LM2599
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SNVS123C – APRIL 1998 – REVISED APRIL 2013
LM2599 SIMPLE SWITCHER® Power Converter 150 kHz 3A Step-Down Voltage Regulator,
with Features
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FEATURES
DESCRIPTION
•
•
The LM2599 series of regulators are monolithic
integrated circuits that provide all the active functions
for a step-down (buck) switching regulator, capable of
driving a 3A 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.
1
23
•
•
•
•
•
•
•
•
•
•
•
3.3V, 5V, 12V, and Adjustable Output Versions
Adjustable Version Output Voltage Range,
1.2V to 37V ±4% Max Over Line and Load
Conditions
Ensured 3A Output Current
Available in 7-Pin TO-220 and TO-263 (Surface
Mount) Package
Input Voltage Range Up to 40V
150 kHz Fixed Frequency Internal Oscillator
Shutdown/Soft-start
Out of Regulation Error Flag
Error Output Delay
Low Power Standby Mode, IQ Typically 80 μA
High Efficiency
Uses Readily Available Standard Inductors
Thermal Shutdown and Current Limit
Protection
APPLICATIONS
•
•
•
•
Simple High-Efficiency Step-down (Buck)
Regulator
Efficient Pre-Regulator for Linear Regulators
On-Card Switching Regulators
Positive to Negative Converter
This series of switching regulators is similar to the
LM2596 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 LM2599 series operates at a switching frequency
of 150 kHz thus allowing smaller sized filter
components than what would be needed with lower
frequency switching regulators. Available in a
standard 7-lead TO-220 package with several
different lead bend options, and a 7-lead TO-263
Surface mount package.
A standard series of inductors (both through hole and
surface mount types) are available from several
different manufacturers optimized for use with the
LM2599 series. This feature greatly simplifies the
design of switch-mode power supplies.
Other features include an ensured ±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 80 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. (1)
(1)
† Patent Number 5,382,918.
1
2
3
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
SIMPLE SWITCHER is a registered trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 1998–2013, Texas Instruments Incorporated
LM2599
SNVS123C – APRIL 1998 – REVISED APRIL 2013
www.ti.com
Typical Application
(Fixed Output Voltage Versions)
Connection Diagrams
Figure 1. Bent and Staggered Leads, Through
Hole Package
7-Lead TO-220 (NDZ)
Package Number NDZ0007B
Figure 2. Surface Mount Package
7-Lead TO-263 (KTW)
Package Number KTW0007B
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
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Absolute Maximum Ratings
(1) (2)
Maximum Supply Voltage (VIN)
SD /SS Pin Input Voltage
Delay Pin Voltage
45V
(3)
6V
(3)
1.5V
−0.3 ≤ V ≤45V
Flag Pin Voltage
−0.3 ≤ V ≤+25V
Feedback Pin Voltage
Output Voltage to Ground
−1V
(Steady State)
Power Dissipation
Internally limited
−65°C to +150°C
Storage Temperature Range
ESD Susceptibility
Human Body Model
(4)
2 kV
Lead Temperature
KTW Package
Vapor Phase (60 sec.)
+215°C
Infrared (10 sec.)
+245°C
NDZ Package (Soldering, 10 sec.)
+260°C
Maximum Junction Temperature
(1)
(2)
(3)
(4)
+150°C
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 ensure specific performance limits. For ensured specifications and test
conditions, see the Electrical Characteristics section.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
Voltage internally clamped. If clamp voltage is exceeded, limit current to a maximum of 1 mA.
The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.
Operating Conditions
−40°C ≤ TJ ≤ +125°C
Temperature Range
Supply Voltage
4.5V to 40V
LM2599-3.3 Electrical Characteristics
Specifications with standard type face are for TJ = 25°C, and those with boldface type apply over full Operating
Temperature Range.
Symbol
Parameter
Conditions
LM2599-3.3
Typ
(1)
SYSTEM PARAMETERS
VOUT
η
(1)
(2)
(3)
(3)
Output Voltage
Efficiency
Limit
Units
(Limits)
(2)
Test Circuit Figure 24
4.75V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 3A
VIN = 12V, ILOAD = 3A
3.3
V
3.168/3.135
V(min)
3.432/3.465
V(max)
73
%
Typical numbers are at 25°C and represent the most likely norm.
All limits ensured 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 specified via correlation using standard Statistical Quality Control
(SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2599 is used as shown in the Figure 24 test circuit, system performance will be as shown in system parameters of
Electrical Characteristics section.
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LM2599
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LM2599-5.0 Electrical Characteristics
Specifications with standard type face are for TJ = 25°C, and those with boldface type apply over full Operating
Temperature Range.
Symbol
Parameter
Conditions
LM2599-5.0
Typ
Limit
(1)
SYSTEM PARAMETERS
VOUT
(3)
7V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 3A
Efficiency
(1)
(2)
(3)
(2)
Test Circuit Figure 24
Output Voltage
η
Units
(Limits)
VIN = 12V, ILOAD = 3A
5
V
4.800/4.750
V(min)
5.200/5.250
V(max)
80
%
Typical numbers are at 25°C and represent the most likely norm.
All limits ensured 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 specified via correlation using standard Statistical Quality Control
(SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2599 is used as shown in the Figure 24 test circuit, system performance will be as shown in system parameters of
Electrical Characteristics section.
LM2599-12 Electrical Characteristics
Specifications with standard type face are for TJ = 25°C, and those with boldface type apply over full Operating
Temperature Range.
Symbol
Parameter
Conditions
LM2599-12
Typ
(1)
SYSTEM PARAMETERS
VOUT
η
(1)
(2)
(3)
4
(3)
Output Voltage
Efficiency
Limit
Units
(Limits)
(2)
Test Circuit Figure 24
15V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 3A
VIN = 25V, ILOAD = 3A
12
90
V
11.52/11.40
V(min)
12.48/12.60
V(max)
%
Typical numbers are at 25°C and represent the most likely norm.
All limits ensured 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 specified via correlation using standard Statistical Quality Control
(SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2599 is used as shown in the Figure 24 test circuit, system performance will be as shown in system parameters of
Electrical Characteristics section.
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LM2599-ADJ Electrical Characteristics
Specifications with standard type face are for TJ = 25°C, and those with boldface type apply over full Operating
Temperature Range.
Symbol
Parameter
Conditions
LM2599-ADJ
Typ
(1)
SYSTEM PARAMETERS
VFB
(3)
4.5V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 3A
1.230
VOUT programmed for 3V. Circuit of Figure 24.
(1)
(2)
(3)
(2)
Test Circuit Figure 24
Feedback Voltage
η
Units
(Limits)
Limit
Efficiency
VIN = 12V, VOUT = 3V, ILOAD = 3A
V
1.193/1.180
V(min)
1.267/1.280
V(max)
73
%
Typical numbers are at 25°C and represent the most likely norm.
All limits ensured 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 specified via correlation using standard Statistical Quality Control
(SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2599 is used as shown in the Figure 24 test circuit, system performance will be as shown in system parameters of
Electrical Characteristics section.
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 = 500 mA
Symbol
Parameter
Conditions
LM2599-XX
Typ
(1)
Limit
Units
(Limits)
(2)
DEVICE PARAMETERS
Ib
Feedback Bias Current
fO
Oscillator Frequency
VSAT
DC
ICL
IL
Saturation Voltage
Adjustable Version Only, VFB = 1.3V
See
10
(3)
IOUT = 3A
(4) (5)
(5)
100
Min Duty Cycle (OFF)
See
(6)
0
Current Limit
Peak Current,
(4) (5)
Operating Quiescent
(4) (6) (7)
Output = 0V
SD /SS Pin Open
(6)
Current
(1)
(2)
(3)
(4)
(5)
(6)
(7)
kHz(min)
173/173
kHz(max)
1.4/1.5
V(max)
kHz
V
%
4.5
Output = −1V
IQ
127/110
1.16
See
See
nA (max)
150
Max Duty Cycle (ON)
Output Leakage Current
nA
50/100
A
3.6/3.4
A(min)
6.9/7.5
A(max)
50
μA(max)
30
mA(max)
10
mA(max)
2
mA
5
mA
Typical numbers are at 25°C and represent the most likely norm.
All limits ensured 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 specified via correlation using standard Statistical Quality Control
(SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
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.
No diode, inductor or capacitor connected to output pin.
Feedback pin removed from output and connected to 0V to force the output transistor switch ON.
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.
VIN = 40V.
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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 = 500 mA
Symbol
Parameter
Conditions
LM2599-XX
Typ
(1)
ISTBY
Standby Quiescent
SD /SS pin = 0V
(7)
Thermal Resistance
μA
200/250
μA(max)
2
°C/W
TO220 Package, Juncton to Ambient
(8)
50
°C/W
θJA
TO263 Package, Juncton to Ambient
(9)
50
°C/W
θJA
TO263 Package, Juncton to Ambient
(10)
30
°C/W
θJA
TO263 Package, Juncton to Ambient
(11)
20
°C/W
θJA
TO220 or TO263 Package, Junction to Case
Units
(Limits)
(2)
80
Current
θJC
Limit
SHUTDOWN/SOFT-START CONTROL Test Circuit of Figure 24
VSD
Shutdown Threshold
Voltage
1.3
Low, (Shutdown Mode)
High, (Soft-start Mode)
VSS
Soft-start Voltage
VOUT = 20% of Nominal Output Voltage
2
VOUT = 100% of Nominal Output Voltage
3
5
ISD
Shutdown Current
VSHUTDOWN = 0.5V
ISS
Soft-start Current
VSoft-start = 2.5V
V
0.6
V(max)
2
V(min)
V
μA
10
μA(max)
5
μA(max)
92
%(min)
98
%(max)
0.7/1.0
V(max)
μA
1.6
FLAG/DELAY CONTROL Test Circuit of Figure 24
Regulator Dropout Detector
Low (Flag ON)
96
Threshold Voltage
VFSAT
IFL
Flag Output Saturation
ISINK = 3 mA
Voltage
VDELAY = 0.5V
Flag Output Leakage Current
VFLAG = 40V
Delay Pin Threshold
Voltage
Delay Pin Source Current
Delay Pin Saturation
%
0.3
V
μA
0.3
1.25
V
Low (Flag ON)
1.21
V(min)
High (Flag OFF) and VOUT Regulated
1.29
V(max)
6
μA(max)
VDELAY = 0.5V
Low (Flag ON)
μA
3
55
mV
350/400
mV(max)
(8)
Junction to ambient thermal resistance (no external heat sink) for the package mounted TO-220 package mounted vertically, with the
leads soldered to a printed circuit board with (1 oz.) copper area of approximately 1 in2.
(9) Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 0.5 in2 of (1
oz.) copper area.
(10) Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 2.5 in2 of (1
oz.) copper area.
(11) Junction to ambient thermal resistance with the TO-263 package tab soldered to a double sided printed circuit board with 3 in2 of (1 oz.)
copper area on the LM2599S side of the board, and approximately 16 in2 of copper on the other side of the p-c board. See Application
Information in this data sheet and the thermal model in Switchers Made Simple version 4.2.1 (or later) software.
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Typical Performance Characteristics
(Circuit of Figure 24)
Normalized
Output Voltage
Line Regulation
Figure 3.
Figure 4.
Efficiency
Switch Saturation
Voltage
Figure 5.
Figure 6.
Switch Current Limit
Dropout Voltage
Figure 7.
Figure 8.
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Typical Performance Characteristics (continued)
(Circuit of Figure 24)
8
Operating
Quiescent Current
Shutdown
Quiescent Current
Figure 9.
Figure 10.
Minimum Operating
Supply Voltage
Feedback Pin
Bias Current
Figure 11.
Figure 12.
Flag Saturation
Voltage
Switching Frequency
Figure 13.
Figure 14.
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Typical Performance Characteristics (continued)
(Circuit of Figure 24)
Soft-start
Shutdown /Soft-start
Current
Figure 15.
Figure 16.
Daisy Pin Current
Soft-start Response
Figure 17.
Figure 18.
Shutdown/Soft-start
Threshold Voltage
Continuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 2A
L = 32 μH, COUT = 220 μF, COUT ESR = 50 mΩ
Figure 19.
A: Output Pin Voltage, 10V/div.
B: Inductor Current 1A/div.
C: Output Ripple Voltage, 50 mV/div.
Figure 20. Horizontal Time Base: 2 μs/div.
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Typical Performance Characteristics (continued)
(Circuit of Figure 24)
Discontinuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 500 mA
L = 10 μH, COUT = 330 μF, COUT ESR = 45 mΩ
A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.5A/div.
C: Output Ripple Voltage, 100 mV/div.
Figure 21. Horizontal Time Base: 2 μs/div.
Load Transient Response for Continuous Mode
VIN = 20V, VOUT = 5V, ILOAD = 500 mA to 2A
L = 32 μH, COUT = 220 μF, COUT ESR = 50 mΩ
A: Output Voltage, 100 mV/div. (AC)
B: 500 mA to 2A Load Pulse
Figure 22. Horizontal Time Base: 50 μs/div.
Load Transient Response for Discontinuous Mode
VIN = 20V, VOUT = 5V, ILOAD = 500 mA to 2A
L = 10 μH, COUT = 330 μF, COUT ESR = 45 mΩ
A: Output Voltage, 100 mV/div. (AC)
B: 500 mA to 2A Load Pulse
Figure 23. Horizontal Time Base: 200 μs/div.
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Test Circuit and Layout Guidelines
Fixed Output Voltage Versions
Component Values shown are for VIN = 15V,
VOUT = 5V, ILOAD = 3A.
CIN — 470 μF, 50V, Aluminum Electrolytic Nichicon “PL Series”
COUT — 220 μF, 25V Aluminum Electrolytic, Nichicon “PL Series”
D1 — 5A, 40V Schottky Rectifier, 1N5825
L1 — 68 μH, L38
Typical Values
CSS — 0.1 μF
CDELAY — 0.1 μF
RPull Up — 4.7k
Adjustable Output Voltage Versions
where VREF = 1.23V
Select R1 to be approximately 1 kΩ, use a 1% resistor for best stability.
Component Values shown are for VIN = 20V,
VOUT = 10V, ILOAD = 3A.
CIN: — 470 μF, 35V, Aluminum Electrolytic Nichicon “PL Series”
COUT: — 220 μF, 35V Aluminum Electrolytic, Nichicon “PL Series”
D1 — 5A, 30V Schottky Rectifier, 1N5824
L1 — 68 μH, L38
R1 — 1 kΩ, 1%
R2 — 7.15k, 1%
CFF — 3.3 nF, See Application Information Section
RFF — 3 kΩ, See Application Information Section
Typical Values
CSS—0.1 μF
CDELAY—0.1 μF
RPULL UP—4.7k
Figure 24. Standard Test Circuits and Layout Guides
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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 Information section for more information.)
Table 1. LM2599 Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version)
EXAMPLE (Fixed Output Voltage Version)
Given:
VOUT = Regulated Output Voltage (3.3V, 5V or 12V)
Given:
VOUT = 5V
VIN(max) = Maximum DC Input Voltage
ILOAD(max) = Maximum Load Current
VIN(max) = 12V
ILOAD(max) = 3A
1. Inductor Selection (L1)
A. Select the correct inductor value selection guide from Figure 25,
Figure 26, or 6. (Output voltages of 3.3V, 5V, or 12V respectively.)
For all other voltages, see the Design Procedure for the adjustable
version.
B. From the inductor value selection guide, identify the inductance
region intersected by the Maximum Input Voltage line and the
Maximum Load Current line. Each region is identified by an
inductance value and an inductor code (LXX).
C. Select an appropriate inductor from the four manufacturer's part
numbers listed in Table 5.
1. Inductor Selection (L1)
A. Use the inductor selection guide for the 5V version shown in
Figure 26.
B. From the inductor value selection guide shown in Figure 26, the
inductance region intersected by the 12V horizontal line and the 3A
vertical line is 33 μH, and the inductor code is L40.
C. The inductance value required is 33 μH. From the table in
Table 5, go to the L40 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 820 μF and
low ESR solid tantalum capacitors between 10 μF and 470 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 820 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 Table 2. This table
contains different input voltages, output voltages, and load currents,
and lists various inductors and output capacitors that will provide the
best design solutions.
C. The capacitor voltage rating for electrolytic capacitors should be
at least 1.5 times greater than the output voltage, and often much
higher voltage ratings are needed to satisfy the low ESR
requirements for low output ripple voltage.
D. For computer aided design software, see Switchers Made
Simple (version 4.2.1 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
Table 2, locate the 5V output voltage section. In the load current
column, choose the load current line that is closest to the current
needed in your application, for this example, use the 3A 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.
330 μF 35V Panasonic HFQ Series
330 μF 35V Nichicon PL Series
C. For a 5V output, a capacitor voltage rating at least 7.5V or more
is needed. But even a low ESR, switching grade, 220 μF 10V
aluminum electrolytic capacitor would exhibit approximately 225 mΩ
of ESR (see the curve in Figure 33 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 value or with a higher voltage rating (lower
ESR) should be selected. A 16V or 25V capacitor will reduce the
ripple voltage by approximately half.
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Table 1. LM2599 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 LM2599. 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 LM2599 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
IN5400 series are much too slow and should not be used.
3. Catch Diode Selection (D1)
A. Refer to the table shown in Table 8. In this example, a 5A, 20V,
1N5823 Schottky diode will provide the best performance, and will
not be overstressed even for a shorted output.
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 ½ 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 32 shows typical RMS current ratings for several different
aluminum electrolytic capacitor values.
This capacitor should be located close to the IC using short leads
and the voltage rating should be approximately 1.5 times the
maximum input voltage.
If solid tantalum input capacitors are used, it is recomended that they
be surge current tested by the manufacturer.
Use caution when using ceramic capacitors for input bypassing,
because it may cause severe ringing at the VIN pin.
For additional information, see section on input capacitors in
Application Information section.
4. Input Capacitor (CIN)
The important parameters for the Input capacitor are the input
voltage rating and the RMS current rating. With a nominal input
voltage of 12V, an aluminum electrolytic capacitor with a voltage
rating greater than 18V (1.5 × 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 ½ the DC load current. In this example,
with a 3A load, a capacitor with a RMS current rating of at least 1.5A
is needed. The curves shown in Figure 32 can be used to select an
appropriate input capacitor. From the curves, locate the 35V line and
note which capacitor values have RMS current ratings greater than
1.5A. A 680 μF, 35V capacitor could be used.
For a through hole design, a 680 μF/35V 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.
Table 2. LM2599 Fixed Voltage Quick Design Component Selection Table
Conditions
Inductor
Output Capacitor
Through Hole Electrolytic
Output
Voltage
(V)
Load
Current
(A)
3
3.3
2
Max Input
Voltage (V)
Inductance
(μH)
Inductor
(#)
Surface Mount Tantalum
Panasonic
Nichicon
AVX TPS
Sprague
HFQ Series
PL Series
Series
595D Series
(μF/V)
(μF/V)
(μF/V)
(μF/V)
5
22
L41
470/25
560/16
330/6.3
390/6.3
7
22
L41
560/35
560/35
330/6.3
390/6.3
10
22
L41
680/35
680/35
330/6.3
390/6.3
40
33
L40
560/35
470/35
330/6.3
390/6.3
6
22
L33
470/25
470/35
330/6.3
390/6.3
10
33
L32
330/35
330/35
330/6.3
390/6.3
40
47
L39
330/35
270/50
220/10
330/10
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Table 2. LM2599 Fixed Voltage Quick Design Component Selection Table (continued)
Conditions
Inductor
Output Capacitor
Through Hole Electrolytic
Output
Voltage
(V)
Load
Current
(A)
3
5
2
3
12
2
Max Input
Voltage (V)
Inductance
(μH)
Inductor
(#)
Surface Mount Tantalum
Panasonic
Nichicon
AVX TPS
Sprague
HFQ Series
PL Series
Series
595D Series
(μF/V)
(μF/V)
(μF/V)
(μF/V)
8
22
L41
470/25
560/16
220/10
330/10
10
22
L41
560/25
560/25
220/10
330/10
15
33
L40
330/35
330/35
220/10
330/10
40
47
L39
330/35
270/35
220/10
330/10
9
22
L33
470/25
560/16
220/10
330/10
20
68
L38
180/35
180/35
100/10
270/10
40
68
L38
180/35
180/35
100/10
270/10
15
22
L41
470/25
470/25
100/16
180/16
18
33
L40
330/25
330/25
100/16
180/16
30
68
L44
180/25
180/25
100/16
120/20
40
68
L44
180/35
180/35
100/16
120/20
15
33
L32
330/25
330/25
100/16
180/16
20
68
L38
180/25
180/25
100/16
120/20
40
150
L42
82/25
82/25
68/20
68/25
Table 3. LM2599 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
ILOAD(max) = Maximum Load Current
F = Switching Frequency (Fixed at a nominal 150 kHz).
Given:
VOUT = 20V
VIN(max) = 28V
ILOAD(max) = 3A
F = Switching Frequency (Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R1 and R2, as shown in 1. Programming Output Voltage (Selecting R1 and R2, as shown in
Figure 24)
Figure 24)
Use the following formula to select the appropriate resistor values.
Select R1 to be 1 kΩ, 1%. Solve for R2.
(1)
(3)
Select a value for R1 between 240Ω and 1.5 kΩ. The lower resistor R = 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ.
2
values minimize noise pickup in the sensitive feedback pin. (For the
R
lowest temperature coefficient and the best stability with time, use 2 = 15.4 kΩ.
1% metal film resistors.)
(2)
14
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Table 3. LM2599 Series Buck Regulator Design Procedure (Adjustable Output) (continued)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
2. Inductor Selection (L1)
2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant E • T (V • μs), A. Calculate the inductor Volt • microsecond constant (E • T),
from the following formula:
(4)
where VSAT = internal switch saturation voltage = 1.16V and VD =
diode forward voltage drop = 0.5V
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 28.
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 Table 5.
(5)
B. E • T = 34.2 (V • μs)
C. ILOAD(max) = 3A
D. From the inductor value selection guide shown in Figure 28, the
inductance region intersected by the 34 (V • μs) horizontal line and
the 3A vertical line is 47 μH, and the inductor code is L39.
E. From the table in Table 5, locate line L39, and select an inductor
part number from the list of manufacturers part numbers.
3.Output Capacitor Selection (COUT)
A. In the majority of applications, low ESR electrolytic or solid
tantalum capacitors between 82 μF and 820 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 820 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 Table 4. 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.
3. Output Capacitor SeIection (COUT)
A. See section on COUT in Application Information section.
B. From the quick design table shown in Table 4, 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.
220/35 Panasonic HFQ Series
150/35 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 24)
For output voltages greater than approximately 10V, an additional
capacitor is required. The compensation capacitor is typically
between 100 pF and 33 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 Table 4 contains feed forward capacitor values
for various output voltages. In this example, a 560 pF capacitor is
needed.
(6)
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.)
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Table 3. LM2599 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 LM2599. 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 LM2599 using short leads and short printed
circuit traces. Because of their fast switching speed and low forward
voltage drop, Schottky diodes provide the best performance and
efficiency, and should be the first choice, especially in low output
voltage applications. Ultra-fast recovery, or High-Efficiency rectifiers
are also a good choice, but some types with an abrupt turn-off
characteristic may cause instability or EMl problems. Ultra-fast
recovery diodes typically have reverse recovery times of 50 ns or
less. Rectifiers such as the 1N4001 series are much too slow and
should not be used.
5. Catch Diode Selection (D1)
A. Refer to the table shown in Table 8. Schottky diodes provide the
best performance, and in this example a 3A, 40V, 1N5825 Schottky
diode would be a good choice. The 3A diode rating is more than
adequate and will not be overstressed even for a shorted output.
6. Input Capacitor (CIN)
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 ½ 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 32 shows typical RMS current ratings for several different
aluminum electrolytic capacitor values.
This capacitor should be located close to the IC using short leads
and the voltage rating should be approximately 1.5 times the
maximum input voltage.
If solid tantalum input capacitors are used, it is recomended that they
be surge current tested by the manufacturer.
Use caution when using a high dielectric constant ceramic capacitor
for input bypassing, because it may cause severe ringing at the VIN
pin.
For additional information, see section on input capacitor in
Application Information section.
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 × 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 ×
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 ½ the DC load current. In this example,
with a 3A load, a capacitor with a RMS current rating of at least 1.5A
is needed.
The curves shown in Figure 32 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 1.5A. Either
a 470 μF or 680 μF, 50V capacitor could be used.
For a through hole design, a 680 μF/50V electrolytic capacitor
(Panasonic HFQ series or Nichicon PL series or equivalent) would
be adequate. Other types or other manufacturers capacitors can be
used provided the RMS ripple current ratings are adequate.
For surface mount designs, solid tantalum capacitors can be used,
but caution must be exercised with regard to the capacitor sure
current rating (see Application Information or input capacitors in this
data sheet). The TPS series available from AVX, and the 593D
series from Sprague are both surge current tested.
To further simplify the buck regulator design procedure, Texas
Instruments is making available computer design software to be
used with the Simple Switcher line ot switching regulators.Switchers
Made Simple (version 4.2.1 or later) is available on a 3½″ diskette
for IBM compatible computers.
Table 4. Output Capacitor and Feedforward Capacitor Selection Table
Output
Voltage
(V)
16
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)
2
820/35
820/35
33 nF
330/6.3
470/4
4
560/35
470/35
10 nF
330/6.3
390/6.3
10 nF
6
470/25
470/25
3.3 nF
220/10
330/10
3.3 nF
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Table 4. Output Capacitor and Feedforward Capacitor Selection Table (continued)
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)
9
330/25
330/25
1.5 nF
100/16
180/16
12
330/25
330/25
1 nF
100/16
180/16
1 nF
15
220/35
220/35
680 pF
68/20
120/20
680 pF
24
220/35
150/35
560 pF
33/25
33/25
220 pF
28
100/50
100/50
390 pF
10/35
15/50
220 pF
1.5 nF
LM2599 Series Buck Regulator Design Procedure
INDUCTOR VALUE SELECTION GUIDES
(For Continuous Mode Operation)
Figure 25. LM2599-3.3
Figure 26. LM2599-5.0
Figure 27. LM2599-12
Figure 28. LM2599-ADJ
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Table 5. Inductor Manufacturers Part Numbers
Inductance
(μH)
Current
(A)
L15
22
L21
68
L22
Schott
Renco
Through
Hole
Pulse Engineering
Surface
Mount
Through
Hole
Surface
Mount
Coilcraft
Through
Hole
Surface
Mount
Surface
Mount
0.99
67148350
67148460
RL-1284-22-43
RL1500-22
PE-53815
PE-53815-S
DO3308-223
0.99
67144070
67144450
RL-5471-5
RL1500-68
PE-53821
PE-53821-S
DO3316-683
47
1.17
67144080
67144460
RL-5471-6
—
PE-53822
PE-53822-S
DO3316-473
L23
33
1.40
67144090
67144470
RL-5471-7
—
PE-53823
PE-53823-S
DO3316-333
L24
22
1.70
67148370
67148480
RL-1283-22-43
—
PE-53824
PE-53825-S
DO3316-223
L25
15
2.1
67148380
67148490
RL-1283-15-43
—
PE-53825
PE-53824-S
DO3316-153
L26
330
0.80
67144100
67144480
RL-5471-1
—
PE-53826
PE-53826-S
DOS022P-334
L27
220
1.00
67144110
67144490
RL-5471-2
—
PE-53827
PE-53827-S
DOS022P-224
L28
150
1.20
67144120
67144500
RL-5471-3
—
PE-53828
PE-53828-S
DOS022P-154
L29
100
1.47
67144130
67144510
RL-5471-4
—
PE-53829
PE-53829-S
DOS022P-104
L30
68
1.78
67144140
67144520
RL-5471-5
—
PE-53830
PE-53830-S
DOS022P-683
L31
47
2.2
67144150
67144530
RL-5471-6
—
PE-53831
PE-53831-S
DOS022P-473
L32
33
2.5
67144160
67144540
RL-5471-7
—
PE-53932
PE-53932-S
DOS022P-333
L33
22
3.1
67148390
67148500
RL-1283-22-43
—
PE-53933
PE-53933-S
DOS022P-223
L34
15
3.4
67148400
67148790
RL-1283-15-43
—
PE-53934
PE-53934-S
DOS022P-153
L35
220
1.70
67144170
—
RL-5473-1
—
PE-53935
PE-53935-S
—
L36
150
2.1
67144180
—
RL-5473-4
—
PE-54036
PE-54036-S
—
L37
100
2.5
67144190
—
RL-5472-1
—
PE-54037
PE-54037-S
—
L38
68
3.1
67144200
—
RL-5472-2
—
PE-54038
PE-54038-S
—
L39
47
3.5
67144210
—
RL-5472-3
—
PE-54039
PE-54039-S
—
L40
33
3.5
67144220
67148290
RL-5472-4
—
PE-54040
PE-54040-S
—
L41
22
3.5
67144230
67148300
RL-5472-5
—
PE-54041
PE-54041-S
—
L42
150
2.7
67148410
—
RL-5473-4
—
PE-54042
PE-54042-S
—
L43
100
3.4
67144240
—
RL-5473-2
—
PE-54043
—
—
L44
68
3.4
67144250
—
RL-5473-3
—
PE-54044
—
—
Table 6. Inductor Manufacturers Phone Numbers
Coilcraft Inc.
Coilcraft Inc., Europe
Pulse Engineering Inc.
Phone
(800) 322-2645
FAX
(708) 639-1469
Phone
+11 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
Phone
(612) 475-1173
FAX
(612) 475-1786
Schott Corp.
Table 7. Capacitor Manufacturers Phone Numbers
Nichicon Corp.
Panasonic
18
Phone
(708) 843-7500
FAX
(708) 843-2798
Phone
(714) 373-7857
FAX
(714) 373-7102
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Table 7. Capacitor Manufacturers Phone Numbers (continued)
AVX Corp.
Sprague/Vishay
Phone
(803) 448-9411
FAX
(803) 448-1943
Phone
(207) 324-4140
FAX
(207) 324-7223
Table 8. Diode Selection Table
3 Amp Diodes
VR
Surface Mount
Schottky
Ultra Fast
4 to 6 Amp Diodes
Through Hole
Schottky
Recovery
20V
30V
40V
50V
or
more
Ultra Fast
Surface Mount
Schottky
Recovery
Ultra Fast
Through Hole
Schottky
Recovery
Ultra Fast
Recovery
All of
1N5820
All of
All of
SR502
All of
SK32
these
SR302
these
these
1N5823
these
diodes
MBR320
diodes
diodes
SB520
30WQ03
are rated
1N5821
are rated
are rated
SK33
to at
MBR330
to at
to at
SR503
to at
least
31DQ03
least
least
1N5824
least
50V.
1N5822
50V.
50V.
SB530
50V.
SK34
SR304
MBRS340
MBR340
30WQ04
MURS320
SK35
30WF10
31DQ04
50WQ03
diodes
are rated
50WQ04
MUR320
MURS620
SR305
MBRS360
MBR350
30WQ05
31DQ05
SR504
1N5825
SB540
50WF10
50WQ05
MUR620
HER601
SB550
50SQ080
Block Diagram
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APPLICATION INFORMATION
PIN DESCRIPTIONS
+VIN (Pin 1) This is the positive input supply for the IC switching regulator. A suitable input bypass capacitor must
be present at this pin to minimize voltage transients and to supply the switching currents needed by the
regulator.
Ground (Pin 4) Circuit ground.
Output (Pin 2) Internal switch. The voltage at this pin switches between approximately (+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 6) Senses the regulated output voltage to complete the feedback loop.
Shutdown /Soft-start (Pin 7) This dual function pin provides the following features: (a) Allows the switching
regulator circuit to be shut down using logic level signals thus dropping the total input supply current to
approximately 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 3) Open collector output that provides a low signal (flag transistor ON) when the regulated output
voltage drops more than 5% from the nominal output voltage. On start up, Error Flag is low until VOUT
reaches 95% of the nominal output voltage and a delay time determined by the Delay pin capacitor. This
signal can be used as a reset to a microprocessor on power-up.
Delay (Pin 5) At power-up, this pin can be used to provide a time delay between the time the regulated output
voltage reaches 95% of the nominal output voltage, and the time the error flag output goes high.
Special Note If any of the above three features (Shutdown /Soft-start, Error Flag, or Delay) are not used, the
respective pins should be left open.
EXTERNAL COMPONENTS
SOFT-START CAPACITOR
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 29).
1. Regulator in Shutdown. When the SD /SS pin voltage is between 0V and 1.3V, the regulator is in
shutdown, the output voltage is zero, and the IC quiescent current is approximately 85 μA.
2. Regulator ON, but the output voltage is zero. With the SD /SS pin voltage between approximately 1.3V
and 1.8V, the internal regulator circuitry is operating, the quiescent current rises to approximately 5 mA, but
the output voltage is still zero. Also, as the 1.3V threshold is exceeded, the Soft-start capacitor charging
current decreases from 5 μA down to approximately 1.6 μA. This decreases the slope of capacitor voltage
ramp.
3. Soft-start Region. When the SD /SS pin voltage is between 1.8V and 2.8V (@ 25°C), the regulator is in a
Soft-start condition. The switch (Pin 2) 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 29. 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 Figure 29 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.
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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 31 is an example of
an external 3.7V (approx.) clamp that prevents a line-step related glitch but does not interfere with the soft-start
behavior of the device.
Figure 29. Soft-start, Delay, Error, Output
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Figure 30. Timing Diagram for 5V Output
VIN
LM2599
5
Q1
SD/SS
CSS
Z1
3V
Figure 31. External 3.7V Soft-Start Clamp
DELAY CAPACITOR
CDELAY — Provides delay for the error flag output. See the upper curve in Figure 29, and also refer to timing
diagrams in Figure 30. A capacitor on this pin provides a time delay between the time the regulated output
voltage (when it is increasing in value) reaches 95% of the nominal output voltage, and the time the error flag
output goes high. A 3 μA constant current from the delay pin charges the delay capacitor resulting in a voltage
ramp. When this voltage reaches a threshold of approximately 1.3V, the open collector error flag output (or
power OK) goes high. This signal can be used to indicate that the regulated output has reached the correct
voltage and has stabilized.
If, for any reason, the regulated output voltage drops by 5% or more, the error output flag (Pin 3) immediately
goes low (internal transistor turns on). The delay capacitor provides very little delay if the regulated output is
dropping out of regulation. The delay time for an output that is decreasing is approximately a 1000 times less
than the delay for the rising output. For a 0.1 μF delay capacitor, the delay time would be approximately 50 ms
when the output is rising and passes through the 95% threshold, but the delay for the output dropping would only
be approximately 50 μs.
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.
FEEDFORWARD CAPACITOR
(Adjustable Output Voltage Version)
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CFF — A Feedforward Capacitor CFF, shown across R2 in Figure 24 is used when the output voltage is greater
than 10V or when COUT has a very low ESR. This capacitor adds lead compensation to the feedback loop and
increases the phase margin for better loop stability. For CFF selection, see the Design Procedure section.
If the output ripple is large (> 5% of the nominal output voltage), this ripple can be coupled to the feedback pin
through the feedforward capacitor and cause the error comparator to trigger the error flag. In this situation,
adding a resistor, RFF, in series with the feedforward capacitor, approximately 3 times R1, will attenuate the
ripple voltage at the feedback pin.
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 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.
Figure 32. RMS Current Ratings for Low
ESR Electrolytic Capacitors (Typical)
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Figure 33. Capacitor ESR vs Capacitor Voltage Rating (Typical Low ESR Electrolytic Capacitor)
The consequences of operating an electrolytic capacitor above the RMS current rating is a shortened operating
life. The higher temperature speeds up the evaporation of the capacitor's electrolyte, resulting in eventual failure.
Selecting an input capacitor requires consulting the manufacturers data sheet for maximum allowable RMS ripple
current. For a maximum ambient temperature of 40°C, a general guideline would be to select a capacitor with a
ripple current rating of approximately 50% of the DC load current. For ambient temperatures up to 70°C, a
current rating of 75% of the DC load current would be a good choice for a conservative design. The capacitor
voltage rating must be at least 1.25 times greater than the maximum input voltage, and often a much higher
voltage capacitor is needed to satisfy the RMS current requirements.
A graph shown in Figure 32 shows the relationship between an electrolytic capacitor value, its voltage rating, and
the RMS current it is rated for. These curves were obtained from the Nichicon “PL” series of low ESR, high
reliability electrolytic capacitors designed for switching regulator applications. Other capacitor manufacturers offer
similar types of capacitors, but always check the capacitor data sheet.
“Standard” electrolytic capacitors typically have much higher ESR numbers, lower RMS current ratings and
typically have a shorter operating lifetime.
Because of their small size and excellent performance, surface mount solid tantalum capacitors are often used
for input bypassing, but several precautions must be observed. A small percentage of solid tantalum capacitors
can short if the inrush current rating is exceeded. This can happen at turn on when the input voltage is suddenly
applied, and of course, higher input voltages produce higher inrush currents. Several capacitor manufacturers do
a 100% surge current testing on their products to minimize this potential problem. If high turn on currents are
expected, it may be necessary to limit this current by adding either some resistance or inductance before the
tantalum capacitor, or select a higher voltage capacitor. As with aluminum electrolytic capacitors, the RMS ripple
current rating must be sized to the load current.
OUTPUT CAPACITOR
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.
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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 33). Often, capacitors with much
higher voltage ratings may be needed to provide the low ESR values required for low output ripple voltage.
The output capacitor for many different switcher designs often can be satisfied with only three or four different
capacitor values and several different voltage ratings. See the quick design component selection tables in
Table 2 and Table 4 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 34.
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 LM2599 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 IN5400 series are much too slow and should not be used.
Figure 34. 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 LM2599 (or any of the Simple Switcher family) can be used for both continuous or discontinuous modes of
operation.
In many cases the preferred mode of operation is the continuous mode. It offers greater output power, lower
peak switch, inductor and diode currents, and can have lower output ripple voltage. But it does require larger
inductor values to keep the inductor current flowing continuously, especially at low output load currents and/or
high input voltages.
To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Figure 25
through 7). 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 35).
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Figure 35. (ΔIIND) Peak-to-Peak Inductor Ripple
Current (as a Percentage of the
Load Current) vs Load Current
By allowing the percentage of inductor ripple current to increase for low load currents, the inductor value and size
can be kept relatively low.
When operating in the continuous mode, the inductor current waveform ranges from a triangular to a sawtooth
type of waveform (depending on the input voltage), with the average value of this current waveform equal to the
DC output load current.
Inductors are available in different styles such as pot core, toroid, E-core, bobbin core, etc., as well as different
core materials, such as ferrites and powdered iron. The least expensive, the bobbin, rod or stick core, consists of
wire wound on a ferrite bobbin. This type of construction makes for an inexpensive inductor, but since the
magnetic flux is not completely contained within the core, it generates more Electro-Magnetic Interference (EMl).
This magnetic flux can induce voltages into nearby printed circuit traces, thus causing problems with both the
switching regulator operation and nearby sensitive circuitry, and can give incorrect scope readings because of
induced voltages in the scope probe. Also see section on OPEN CORE INDUCTORS.
When multiple switching regulators are located on the same PC board, open core magnetics can cause
interference between two or more of the regulator circuits, especially at high currents. A torroid or E-core inductor
(closed magnetic structure) should be used in these situations.
The inductors listed in the selection chart include ferrite E-core construction for Schott, ferrite bobbin core for
Renco and Coilcraft, and powdered iron toroid for Pulse Engineering.
Exceeding an inductor's maximum current rating may cause the inductor to overheat because of the copper wire
losses, or the core may saturate. If the inductor begins to saturate, the inductance decreases rapidly and the
inductor begins to look mainly resistive (the DC resistance of the winding). This can cause the switch current to
rise very rapidly and force the switch into a cycle-by-cycle current limit, thus reducing the DC output load current.
This can also result in overheating of the inductor and/or the LM2599. Different inductor types have different
saturation characteristics, and this should be kept in mind when selecting an inductor.
The inductor manufacturer's data sheets include current and energy limits to avoid inductor saturation.
DISCONTINUOUS MODE OPERATION
The selection guide chooses inductor values suitable for continuous mode operation, but for low current
applications and/or high input voltages, a discontinuous mode design may be a better choice. It would use an
inductor that would be physically smaller, and would need only one half to one third the inductance value needed
for a continuous mode design. The peak switch and inductor currents will be higher in a discontinuous design,
but at these low load currents (1A and below), the maximum switch current will still be less than the switch
current limit.
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Discontinuous operation can have voltage waveforms that are considerable different than a continuous design.
The output pin (switch) waveform can have some damped sinusoidal ringing present. (See Typical Performance
Characteristics photo titled Discontinuous Mode Switching Waveforms) This ringing is normal for discontinuous
operation, and is not caused by feedback loop instabilities. In discontinuous operation, there is a period of time
where neither the switch or the diode are conducting, and the inductor current has dropped to zero. During this
time, a small amount of energy can circulate between the inductor and the switch/diode parasitic capacitance
causing this characteristic ringing. Normally this ringing is not a problem, unless the amplitude becomes great
enough to exceed the input voltage, and even then, there is very little energy present to cause damage.
Different inductor types and/or core materials produce different amounts of this characteristic ringing. Ferrite core
inductors have very little core loss and therefore produce the most ringing. The higher core loss of powdered iron
inductors produce less ringing. If desired, a series RC could be placed in parallel with the inductor to dampen the
ringing. The computer aided design software Switchers Made Simple (version 4.3) will provide all component
values for continuous and discontinuous modes of operation.
Figure 36. 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 voltage, the ESR of the output capacitor must be low, however, caution must be exercised when
using extremely low ESR capacitors because they can affect the loop stability, resulting in oscillation problems. If
very low output ripple voltage is needed (less than 20 mV), a post ripple filter is recommended (See Figure 24).
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 36 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.
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Figure 37. Peak-to-Peak Inductor
Ripple Current vs Load Current
When a switching regulator is operating in the continuous mode, the inductor current waveform ranges from a
triangular to a sawtooth type of waveform (depending on the input voltage). For a given input and output voltage,
the peak-to-peak amplitude of this inductor current waveform remains constant. As the load current increases or
decreases, the entire sawtooth current waveform also rises and falls. The average value (or the center) of this
current waveform is equal to the DC load current.
If the load current drops to a low enough level, the bottom of the sawtooth current waveform will reach zero, and
the switcher will smoothly change from a continuous to a discontinuous mode of operation. Most switcher
designs (irregardless how large the inductor value is) will be forced to run discontinuous if the output is lightly
loaded. This is a perfectly acceptable mode of operation.
In a switching regulator design, knowing the value of the peak-to-peak inductor ripple current (ΔIIND) 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 ΔIIND. When the inductor nomographs shown in
Figure 25 through 7 are used to select an inductor value, the peak-to-peak inductor ripple current can
immediately be determined. The curve shown in Figure 37 shows the range of (ΔIIND) that can be expected for
different load currents. The curve also shows how the peak-to-peak inductor ripple current (ΔIIND) 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 2.5A
VIN = 12V, nominal, varying between 10V and 16V.
The selection guide in Figure 26 shows that the vertical line for a 2.5A load current, and the horizontal line for the
12V input voltage intersect approximately midway between the upper and lower borders of the 33 μH inductance
region. A 33 μH inductor will allow a peak-to-peak inductor current (ΔIIND) to flow that will be a percentage of the
maximum load current. Referring to Figure 37, follow the 2.5A line approximately midway into the inductance
region, and read the peak-to-peak inductor ripple current (ΔIIND) on the left hand axis (approximately 620 mA pp).
As the input voltage increases to 16V, it approaches the upper border of the inductance region, and the inductor
ripple current increases. Referring to the curve in Figure 37, it can be seen that for a load current of 2.5A, the
peak-to-peak inductor ripple current (ΔIIND) is 620 mA with 12V in, and can range from 740 mA at the upper
border (16V in) to 500 mA at the lower border (10V in).
Once the ΔIIND value is known, the following formulas can be used to calculate additional information about the
switching regulator circuit.
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1. Peak Inductor or peak switch current
2. Minimum load current before the circuit becomes discontinuous
3. Output Ripple Voltage = (ΔIIND)×(ESR of COUT)
– = 0.62A×0.1Ω=62 mV p-p
4. added
for
line
break
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 in- ductor will provide good results, provided it is exactly in
the center of the inductor (because the induced voltages cancel themselves out), but if it is off center one
direction or the other, then problems could arise. If flux problems are present, even the direction of the inductor
winding can make a difference in some circuits.
This discussion on open core inductors is not to frighten the user, but to alert the user on what kind of problems
to watch out for when using them. Open core bobbin or “stick” inductors are an inexpensive, simple way of
making a compact efficient inductor, and they are used by the millions in many different applications.
THERMAL CONSIDERATIONS
The LM2599 is available in two packages, a 7-pin TO-220 (NDZ) and a 7-pin surface mount TO-263 (KTW).
The TO-220 package needs a heat sink under most conditions. The size of the heat sink depends on the input
voltage, the output voltage, the load current and the ambient temperature. The curves in Figure 38 show the
LM2599T junction temperature rises above ambient temperature for a 3A load and different input and output
voltages. The data for these curves was taken with the LM2599T (TO-220 package) operating as a buck
switching regulator in an ambient temperature of 25°C (still air). These temperature rise numbers are all
approximate and there are many factors that can affect these temperatures. Higher ambient temperatures require
more heat sinking.
The TO-263 surface mount package tab is designed to be soldered to the copper on a printed circuit board. The
copper and the board are the heat sink for this package and the other heat producing components, such as the
catch diode and inductor. The pc board copper area that the package is soldered to should be at least 0.4 in2,
and ideally should have 2 or more square inches of 2 oz. (0.0028 in) copper. Additional copper area improves
the thermal characteristics, but with copper areas greater than approximately 6 in2, only small improvements in
heat dissipation are realized. If further thermal improvements are needed, double sided, multilayer pc-board with
large copper areas and/or airflow are recommended.
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The curves shown in Figure 39 show the LM2599S (TO-263 package) junction temperature rise above ambient
temperature with a 2A load for various input and output voltages. This data was taken with the circuit operating
as a buck switching regulator with all components mounted on a pc board to simulate the junction temperature
under actual operating conditions. This curve can be used for a quick check for the approximate junction
temperature for various conditions, but be aware that there are many factors that can affect the junction
temperature. When load currents higher than 2A are used, double sided or multilayer pc-boards with large
copper areas and/or airflow might be needed, especially for high ambient temperatures and high output voltages.
Circuit Data for Temperature Rise Curve TO-220 Package (NDZ)
Capacitors
Through hole electrolytic
Inductor
Through hole Renco
Diode
Through hole, 5A 40V, Schottky
PC board
3 square inches single sided 2 oz. copper (0.0028″)
Figure 38. Junction Temperature Rise, TO-220
Circuit Data for Temperature Rise Curve TO-263 Package (KTW)
Capacitors
Surface mount tantalum, molded “D” size
Inductor
Surface mount, Pulse engineering, 68 μH
Diode
Surface mount, 5A 40V, Schottky
PC board
9 square inches single sided 2 oz. copper (0.0028″)
Figure 39. Junction Temperature Rise, TO-263
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For the best thermal performance, wide copper traces and generous amounts of printed circuit board copper
should be used in the board layout. (One exception to this is the output (switch) pin, which should not have large
areas of copper.) Large areas of copper provide the best transfer of heat (lower thermal resistance) to the
surrounding air, and moving air lowers the thermal resistance even further.
Package thermal resistance and junction temperature rise numbers are all approximate, and there are many
factors that will affect these numbers. Some of these factors include board size, shape, thickness, position,
location, and even board temperature. Other factors are, trace width, total printed circuit copper area, copper
thickness, single- or double-sided, multilayer board and the amount of solder on the board. The effectiveness of
the pc board to dissipate heat also depends on the size, quantity and spacing of other components on the board,
as well as whether the surrounding air is still or moving. Furthermore, some of these components such as the
catch diode will add heat to the pc board and the heat can vary as the input voltage changes. For the inductor,
depending on the physical size, type of core material and the DC resistance, it could either act as a heat sink
taking heat away from the board, or it could add heat to the board.
SHUTDOWN /SOFT-START
The circuit shown in Figure 42 is a standard buck regulator with 20V in, 12V out, 1A load, and using a 0.068 μF
Soft-start capacitor. The photo in Figure 40 Figure 41 show the effects of Soft-start on the output voltage, the
input current, with, and without a Soft-start capacitor. The reduced input current required at startup is very
evident when comparing the two photos. The Soft-start feature reduces the startup current from 2.6A down to
650 mA, and delays and slows down the output voltage rise time.
Figure 40. Output Voltage, Input Current,
at Start-Up, WITH Soft-start
Figure 41. Output Voltage, Input Current,
at Start-Up, WITHOUT Soft-start
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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 42. Typical Circuit Using Shutdown /Soft-start and Error Flag Features
Figure 43. Inverting −5V Regulator With Shutdown and Soft-start
lNVERTING REGULATOR
The circuit in Figure 43 converts a positive input voltage to a negative output voltage with a common ground. The
circuit operates by bootstrapping the regulator's 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 LM2599-5 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 45 provides a guide as to the amount
of output load current possible for the different input and output voltage conditions.
32
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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 LM2599 has a maximum input voltage rating of 40V.
Figure 44. Inverting Regulator
Figure 45. Maximum Load Current for
Inverting Regulator Circuit
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 IN5400 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 33 μH, 3.5A inductor is the best choice. Capacitor
selection can also be narrowed down to just a few values. Using the values shown in Figure 43 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 LM2599 current limit (approximately 4.5A) are needed for 2 ms or more,
until the output reaches its nominal output voltage. The actual time depends on the output voltage and the size of
the output capacitor. Input power sources that are current limited or sources that can not deliver these currents
without getting loaded down, may not work correctly. Because of the relatively high startup currents required by
the inverting topology, the Soft-start feature shown in Figure 43 is recommended.
Also shown in Figure 43 are several shutdown methods for the inverting configuration. With the inverting
configuration, some level shifting is required, because the ground pin of the regulator is no longer at ground, but
is now at the negative output voltage. The shutdown methods shown accept ground referenced shutdown
signals.
UNDERVOLTAGE LOCKOUT
Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage.
Figure 46 contains a undervoltage lockout circuit for a buck configuration, while Figure 47 and 30 are for the
inverting types (only the circuitry pertaining to the undervoltage lockout is shown). Figure 46 uses a zener diode
to establish the threshold voltage when the switcher begins operating. When the input voltage is less than the
zener voltage, resistors R1 and R2 hold the Shutdown /Soft-start pin low, keeping the regulator in the shutdown
mode. As the input voltage exceeds the zener voltage, the zener conducts, pulling the Shutdown /Soft-start pin
high, allowing the regulator to begin switching. The threshold voltage for the undervoltage lockout feature is
approximately 1.5V greater than the zener voltage.
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Figure 46. Undervoltage Lockout for a Buck Regulator
Figure 47 and 30 apply the same feature to an inverting circuit. Figure 47 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 48 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.
Figure 47. Undervoltage Lockout Without
Hysteresis for an Inverting Regulator
Figure 48. 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 49. This unregulated negative voltage is approximately equal to the positive input voltage
(minus a few volts), and can supply up to a 600 mA of output current. There is a requirement however, that there
be a minimum load of 1.2A 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 LM2599 current limit (typically
4.5A).
This method of generating a negative output voltage without an additional inductor can be used with other
members of the Simple Switcher Family, using either the buck or boost topology.
34
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SNVS123C – APRIL 1998 – REVISED APRIL 2013
Figure 49. Charge Pump for Generating a
Low Current, Negative Output Voltage
TYPICAL THROUGH HOLE PC BOARD LAYOUT, FIXED OUTPUT (1X SIZE), DOUBLE SIDED
CIN: — 470 μF, 50V, Aluminum Electrolytic Panasonic, “HFQ Series”
COUT: — 330 μF, 35V, Aluminum Electrolytic Panasonic, “HFQ Series”
D1: — 5A, 40V Schottky Rectifier, 1N5825
L1: — 47 μH, L39, Renco, Through Hole
RPULL UP: — 10k
CDELAY: — 0.1 μF
CSD/SS: — 0.1 μF
Thermalloy Heat Sink #7020
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LM2599
SNVS123C – APRIL 1998 – REVISED APRIL 2013
www.ti.com
TYPICAL THROUGH HOLE PC BOARD LAYOUT, ADJUSTABLE OUTPUT (1X SIZE), DOUBLE SIDED
CIN: — 470 μF, 50V, Aluminum Electrolytic Panasonic, “HFQ Series”
COUT: — 220 μF, 35V Aluminum Electrolytic Panasonic, “HFQ Series”
D1: — 5A, 40V Schottky Rectifier, 1N5825
L1: — 47 μH, L39, Renco, Through Hole
R1: — 1 kΩ, 1%
R2: — Use formula in Design Procedure
CFF: — See Figure 25.
RFF: — See Application Information Section (CFF Section)
RPULL UP: — 10k
CDELAY: — 0.1 μF
CSD/SS: — 0.1 μF
Thermalloy Heat Sink #7020
Figure 50. PC Board Layout
36
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SNVS123C – APRIL 1998 – REVISED APRIL 2013
REVISION HISTORY
Changes from Revision B (April 2013) to Revision C
•
Page
Changed layout of National Data Sheet to TI format .......................................................................................................... 36
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PACKAGE OPTION ADDENDUM
www.ti.com
23-Sep-2013
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
LM2599S-12
ACTIVE
DDPAK/
TO-263
KTW
7
LM2599S-12/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
LM2599S-3.3
ACTIVE
DDPAK/
TO-263
KTW
LM2599S-3.3/NOPB
ACTIVE
DDPAK/
TO-263
LM2599S-5.0
ACTIVE
LM2599S-5.0/NOPB
Eco Plan
Lead/Ball Finish
(2)
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
TBD
Call TI
Call TI
-40 to 125
LM2599S
-12 P+
45
Pb-Free (RoHS
Exempt)
CU SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-12 P+
7
45
TBD
Call TI
Call TI
-40 to 125
LM2599S
-3.3 P+
KTW
7
45
Pb-Free (RoHS
Exempt)
CU SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-3.3 P+
DDPAK/
TO-263
KTW
7
45
TBD
Call TI
Call TI
-40 to 125
LM2599S
-5.0 P+
ACTIVE
DDPAK/
TO-263
KTW
7
45
Pb-Free (RoHS
Exempt)
CU SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-5.0 P+
LM2599S-ADJ
ACTIVE
DDPAK/
TO-263
KTW
7
45
TBD
Call TI
Call TI
-40 to 125
LM2599S
-ADJ P+
LM2599S-ADJ/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
45
Pb-Free (RoHS
Exempt)
CU SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-ADJ P+
LM2599SX-12
ACTIVE
DDPAK/
TO-263
KTW
7
TBD
Call TI
Call TI
-40 to 125
LM2599S
-12 P+
LM2599SX-12/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
Pb-Free (RoHS
Exempt)
CU SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-12 P+
LM2599SX-3.3
ACTIVE
DDPAK/
TO-263
KTW
7
TBD
Call TI
Call TI
-40 to 125
LM2599S
-3.3 P+
LM2599SX-3.3/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
Pb-Free (RoHS
Exempt)
CU SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-3.3 P+
LM2599SX-5.0
ACTIVE
DDPAK/
TO-263
KTW
7
TBD
Call TI
Call TI
-40 to 125
LM2599S
-5.0 P+
LM2599SX-5.0/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
Pb-Free (RoHS
Exempt)
CU SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-5.0 P+
LM2599SX-ADJ
ACTIVE
DDPAK/
TO-263
KTW
7
TBD
Call TI
Call TI
-40 to 125
LM2599S
-ADJ P+
LM2599SX-ADJ/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
Pb-Free (RoHS
Exempt)
CU SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-ADJ P+
LM2599T-12
ACTIVE
TO-220
NDZ
7
TBD
Call TI
Call TI
-40 to 125
LM2599T
-12 P+
500
500
500
500
Addendum-Page 1
Samples
PACKAGE OPTION ADDENDUM
www.ti.com
23-Sep-2013
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
LM2599T-12/NOPB
ACTIVE
TO-220
NDZ
7
LM2599T-3.3
ACTIVE
TO-220
NDZ
7
LM2599T-3.3/NOPB
ACTIVE
TO-220
NDZ
7
LM2599T-5.0
ACTIVE
TO-220
NDZ
LM2599T-5.0/NOPB
ACTIVE
TO-220
LM2599T-ADJ
ACTIVE
LM2599T-ADJ/NOPB
ACTIVE
45
Eco Plan
Lead/Ball Finish
(2)
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
Green (RoHS
& no Sb/Br)
CU SN
Level-1-NA-UNLIM
-40 to 125
LM2599T
-12 P+
TBD
Call TI
Call TI
-40 to 125
LM2599T
-3.3 P+
45
Green (RoHS
& no Sb/Br)
CU SN
Level-1-NA-UNLIM
-40 to 125
LM2599T
-3.3 P+
7
45
TBD
Call TI
Call TI
-40 to 125
LM2599T
-5.0 P+
NDZ
7
45
Green (RoHS
& no Sb/Br)
CU SN
Level-1-NA-UNLIM
-40 to 125
LM2599T
-5.0 P+
TO-220
NDZ
7
45
TBD
Call TI
Call TI
-40 to 125
LM2599T
-ADJ P+
TO-220
NDZ
7
45
Green (RoHS
& no Sb/Br)
CU SN
Level-1-NA-UNLIM
-40 to 125
LM2599T
-ADJ P+
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability
information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that
lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between
the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight
in homogeneous material)
(3)
MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.
(4)
There may be additional marking, which relates to the logo, the lot trace code information, or the environmental category on the device.
(5)
Multiple Device Markings will be inside parentheses. Only one Device Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a continuation
of the previous line and the two combined represent the entire Device Marking for that device.
Addendum-Page 2
Samples
PACKAGE OPTION ADDENDUM
www.ti.com
23-Sep-2013
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information
provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and
continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.
TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.
Addendum-Page 3
PACKAGE MATERIALS INFORMATION
www.ti.com
23-Sep-2013
TAPE AND REEL INFORMATION
*All dimensions are nominal
Device
Package Package Pins
Type Drawing
SPQ
Reel
Reel
A0
Diameter Width (mm)
(mm) W1 (mm)
B0
(mm)
K0
(mm)
P1
(mm)
W
Pin1
(mm) Quadrant
LM2599SX-12/NOPB
DDPAK/
TO-263
KTW
7
500
330.0
24.4
10.75
14.85
5.0
16.0
24.0
Q2
LM2599SX-3.3/NOPB
DDPAK/
TO-263
KTW
7
500
330.0
24.4
10.75
14.85
5.0
16.0
24.0
Q2
LM2599SX-5.0/NOPB
DDPAK/
TO-263
KTW
7
500
330.0
24.4
10.75
14.85
5.0
16.0
24.0
Q2
LM2599SX-ADJ/NOPB
DDPAK/
TO-263
KTW
7
500
330.0
24.4
10.75
14.85
5.0
16.0
24.0
Q2
Pack Materials-Page 1
PACKAGE MATERIALS INFORMATION
www.ti.com
23-Sep-2013
*All dimensions are nominal
Device
Package Type
Package Drawing
Pins
SPQ
Length (mm)
Width (mm)
Height (mm)
LM2599SX-12/NOPB
DDPAK/TO-263
KTW
7
500
367.0
367.0
45.0
LM2599SX-3.3/NOPB
DDPAK/TO-263
KTW
7
500
367.0
367.0
45.0
LM2599SX-5.0/NOPB
DDPAK/TO-263
KTW
7
500
367.0
367.0
45.0
LM2599SX-ADJ/NOPB
DDPAK/TO-263
KTW
7
500
367.0
367.0
45.0
Pack Materials-Page 2
MECHANICAL DATA
NDZ0007B
TA07B (Rev E)
www.ti.com
MECHANICAL DATA
KTW0007B
TS7B (Rev E)
BOTTOM SIDE OF PACKAGE
www.ti.com
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