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An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
LM2596
SNVS124E NOVEMBER 1999REVISED FEBRUARY 2020
LM2596 SIMPLE SWITCHER
®
Power Converter 150-kHz
3-A Step-Down Voltage Regulator
1
1 Features
1 New product available: LMR33630 36-V, 3-A, 400-
kHz synchronous converter
3.3-V, 5-V, 12-V, and adjustable output versions
Adjustable version output voltage range: 1.2-V to
37-V ±4% maximum over line and load conditions
Available in TO-220 and TO-263 packages
3-A output load current
Input voltage range up to 40 V
Requires only four external components
Excellent line and load regulation specifications
150-kHz Fixed-frequency internal oscillator
TTL shutdown capability
Low power standby mode, IQ, typically 80 μA
High efficiency
Uses readily available standard inductors
Thermal shutdown and current-limit protection
Create a custom design using the LM2596 with
the WEBENCH Power Designer
2 Applications
Appliances
Grid infrastructure
EPOS
Home theater
3 Description
The LM2596 series of regulators are monolithic
integrated circuits that provide all the active functions
for a step-down (buck) switching regulator, capable of
driving a 3-A load with excellent line and load
regulation. These devices are available in fixed output
voltages of 3.3 V, 5 V, 12 V, and an adjustable output
version.
Requiring a minimum number of external
components, these regulators are simple to use and
include internal frequency compensation, and a fixed-
frequency oscillator.
The LM2596 series operates at a switching frequency
of 150 kHz, thus allowing smaller sized filter
components than what would be required with lower
frequency switching regulators. Available in a
standard 5-pin TO-220 package with several different
lead bend options, and a 5-pin TO-263 surface mount
package.
The new product, LMR33630, offers reduced BOM
cost, higher efficiency, and an 85% reduction in
solution size among many other features. See the
device comparison table to compare specs. Start
WEBENCH Design with LMR33630.
Device Information(1)
PART NUMBER PACKAGE BODY SIZE (NOM)
LM2596 TO-220 (5) 14.986 mm × 10.16 mm
TO-263 (5) 10.10 mm × 8.89 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Typical Application
(Fixed Output Voltage Versions)
2
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Table of Contents
1 Features.................................................................. 1
2 Applications ........................................................... 1
3 Description............................................................. 1
4 Revision History..................................................... 2
5 Description (continued)......................................... 3
6 Pin Configuration and Functions......................... 4
7 Specifications......................................................... 5
7.1 Absolute Maximum Ratings ..................................... 5
7.2 ESD Ratings.............................................................. 5
7.3 Operating Conditions ................................................ 5
7.4 Thermal Information.................................................. 5
7.5 Electrical Characteristics 3.3-V Version................. 6
7.6 Electrical Characteristics 5-V Version.................... 6
7.7 Electrical Characteristics 12-V Version.................. 6
7.8 Electrical Characteristics Adjustable Voltage
Version....................................................................... 6
7.9 Electrical Characteristics All Output Voltage
Versions..................................................................... 7
7.10 Typical Characteristics............................................ 8
8 Detailed Description............................................ 11
8.1 Overview................................................................. 11
8.2 Functional Block Diagram....................................... 11
8.3 Feature Description................................................. 11
8.4 Device Functional Modes........................................ 15
9 Application and Implementation ........................ 16
9.1 Application Information............................................ 16
9.2 Typical Applications ................................................ 23
10 Power Supply Recommendations ..................... 32
11 Layout................................................................... 32
11.1 Layout Guidelines ................................................. 32
11.2 Layout Examples................................................... 32
11.3 Thermal Considerations........................................ 34
12 Device and Documentation Support ................. 36
12.1 Device Support...................................................... 36
12.2 Receiving Notification of Documentation Updates 36
12.3 Support Resources ............................................... 36
12.4 Trademarks........................................................... 36
12.5 Electrostatic Discharge Caution............................ 36
12.6 Glossary................................................................ 36
13 Mechanical, Packaging, and Orderable
Information........................................................... 36
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision D (May 2016) to Revision E Page
Added link to the LMR33630 product folder in the Features ................................................................................................. 1
Updated Description to include the LMR33630 product page, device comparison table, and WEBENCH link .................... 1
Changed the package from 7 pins to 5 pins .......................................................................................................................... 1
Changes from Revision C (April 2013) to Revision D Page
Added ESD Ratings table, Feature Description section, Device Functional Modes,Application and Implementation
section, Power Supply Recommendations section, Layout section, Device and Documentation Support section, and
Mechanical, Packaging, and Orderable Information section.................................................................................................. 1
Removed all references to design software Switchers Made Simple .................................................................................... 1
Changes from Revision B (April 2013) to Revision C Page
Changed layout of National Semiconductor Data Sheet to TI format .................................................................................. 11
3
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5 Description (continued)
A standard series of inductors are available from several different manufacturers optimized for use with the
LM2596 series. This feature greatly simplifies the design of switch-mode power supplies.
Other features include a ±4% tolerance on output voltage under specified 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 frequency reducing current limit for the output
switch and an overtemperature shutdown for complete protection under fault conditions.
4
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6 Pin Configuration and Functions
NDH Package
5-Pin TO-220
Top View KTT Package
5-Pin TO-263
Top View
Pin Functions
PIN I/O DESCRIPTION
NO. NAME
1 VIN IThis 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 required by the regulator.
2 Output O Internal switch. The voltage at this pin switches between approximately (+VIN VSAT) and
approximately 0.5 V, with a duty cycle of VOUT / VIN. To minimize coupling to sensitive
circuitry, the PCB copper area connected to this pin must be kept to a minimum.
3 Ground Circuit ground
4 Feedback I Senses the regulated output voltage to complete the feedback loop.
5 ON/OFF I
Allows the switching regulator circuit to be shut down using logic signals thus dropping the
total input supply current to approximately 80 µA. Pulling this pin below a threshold voltage
of approximately 1.3 V turns the regulator on, and pulling this pin above 1.3 V (up to a
maximum of 25 V) shuts the regulator down. If this shutdown feature is not required, the
ON/OFF pin can be wired to the ground pin or it can be left open. In either case, the
regulator will be in the ON condition.
5
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(1) Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
(2) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
(3) Voltage internally clamped. If clamp voltage is exceeded, limit current to a maximum of 1 mA.
7 Specifications
7.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted)(1)(2)
MIN MAX UNIT
Maximum supply voltage (VIN) 45 V
SD/SS pin input voltage(3) 6 V
Delay pin voltage(3) 1.5 V
Flag pin voltage –0.3 45 V
Feedback pin voltage –0.3 25 V
Output voltage to ground, steady-state –1 V
Power dissipation Internally limited
Lead temperature KTW package Vapor phase (60 s) 215 °CInfrared (10 s) 245
NDZ package, soldering (10 s) 260
Maximum junction temperature 150 °C
Storage temperature, Tstg –65 150 °C
(1) JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
7.2 ESD Ratings VALUE UNIT
V(ESD) Electrostatic discharge Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001(1) ±2000 V
7.3 Operating Conditions MIN MAX UNIT
Supply voltage 4.5 40 V
Temperature –40 125 °C
(1) For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report.
(2) The package thermal impedance is calculated in accordance to JESD 51-7.
(3) Thermal Resistances were simulated on a 4-layer, JEDEC board.
(4) 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.
(5) Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 0.5 in2of 1-oz
copper area.
(6) Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 2.5 in2of 1-oz
copper area.
(7) Junction to ambient thermal resistance with the TO-263 package tab soldered to a double sided printed circuit board with 3 in2of 1-oz
copper area on the LM2596S side of the board, and approximately 16 in2of copper on the other side of the PCB.
7.4 Thermal Information
THERMAL METRIC(1)
LM2596
UNITKTW (TO-263) NDZ (TO-220)
5 PINS 5 PINS
RθJA Junction-to-ambient thermal resistance(2)(3)
See(4) 50
°C/W
See(5) 50
See(6) 30
See(7) 20
RθJC(top) Junction-to-case (top) thermal resistance 2 2 °C/W
6
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(1) 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).
(2) Typical numbers are at 25°C and represent the most likely norm.
(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2596 is used as shown in Figure 35, system performance is shown in the test conditions column.
7.5 Electrical Characteristics 3.3-V Version
Specifications are for TJ= 25°C (unless otherwise noted)
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT
SYSTEM PARAMETERS(3) (see Figure 35 for test circuit)
VOUT Output voltage 4.75 V VIN 40 V,
0.2 A ILOAD 3 A TJ= 25°C 3.168 3.3 3.432 V
–40°C TJ125°C 3.135 3.465
ηEfficiency VIN = 12 V, ILOAD = 3 A 73%
(1) 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).
(2) Typical numbers are at 25°C and represent the most likely norm.
(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2596 is used as shown in Figure 35, system performance is shown in the test conditions column.
7.6 Electrical Characteristics 5-V Version
Specifications are for TJ= 25°C (unless otherwise noted)
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT
SYSTEM PARAMETERS(3) (see Figure 35 for test circuit)
VOUT Output voltage 7 V VIN 40 V,
0.2 A ILOAD 3 A TJ= 25°C 4.8 5 5.2 V
–40°C TJ125°C 4.75 5.25
ηEfficiency VIN = 12 V, ILOAD = 3 A 80%
(1) 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).
(2) Typical numbers are at 25°C and represent the most likely norm.
(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2596 is used as shown in Figure 35, system performance is shown in the test conditions column.
7.7 Electrical Characteristics 12-V Version
Specifications are for TJ= 25°C (unless otherwise noted)
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT
SYSTEM PARAMETERS(3) (see Figure 35 for test circuit)
VOUT Output voltage 15 V VIN 40 V,
0.2 A ILOAD 3 A TJ= 25°C 11.52 12 12.48 V
–40°C TJ125°C 11.4 12.6
ηEfficiency VIN = 25 V, ILOAD = 3 A 90%
(1) 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).
(2) Typical numbers are at 25°C and represent the most likely norm.
(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2596 is used as shown in Figure 35, system performance is shown in the test conditions column.
7.8 Electrical Characteristics Adjustable Voltage Version
Specifications are for TJ= 25°C (unless otherwise noted)
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT
SYSTEM PARAMETERS(3) (see Figure 35 for test circuit)
VFB Feedback voltage
4.5 V VIN 40 V, 0.2 A ILOAD 3 A 1.23
V
VOUT programmed for 3 V
(see Figure 35 for test circuit) TJ= 25°C 1.193 1.267
–40°C TJ125°C 1.18 1.28
ηEfficiency VIN = 12 V, VOUT = 3 V, ILOAD = 3 A 73%
7
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(1) 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).
(2) Typical numbers are at 25°C and represent the most likely norm.
(3) 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.
(4) No diode, inductor, or capacitor connected to output pin.
(5) Feedback pin removed from output and connected to 0 V to force the output transistor switch ON.
(6) Feedback pin removed from output and connected to 12 V for the 3.3-V, 5-V, and the adjustable versions, and 15 V for the 12-V
version, to force the output transistor switch OFF.
(7) VIN = 40 V.
7.9 Electrical Characteristics All Output Voltage Versions
Specifications are for TJ= 25°C, ILOAD = 500 mA, VIN = 12 V for the 3.3-V, 5-V, and adjustable version, and VIN = 24 V for the
12-V version (unless otherwise noted).
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT
DEVICE PARAMETERS
IbFeedback bias current Adjustable version only,
VFB = 1.3 V TJ= 25°C 10 50 nA
–40°C TJ125°C 100
fOOscillator frequency(3) TJ= 25°C 127 150 173 kHz
–40°C TJ125°C 110 173
VSAT Saturation voltage(4) (5) IOUT = 3 A TJ= 25°C 1.16 1.4 V
–40°C TJ125°C 1.5
DC Max duty cycle (ON)(5) 100%
Min duty cycle (OFF)(6) 0%
ICL Current limit(4) (5) Peak current TJ= 25°C 3.6 4.5 6.9 A
–40°C TJ125°C 3.4 7.5
ILOutput leakage
current(4) (6) Output = 0 V, VIN = 40 V 50 μA
Output = –1 V 2 30 mA
IQOperating quiescent
current(6) See (6) 5 10 mA
ISTBY Current standby
quiescent ON/OFF pin = 5 V (OFF)(7) TJ= 25°C 80 200 μA
–40°C TJ125°C 250 μA
SHUTDOWN/SOFT-START CONTROL (see Figure 35 for test circuit)
VIH ON/OFF pin logic input
threshold voltage
Low (regulator ON) TJ= 25°C 1.3 V
–40°C TJ125°C 0.6
VIL High (regulator OFF) TJ= 25°C 1.3 V
–40°C TJ125°C 2
IHON/OFF pin input
current VLOGIC = 2.5 V (regulator OFF) 5 15 μA
ILVLOGIC = 0.5 V (regulator ON) 0.02 5 μA
8
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7.10 Typical Characteristics
See Figure 35 for test circuit
Figure 1. Normalized Output Voltage Figure 2. Line Regulation
Figure 3. Efficiency Figure 4. Switch Saturation Voltage
Figure 5. Switch Current Limit Figure 6. Dropout Voltage
9
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Typical Characteristics (continued)
See Figure 35 for test circuit
Figure 7. Operating Quiescent Current Figure 8. Shutdown Quiescent Current
Figure 9. Minimum Operating Supply Voltage Figure 10. ON/OFF Threshold Voltage
Figure 11. ON/OFF Pin Current (Sinking) Figure 12. Switching Frequency
10
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Typical Characteristics (continued)
See Figure 35 for test circuit
Figure 13. Feedback Pin Bias Current
11
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8 Detailed Description
8.1 Overview
The LM2596 SIMPLE SWITCHER®regulator is an easy-to-use, nonsynchronous, step-down DC-DC converter
with a wide input voltage range up to 40 V. The regulator is capable of delivering up to 3-A DC load current with
excellent line and load regulation. These devices are available in fixed output voltages of 3.3-V, 5-V, 12-V, and
an adjustable output version. The family requires few external components, and the pin arrangement was
designed for simple, optimum PCB layout.
8.2 Functional Block Diagram
8.3 Feature Description
8.3.1 Delayed Start-Up
The circuit in Figure 14 uses the ON/OFF pin to provide a time delay between the time the input voltage is
applied and the time the output voltage comes up (only the circuitry pertaining to the delayed start-up is shown).
As the input voltage rises, the charging of capacitor C1 pulls the ON/OFF pin high, keeping the regulator OFF.
Once the input voltage reaches its final value and the capacitor stops charging, resistor R2pulls the ON/OFF pin
low, thus allowing the circuit to start switching. Resistor R1is included to limit the maximum voltage applied to the
ON/OFF pin (maximum of 25 V), reduces power supply noise sensitivity, and also limits the capacitor C1
discharge current. When high input ripple voltage exists, avoid long delay time, because this ripple can be
coupled into the ON/OFF pin and cause problems.
This delayed start-up feature is useful in situations where the input power source is limited in the amount of
current it can deliver. It allows the input voltage to rise to a higher voltage before the regulator starts operating.
Buck regulators require less input current at higher input voltages.
12
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Feature Description (continued)
Figure 14. Delayed Start-Up
8.3.2 Undervoltage Lockout
Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage.
Figure 15 shows an undervoltage lockout feature applied to a buck regulator, while Figure 16 and Figure 17
apply the same feature to an inverting circuit. The circuit in Figure 16 features a constant threshold voltage for
turnon and turnoff (Zener voltage plus approximately one volt). If hysteresis is required, the circuit in Figure 17
has a turnon voltage which is different than the turnoff voltage. The amount of hysteresis is approximately equal
to the value of the output voltage. If Zener voltages greater than 25 V are used, an additional 47-kΩresistor is
required from the ON/OFF pin to the ground pin to stay within the 25 V maximum limit of the ON/OFF pin.
Figure 15. Undervoltage Lockout
for Buck Regulator
8.3.3 Inverting Regulator
The circuit in Figure 18 converts a positive input voltage to a negative output voltage with a common ground. The
circuit operates by bootstrapping the ground pin of the regulator to the negative output voltage, then grounding
the feedback pin, the regulator senses the inverted output voltage and regulates it.
This circuit has an ON/OFF threshold of approximately 13 V.
Figure 16. Undervoltage Lockout
for Inverting Regulator
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Feature Description (continued)
This example uses the LM2596-5.0 to generate a 5-V output, but other output voltages are possible by
selecting other output voltage versions, including the adjustable version. Because 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. Figure 19 provides a guide as to the amount of output load
current possible for the different input and output voltage conditions.
The maximum voltage appearing across the regulator is the absolute sum of the input and output voltage, and
this must be limited to a maximum of 40 V. For example, when converting +20 V to 12 V, the regulator would
see 32 V between the input pin and ground pin. The LM2596 has a maximum input voltage spec of 40 V.
Additional diodes are 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.
TI recommends using a Schottky diode for low input voltages, (because of its lower voltage drop) but for higher
input voltages, a fast recovery diode could be used.
Without diode D3, when the input voltage is first applied, the charging current of CIN can pull the output positive
by several volts for a short period of time. Adding D3 prevents the output from going positive by more than a
diode voltage.
This circuit has hysteresis
Regulator starts switching at VIN = 13 V
Regulator stops switching at VIN = 8 V
Figure 17. Undervoltage Lockout With Hysteresis for Inverting Regulator
CIN 68-μF, 25-V Tant. Sprague 595D
470 -μF, 50-V Elec. Panasonic HFQ
COUT 47-μF, 20-V Tant. Sprague 595D
220-μF, 25-V Elec. Panasonic HFQ
Figure 18. Inverting 5-V Regulator With Delayed Start-Up
14
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Feature Description (continued)
Figure 19. Inverting Regulator Typical Load Current
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, 3.5-A inductor is the best choice. Capacitor
selection can also be narrowed down to just a few values. Using the values shown in Figure 18 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 LM2596 current limit (approximately 4.5 A) are required for at least 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 start-up currents
required by the inverting topology, the delayed start-up feature (C1, R1, and R2) shown in Figure 18 is
recommended. By delaying the regulator start-up, the input capacitor is allowed to charge up to a higher voltage
before the switcher begins operating. A portion of the high input current required for start-up is now supplied by
the input capacitor (CIN). For severe start-up conditions, the input capacitor can be made much larger than
normal.
8.3.4 Inverting Regulator Shutdown Methods
Using the ON/OFF pin in a standard buck configuration is simple. To turn the regulator ON, pull the ON/OFF pin
below 1.3 V (at 25°C, referenced to ground). To turn the regulator OFF, pull the ON/OFF pin above 1.3 V. With
the inverting configuration, some level shifting is required, because the ground pin of the regulator is no longer at
ground, but is now setting at the negative output voltage level. Two different shutdown methods for inverting
regulators are shown in Figure 20 and Figure 21.
Figure 20. Inverting Regulator Ground Referenced Shutdown
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Feature Description (continued)
Figure 21. Inverting Regulator Ground Referenced Shutdown Using Opto Device
8.4 Device Functional Modes
8.4.1 Discontinuous Mode Operation
The selection guide chooses inductor values suitable for continuous mode operation, but for low current
applications or high input voltages, a discontinuous mode design can be a better choice. A discontinuous mode
design would use an inductor that would be physically smaller, and would require only one half to one third the
inductance value required for a continuous mode design. The peak switch and inductor currents will be higher in
a discontinuous design, but at these low load currents (1 A and below), the maximum switch current will still be
less than the switch current limit.
Discontinuous operation can have voltage waveforms that are considerably different than a continuous design.
The output pin (switch) waveform can have some damped sinusoidal ringing present (see Figure 36). 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 nor 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 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.
Figure 22. Post Ripple Filter Waveform
16
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9 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
9.1.1 Input Capacitor (CIN)
A low ESR aluminum or tantalum bypass capacitor is required between the input pin and ground pin. It must be
placed near the regulator using short leads. This capacitor prevents large voltage transients from occuring at the
input, and provides the instantaneous current required 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 input capacitor of the buck converter, this capacitor must 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 power rating of the capacitor. 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.
For a given capacitor value, a higher voltage electrolytic capacitor is 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.
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 required to satisfy the RMS current requirements.
Figure 23 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 turnon 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 turnon currents are
expected, it may be necessary to limit this current by adding either some resistance or inductance before the
tantalum capacitor, or select a higher voltage capacitor. As with aluminum electrolytic capacitors, the RMS ripple
current rating must be sized to the load current.
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9.1.2 Feedforward Capacitor (CFF)
NOTE
For adjustable output voltage version only.
A feedforward capacitor, shown across R2 in Table 6, is used when the output voltage is greater than 10 V 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 Detailed Design Procedure section.
Figure 23. RMS Current Ratings for Low ESR Electrolytic Capacitors (Typical)
9.1.3 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 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 required. 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, see Output Voltage Ripple and Transients for a
post ripple filter.
An ESR value of the aluminum electrolytic capacitor is related to the capacitance value and its voltage rating. In
most cases, higher voltage electrolytic capacitors have lower ESR values (see Figure 24). Often, capacitors with
much higher voltage ratings may be required 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 Table 3 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 is typically three times as large at 25°C and as much as 10 times as large at 40°C. See
Figure 25.
Solid tantalum capacitors have a much better ESR specifications for cold temperatures and are recommended
for temperatures below 25°C.
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Figure 24. Capacitor ESR versus Capacitor Voltage Rating (Typical Low-ESR Electrolytic Capacitor)
9.1.4 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 placed close to the LM2596 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 (5 V 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 1N5400 series are much too slow and should not be used.
Figure 25. Capacitor ESR Change versus Temperature
9.1.5 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 LM2596 (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, which offers greater output power, lower
peak switch, lower inductor and diode currents, and can have lower output ripple voltage. However, the
continuous mode does require larger inductor values to keep the inductor current flowing continuously, especially
at low output load currents or high input voltages.
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To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Figure 27
through Figure 30). 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 26.)
Figure 26. (ΔIIND) Peak-to-Peak Inductor
Ripple Current (as a Percentage of the Load Current)
versus 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, and so forth, 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
because 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 (see Open-Core Inductors).
When multiple switching regulators are located on the same PCB, 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 Schottky, ferrite bobbin core for
Renco and Coilcraft, and powdered iron toroid for Pulse Engineering.
Exceeding the maximum current rating of the inductor can 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 or the LM2596. Different inductor types have different
saturation characteristics, so consider this when selecting an inductor.
The inductor manufacturer's data sheets include current and energy limits to avoid inductor saturation.
For continuous mode operation, see the inductor selection graphs in Figure 27 through Figure 30.