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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.
LM2576
,
LM2576HV
SNVS107D JUNE 1999REVISED MAY 2016
LM2576xx Series SIMPLE SWITCHER
®
3-A Step-Down Voltage Regulator
1
1 Features
1 3.3-V, 5-V, 12-V, 15-V, and Adjustable Output
Versions
Adjustable Version Output Voltage Range,1.23 V
to 37 V (57 V for HV Version) ±4% Maximum Over
Line and Load Conditions
Specified 3-A Output Current
Wide Input Voltage Range: 40 V Up to 60 V for
HV Version
Requires Only 4 External Components
52-kHz Fixed-Frequency Internal Oscillator
TTL-Shutdown Capability, Low-Power Standby
Mode
High Efficiency
Uses Readily Available Standard Inductors
Thermal Shutdown and Current Limit Protection
2 Applications
Simple High-Efficiency Step-Down (Buck)
Regulator
Efficient Preregulator for Linear Regulators
On-Card Switching Regulators
Positive-to-Negative Converter (Buck-Boost)
3 Description
The LM2576 series of regulators are monolithic
integrated circuits that provide all the active functions
for a step-down (buck) switching regulator, capable of
driving 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, 15 V, and an adjustable
output version.
Requiring a minimum number of external
components, these regulators are simple to use and
include fault protection and a fixed-frequency
oscillator.
The LM2576 series offers a high-efficiency
replacement for popular three-terminal linear
regulators. It substantially reduces the size of the
heat sink, and in some cases no heat sink is
required.
A standard series of inductors optimized for use with
the LM2576 are available from several different
manufacturers. This feature greatly simplifies the
design of switch-mode power supplies.
Other features include a ±4% tolerance on output
voltage within specified input voltages and output
load conditions, and ±10% on the oscillator
frequency. External shutdown is included, featuring
50-μA (typical) standby current. The output switch
includes cycle-by-cycle current limiting, as well as
thermal shutdown for full protection under fault
conditions.
Device Information(1)
PART NUMBER PACKAGE BODY SIZE (NOM)
LM2576
LM2576HV TO-220 (5) 10.16 mm × 8.51 mm
DDPAK/TO-263 (5) 10.16 mm × 8.42 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Fixed Output Voltage Version Typical Application Diagram
2
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Table of Contents
1 Features.................................................................. 1
2 Applications ........................................................... 1
3 Description............................................................. 1
4 Revision History..................................................... 2
5 Pin Configuration and Functions......................... 3
6 Specifications......................................................... 4
6.1 Absolute Maximum Ratings ..................................... 4
6.2 ESD Ratings.............................................................. 4
6.3 Recommended Operating Conditions....................... 4
6.4 Thermal Information.................................................. 4
6.5 Electrical Characteristics: 3.3 V................................ 5
6.6 Electrical Characteristics: 5 V................................... 5
6.7 Electrical Characteristics: 12 V................................. 5
6.8 Electrical Characteristics: 15 V................................. 6
6.9 Electrical Characteristics: Adjustable Output
Voltage....................................................................... 6
6.10 Electrical Characteristics: All Output Voltage
Versions..................................................................... 6
6.11 Typical Characteristics............................................ 8
7 Detailed Description............................................ 12
7.1 Overview................................................................. 12
7.2 Functional Block Diagram....................................... 12
7.3 Feature Description................................................. 12
7.4 Device Functional Modes........................................ 14
8 Application and Implementation ........................ 15
8.1 Application Information............................................ 15
8.2 Typical Applications ................................................ 19
9 Power Supply Recommendations...................... 24
10 Layout................................................................... 25
10.1 Layout Guidelines ................................................. 25
10.2 Layout Example .................................................... 26
10.3 Grounding ............................................................. 26
10.4 Heat Sink and Thermal Considerations................ 26
11 Device and Documentation Support................. 28
11.1 Device Support .................................................... 28
11.2 Documentation Suuport ........................................ 29
11.3 Related Links ........................................................ 29
11.4 Community Resources.......................................... 29
11.5 Trademarks........................................................... 29
11.6 Electrostatic Discharge Caution............................ 29
11.7 Glossary................................................................ 29
12 Mechanical, Packaging, and Orderable
Information........................................................... 30
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
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
Moved the thermal resistance data from the Electrical Characteristics: All Output Voltage Versions table to the
Thermal Information table....................................................................................................................................................... 4
Changes from Revision B (April 2013) to Revision C Page
Changed layout of National Data Sheet to TI format ............................................................................................................. 3
3
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,
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5 Pin Configuration and Functions
KC Package
5-Pin TO-220
Top View
KTT Package
5-PIN DDPAK/TO-263
Top View DDPAK/TO-263 (S) Package
5-Lead Surface-Mount Package
Top View
(1) I = INPUT, O = OUTPUT
Pin Functions
PIN I/O(1) DESCRIPTION
NO. NAME
1 VIN ISupply input pin to collector pin of high-side transistor. Connect to power supply and input
bypass capacitors CIN. Path from VIN pin to high frequency bypass CIN and GND must be as
short as possible.
2 OUTPUT O Emitter pin of the power transistor. This is a switching node. Attached this pin to an inductor
and the cathode of the external diode.
3 GROUND Ground pin. Path to CIN must be as short as possible.
4 FEEDBACK I Feedback sense input pin. Connect to the midpoint of feedback divider to set VOUT for ADJ
version or connect this pin directly to the output capacitor for a fixed output version.
5 ON/OFF I Enable input to the voltage regulator. High = OFF and low = ON. Connect to GND to enable
the voltage regulator. Do not leave this pin float.
TAB Connected to GND. Attached to heatsink for thermal relief for TO-220 package or put a
copper plane connected to this pin as a thermal relief for DDPAK package.
4
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,
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SNVS107D JUNE 1999REVISED MAY 2016
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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 TI Sales Office/ Distributors for availability and specifications.
6 Specifications
6.1 Absolute Maximum Ratings
over the recommended operating junction temperature range of -40°C to 125°C (unless otherwise noted)(1)(2)
MIN MAX UNIT
Maximum supply voltage LM2576 45 V
LM2576HV 63
ON /OFF pin input voltage 0.3V V+VIN V
Output voltage to ground (Steady-state) 1 V
Power dissipation Internally Limited
Maximum junction temperature, TJ150 °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.
6.2 ESD Ratings VALUE UNIT
V(ESD) Electrostatic discharge Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001(1) ±2000 V
6.3 Recommended Operating Conditions
over the recommended operating junction temperature range of -40°C to 125°C (unless otherwise noted)
MIN MAX UNIT
Temperature LM2576, LM2576HV 40 125 °C
Supply voltage LM2576 40 V
LM2576HV 60
(1) For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report, SPRA953 and the Using New Thermal Metrics applications report, SBVA025.
(2) The package thermal impedance is calculated in accordance with JESD 51-7
(3) Thermal Resistances were simulated on a 4-layer, JEDEC board.
6.4 Thermal Information
THERMAL METRIC(1)(2)(3) LM2576, LM2576HV
UNITKTT (TO-263) KC (TO-220)
5 PINS 5 PINS
RθJA Junction-to-ambient thermal resistance 42.6 32.4 °C/W
RθJC(top) Junction-to-case (top) thermal resistance 43.3 41.2 °C/W
RθJB Junction-to-board thermal resistance 22.4 17.6 °C/W
ψJT Junction-to-top characterization parameter 10.7 7.8 °C/W
ψJB Junction-to-board characterization parameter 21.3 17 °C/W
RθJC(bot) Junction-to-case (bottom) thermal resistance 0.4 0.4 °C/W
5
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(1) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2576/LM2576HV is used as shown in Figure 26 and Figure 32, system performance is as shown in Electrical
Characteristics: All Output Voltage Versions.
6.5 Electrical Characteristics: 3.3 V
Specifications are for TJ= 25°C (unless otherwise noted).
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
SYSTEM PARAMETERS TEST CIRCUIT Figure 26 and Figure 32(1)
VOUT
Output Voltage VIN = 12 V, ILOAD = 0.5 A
Circuit of Figure 26 and Figure 32 3.234 3.3 3.366 V
Output Voltage: LM2576 6 V VIN 40 V, 0.5 A
ILOAD 3 A
Circuit of Figure 26 and
Figure 32
TJ= 25°C 3.168 3.3 3.432
V
Applies over full
operating
temperature range 3.135 3.465
Output Voltage: LM2576HV 6 V VIN 60 V, 0.5 A
ILOAD 3 A
Circuit of Figure 26 and
Figure 32
TJ= 25°C 3.168 3.3 3.45
V
Applies over full
operating
temperature range 3.135 3.482
ηEfficiency VIN = 12 V, ILOAD = 3 A 75%
(1) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2576/LM2576HV is used as shown in Figure 26 and Figure 32, system performance is as shown in Electrical
Characteristics: All Output Voltage Versions.
6.6 Electrical Characteristics: 5 V
Specifications are for TJ= 25°C for the Figure 26 and Figure 32 (unless otherwise noted).
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
SYSTEM PARAMETERS TEST CIRCUIT Figure 26 and Figure 32(1)
VOUT Output Voltage VIN = 12 V, ILOAD = 0.5 A
Circuit of Figure 26 and Figure 32 4.9 5 5.1 V
VOUT Output Voltage
LM2576
0.5 A ILOAD 3 A,
8 V VIN 40 V
Circuit of Figure 26 and
Figure 32
TJ= 25°C 4.8 5 5.2
V
Applies over full
operating
temperature range 4.75 5.25
VOUT Output Voltage
LM2576HV
0.5 A ILOAD 3 A,
8 V VIN 60 V
Circuit of Figure 26 and
Figure 32
TJ= 25°C 4.8 5 4.75
V
Applies over full
operating
temperature range 5.225 5.275
ηEfficiency VIN = 12 V, ILOAD = 3 A 77%
(1) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2576/LM2576HV is used as shown in Figure 26 and Figure 32, system performance is as shown in Electrical
Characteristics: All Output Voltage Versions.
6.7 Electrical Characteristics: 12 V
Specifications are for TJ= 25°C (unless otherwise noted).
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
SYSTEM PARAMETERS TEST CIRCUIT Figure 26 and Figure 32(1)
VOUT Output Voltage VIN = 25 V, ILOAD = 0.5 A
Circuit of Figure 26 and Figure 32 11.76 12 12.24 V
VOUT Output Voltage
LM2576
0.5 A ILOAD 3 A,
15 V VIN 40 V
Circuit of Figure 26 and
Figure 32 and
TJ= 25°C 11.52 12 12.48
V
Applies over full
operating
temperature range 11.4 12.6
VOUT Output Voltage
LM2576HV
0.5 A ILOAD 3 A,
15 V VIN 60 V
Circuit of Figure 26 and
Figure 32
TJ= 25°C 11.52 12 12.54
V
Applies over full
operating
temperature range 11.4 12.66
ηEfficiency VIN = 15 V, ILOAD = 3 A 88%
6
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,
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(1) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2576/LM2576HV is used as shown in Figure 26 and Figure 32, system performance is as shown in Electrical
Characteristics: All Output Voltage Versions.
6.8 Electrical Characteristics: 15 V
over operating free-air temperature range (unless otherwise noted).
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
SYSTEM PARAMETERS TEST CIRCUIT Figure 26 and Figure 32(1)
VOUT Output Voltage VIN = 25 V, ILOAD = 0.5 A
Circuit of Figure 26 and Figure 32 14.7 15 15.3 V
VOUT Output Voltage
LM2576
0.5 A ILOAD 3 A,
18 V VIN 40 V
Circuit of Figure 26 and
Figure 32
TJ= 25°C 14.4 15 15.6
V
Applies over full
operating
temperature range 14.25 15.75
VOUT Output Voltage
LM2576HV
0.5 A ILOAD 3 A,
18 V VIN 60 V
Circuit of Figure 26 and
Figure 32
TJ= 25°C 14.4 15 14.25
V
Applies over full
operating
temperature range 15.68 15.83
ηEfficiency VIN = 18 V, ILOAD = 3 A 88%
(1) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2576/LM2576HV is used as shown in Figure 26 and Figure 32, system performance is as shown in Electrical
Characteristics: All Output Voltage Versions.
6.9 Electrical Characteristics: Adjustable Output Voltage
over operating free-air temperature range (unless otherwise noted).
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
SYSTEM PARAMETERS TEST CIRCUIT Figure 26 and Figure 32(1)
VOUT Feedback voltage VIN = 12 V, ILOAD = 0.5 A
VOUT = 5 V,
Circuit of Figure 26 and Figure 32 1.217 1.23 1.243 V
VOUT Feedback Voltage
LM2576
0.5 A ILOAD 3 A,
8 V VIN 40 V
VOUT = 5 V, Circuit of
Figure 26 and Figure 32
TJ= 25°C 1.193 1.23 1.267
V
Applies over full
operating
temperature range 1.18 1.28
VOUT Feedback Voltage
LM2576HV
0.5 A ILOAD 3 A,
8 V VIN 60 V
VOUT = 5 V, Circuit of
Figure 26 and Figure 32
TJ= 25°C 1.193 1.23 1.273
V
Applies over full
operating
temperature range 1.18 1.286
ηEfficiency VIN = 12 V, ILOAD = 3 A, VOUT = 5 V 77%
(1) All limits specified at room temperature (25°C) unless otherwise noted. All room temperature limits are 100% production tested. All limits
at temperature extremes are specified through correlation using standard Statistical Quality Control (SQC) methods.
(2) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2576/LM2576HV is used as shown in Figure 26 and Figure 32, system performance is as shown in Electrical
Characteristics: All Output Voltage Versions.
(3) The oscillator frequency reduces to approximately 11 kHz in the event of an output short or an overload which causes the regulated
output voltage to drop approximately 40% from the nominal output voltage. This self protection feature lowers the average power
dissipation of the IC by lowering the minimum duty cycle from 5% down to approximately 2%.
6.10 Electrical Characteristics: All Output Voltage Versions
over operating free-air temperature range (unless otherwise noted)
PARAMETER TEST CONDITIONS MIN TYP(1) MAX UNIT
SYSTEM PARAMETERS TEST CIRCUIT Figure 26 and Figure 32(2)
IbFeedback Bias Current VOUT = 5 V (Adjustable
Version Only)
TJ= 25°C 100 50
nA
Applies over full
operating
temperature range 500
fOOscillator Frequency(3) TJ= 25°C 47 52 58 kHz
Applies over full operating temperature range 42 63
7
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Electrical Characteristics: All Output Voltage Versions (continued)
over operating free-air temperature range (unless otherwise noted)
PARAMETER TEST CONDITIONS MIN TYP(1) MAX UNIT
(4) Output pin sourcing current. No diode, inductor or capacitor connected to output.
(5) Feedback pin removed from output and connected to 0V.
(6) Feedback pin removed from output and connected to +12 V for the Adjustable, 3.3-V, and 5-V versions, and +25 V for the 12-V and 15-
V versions, to force the output transistor OFF.
(7) VIN = 40 V (60 V for high voltage version).
VSAT Saturation Voltage IOUT = 3 A (4)
TJ= 25°C 1.4 1.8
V
Applies over full
operating
temperature range 2
DC Max Duty Cycle (ON)(5) 93% 98%
ICL Current Limit(4)(3) TJ= 25°C 4.2 5.8 6.9 A
Applies over full operating temperature range 3.5 7.5
ILOutput Leakage Current Output = 0 V
Output = 1 V
Output = 1 V (6)(7) 2 7.5 30 mA
IQQuiescent Current(6) 5 10 mA
ISTBY Standby Quiescent
Current ON /OFF Pin = 5 V (OFF) 50 200 μA
ON /OFF CONTROL TEST CIRCUIT Figure 26 and Figure 32
VIH
ON /OFF Pin
Logic Input Level
VOUT = 0 V TJ= 25°C 2.2 1.4
V
Applies over full
operating
temperature range 2.4
VIL
VOUT = Nominal Output
Voltage TJ= 25°C 1.2 1
V
Applies over full
operating
temperature range 0.8
IIH ON /OFF Pin Input
Current ON /OFF Pin = 5 V (OFF) 12 30 μA
IIL ON /OFF Pin = 0 V (ON) 0 10 μA
8
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6.11 Typical Characteristics
(Circuit of Figure 26 and Figure 32)
Figure 1. Normalized Output Voltage Figure 2. Line Regulation
Figure 3. Dropout Voltage Figure 4. Current Limit
Figure 5. Quiescent Current Figure 6. Standby Quiescent Current
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Typical Characteristics (continued)
(Circuit of Figure 26 and Figure 32)
Figure 7. Oscillator Frequency Figure 8. Switch Saturation Voltage
Figure 9. Efficiency Figure 10. Minimum Operating Voltage
Figure 11. Quiescent Current vs Duty Cycle Figure 12. Feedback Voltage vs Duty Cycle
10
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Typical Characteristics (continued)
(Circuit of Figure 26 and Figure 32)
Figure 13. Minimum Operating Voltage Figure 14. Quiescent Current vs Duty Cycle
Figure 15. Feedback Voltage vs Duty Cycle Figure 16. Feedback Pin Current
If the DDPAK/TO-263 package is used, the thermal resistance can be
reduced by increasing the PCB copper area thermally connected to
the package. Using 0.5 square inches of copper area, θJA is 50°C/W,
with 1 square inch of copper area, θJA is 37°C/W, and with 1.6 or
more square inches of copper area, θJA is 32°C/W.
Figure 17. Maximum Power Dissipation (DDPAK/TO-263)
VOUT = 15 V
A: Output Pin Voltage, 50 V/div
B: Output Pin Current, 2 A/div
C: Inductor Current, 2 A/div
D: Output Ripple Voltage, 50 mV/div,
AC-Coupled
Horizontal Time Base: 5 μs/div
Figure 18. Switching Waveforms
11
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Typical Characteristics (continued)
(Circuit of Figure 26 and Figure 32)
Figure 19. Load Transient Response
12
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7 Detailed Description
7.1 Overview
The LM2576 SIMPLE SWITCHER® regulator is an easy-to-use, non-synchronous step-down DC-DC converter
with a wide input voltage range from 40 V to up to 60 V for a HV version. It 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, 15 V, and an adjustable output version. The family requires few external components, and the pin
arrangement was designed for simple, optimum PCB layout.
7.2 Functional Block Diagram
3.3 V R2 = 1.7 k
5 V, R2 = 3.1 k
12 V, R2 = 8.84 k
15 V, R2 = 11.3 k
For ADJ. Version
R1 = Open, R2 = 0 Ω
Patent Pending
7.3 Feature Description
7.3.1 Undervoltage Lockout
In some applications it is desirable to keep the regulator off until the input voltage reaches a certain threshold.
Figure 20 shows an undervoltage lockout circuit that accomplishes this task, while Figure 21 shows the same
circuit applied to a buck-boost configuration. These circuits keep the regulator off until the input voltage reaches
a predetermined level.
VTH VZ1 + 2VBE(Q1) (1)
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Feature Description (continued)
Complete circuit not shown.
Figure 20. Undervoltage Lockout for Buck Circuit
Complete circuit not shown (see Figure 24).
Figure 21. Undervoltage Lockout
for Buck-Boost Circuit
7.3.2 Delayed Start-Up
The ON /OFF pin can be used to provide a delayed start-up feature as shown in Figure 22. With an input voltage
of 20 V and for the part values shown, the circuit provides approximately 10 ms of delay time before the circuit
begins switching. Increasing the RC time constant can provide longer delay times. But excessively large RC time
constants can cause problems with input voltages that are high in 60-Hz or 120-Hz ripple, by coupling the ripple
into the ON /OFF pin.
7.3.3 Adjustable Output, Low-Ripple Power Supply
Figure 23 shows a 3-A power supply that features an adjustable output voltage. An additional LC filter that
reduces the output ripple by a factor of 10 or more is included in this circuit.
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Feature Description (continued)
Complete circuit not shown.
Figure 22. Delayed Start-Up
Figure 23. 1.2-V to 55-V Adjustable 3-A Power Supply With Low Output Ripple
7.4 Device Functional Modes
7.4.1 Shutdown Mode
The ON/OFF pin provides electrical ON and OFF control for the LM2576. When the voltage of this pin is higher
than 1.4 V, the device is in shutdown mode. The typical standby current in this mode is 50 μA.
7.4.2 Active Mode
When the voltage of the ON/OFF pin is below 1.2 V, the device starts switching, and the output voltage rises until
it reaches the normal regulation voltage.
7.4.3 Current Limit
The LM2576 device has current limiting to prevent the switch current from exceeding safe values during an
accidental overload on the output. This current limit value can be found in Electrical Characteristics: All Output
Voltage Versions under the heading of ICL.
The LM2576 uses cycle-by-cycle peak current limit for overload protection. This helps to prevent damage to the
device and external components. The regulator operates in current limit mode whenever the inductor current
exceeds the value of ICL given in Electrical Characteristics: All Output Voltage Versions. This occurs if the load
current is greater than 3 A, or the converter is starting up. Keep in mind that the maximum available load current
depends on the input voltage, output voltage, and inductor value. The regulator also incorporates short-circuit
protection to prevent inductor current run-away. When the voltage on the FB pin (ADJ) falls below about 0.58 V
the switching frequency is dropped to about 11 kHz. This allows the inductor current to ramp down sufficiently
during the switch OFF-time to prevent saturation.
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8 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.
8.1 Application Information
8.1.1 Input Capacitor (CIN)
To maintain stability, the regulator input pin must be bypassed with at least a 100-μF electrolytic capacitor. The
capacitor's leads must be kept short, and placed near the regulator.
If the operating temperature range includes temperatures below 25°C, the input capacitor value may need to be
larger. With most electrolytic capacitors, the capacitance value decreases and the ESR increases with lower
temperatures and age. Paralleling a ceramic or solid tantalum capacitor increases the regulator stability at cold
temperatures. For maximum capacitor operating lifetime, the RMS ripple current rating of the capacitor must be
greater than:
(2)
8.1.2 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 regulator performance and requirements.
The LM2576 (or any of the SIMPLE SWITCHER® family can be used for both continuous and discontinuous
modes of operation.
The inductor value selection guides in Figure 27 through Figure 31 are designed for buck regulator designs of
the continuous inductor current type. When using inductor values shown in the inductor selection guide, the
peak-to-peak inductor ripple current is approximately 20% to 30% of the maximum DC current. With relatively
heavy load currents, the circuit operates in the continuous mode (inductor current always flowing), but under light
load conditions, the circuit is forced to the discontinuous mode (inductor current falls to zero for a period of time).
This discontinuous mode of operation is perfectly acceptable. For light loads (less than approximately 300 mA), it
may be desirable to operate the regulator in the discontinuous mode, primarily because of the lower inductor
values required for the discontinuous mode.
The selection guide chooses inductor values suitable for continuous mode operation, but if the inductor value
chosen is prohibitively high, the designer should investigate the possibility of discontinuous operation.
Inductors are available in different styles such as pot core, toriod, E-frame, bobbin core, and so on, as well as
different core materials, such as ferrites and powdered iron. The bobbin core is the least expensive type, and
consists of wire wrapped on a ferrite rod core. This type of construction makes for an inexpensive inductor;
however, because the magnetic flux is not completely contained within the core, the bobbin core generates more
electromagnetic interference (EMI). This EMI can cause problems in sensitive circuits, or can give incorrect
scope readings because of induced voltages in the scope probe.
The inductors listed in the selection chart include ferrite pot core construction for AIE, powdered iron toroid for
Pulse Engineering, and ferrite bobbin core for Renco.
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An inductor must not operate beyond its maximum-rated current because it may saturate. When an inductor
begins to saturate, the inductance decreases rapidly, and the inductor begins to look mainly resistive (the DC
resistance of the winding), causing the switch current to rise very rapidly. Different inductor types have different
saturation characteristics, and this must be considered when selecting an inductor.
The inductor manufacturer's data sheets include current and energy limits to avoid inductor saturation.
8.1.3 Inductor Ripple Current
When the switcher 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 voltage and output voltage,
the peak-to-peak amplitude of this inductor current waveform remains constant. As the load current rises or falls,
the entire sawtooth current waveform also rises or falls. The average DC value of this waveform is equal to the
DC load current (in the buck regulator configuration).
If the load current drops to a low enough level, the bottom of the sawtooth current waveform reaches zero, and
the switcher changes to a discontinuous mode of operation. This is a perfectly acceptable mode of operation.
Any buck switching regulator (no matter how large the inductor value is) is forced to run discontinuous if the load
current is light enough.
8.1.4 Output Capacitor
An output capacitor is required to filter the output voltage and is needed for loop stability. The capacitor must be
placed near the LM2576 using short PCB traces. Standard aluminum electrolytics are usually adequate, but TI
recommends low ESR types for low output ripple voltage and good stability. The ESR of a capacitor depends on
many factors, including: the value, the voltage rating, physical size, and the type of construction. In general, low
value or low voltage (less than 12 V) electrolytic capacitors usually have higher ESR numbers.
The amount of output ripple voltage is primarily a function of the ESR (Equivalent Series Resistance) of the
output capacitor and the amplitude of the inductor ripple current (ΔIIND). See Inductor Ripple Current.
The lower capacitor values (220 μF to 1000 μF) allows typically 50 mV to 150 mV of output ripple voltage, while
larger-value capacitors reduces the ripple to approximately 20 mV to 50 mV.
Output Ripple Voltage = (ΔIIND) (ESR of COUT) (3)
To further reduce the output ripple voltage, several standard electrolytic capacitors may be paralleled, or a
higher-grade capacitor may be used. Such capacitors are often called high-frequency,low-inductance, or low-
ESR. These reduces the output ripple to 10 mV or 20 mV. However, when operating in the continuous mode,
reducing the ESR below 0.03 Ωcan cause instability in the regulator.
Tantalum capacitors can have a very low ESR, and must be carefully evaluated if it is the only output capacitor.
Because of their good low temperature characteristics, a tantalum can be used in parallel with aluminum
electrolytics, with the tantalum making up 10% or 20% of the total capacitance.
The ripple current rating of the capacitor at 52 kHz should be at least 50% higher than the peak-to-peak inductor
ripple current.
8.1.5 Catch Diode
Buck regulators require a diode to provide a return path for the inductor current when the switch is off. This diode
must be placed close to the LM2576 using short leads and short printed-circuit traces.
Because of their fast switching speed and low forward voltage drop, Schottky diodes provide the best efficiency,
especially in low output voltage switching regulators (less than 5 V). Fast-recovery, high-efficiency, or ultra-fast
recovery diodes are also suitable, but some types with an abrupt turnoff characteristic may cause instability and
EMI problems. A fast-recovery diode with soft recovery characteristics is a better choice. Standard 60-Hz diodes
(for example, 1N4001 or 1N5400, and so on) are also not suitable. See Table 3 for Schottky and soft fast-
recovery diode selection guide.
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Application Information (continued)
8.1.6 Output Voltage Ripple and Transients
The output voltage of a switching power supply contains a sawtooth ripple voltage at the switcher frequency,
typically about 1% of the output voltage, and may also contain short voltage spikes at the peaks of the sawtooth
waveform.
The output ripple voltage is due mainly to the inductor sawtooth ripple current multiplied by the ESR of the output
capacitor (see Inductor Selection).
The voltage spikes are present because of the fast switching action of the output switch, and the parasitic
inductance of the output filter capacitor. To minimize these voltage spikes, special low inductance capacitors can
be used, and their lead lengths must be kept short. Wiring inductance, stray capacitance, as well as the scope
probe used to evaluate these transients, all contribute to the amplitude of these spikes.
An additional small LC filter (20 μH and 100 μF) can be added to the output (as shown in Figure 23) to further
reduce the amount of output ripple and transients. A 10 × reduction in output ripple voltage and transients is
possible with this filter.
8.1.7 Feedback Connection
The LM2576 (fixed voltage versions) feedback pin must be wired to the output voltage point of the switching
power supply. When using the adjustable version, physically locate both output voltage programming resistors
near the LM2576 to avoid picking up unwanted noise. Avoid using resistors greater than 100 kΩbecause of the
increased chance of noise pickup.
8.1.8 ON /OFF INPUT
For normal operation, the ON /OFF pin must be grounded or driven with a low-level TTL voltage (typically below
1.6 V). To put the regulator into standby mode, drive this pin with a high-level TTL or CMOS signal. The ON
/OFF pin can be safely pulled up to +VIN without a resistor in series with it. The ON /OFF pin must not be left
open.
8.1.9 Inverting Regulator
Figure 24 shows a LM2576-12 in a buck-boost configuration to generate a negative 12-V output from a positive
input voltage. This circuit bootstraps the ground pin of the regulator to the negative output voltage, then by
grounding the feedback pin, the regulator senses the inverted output voltage and regulates it to 12 V.
For an input voltage of 12 V or more, the maximum available output current in this configuration is approximately
700 mA. At lighter loads, the minimum input voltage required drops to approximately 4.7 V.
The switch currents in this buck-boost configuration are higher than in the standard buck-mode design, thus
lowering the available output current. Also, the start-up input current of the buck-boost converter is higher than
the standard buck-mode regulator, and this may overload an input power source with a current limit less than
5 A. Using a delayed turn-on or an undervoltage lockout circuit (described in Negative Boost Regulator) would
allow the input voltage to rise to a high enough level before the switcher would be allowed to turn on.
Because of the structural differences between the buck and the buck-boost regulator topologies, the buck
regulator design procedure section can not be used to select the inductor or the output capacitor. The
recommended range of inductor values for the buck-boost design is between 68 μH and 220 μH, and the output
capacitor values must be larger than what is normally required for buck designs. Low input voltages or high
output currents require a large value output capacitor (in the thousands of micro Farads).
The peak inductor current, which is the same as the peak switch current, can be calculated in Equation 4:
where
fosc = 52 kHz (4)
Under normal continuous inductor current operating conditions, the minimum VIN represents the worst case.
Select an inductor that is rated for the peak current anticipated.
Copyright © 2016, Texas Instruments Incorporated
LM2576-12
2
4
1
3 5
+1N5820
+COUT
2200 PF
Feedback
Output
VIN
CIN
100 PFGND
100 PH
VOUT = -12V
-VIN
-5V to -12V
LOW ESR
ON/OFF
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Application Information (continued)
Figure 24. Inverting Buck-Boost Develops 12 V
Also, the maximum voltage appearing across the regulator is the absolute sum of the input and output voltage.
For a 12-V output, the maximum input voltage for the LM2576 is +28 V, or +48 V for the LM2576HV.
8.1.10 Negative Boost Regulator
Another variation on the buck-boost topology is the negative boost configuration. The circuit in Figure 25 accepts
an input voltage ranging from 5 V to 12 V and </