Voltage
5V/5A
COUT
2 x 180 PF/16V
22 PH
L
Feedback
Boost
6TQ045S
Ground
Current
VIN
Softstart
1 nF
+
Voltage
8V to 40V
Input LM2679 - 5.0 Output
Switch
Output
0.01 PF
+
Limit
Adjust
5.6k
0.47 PF
+++
CIN
3 x 15 PF/50V
= 37,125
RADJ
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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.
LM2679
SNVS026O MARCH 2000REVISED JUNE 2016
LM2679 SIMPLE SWITCHER
®
5-A Step-Down Voltage Regulator
With Adjustable Current Limit
1
1 Features
1 Efficiency Up to 92%
Simple and Easy to Design Using Off-The-Shelf
External Components
Resistor Programmable Peak Current Limit Over a
Range of 3 A to 7 A
120-mΩDMOS Output Switch
3.3-V, 5-V, 12-V Fixed Output and Adjustable
(1.2 V to 37 V) Versions
±2% Maximum Output Tolerance Over Full Line
and Load Conditions
Wide Input Voltage Range: 8 V to 40 V
260-kHz Fixed Frequency Internal Oscillator
Soft-Start Capability
40 to 125°C Operating Junction Temperature
Range
2 Applications
Simple-to-Design, High Efficiency (>90%)
Step-Down Switching Regulators
Efficient System Preregulator for Linear Voltage
Regulators
Battery Chargers
3 Description
The LM2679 series of regulators are monolithic
integrated circuits which provide all of the active
functions for a step-down (buck) switching regulator
capable of driving up to 5-A loads with excellent line
and load regulation characteristics. High efficiency
(>90%) is obtained through the use of a low ON-
resistance DMOS power switch. The series consists
of fixed output voltages of 3.3 V, 5 V, and 12 V and
an adjustable output version.
The SIMPLE SWITCHER®concept provides for a
complete design using a minimum number of external
components. A high fixed frequency oscillator
(260 kHz) allows the use of physically smaller sized
components. A family of standard inductors for use
with the LM2679 are available from several
manufacturers to greatly simplify the design process.
Other features include the ability to reduce the input
surge current at power on by adding a soft-start
timing capacitor to gradually turn on the regulator.
The LM2679 series also has built-in thermal
shutdown and resistor programmable current limit of
the power MOSFET switch to protect the device and
load circuitry under fault conditions. The output
voltage is specified to a ±2% tolerance. The clock
frequency is controlled to within a ±11% tolerance.
Device Information(1)
PART NUMBER PACKAGE BODY SIZE (NOM)
LM2679 TO-263 (7) 10.10 mm × 8.89 mm
TO-220 (7) 14.986 mm × 10.16 mm
VSON (14) 6.00 mm × 5.00 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Typical Application
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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 Specification........................................................... 4
6.1 Absolute Maximum Ratings ...................................... 4
6.2 ESD Ratings.............................................................. 4
6.3 Recommended Operating Conditions....................... 4
6.4 Thermal Information.................................................. 5
6.5 Electrical Characteristics 3.3 V.............................. 5
6.6 Electrical Characteristics 5 V................................. 5
6.7 Electrical Characteristics 12 V............................... 6
6.8 Electrical Characteristics Adjustable...................... 6
6.9 Electrical Characteristics All Output Voltage
Versions..................................................................... 6
6.10 Typical Characteristics............................................ 7
7 Detailed Description.............................................. 9
7.1 Overview................................................................... 9
7.2 Functional Block Diagram......................................... 9
7.3 Feature Description................................................... 9
7.4 Device Functional Modes........................................ 10
8 Application and Implementation ........................ 11
8.1 Application Information............................................ 11
8.2 Typical Application.................................................. 14
9 Power Supply Recommendations...................... 26
10 Layout................................................................... 26
10.1 Layout Guidelines ................................................. 26
10.2 Layout Example .................................................... 27
11 Device and Documentation Support................. 28
11.1 Related Documentation......................................... 28
11.2 Receiving Notification of Documentation Updates 28
11.3 Community Resources.......................................... 28
11.4 Trademarks........................................................... 28
11.5 Electrostatic Discharge Caution............................ 28
11.6 Glossary................................................................ 28
12 Mechanical, Packaging, and Orderable
Information........................................................... 28
12.1 VSON Package Devices....................................... 28
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision N (April 2013) to Revision O 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 Computer Design Software LM267X Made Simple (Version 6.0).............................................. 1
Changes from Revision M (April 2013) to Revision N Page
Changed layout of National Data Sheet to TI format ........................................................................................................... 15
Not to scale
DAP
1NC 14 Switch_output
2Input 13 Switch_output
3Input 12 Switch_output
4CB 11 NC
5NC 10 NC
6Current_adjust 9 GND
7FB 8 SS
Not to scale
1Switch_output
2Input
3CB
4GND
5Current_adjust
6FB
7SS
1 Switch_output
2 Input
3 CB
4 GND
5 Current_adjust
6 FB
7 SS
Not to scale
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5 Pin Configuration and Functions
KTW Package
7-Pin TO-263
Top View NDZ Package
7-Pin TO-220
Top View
NHM Package
14-Pin VSON
Top View
Connect DAP to pin 9
Pin Functions
PIN I/O DESCRIPTION
NAME TO-263, TO-220 VSON
Switch output 1 12, 13, 14 O Source pin of the internal high side FET. This is a switching node. Attached
this pin to an inductor and the cathode of the external diode.
Input 2 2, 3 I Supply input pin to collector pin of high side FET. 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.
CB 3 4 I Boot-strap capacitor connection for high-side driver. Connect a high quality
100-nF capacitor from CB to VSW pin.
GND 4 9 Power ground pins. Connect to system ground. Ground pins of CIN and
COUT. Path to CIN must be as short as possible.
Current adjust 5 6 I Current limit adjust pin. Connect a resistor from this pin to GND to set the
current limit of the part.
FB 6 7 I Feedback sense input pin. Connect to the midpoint of feedback divider to
set VOUT for adjustable version or connect this pin directly to the output
capacitor for a fixed output version.
SS 7 8 I Soft-start pin. Connect a capacitor from this pin to GND to control the output
voltage ramp. If the feature not desired, the pin can be left floating
NC 1, 5, 10, 11 No connect pins
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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) The absolute maximum specification of the switch voltage to ground applies to DC voltage. An extended negative voltage limit of –10 V
applies to a pulse of up to 20 ns, –6 V of 60 ns and –3 V of up to 100 ns.
6 Specification
6.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted)(1)(2)
MIN MAX UNIT
Input supply voltage 45 V
Soft-start pin voltage –0.1 6 V
Switch voltage to ground(3) –1 VIN V
Boost pin voltage VSW + 8 V V
Feedback pin voltage –0.3 14 V
Power dissipation Internally limited
Soldering temperature Wave (4 s) 260 °CInfrared (10 s) 240
Vapor phase (75 s) 219
Storage Temperature, Tstg –65 150 °C
(1) JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
(2) ESD was applied using the human-body model, a 100-pF capacitor discharged through a 1.5-kΩresistor into each pin.
6.2 ESD Ratings VALUE UNIT
V(ESD) Electrostatic discharge Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001(1)(2) ±2000 V
6.3 Recommended Operating Conditions MIN MAX UNIT
Supply voltage 8 40 V
Junction temperature, TJ–40 125 °C
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(1) For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report.
(2) Junction to ambient thermal resistance (no external heat sink) for the 7-lead TO-220 package mounted vertically, with ½ inch leads in a
socket, or on a PCB with minimum copper area.
(3) Junction to ambient thermal resistance (no external heat sink) for the 7-lead TO-220 package mounted vertically, with ½ inch leads
soldered to a PCB containing approximately 4 square inches of (1 oz.) copper area surrounding the leads.
(4) Junction to ambient thermal resistance for the 7-lead DDPAK mounted horizontally against a PCB area of 0.136 square inches (the
same size as the DDPAK package) of 1 oz. (0.0014 in. thick) copper.
(5) Junction to ambient thermal resistance for the 7-lead DDPAK mounted horizontally against a PCB area of 0.4896 square inches
(3.6 times the area of the DDPAK package) of 1 oz. (0.0014 in. thick) copper.
(6) Junction to ambient thermal resistance for the 7-lead DDPAK mounted horizontally against a PCB copper area of 1.0064 square inches
(7.4 times the area of the DDPAK 3 package) of 1 oz. (0.0014 in. thick) copper. Additional copper area reduces thermal resistance
further.
(7) Junction to ambient thermal resistance for the 14-lead VSON mounted on a PCB copper area equal to the die attach paddle.
(8) Junction to ambient thermal resistance for the 14-lead VSON mounted on a PCB copper area using 12 vias to a second layer of copper
equal to die attach paddle. Additional copper area reduces thermal resistance further. For layout recommendations, see AN-1187
Leadless Leadfram Package (LLP).
6.4 Thermal Information
THERMAL METRIC(1)
LM2679
UNIT
NDZ
(TO-220) KTW
(TO-263) NHM
(VSON)
7 PINS 7 PINS 14 PINS
RθJA Junction-to-ambient thermal resistance
See(2) 65
°C/W
See(3) 45
See(4) 56
See(5) 35
See(6) 26
See(7) 55
See(8) 29
RθJC(top) Junction-to-case (top) thermal resistance 2 2 °C/W
(1) All room temperature limits are 100% tested during production with TA= TJ= 25°C. All limits at temperature extremes are specified
through correlation using standard Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level
(AOQL).
(2) Typical values are determined with TA= TJ= 25°C and represent the most likely norm.
6.5 Electrical Characteristics 3.3 V
Specifications apply for TA= TJ= 25°C and RADJ = 5.6 k(unless otherwise noted).
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT
VOUT Output voltage VIN = 8 V to 40 V,
100 mA IOUT 5 A TJ= 25°C 3.234 3.3 3.366 V
TJ= –40°C to 125°C 3.201 3.399
ηEfficiency VIN = 12 V, ILOAD = 5 A 82%
(1) All room temperature limits are 100% tested during production with TA= TJ= 25°C. All limits at temperature extremes are specified
through correlation using standard Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level
(AOQL).
(2) Typical values are determined with TA= TJ= 25°C and represent the most likely norm.
6.6 Electrical Characteristics 5 V
Specifications apply for TA= TJ= 25°C and RADJ = 5.6 k(unless otherwise noted).
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT
VOUT Output voltage VIN = 8 V to 40 V,
100 mA IOUT 5 A TJ= 25°C 4.9 5 5.1 V
TJ= –40°C to 125°C 4.85 5.15
ηEfficiency VIN = 12 V, ILOAD = 5 A 84%
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(1) All room temperature limits are 100% tested during production with TA= TJ= 25°C. All limits at temperature extremes are specified
through correlation using standard Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level
(AOQL).
(2) Typical values are determined with TA= TJ= 25°C and represent the most likely norm.
6.7 Electrical Characteristics 12 V
Specifications apply for TA= TJ= 25°C and RADJ = 5.6 k(unless otherwise noted).
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT
VOUT Output voltage VIN = 15 V to 40 V,
100 mA IOUT 5 A TJ= 25°C 11.76 12 12.24 V
TJ= –40°C to 125°C 11.64 12.36
ηEfficiency VIN = 24 V, ILOAD = 5 A 92%
(1) All room temperature limits are 100% tested during production with TA= TJ= 25°C. All limits at temperature extremes are specified
through correlation using standard Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level
(AOQL).
(2) Typical values are determined with TA= TJ= 25°C and represent the most likely norm.
6.8 Electrical Characteristics Adjustable
Specifications apply for TA= TJ= 25°C and RADJ = 5.6 k(unless otherwise noted).
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT
VFB Feedback voltage VIN = 8 V to 40 V,
100 mA IOUT 5 A,
VOUT programmed for 5 V
TJ= 25°C 1.186 1.21 1.234 V
TJ= –40°C to 125°C 1.174 1.246
ηEfficiency VIN = 12 V, ILOAD = 5 A 84%
(1) The peak switch current limit is determined by the following relationship: ICL = 37,125 / RADJ
6.9 Electrical Characteristics All Output Voltage Versions
Specifications are for TA= TJ= 25°C, VIN = 12 V for the 3.3 V, 5-V, and adjustable versions, and VIN = 24 V for the 12-V
version (unless otherwise specified).
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
IQQuiescent current VFEEDBACK = 8 V for 3.3-V, 5-V, and adjustable versions,
VFEEDBACK = 15 V for 12-V version 4.2 6 mA
VADJ Current limit adjust
voltage TJ= 25°C 1.181 1.21 1.229 V
TJ= –40°C to 125°C 1.169 1.246
ICL Current limit RADJ = 5.6 kΩ(1) TJ= 25°C 5.5 6.3 7.6 A
TJ= –40°C to 125°C 5.3 8.1
ILOutput leakage current VIN = 40 V, soft-start pin = 0 V VSWITCH = 0 V 1 1.5 mA
VSWITCH =1 V 6 15
RDS(ON) Switch ON-resistance ISWITCH = 5 A TJ= 25°C 0.12 0.14 Ω
TJ= –40°C to 125°C 0.225
fOOscillator frequency Measured at switch pin TJ= 25°C 260 kHz
TJ= –40°C to 125°C 225 280
D Duty cycle Maximum duty cycle 91%
Minimum duty cycle 0%
IBIAS Feedback bias
current VFEEDBACK = 1.3 V (adjustable version only) 85 nA
VSFST Soft-start threshold
voltage TJ= 25°C 0.63 V
TJ= –40°C to 125°C 0.53 0.74
ISFST Soft-start pin current Soft-start pin = 0 V TJ= 25°C 3.7 μA
TJ= –40°C to 125°C 6.9
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6.10 Typical Characteristics
Figure 1. Normalized Output Voltage Figure 2. Line Regulation
Figure 3. Efficiency vs Input Voltage Figure 4. Efficiency vs ILOAD
Figure 5. Switch Current Limit Figure 6. Operating Quiescent Current
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Typical Characteristics (continued)
Figure 7. Switching Frequency Figure 8. Feedback Pin Bias Current
Continuous Mode Switching Waveforms VIN = 20 V, VOUT = 5 V,
ILOAD = 5 A, L = 10 μH, COUT = 400 μF, COUTESR = 13 mΩ
A. VSW pin voltage, 10 V/div
B. Inductor current, 2 A/div
C. Output ripple voltage, 20 mV/div AC-coupled
Figure 9. Horizontal Time Base: 1 μs/div
Discontinuous Mode Switching Waveforms VIN = 20 V, VOUT = 5 V,
ILOAD = 500 mA, L = 10 μH, COUT = 400 μF, COUTESR = 13 mΩ
A. VSW pin voltage, 10 V/div
B. Inductor current, 1 A/div
C. Output ripple voltage, 20 mV/div AC-coupled
Figure 10. Horizontal Time Base: 1 μs/div
Load Transient Response for Continuous Mode VIN = 20 V,
VOUT = 5 V, L = 10 μH, COUT = 400 μF, COUTESR = 13 mΩ
A. Output voltage, 100 mV/div, AC-coupled
B. Load current: 500-mA to 5-A load pulse
Figure 11. Horizontal Time Base: 100 μs/div
Load Transient Response for Discontinuous Mode VIN = 20 V,
VOUT = 5 V, L = 10 μH, COUT = 400 μF, COUTESR = 13 mΩ
A. Output voltage, 100 mV/div, AC-coupled
B. Load current: 200-mA to 3-A load pulse
Figure 12. Horizontal Time Base: 200 μs/div
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7 Detailed Description
7.1 Overview
The LM2679 provides all of the active functions required for a step-down (buck) switching regulator. The internal
power switch is a DMOS power MOSFET to provide power supply designs with high current capability, up to 5 A,
and highly efficient operation.
The LM2679 is part of the SIMPLE SWITCHER®family of power converters. A complete design uses a minimum
number of external components, which have been predetermined from a variety of manufacturers. The software
is provided free of charge and can be downloaded from Texas Instruments Internet site: www.ti.com.
7.2 Functional Block Diagram
7.3 Feature Description
7.3.1 Switch Output
This is the output of a power MOSFET switch connected directly to the input voltage. The switch provides energy
to an inductor, an output capacitor and the load circuitry under control of an internal pulse-width-modulator
(PWM). The PWM controller is internally clocked by a fixed 260-kHz oscillator. In a standard step-down
application the duty cycle (Time ON/Time OFF) of the power switch is proportional to the ratio of the power
supply output voltage to the input voltage. The voltage on pin 1 switches between VIN (switch ON) and below
ground by the voltage drop of the external Schottky diode (switch OFF).
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Feature Description (continued)
7.3.2 Input
The input voltage for the power supply is connected to pin 2. In addition to providing energy to the load the input
voltage also provides bias for the internal circuitry of the LM2679. For ensured performance the input voltage
must be in the range of 8 V to 40 V. For best performance of the power supply the input pin must always be
bypassed with an input capacitor placed close to pin 2.
7.3.3 C Boost
A capacitor must be connected from pin 3 to the switch output, pin 1. This capacitor boosts the gate drive to the
internal MOSFET above VIN to fully turn it ON. This minimizes conduction losses in the power switch to maintain
high efficiency. The recommended value for C Boost is 0.01 μF.
7.3.4 Ground
This is the ground reference connection for all components in the power supply. In fast-switching, high-current
applications such as those implemented with the LM2679, TI recommends that a broad ground plane be used to
minimize signal coupling throughout the circuit.
7.3.5 Current Adjust
A key feature of the LM2679 is the ability to tailor the peak switch current limit to a level required by a particular
application. This alleviates the requirement to use external components that must be physically sized to
accommodate current levels (under shorted output conditions for example) that may be much higher than the
normal circuit operating current requirements.
A resistor connected from pin 5 to ground establishes a current (I(pin 5) =1.2V/RADJ) that sets the peak current
through the power switch. The maximum switch current is fixed at a level of 37,125 / RADJ.
7.3.6 Feedback
This is the input to a two-stage high gain amplifier, which drives the PWM controller. It is necessary to connect
pin 6 to the actual output of the power supply to set the DC output voltage. For the fixed output devices (3.3-V,
5-V and 12-V outputs), a direct wire connection to the output is all that is required as internal gain setting
resistors are provided inside the LM2679. For the adjustable output version two external resistors are required to
set the DC output voltage. For stable operation of the power supply it is important to prevent coupling of any
inductor flux to the feedback input.
7.4 Device Functional Modes
7.4.1 Soft Start
A capacitor connected from pin 7 to ground allows for a slow turnon of the switching regulator. The capacitor sets
a time delay to gradually increase the duty cycle of the internal power switch. This can significantly reduce the
amount of surge current required from the input supply during an abrupt application of the input voltage. If soft
start is not required this pin must be left open circuited. See Soft-Start Capacitor, CSS for further information
regarding soft-start capacitor values.
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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 Design Considerations
Power supply design using the LM2679 is greatly simplified by using recommended external components. A wide
range of inductors, capacitors and Schottky diodes from several manufacturers have been evaluated for use in
designs that cover the full range of capabilities (input voltage, output voltage, and load current) of the LM2679. A
simple design procedure using nomographs and component tables provided in this data sheet leads to a working
design with very little effort.
The individual components from the various manufacturers called out for use are still just a small sample of the
vast array of components available in the industry. While these components are recommended, they are not
exclusively the only components for use in a design. After a close comparison of component specifications,
equivalent devices from other manufacturers could be substituted for use in an application.
Important considerations for each external component and an explanation of how the nomographs and selection
tables were developed follows.
8.1.2 Inductor
The inductor is the key component in a switching regulator. For efficiency the inductor stores energy during the
switch ON time and then transfers energy to the load while the switch is OFF.
Nomographs are used to select the inductance value required for a given set of operating conditions. The
nomographs assume that the circuit is operating in continuous mode (the current flowing through the inductor
never falls to zero). The magnitude of inductance is selected to maintain a maximum ripple current of 30% of the
maximum load current. If the ripple current exceeds this 30% limit the next larger value is selected.
The inductors offered have been specifically manufactured to provide proper operation under all operating
conditions of input and output voltage and load current. Several part types are offered for a given amount of
inductance. Both surface mount and through-hole devices are available. The inductors from each of the three
manufacturers have unique characteristics:
Renco: ferrite stick core inductors; benefits are typically lowest cost and can withstand ripple and transient
peak currents above the rated value. These inductors have an external magnetic field, which may generate
EMI.
Pulse Engineering: powdered iron toroid core inductors; these also can withstand higher than rated currents
and, being toroid inductors, have low EMI.
Coilcraft: ferrite drum core inductors; these are the smallest physical size inductors and are available only as
surface mount components. These inductors also generate EMI but less than stick inductors.
8.1.3 Output Capacitor
The output capacitor acts to smooth the DC output voltage and also provides energy storage. Selection of an
output capacitor, with an associated equivalent series resistance (ESR), impacts both the amount of output ripple
voltage and stability of the control loop.
The output ripple voltage of the power supply is the product of the capacitor ESR and the inductor ripple current.
The capacitor types recommended in the tables were selected for having low ESR ratings.
In addition, both surface mount tantalum capacitors and through-hole aluminum electrolytic capacitors are offered
as solutions.
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Application Information (continued)
Impacting frequency stability of the overall control loop, the output capacitance, in conjunction with the inductor,
creates a double pole inside the feedback loop. In addition the capacitance and the ESR value create a zero.
These frequency response effects together with the internal frequency compensation circuitry of the LM2679
modify the gain and phase shift of the closed-loop system.
As a general rule for stable switching regulator circuits it is desired to have the unity gain bandwidth of the circuit
to be limited to no more than one-sixth of the controller switching frequency. With the fixed 26-kHz switching
frequency of the LM2679, the output capacitor is selected to provide a unity gain bandwidth of 40 kHz maximum.
Each recommended capacitor value has been chosen to achieve this result.
In some cases multiple capacitors are required either to reduce the ESR of the output capacitor, to minimize
output ripple (a ripple voltage of 1% of VOUT or less is the assumed performance condition), or to increase the
output capacitance to reduce the closed loop unity gain bandwidth (to less than 40 kHz). When parallel
combinations of capacitors are required it has been assumed that each capacitor is the exact same part type.
The RMS current and working voltage (WV) ratings of the output capacitor are also important considerations. In a
typical step-down switching regulator, the inductor ripple current (set to be no more than 30% of the maximum
load current by the inductor selection) is the current that flows through the output capacitor. The capacitor RMS
current rating must be greater than this ripple current. The voltage rating of the output capacitor must be greater
than 1.3 times the maximum output voltage of the power supply. If operation of the system at elevated
temperatures is required, the capacitor voltage rating may be de-rated to less than the nominal room temperature
rating. Careful inspection of the manufacturer's specification for de-rating of working voltage with temperature is
important.
8.1.4 Input Capacitor
Fast changing currents in high current switching regulators place a significant dynamic load on the unregulated
power source. An input capacitor helps to provide additional current to the power supply as well as smooth out
input voltage variations.
Like the output capacitor, the key specifications for the input capacitor are RMS current rating and working
voltage. The RMS current flowing through the input capacitor is equal to one-half of the maximum DC load
current so the capacitor must be rated to handle this. Paralleling multiple capacitors proportionally increases the
current rating of the total capacitance. The voltage rating must also be selected to be 1.3 times the maximum
input voltage. Depending on the unregulated input power source, under light load conditions the maximum input
voltage could be significantly higher than normal operation. Consider this when selecting an input capacitor.
The input capacitor must be placed very close to the input pin of the LM2679. Due to relative high current
operation with fast transient changes, the series inductance of input connecting wires or PCB traces can create
ringing signals at the input terminal which could possibly propagate to the output or other parts of the circuitry. It
may be necessary in some designs to add a small valued (0.1-μF to 0.47-μF) ceramic type capacitor in parallel
with the input capacitor to prevent or minimize any ringing.
8.1.5 Catch Diode
When the power switch in the LM2679 turns OFF, the current through the inductor continues to flow. The path for
this current is through the diode connected between the switch output and ground. This forward biased diode
clamps the switch output to a voltage less than ground. This negative voltage must be greater than 1 V so a low
voltage drop (particularly at high current levels) Schottky diode is recommended. Total efficiency of the entire
power supply is significantly impacted by the power lost in the output catch diode. The average current through
the catch diode is dependent on the switch duty cycle (D) and is equal to the load current times (1-D). Use of a
diode rated for much higher current than is required by the actual application helps to minimize the voltage drop
and power loss in the diode.
During the switch ON time the diode is reversed biased by the input voltage. The reverse voltage rating of the
diode must be at least 1.3 times greater than the maximum input voltage.
8.1.6 Boost Capacitor
The boost capacitor creates a voltage used to overdrive the gate of the internal power MOSFET. This improves
efficiency by minimizing the ON-resistance of the switch and associated power loss. For all applications TI
recommends a 0.01-μF, 50-V ceramic capacitor.
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Application Information (continued)
8.1.7 Adjustable Current Limit, RADJ
A key feature of the LM2679 is the ability to control the peak switch current. Without this feature the peak switch
current would be internally set to 7 A or higher to accommodate 5-A load current designs. This requires that both
the inductor (which could saturate with excessively high currents) and the catch diode be able to safely handle
up to 7 A which would be conducted under load fault conditions.
If an application only requires a load current of 3 A or 4 A the peak switch current can be set to a limit just over
the maximum load current with the addition of a single programming resistor. This allows the use of less powerful
and more cost-effective inductors and diodes.
The peak switch current is equal to a factor of 37,125 divided by RADJ. A resistance of 5.6 kΩsets the current
limit to typically 6.3 A and an RADJ of 8.25 kΩreduces the maximum current to approximately 4.4 A. For
predictable control of the current limit, TI recommends keeping the peak switch current greater than 3 A. For
lower current applications a 3-A switching regulator with adjustable current limit, the LM2673, is available.
When the power switch reaches the current limit threshold it is immediately turned OFF and the internal switching
frequency is reduced. This extends the OFF time of the switch to prevent a steady-state high current condition.
As the switch current falls below the current limit threshold, the switch turns back ON. If a load fault continues,
the switch again exceeds the threshold and switch back OFF. This results in a low duty cycle pulsing of the
power switch to minimize the overall fault condition power dissipation.
8.1.8 Soft-Start Capacitor, CSS
This optional capacitor controls the rate at which the LM2679 starts up at power on. The capacitor is charged
linearly by an internal current source. This voltage ramp gradually increases the duty cycle of the power switch
until it reaches the normal operating duty cycle defined primarily by the ratio of the output voltage to the input
voltage. The soft-start turnon time is programmable by the selection of CSS.
The formula for selecting a soft-start capacitor is Equation 1.
where
ISST = Soft-start current (3.7 μA typical)
tSS = Soft-start time (from Detailed Design Procedure)
VSST = Soft-start threshold voltage (0.63 V typical)
VOUT = Output voltage (from Detailed Design Procedure)
VSCHOTTKY = Schottky diode voltage drop (0.5 V typical)
VIN = Maximum input voltage (from Detailed Design Procedure) (1)
If this feature is not desired, leave the soft-start pin (pin 7) open circuited.
With certain soft-start capacitor values and operating conditions, the LM2679 can exhibit an overshoot on the
output voltage during turnon. Especially when starting up into no load or low load, the soft-start function may not
be effective in preventing a larger voltage overshoot on the output. With larger loads or lower input voltages
during start-up this effect is minimized. In particular, avoid using soft-start capacitors between 0.033 µF and 1 µF.
8.1.9 Additional Application Information
When the output voltage is greater than approximately 6 V, and the duty cycle at minimum input voltage is
greater than approximately 50%, the designer must exercise caution in selection of the output filter components.
When an application designed to these specific operating conditions is subjected to a current limit fault condition,
it may be possible to observe a large hysteresis in the current limit. This can affect the output voltage of the
device until the load current is reduced sufficiently to allow the current limit protection circuit to reset itself.
Under current limiting conditions, the LM267x is designed to respond in the following manner:
1. At the moment when the inductor current reaches the current limit threshold, the ON-pulse is immediately
terminated. This happens for any application condition.
2. However, the current limit block is also designed to momentarily reduce the duty cycle to below 50% to avoid
subharmonic oscillations, which could cause the inductor to saturate.
Voltage
5V/5A
COUT
2 x 180 PF/16V
22 PH
L
Feedback
Boost
6TQ045S
Ground
Current
VIN
Softstart
1 nF
+
Voltage
8V to 40V
Input LM2679 - 5.0 Output
Switch
Output
0.01 PF
+
Limit
Adjust
5.6k
0.47 PF
+++
CIN
3 x 15 PF/50V
= 37,125
RADJ
ICL Copyright © 2016, Texas Instruments Incorporated
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Application Information (continued)
3. Thereafter, once the inductor current falls below the current limit threshold, there is a small relaxation time
during which the duty cycle progressively rises back above 50% to the value required to achieve regulation.
If the output capacitance is sufficiently ‘arge, it may be possible that as the output tries to recover, the output
capacitor charging current is large enough to repeatedly re-trigger the current limit circuit before the output has
fully settled. This condition is exacerbated with higher output voltage settings because the energy requirement of
the output capacitor varies as the square of the output voltage CV2), thus requiring an increased charging
current.
A simple test to determine if this condition might exist for a suspect application is to apply a short circuit across
the output of the converter, and then remove the shorted output condition. In an application with properly
selected external components, the output recovers smoothly.
Practical values of external components that have been experimentally found to work well under these specific
operating conditions are COUT = 47 µF, L = 22 µH. It must be noted that even with these components, for a
device’s current limit of ICLIM, the maximum load current under which the possibility of the large current limit
hysteresis can be minimized is ICLIM/2. For example, if the input is 24 V and the set output voltage is 18 V, then
for a desired maximum current of 1.5 A, the current limit of the chosen switcher must be confirmed to be at least
3 A.
Under extreme overcurrent or short-circuit conditions, the LM267X employs frequency foldback in addition to the
current limit. If the cycle-by-cycle inductor current increases above the current limit threshold (due to short circuit
or inductor saturation for example) the switching frequency is automatically reduced to protect the IC. Frequency
below 100 kHz is typical for an extreme short-circuit condition.
8.2 Typical Application
8.2.1 Typical Application for All Output Voltage Versions
Figure 13. Typical Application Schematic
8.2.1.1 Design Requirements
Select the power supply operating conditions and the maximum output current. Then follow the procedure below
to find external components for LM2679.
8.2.1.2 Detailed Design Procedure
Using the nomographs and tables in this data sheet (or use the available design software at http://www.ti.com) a
complete step-down regulator can be designed in a few simple steps.
Step 1: Define the power supply operating conditions:
Required output voltage
Maximum DC input voltage
Maximum output load current
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Typical Application (continued)
Step 2: Set the output voltage by selecting a fixed output LM2679 (3.3-V, 5-V, or 12-V applications) or determine
the required feedback resistors for use with the adjustable LM2679ADJ
Step 3: Determine the inductor required by using one of the four nomographs, Figure 14 through Figure 17.
Table 3 provides a specific manufacturer and part number for the inductor.
Step 4: Using Table 1 and Table 6 (fixed output voltage) or Table 9 and Table 10 (adjustable output voltage),
determine the output capacitance required for stable operation. Table 1 and Table 2 provide the specific
capacitor type from the manufacturer of choice.
Step 5: Determine an input capacitor from Table 7 and Table 8 for fixed output voltage applications. Use Table 1
and Table 2 to find the specific capacitor type. For adjustable output circuits select a capacitor from Table 1 and
Table 2 with a sufficient working voltage (WV) rating greater than VIN max, and an RMS current rating greater
than one-half the maximum load current (2 or more capacitors in parallel may be required).
Step 6: Select a diode from Table 4. The current rating of the diode must be greater than ILOAD max and the
reverse voltage rating must be greater than VIN max.
Step 7: Include a 0.01-μF, 50-V capacitor for CBOOST in the design and then determine the value of a soft-start
capacitor if desired.
Step 8: Define a value for RADJ to set the peak switch current limit to be at least 20% greater than IOUT max to
allow for at least 30% inductor ripple current (±15% of IOUT). For designs that must operate over the full
temperature range the switch current limit must be set to at least 50% greater than IOUT max (1.5 × IOUT max).
8.2.1.2.1 Capacitor Selection Guides
Table 1. Input and Output Capacitor Codes—Surface Mount
CAPACITOR
REFERENCE
CODE
SURFACE MOUNT
AVX TPS SERIES SPRAGUE 594D SERIES KEMET T495 SERIES
C (µF) WV (V) Irms (A) C (µF) WV (V) Irms (A) C (µF) WV (V) Irms (A)
C1 330 6.3 1.15 120 6.3 1.1 100 6.3 0.82
C2 100 10 1.1 220 6.3 1.4 220 6.3 1.1
C3 220 10 1.15 68 10 1.05 330 6.3 1.1
C4 47 16 0.89 150 10 1.35 100 10 1.1
C5 100 16 1.15 47 16 1 150 10 1.1
C6 33 20 0.77 100 16 1.3 220 10 1.1
C7 68 20 0.94 180 16 1.95 33 20 0.78
C8 22 25 0.77 47 20 1.15 47 20 0.94
C9 10 35 0.63 33 25 1.05 68 20 0.94
C10 22 35 0.66 68 25 1.6 10 35 0.63
C11 15 35 0.75 22 35 0.63
C12 33 35 1 4.7 50 0.66
C13 15 50 0.9
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Table 2. Input and Output Capacitor Codes—Through Hole
CAPACITOR
REFERENCE
CODE
THROUGH HOLE
SANYO OS-CON SA SERIES SANYO MV-GX SERIES NICHICON PL SERIES PANASONIC HFQ SERIES
C (µF) WV (V) Irms (A) C (µF) WV (V) Irms (A) C (µF) WV (V) Irms (A) C (µF) WV (V) Irms (A)
C1 47 6.3 1 1000 6.3 0.8 680 10 0.8 82 35 0.4
C2 150 6.3 1.95 270 16 0.6 820 10 0.98 120 35 0.44
C3 330 6.3 2.45 470 16 0.75 1000 10 1.06 220 35 0.76
C4 100 10 1.87 560 16 0.95 1200 10 1.28 330 35 1.01
C5 220 10 2.36 820 16 1.25 2200 10 1.71 560 35 1.4
C6 33 16 0.96 1000 16 1.3 3300 10 2.18 820 35 1.62
C7 100 16 1.92 150 35 0.65 3900 10 2.36 1000 35 1.73
C8 150 16 2.28 470 35 1.3 6800 10 2.68 2200 35 2.8
C9 100 20 2.25 680 35 1.4 180 16 0.41 56 50 0.36
C10 47 25 2.09 1000 35 1.7 270 16 0.55 100 50 0.5
C11 220 63 0.76 470 16 0.77 220 50 0.92
C12 470 63 1.2 680 16 1.02 470 50 1.44
C13 680 63 1.5 820 16 1.22 560 50 1.68
C14 1000 63 1.75 1800 16 1.88 1200 50 2.22
C15 220 25 0.63 330 63 1.42
C16 220 35 0.79 1500 63 2.51
C17 560 35 1.43
C18 2200 35 2.68
C19 150 50 0.82
C20 220 50 1.04
C21 330 50 1.3
C22 100 63 0.75
C23 390 63 1.62
C24 820 63 2.22
C25 1200 63 2.51
Table 3. Inductor Manufacturer Part Numbers
INDUCTOR
REFERENCE
NUMBER
INDUCTANCE
(µH) CURRENT
(A)
RENCO PULSE ENGINEERING COILCRAFT
THROUGH HOLE SURFACE
MOUNT THROUGH HOLE SURFACE
MOUNT SURFACE MOUNT
L23 33 1.35 RL-5471-7 RL1500-33 PE-53823 PE-53823S DO3316-333
L24 22 1.65 RL-1283-22-43 RL1500-22 PE-53824 PE-53824S DO3316-223
L25 15 2 RL-1283-15-43 RL1500-15 PE-53825 PE-53825S DO3316-153
L29 100 1.41 RL-5471-4 RL-6050-100 PE-53829 PE-53829S DO5022P-104
L30 68 1.71 RL-5471-5 RL6050-68 PE-53830 PE-53830S DO5022P-683
L31 47 2.06 RL-5471-6 RL6050-47 PE-53831 PE-53831S DO5022P-473
L32 33 2.46 RL-5471-7 RL6050-33 PE-53932 PE-53932S DO5022P-333
L33 22 3.02 RL-1283-22-43 RL6050-22 PE-53933 PE-53933S DO5022P-223
L34 15 3.65 RL-1283-15-43 PE-53934 PE-53934S DO5022P-153
L38 68 2.97 RL-5472-2 PE-54038 PE-54038S
L39 47 3.57 RL-5472-3 PE-54039 PE-54039S
L40 33 4.26 RL-1283-33-43 PE-54040 PE-54040S
L41 22 5.22 RL-1283-22-43 PE-54041 P0841
L44 68 3.45 RL-5473-3 PE-54044
L45 10 4.47 RL-1283-10-43 P0845 DO5022P-103HC
L46 15 5.6 RL-1283-15-43 P0846 DO5022P-153HC
L47 10 5.66 RL-1283-10-43 P0847 DO5022P-103HC
L48 47 5.61 RL-1282-47-43 P0848
L49 33 5.61 RL-1282-33-43 P0849