulated 1.29 V(max)
OUT
350/400 mV(max)
400 µA(max)
V
= 4.4V (Note 10) 4 mA
BS
10 mA(max)
VBS= 4.4V , Vinpin current(Note 10) 1 2 mA
(Limits)
www.national.com 4
Page 5
All Output Voltage Versions Electrical Characteristics
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
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: V 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
= 40V for the LM2597 and 60V for the LM2597HV.
IN
Figure 12
(Continued)
test circuit, system performance will be as shown in system parameters section of Electrical Characteristics.
™
software.
Typical Performance Characteristics
LM2597/LM2597HV
Normalized Output Voltage
Switch Saturation Voltage
DS012440-2
Line Regulation
Switch Current Limit
DS012440-3
Efficiency
DS012440-4
Dropout Voltage
DS012440-5
DS012440-6
DS012440-7
www.national.com5
Page 6
Typical Performance Characteristics (Continued)
Quiescent Current
LM2597/LM2597HV
Feedback Pin Bias Current
DS012440-8
Standby Quiescent Current
Flag Saturation Voltage
DS012440-9
Minimum Operating Supply Voltage
DS012440-10
Switching Frequency
Soft-start
DS012440-11
DS012440-14
Shutdown /Soft-start Current
DS012440-12
DS012440-13
Delay Pin Current
DS012440-16
DS012440-15
www.national.com 6
Page 7
Typical Performance Characteristics (Continued)
V
and VBSCurrent vs
IN
V
and Temperature
BS
DS012440-17
Soft-start Response
DS012440-18
LM2597/LM2597HV
Shutdown /Soft-start Threshold Voltage
DS012440-25
Continuous Mode Switching Waveforms V
= 20V, V
IN
L = 100 µH, C
A: Output Pin Voltage, 10V/div. B: Inductor Current 0.2A/div. C: Output Ripple Voltage, 20 mV/div.
OUT
OUT
=5V,I
LOAD
= 120 µF, C
= 400 mA
ESR = 140 mΩ
OUT
Horizontal Time Base: 2 µs/div.
Load Transient Response for Continuous Mode V
= 20V, V
IN
L = 100 µH, C
OUT
OUT
=5V,I
LOAD
= 120 µF, C
= 200 mA to 500 mA
ESR = 140 mΩ
OUT
DS012440-19
Discontinuous Mode Switching Waveforms
= 20V, V
V
IN
L = 33 µH, C
A: Output Pin Voltage, 10V/div. B: Inductor Current 0.2A/div. C: Output Ripple Voltage, 20 mV/div.
OUT
OUT
=5V,I
LOAD
= 220 µF, C
= 200 mA
ESR=60mΩ
OUT
DS012440-20
Horizontal Time Base: 2 µs/div.
Load Transient Response for Discontinuous Mode
= 20V, V
V
IN
L = 33 µH, C
OUT
OUT
=5V,I
LOAD
= 220 µF, C
= 100 mA to 200 mA
ESR=60mΩ
OUT
A: Output Voltage, 50 mV/div. (AC) B: 200 mA to 500 mA Load Pulse
Horizontal Time Base: 50 µs/div.
DS012440-21
DS012440-22
A: Output Voltage, 50 mV/div. (AC) B: 100 mA to 200 mA Load Pulse
Horizontal Time Base: 200 µs/div.
www.national.com7
Page 8
Connection Diagrams and Ordering Information
LM2597/LM2597HV
Order Number LM2597N-3.3,
LM2597N-5.0, LM2597N-12 or
LM2597HVN-3.3, LM2597HVN-5.0,
LM2597HVN-12 or LM2597HVN-ADJ
See NS Package Number N08E
8–Lead DIP (N)
Top View
LM2597N-ADJ
DS012440-23
8–Lead Surface Mount (M)
DS012440-24
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
www.national.com 8
Page 9
LM2597/LM2597HV Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
Given:
V
= Regulated Output Voltage (3.3V, 5V or 12V)
OUT
V
(max) = Maximum DC Input Voltage
IN
I
(max) = Maximum Load Current
LOAD
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
2. Output Capacitor Selection (C
Figure 7
.
)
OUT
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
®
Simple
version 4.1 or later).
Switchers Made
Given:
=5V
V
OUT
V
(max) = 12V
IN
I
(max) = 0.4A
LOAD
1. Inductor Selection (L1) A. Use the inductor selection guide for the 5V version shown
Figure 4
in
.
B. From the inductor value selection guide shown in 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 (C
OUT
)
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 0.5Aline. 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.
Figure 4
LM2597/LM2597HV
,
www.national.com9
Page 10
LM2597/LM2597HV Series Buck Regulator Design Procedure (Fixed Output)
3. Catch Diode Selection (D1) A. The catch diode current rating must be at least 1.3 times
LM2597/LM2597HV
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.
4. Input Capacitor (C
A low ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground to prevent large voltage transients from appearing at the input. In addition, the RMS current rating of the input capacitor should be selected to be at least data sheet must be checked to assure that this current rating is not exceeded. The curve shown in 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 V
For additional information, see section on input capaci­tors in Application Information section.
(Continued)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
3. Catch Diode Selection (D1) A. Refer to the table shown in
Figure 10
1A, 20V, 1N5817 Schottky diode will provide the best perfor­mance, and will not be overstressed even for a shorted out­put.
)
IN
4. Input Capacitor (C
)
IN
The important parameters for the Input capacitor are the input voltage rating and the RMS current rating. With a nominal input voltage of 12V, an aluminum electrolytic capacitor with a
1
⁄2the DC load current. The capacitor manufacturers
Figure 16
shows typical
voltage rating greater than 18V (1.5 x V The next higher capacitor voltage rating is 25V.
The RMS current rating requirement for the input capacitor in a buck regulator is approximately this example, with a 400 mA load, a capacitor with a RMS current rating of at least 200 mAis needed. The curves shown in
Figure 16
can be used to select an appropriate input capacitor.From the curves, locate the 25Vline 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)
pin.
IN
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.
. In this example, a
) would be needed.
IN
1
⁄2the DC load current. In
www.national.com 10
Page 11
LM2597/LM2597HV Series Buck Regulator Design Procedure (Fixed Output)
Output Load Max Input Inductance Inductor Panasonic Nichicon AVX TPS Sprague
Voltage Current Voltage (µH) (
(V) (A) (V) (µF/V) (µF/V) (µF/V) (µF/V)
3.3 0.5 5 33 L14 220/16 220/16 100/16 100/6.3
5 0.5 8 47 L13 180/16 180/16 100/16 33/25
12 0.5 15 68 L21 82/25 82/25 100/16 15/25
(Continued)
Conditions Inductor Output Capacitor
Through Hole Surface Mount
#
) HFQ Series PL Series Series 595D Series
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
0.2 10 150 L10 120/16 120/16 100/16 100/6.3 40 220 L9 120/16 120/16 100/16 100/6.3
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
0.2 20 220 L9 120/16 120/16 100/16 33/25 40 330 L8 120/16 120/16 100/16 33/25
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
0.2 20 220 L9 82/25 82/25 100/16 15/25 40 330 L17 82/25 82/25 100/16 15/25
LM2597/LM2597HV
FIGURE 1. LM2597/LM2597HV Fixed Voltage Quick Design Component Selection Table
www.national.com11
Page 12
LM2597/LM2597HV Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
Given:
V
= Regulated Output Voltage
OUT
LM2597/LM2597HV
V
(max) = Maximum Input Voltage
IN
I
(max) = Maximum Load Current
LOAD
F = Switching Frequency
(Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R1and R2,as shown in
Figure 12
)
Use the following formula to select the appropriate resistor values.
Given:
V
= 20V
OUT
V
(max) = 28V
IN
I
(max) = 0.5A
LOAD
F = Switching Frequency
(Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R1and R2,as shown in
Select R
Figure 12
to be 1 kΩ, 1%. Solve for R2.
1
)
Select a value for R1between 240Ω and 1.5 kΩ. The lower resistor values minimize noise pickup in the sensitive feed­back pin. (For the lowest temperature coefficient and the best stability with time, use 1% metal film resistors.)
2. Inductor Selection (L1) A. Calculate the inductor Volt microsecond constant E
(V
µs), from the following formula:
•
where V and V B. Use the E
it with the E Value Selection Guide shown in
= internal switch saturation voltage = 0.9V
SAT
= diode forward voltage drop = 0.5V
D
T value from the previous formula and match
•
T number on the vertical axis of the Inductor
•
Figure 6
.
•
C. on the horizontal axis, select the maximum load current. D. Identify the inductance region intersected by the E
•
value and the Maximum Load Current value. Each region is identified by an inductance value and an inductor code (LXX).
E. Select an appropriate inductor from the four manufacturer’s part numbers listed in
Figure 7
.
R2= 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ. R
= 15.4 kΩ.
2
2. Inductor Selection (L1)
T
A. Calculate the inductor Volt
microsecond constant (E•T),
•
B. E•T = 35.2 (V•µs) C. I
(max) = 0.5A
LOAD
D. From the inductor value selection guide shown in the inductance region intersected by the 35 (V line and the 0.5A vertical line is 150 µH, and the inductor code
T
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.
Figure 6
µs) horizontal
•
,
www.national.com 12
Page 13
LM2597/LM2597HV Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
3. Output Capacitor Selection (C A. In the majority of applications, low ESR electrolytic or solid
tantalum capacitors between 82 µF and 220 µF provide the best results. This capacitor should be located close to the IC using short capacitor leads and short copper traces. Do not use capacitors larger than 220 µF. For additional informa-
tion, see section on output capacitors in application in­formation section.
B. To simplify the capacitor selection procedure, refer to the
quick design table shown in different output voltages, and lists various output capacitors that will provide the best design solutions.
C. The capacitor voltage rating should be at least 1.5 times greater than the output voltage, and often much higher volt­age ratings are needed to satisfy the low ESR requirements needed for low output ripple voltage.
4. Feedforward Capacitor (C
For output voltages greater than approximately 10V, an addi­tional capacitor is required. The compensation capacitor is typically between 50 pF and 10 nF,and is wired in parallel with the output voltage setting resistor, R stability for high output voltages, low input-output voltages, and/or very low ESR output capacitors, such as solid tantalum capacitors.
(Continued)
)
OUT
Figure 2
FF
. This table contains
) (See
Figure 12
. It provides additional
2
3. Output Capacitor SeIection (C A. See section on C
in Application Information section.
OUT
B. From the quick design table shown in
OUT
)
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 (C
The table shown in
Figure 2
)
FF
contains feed forward capacitor values for various output voltages. In this example,a1nF capacitor is needed.
LM2597/LM2597HV
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.)
www.national.com13
Page 14
LM2597/LM2597HV Series Buck Regulator Design Procedure (Adjustable Output)
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
LM2597/LM2597HV
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.
6. Input Capacitor (C
A low ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground to prevent large voltage transients from appearing at the input. In addition, the RMS current rating of the input capacitor should be selected to be at least data sheet must be checked to assure that this current rating is not exceeded. The curve shown in RMS current ratings for several different aluminum electrolytic capacitor values.
This capacitor should be located close to the IC using short leads and the voltage rating should be approximately 1.5 times the maximum input voltage.
If solid tantalum input capacitors are used, it is recomended that they be surge current tested by the manufacturer.
Use caution when using ceramic capacitors for input bypass­ing, because it may cause severe ringing at the V
For additional information, see section on input capacitor in application information section.
(Continued)
)
IN
1
⁄2the DC load current. The capacitor manufacturers
Figure 16
shows typical
pin.
IN
5. Catch Diode Selection (D1) A. Refer to the table shown in
Figure 10
provide the best performance, and in this example a 1A, 40V, 1N5819 Schottky diode would be a good choice. The 1Adiode rating is more than adequate and will not be overstressed even for a shorted output.
6. Input Capacitor (C
)
IN
The important parameters for the Input capacitor are the input voltage rating and the RMS current rating. With a nominal input voltage of 28V,an aluminum electrolytic aluminum elec­trolytic capacitor with a voltage rating greater than 42V (1.5 x V
) would be needed. Since the the next higher capacitor
IN
voltage rating is 50V, a 50V capacitor should be used. The capacitor voltage rating of (1.5 x V
) is a conservative guide-
IN
line, and can be modified somewhat if desired. The RMS current rating requirement for the input capacitor of
a buck regulator is approximately
1
⁄2the DC load current. In this example, with a 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.
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.
. Schottky diodes
www.national.com 14
Page 15
LM2597/LM2597HV Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
LM2597/LM2597HV
Output
Voltage
(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
Panasonic Nichicon PL Feedforward AVX TPS Sprague Feedforward
HFQ Series Series Capacitor Series 595D Series Capacitor
Through Hole Output Capacitor Surface Mount Output Capacitor
(µF/V) (µF/V) (µF/V) (µF/V)
FIGURE 2. Output Capacitor and Feedforward Capacitor Selection Table
www.national.com15
Page 16
LM2597/LM2597HV Series Buck Regulator Design Procedure
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)
LM2597/LM2597HV
DS012440-57
FIGURE 3. LM2597/LM2597HV-3.3
DS012440-58
FIGURE 5. LM2597/LM2597HV-12
DS012440-30
FIGURE 4. LM2597/LM2597HV-5.0
DS012440-32
FIGURE 6. LM2597/LM2597HV-ADJ
www.national.com 16
Page 17
LM2597/LM2597HV Series Buck Regulator Design Procedure (Continued)
LM2597/LM2597HV
Induc-
tance
(µH)
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 —
Cur­rent
(A)
Schott Renco Pulse Engineering Coilcraft
Through Surface Through Surface Through Surface Surface
Hole Mount Hole Mount Hole Mount Mount
FIGURE 7. Inductor Manufacturers Part Numbers
Coilcraft Inc. Phone (800) 322-2645
FAX (708) 639-1469
Coilcraft Inc., Europe Phone +44 1236 730 595
FAX +44 1236 730 627
Pulse Engineering Inc. Phone (619) 674-8100
FAX (619) 674-8262
Pulse Engineering Inc., Phone +353 93 24 107 Europe FAX +353 93 24 459 Renco Electronics Inc. Phone (800) 645-5828
FAX (516) 586-5562
Schott Corp. Phone (612) 475-1173
FAX (612) 475-1786
FIGURE 8. Inductor Manufacturers Phone Numbers
Nichicon Corp. Phone (708) 843-7500
FAX (708) 843-2798
Panasonic Phone (714) 373-7857
FAX (714) 373-7102
AVX Corp. Phone (803) 448-9411
FAX (803) 448-1943
Sprague/Vishay Phone (207) 324-7223
FAX (207) 324-4140
FIGURE 9. Capacitor Manufacturers Phone Numbers
www.national.com17
Page 18
LM2597/LM2597HV Series Buck Regulator Design Procedure (Continued)
VR 1A Diodes
Surface Mount Through Hole
Schottky Ultra Fast Schottky Ultra Fast
LM2597/LM2597HV
20V All of these diodes are rated to 1N5817 All of these diodes are rated to
MBRS130 1N5818
30V SR103
MBRS140 MURS120 1N5819 MUR120
40V 10BQ040 10BF10 SR104 HER101
10MQ040 11DQ04 11DF1
50V
MBRS160 SR105
or
more
10BQ050 MBR150 10MQ060 11DQ05
MBRS1100 MBR160
10MQ090 SB160
SGL41-60 11DQ10
SS16
Recovery Recovery
at least 60V. SR102 at least 60V.
11DQ03
FIGURE 10. Diode Selection Table
Block Diagram
FIGURE 11.
www.national.com 18
DS012440-26
Page 19
Typical Circuit and Layout Guidelines
Fixed Output Voltage Versions
Component Values shown are for VIN= 15V, V
— 47 µF, 50V,Aluminum Electrolytic Nichicon “PL Series”
C
IN
— 120 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”
C
OUT
D1 — 1A, 30V Schottky Rectifier, 1N5818 L1 — 100 µH, L20
Typical Values
CSS— 0.1 µF
— 0.1 µF
C
DELAY
— 4.7k
R
Pull Up
*Use Bias Supply pin for 5V and 12V Versions
Adjustable Output Voltage Versions
OUT
=5V,I
LOAD
= 500 mA.
LM2597/LM2597HV
DS012440-27
Select R1to be approximately 1 kΩ, use a 1% resistor for best stability. Component Values shown are for V
= 10V, I
V
OUT
— 68 µF, 35V,Aluminum Electrolytic Nichicon “PL Series”
C
IN
— 120 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”
C
OUT
D1 — 1A, 30V Schottky Rectifier, 1N5818
LOAD
= 500 mA.
IN
= 20V,
L1 — 150 µH, L19
—1kΩ,1%
R
1
— 7.15k, 1%
R
2
— 3.3 nF, See Application Information Section
C
FF
Typical Values
CSS— 0.1 µF
— 0.1 µF
C
DELAY PULL UP
— 4.7k
R *For output voltages between 4V and 20V
FIGURE 12. Typical Circuits and Layout Guides
DS012440-56
www.national.com19
Page 20
Typical Circuit and Layout Guidelines
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
LM2597/LM2597HV
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 C
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.)
(Continued)
wiring can cause problems.
OUT
Application Information
PIN FUNCTIONS +V
(Pin 7)—This is the positive input supply for the IC
IN
switching regulator. A suitable input bypass capacitor must be present at this pin to minimize voltage transients and to supply the switching currents needed by the regulator.
Ground (Pin 6) — Circuit ground. Output (Pin 8)—Internal switch. The voltage at this pin
switches between (+V with a duty cycle of V
IN−VSAT
OUT/VIN
sitive 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 V the nominal output voltage and a delay time determined by the Delay pin capacitor.This signal can be used as a reset to a microprocessor on power-up.
Delay (Pin 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.
) and approximately −0.5V,
. To minimize coupling to sen-
reaches 95% of
OUT
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.
EXTERNAL COMPONENTS
SOFT-START CAPACITOR C
—A capacitor on this pin provides the regulator with a
SS
Soft-start feature (slow start-up). When the DC input voltage is first applied to the regulator, or when the Shutdown /Soft-start pin is allowed to go high, a constant current (approximately 5 µA begins charging this capacitor). As the capacitor voltage rises, the regulator goes through four op­erating regions (See the bottom curve in
1. Regulator in Shutdown.
When the SD /SS pin voltage is
Figure 13
).
between 0V and 1.3V, the regulator is in shutdown, the output voltage is zero, and the IC quiescent current is ap­proximately 85 µA.
2. Regulator ON, but the output voltage is zero.
With the SD /SS pin voltage between approximately 1.3V and 1.8V, the internal 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.
Note: The lower curve shown in
0% to 100%. This is not the duty cycle percentage, but the output voltage percentage. Also, the Soft-start voltage range has a negative temperature coefficient associated with it. See the Soft-start curve in the electrical characteristics section.
4. Normal operation.
Figure 13
shows the Soft-start region from
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.
www.national.com 20
Page 21
Application Information (Continued)
LM2597/LM2597HV
FIGURE 13. Soft-start, Delay, Error, Output
DS012440-33
www.national.com21
Page 22
Application Information (Continued)
LM2597/LM2597HV
DS012440-34
FIGURE 14. Timing Diagram for 5V Output
FIGURE 15. External 3.7V Soft-Start Clamp
DELAY CAPACITOR C
upper curve in
Figure 14
—Provides delay for the error flag output. See the
DELAY
Figure 13
, and also refer to timing diagrams in
. A capacitor on this pin provides a time delay 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
www.national.com 22
DS012440-75
approximately 50 ms when the output is rising and passes through the 95% threshold, but the delay for the output dropping would only be approximately 50 µs.
R
—The error flag output, (or power OK) is the col-
Pull Up
lector of a NPN transistor, with the emitter internally grounded. To use the error flag, a pullup resistor to a positive voltage is needed. The error flag transistor is rated up to a maximum of 45V and can sink approximately 3 mA. If the error flag is not used, it can be left open.
INPUT CAPACITOR C
—A low ESR aluminum or tantalum bypass capacitor is
IN
needed between the input pin and ground pin. It must be located near the regulator using short leads. This capacitor prevents large voltage transients from appearing at the in­put, and provides the instantaneous current needed each time the switch turns on.
The important parameters for the Input capacitor are the voltage rating and the RMS current rating. Because of the relatively high RMS currents flowing in a buck regulator’s input capacitor, this capacitor should be chosen for its RMS
Page 23
Application Information (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 willbe 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.
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.
LM2597/LM2597HV
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
OUTPUT CAPACITOR C
—An output capacitor is required to filter the output
OUT
and provide regulator loop stability. Low impedance or low ESR Electrolytic or solid tantalum capacitors designed for switching regulator applications must be used. When select­ing an output capacitor, the important capacitor parameters are; the 100 kHz Equivalent Series Resistance (ESR), the RMS ripple current rating, voltage rating, and capacitance value. For the output capacitor, the ESR value is the most important parameter.
The output capacitor requires an ESR value that has an upper and lower limit. For low output ripple voltage, a low ESR value is needed. This value is determined by the maxi­mum allowable output ripple voltage, typically 1% to 2% of the output voltage. But if the selected capacitor’s ESR is extremely low, there is a possibility of an unstable feedback loop, resulting in an oscillation at the output. Using the capacitors listed in the tables, or similar types, will provide design solutions under all conditions.
If very low output ripple voltage (less than 15 mV) is re­quired, refer to the section on Output Voltage Ripple and Transients for a post ripple filter.
An aluminum electrolytic capacitor’s ESR value is related to the capacitance value and its voltage rating. In most cases, Higher voltage electrolytic capacitors have lower ESR values (see
Figure 17
). Often, capacitors with much higher voltage ratings may be needed to provide the low ESR values re­quired for low output ripple voltage.
The output capacitor for many different switcher designs often can be satisfied with only three or four different capaci­tor values and several different voltage ratings. See the
www.national.com23
Page 24
Application Information (Continued)
quick design component selection tables in
Figure 2
manufacturers capacitor types. Electrolytic capacitors are not recommended for tempera-
tures below −25˚C. The ESR rises dramatically at cold tem­peratures and typically rises 3X
LM2597/LM2597HV
10X at −40˚C. See curve shown in 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.
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.
for typical capacitor values, voltage ratings, and
@
−25˚C and as much as
Figure 18
.
DS012440-37
FIGURE 18. Capacitor ESR Change vs Temperature
Figure 1
and
To simplify the inductor selection process, an inductor selec­tion guide (nomograph) was designed (see
Figure 6
). This guide assumes that the regulator is operating
Figure 3
through
in the continuous mode, and selects an inductor that will allow a peak-to-peak inductor ripple current to be a certain percentage of the maximum design load current. This peak-to-peak inductor ripple current percentage is not fixed, but is allowed to change as different design load currents are selected. (See
Figure 19
FIGURE 19. (∆I
.)
) Peak-to-Peak Inductor
IND
DS012440-31
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.
www.national.com 24
Page 25
Application Information (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 (version 4.1) will provide all component values for continu­ous and discontinuous modes of operation.
Switchers Made Simple
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.
LM2597/LM2597HV
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.
0.5% to 3% of the output voltage. To obtain low ripple
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.
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-
www.national.com25
Page 26
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 (∆I determining a number of other circuit parameters. Param-
LM2597/LM2597HV
eters such as, peak inductor or peak switch current, mini­mum load current before the circuit becomes discontinuous, output ripple voltage and output capacitor ESR can all be calculated from the peak-to-peak ∆I nomographs shown in
Figure 3
through select an inductor value, the peak-to-peak inductor ripple current can immediately be determined. The curve shown in
Figure 21
shows the range of (∆I
IND
for different load currents. The curve also shows how the peak-to-peak inductor ripple current (∆I go from the lower border to the upper border (for a given load current) within an inductance region. The upper border rep­resents a higher input voltage, while the lower border repre­sents a lower input voltage (see Inductor Selection Guides).
These curves are only correct for continuous mode opera­tion, and only if the inductor selection guides are used to select the inductor value
Consider the following example:
V
= 5V, maximum load current of 300 mA
OUT
V
= 15V, nominal, varying between 11V and 20V.
IN
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 (∆I
) to flow that will be a percentage of the maximum load
IND
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 (∆I 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 it can be seen that for a load current of 0.3A, the peak-to-peak inductor ripple current (∆I 15V in, and can range from 175 mAat the upper border (20V in) to 120 mA at the lower border (11V in).
Once the ∆I
value is known, the following formulas can be
IND
used to calculate additional information about the switching regulator circuit.
1. Peak Inductor or peak switch current
) can be useful for
IND
. When the inductor
IND
Figure 6
) that can be expected
) changes as you
IND
) on the left hand
IND
) is 150 mA with
IND
are used to
Figure 21
OPEN CORE INDUCTORS
Another possible source of increased output ripple voltage or unstable operation is from an open core inductor. Ferrite bobbin or stick inductors have magnetic lines of flux flowing through the air from one end of the bobbin to the other end. These magnetic lines of flux will induce a voltage into any wire or PC board copper trace that comes within the induc­tor’s magnetic field. The strength of the magnetic field, the orientation and location of the PC copper trace to the mag­netic field, and the distance between the copper trace and the inductor, determine the amount of voltage generated in the copper trace. Another way of looking at this inductive coupling is to consider the PC board copper trace as one turn of a transformer (secondary) with the inductor winding as the primary. Many millivolts can be generated in a copper trace located near an open core inductor which can cause stability problems or high output ripple voltage problems.
If unstable operation is seen, and an open core inductor is used, it’s possible that the location of the inductor with respect to other PC traces may be the problem. To deter­mine if this is the problem, temporarily raise the inductor away from the board by several inches and then check circuit operation. If the circuit now operates correctly, then the magnetic flux from the open core inductor is causing the problem. Substituting a closed core inductor such as a tor­roid or E-core will correct the problem, or re-arranging the PC layout may be necessary. Magnetic flux cutting the IC device ground trace, feedback trace, or the positive or nega­tive traces of the output capacitor should be minimized.
Sometimes, locating a trace directly beneath a bobbin in­ductor will provide good results, provided it is exactly in the center of the inductor (because the induced voltages cancel themselves out), but if it is off center one direction or the other, then problems could arise. If flux problems are
,
present, even the direction of the inductor winding can make a difference in some circuits.
This discussion on open core inductors is not to frighten the user, but to alert the user on what kind of problems to watch out for when using them. Open core bobbin or “stick” induc­tors are an inexpensive, simple way of making a compact efficient inductor, and they are used by the millions in many different applications.
2. Minimum load current before the circuit becomes dis-
continuous
3. Output Ripple Voltage
=(∆I
)x(ESR of C
IND
OUT
)
= 0.150Ax0.240Ω=36mVp-p
4.
www.national.com 26
Page 27
Application Information (Continued)
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
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
THERMAL CONSIDERATIONS
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.
BIAS SUPPLY FEATURE
The bias supply (V circuitry to be powered from a power source, other than V
) pin allows the LM2597’s internal
BS
IN
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 25
. The curves with solid lines are with the VBSpin
Figure 24
and
connected to the regulated output voltage, while the curves with dashed lines are with the V
pin open.
BS
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 V
pin is typically 4 mA.
IN
LM2597/LM2597HV
,
www.national.com27
Page 28
Application Information (Continued)
To use the bias supply feature with output voltages between 4V and 15V, wire the bias pin to the regulated output. Since the V part cannot be used this way. When the V the intemal regulator circuitry is powered from the input voltage.
LM2597/LM2597HV
pin requires a minimum of 4V to operate, the 3.3V
BS
pin is left open,
BS
DS012440-43
FIGURE 24. Effects of Bias Supply Feature on 5V
Regulator Efficiency
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 mAdown to 160 mA, and delays and slows down the output voltage rise time.
DS012440-44
FIGURE 26. Output Voltage, Input Current, Error Flag
Signal, at Start-Up, WITH Soft-start
DS012440-45
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
DS012440-46
FIGURE 27. Output Voltage, Input Current, at Start-Up,
WITHOUT Soft-start
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.
www.national.com 28
Page 29
Application Information (Continued)
FIGURE 28. Typical Circuit Using Shutdown /Soft-start and Error Flag Features
LM2597/LM2597HV
DS012440-47
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.
DS012440-48
www.national.com29
Page 30
Application Information (Continued)
LM2597/LM2597HV
DS012440-49
FIGURE 30. Maximum Load Current for Inverting
Regulator Circuit
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 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
Also shown in
Figure 29
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.
capacitor to the output, under light
IN
Figure 29
will provide good results in the
Figure 29
is recommended.
are several shutdown methods for
DS012440-50
FIGURE 31. Undervoltage Lockout for a Buck
Regulator
Figure 32
inverting circuit.
and
Figure 33
Figure 32
apply the same feature to an
features a constant threshold voltage for turn on and turn off (zener voltage plus approxi­mately one volt). If hysteresis is needed, the circuit in
33
has a turn ON voltage which is different than the turn OFF
Figure
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.
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.
31
contains a undervoltage lockout circuit for a buck configu-
ration, while
Figure 32
and
Figure 33
are for the inverting
Figure
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
www.national.com 30
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 sev-
Page 31
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
LM2597/LM2597HV
www.national.com31
Page 32
Application Information (Continued)
TYPICAL SURFACE MOUNT PC BOARD LAYOUT, FIXED OUTPUT (2X size)
LM2597/LM2597HV
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)
C
OUT
D1 — 1A, 40V Surface Mount Schottky Rectifier L1 — Surface Mount Inductor, Coilcraft DO33
— Soft-start Capacitor (surface mount ceramic chip capacitor)
C
SS
— Delay Capacitor (surface mount ceramic chip capacitor)
C
D
R3 — Error Flag Pullup Resistor (surface mount chip resistor)
TYPICAL SURFACE MOUNT PC BOARD LAYOUT, ADJUSTABLE OUTPUT (2X size)
DS012440-54
CIN— 10 µF, 35V, Solid Tantalum, AVX, “TPS Series” (surface mount, “D” size)
— 68 µF, 20V Solid Tantalum, AVX, “TPS Series” (surface mount, “D” size)
C
OUT
D1 — 1A, 40V Surface Mount Schottky Rectifier L1 — Surface Mount Inductor, Coilcraft DO33
— Soft-start Capacitor (surface mount ceramic chip capacitor)
C
SS
— Delay Capacitor (surface mount ceramic chip capacitor)
C
D
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
www.national.com 32
DS012440-55
Page 33
Physical Dimensions inches (millimeters) unless otherwise noted
LM2597/LM2597HV
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
www.national.com33
Page 34
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
Regulator, with Features
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
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the
2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness.
labeling, can be reasonably expected to result in a significant injury to the user.
LM2597/LM2597HV SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down Voltage
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
National Semiconductor Corporation
Americas Tel: 1-800-272-9959 Fax: 1-800-737-7018 Email: support@nsc.com
www.national.com
National Semiconductor Europe
Fax: +49 (0) 180-530 85 86
Email: europe.support@nsc.com Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790
National Semiconductor Asia Pacific Customer Response Group
Tel: 65-2544466 Fax: 65-2504466 Email: ap.support@nsc.com
National Semiconductor Japan Ltd.
Tel: 81-3-5639-7560 Fax: 81-3-5639-7507
Loading...
Buy Points How it Works FAQ Contact Us Questions and Suggestions Privacy Policy Cookie Policy Terms of Use