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
LM2679
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LM2679 SIMPLE SWITCHER
®
5A Step-Down Voltage Regulator with Adjustable Current
Limit
Check for Samples: LM2679
1FEATURES DESCRIPTION
The LM2679 series of regulators are monolithic
2 Efficiency Up to 92% integrated circuits which provide all of the active
Simple and Easy to Design with (Using Off- functions for a step-down (buck) switching regulator
The-Shelf External Components) capable of driving up to 5A loads with excellent line
Resistor Programmable Peak Current Limit and load regulation characteristics. High efficiency
(>90%) is obtained through the use of a low ON-
Over a Range of 3A to 7A. resistance DMOS power switch. The series consists
120 mΩDMOS Output Switch of fixed output voltages of 3.3V, 5V and 12V and an
3.3V, 5V and 12V Fixed Output and Adjustable adjustable output version.
(1.2V to 37V ) Versions The SIMPLE SWITCHER®concept provides for a
±2% Maximum Output Tolerance Over Full complete design using a minimum number of external
Line and Load Conditions components. A high fixed frequency oscillator
Wide Input Voltage Range: 8V to 40V (260KHz) allows the use of physically smaller sized
components. A family of standard inductors for use
260 KHz Fixed Frequency Internal Oscillator with the LM2679 are available from several
Softstart Capability manufacturers to greatly simplify the design process.
40 to +125°C Operating Junction Temperature Other features include the ability to reduce the input
Range surge current at power-ON by adding a softstart
timing capacitor to gradually turn on the regulator.
APPLICATIONS The LM2679 series also has built in thermal
shutdown and resistor programmable current limit of
Simple to Design, High Efficiency (>90%) Step- the power MOSFET switch to protect the device and
Down Switching Regulators load circuitry under fault conditions. The output
Efficient System Pre-Regulator for Linear voltage is specified to a ±2% tolerance. The clock
Voltage Regulators frequency is controlled to within a ±11% tolerance.
Battery Chargers
Typical Application
1Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
2SIMPLE SWITCHER, Switchers Made Simple are registered trademarks of Texas Instruments.
PRODUCTION DATA information is current as of publication date. Copyright © 2000–2013, Texas Instruments Incorporated
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
1
2
3
4
7
9
10
VSW
CB
*
VIN
5
6
CURRENT ADJ
11
12
13
14
VSW
GND
SOFTSTART
*
*
*
FB 8
VIN VSW
DAP**
*No Connections
** Connect to Pin 9 on PCB
LM2679
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Connection Diagrams
Figure 1. DDPAK Package Figure 2. TO-220 Package
Top View Top View
See Package Number KTW0007B See Package Number NDZ0007B
Figure 3. VSON-14
Top View
See Package Number NHM0014A
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
Absolute Maximum Ratings(1)(2)
Input Supply Voltage 45V
Softstart Pin Voltage 0.1V to 6V
Switch Voltage to Ground(3) 1V to VIN
Boost Pin Voltage VSW + 8V
Feedback Pin Voltage 0.3V to 14V
Power Dissipation Internally Limited
ESD(4) 2 kV
Storage Temperature Range 65°C to 150°C
Soldering Temperature Wave 4 sec, 260°C
Infrared 10 sec, 240°C
Vapor Phase 75 sec, 219°C
(1) Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings indicate conditions under
which of the device is specified. Operating Ratings do not imply ensure performance limits. For specific performance limits and
associated test condition, see the electrical Characteristics tables.
(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 -10V
applies to a pulse of up to 20 ns, -6V of 60 ns and -3V of up to 100 ns.
(4) ESD was applied using the human-body model, a 100pF capacitor discharged through a 1.5 kΩresistor into each pin.
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Operating Ratings
Supply Voltage 8V to 40V
Junction Temperature Range (TJ)40°C to 125°C
Electrical Characteristics
LM2679-3.3
Limits appearing in bold type face apply over the entire junction temperature range of operation, 40°C to 125°C.
Specifications appearing in normal type apply for TA= TJ= 25°C. RADJ = 5.6K.
Symbol Parameter Conditions Min(1) Typ(2) Max(1) Units
VOUT Output Voltage VIN = 8V to 40V, 100mA IOUT 5A 3.234/3.201 3.3 3.366/3.399 V
ηEfficiency VIN = 12V, ILOAD = 5A 82 %
(1) All limits are specified at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature
limits are 100% tested during production with TA= TJ= 25°C. All limits at temperature extremes are specified via 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.
LM2679-5.0
Symbol Parameter Conditions Min(1) Typ(2) Max(1) Units
VOUT Output Voltage VIN = 8V to 40V, 100mA IOUT 5A 4.900/4.850 5.0 5.100/5.150 V
ηEfficiency VIN = 12V, ILOAD = 5A 84 %
(1) All limits are specified at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature
limits are 100% tested during production with TA= TJ= 25°C. All limits at temperature extremes are specified via 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.
LM2679-12
Symbol Parameter Conditions Min(1) Typ(2) Max(1) Units
VOUT Output Voltage VIN = 15V to 40V, 100mA IOUT 5A 11.76/11.64 12 12.24/12.36 V
ηEfficiency VIN = 24V, ILOAD = 5A 92 %
(1) All limits are specified at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature
limits are 100% tested during production with TA= TJ= 25°C. All limits at temperature extremes are specified via 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.
LM2679-ADJ
Symbol Parameter Conditions Min(1) Typ(2) Max(1) Units
VFB Feedback VIN = 8V to 40V, 100mA IOUT 5A 1.186/1.174 1.21 1.234/1.246 V
Voltage VOUT Programmed for 5V
ηEfficiency VIN = 12V, ILOAD = 5A 84 %
(1) All limits are specified at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature
limits are 100% tested during production with TA= TJ= 25°C. All limits at temperature extremes are specified via 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.
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All Output Voltage Versions
Electrical Characteristics
Limits appearing in bold type face apply over the entire junction temperature range of operation, 40°C to 125°C.
Specifications appearing in normal type apply for TA= TJ= 25°C. Unless otherwise specified VIN=12V for the 3.3V, 5V and
Adjustable versions and VIN=24V for the 12V version.
Symbol Parameter Conditions Min Typ Max Units
DEVICE PARAMETERS
IQQuiescent Current VFEEDBACK = 8V 4.2 6 mA
For 3.3V, 5.0V, and ADJ Versions
VFEEDBACK = 15V
For 12V Versions
VADJ Current Limit Adjust 1.181/1.169 1.21 1.229/1.246 V
Voltage
ICL Current Limit RADJ = 5.6KΩ,(1) 5.5/5.3 6.3 7.6/8.1 A
ILOutput Leakage VIN = 40V, Softstart Pin = 0V mA
Current VSWITCH = 0V 1.0 1.5 mA
VSWITCH =1V 6 15
RDS(ON) Switch On-Resistance ISWITCH = 5A 0.12 0.14/0.225 Ω
fOOscillator Frequency Measured at Switch Pin 225 260 280 kHz
D Duty Cycle Maximum Duty Cycle 91 %
Minimum Duty Cycle 0 %
IBIAS Feedback Bias VFEEDBACK = 1.3V 85 nA
Current ADJ Version Only
VSFST Softstart Threshold 0.53 0.63 0.74 V
Voltage
ISFST Softstart Pin Current Softstart Pin = 0V 3.7 6.9 μA
θJA Thermal Resistance NDZ Package, Junction to Ambient(2) 65
θJA NDZ Package, Junction to Ambient(3) 45
θJC NDZ Package, Junction to Case 2
θJA KTW Package, Junction to Ambient(4) 56 °C/W
θJA KTW Package, Junction to Ambient(5) 35
θJA KTW Package, Junction to Ambient(6) 26
θJC KTW Package, Junction to Case 2 ++
θJA NHM Package, Junction to Ambient(7) 55 °C/W
θJA NHM Package, Junction to Ambient(8) 29
(1) The peak switch current limit is determined by the following relationship: ICL=37,125/ RADJ.
(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 PC board 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 PC board 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 PC board 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 PC board 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 PC board 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 will reduce thermal
resistance further. See the thermal model in Switchers Made Simple®software.
(7) Junction to ambient thermal resistance for the 14-lead VSON mounted on a PC board copper area equal to the die attach paddle.
(8) Junction to ambient thermal resistance for the 14-lead VSON mounted on a PC board copper area using 12 vias to a second layer of
copper equal to die attach paddle. Additional copper area will reduce thermal resistance further. For layout recommendations, refer to
Application Note AN-1187 at www.ti.com/lsds/ti/analog/powermanagement/power_portal.page.
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Typical Performance Characteristics
Normalized Output Voltage Line Regulation
Figure 4. Figure 5.
Efficiency vs Input Voltage Efficiency vs ILOAD
Figure 6. Figure 7.
Switch Current Limit Operating Quiescent Current
Figure 8. Figure 9.
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Typical Performance Characteristics (continued)
Switching Frequency Feedback Pin Bias Current
Figure 10. Figure 11.
Continuous Mode Switching Waveforms Discontinuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 5A VIN = 20V, VOUT = 5V, ILOAD = 500 mA
L = 10 μH, COUT = 400 μF, COUTESR = 13 mΩL = 10 μH, COUT = 400 μF, COUTESR = 13 mΩ
A. VSW Pin Voltage, 10 V/div. A. VSW Pin Voltage, 10 V/div.
B. Inductor Current, 2 A/div B. Inductor Current, 1 A/div
C. Output Ripple Voltage, 20 mV/div AC-Coupled C. Output Ripple Voltage, 20 mV/div AC-Coupled
Figure 12. Horizontal Time Base: 1 μs/div Figure 13. Horizontal Time Base: 1 μs/div
Load Transient Response for Continuous Mode Load Transient Response for Discontinuous Mode
VIN = 20V, VOUT = 5V VIN = 20V, VOUT = 5V,
L = 10 μH, COUT = 400 μF, COUTESR = 13 mΩL = 10 μH, COUT = 400 μF, COUTESR = 13 mΩ
A. Output Voltage, 100 mV/div, AC-Coupled. A. Output Voltage, 100 mV/div, AC-Coupled.
B. Load Current: 500 mA to 5A Load Pulse B. Load Current: 200 mA to 3A Load Pulse
Figure 14. Horizontal Time Base: 100 μs/div Figure 15. Horizontal Time Base: 200 μs/div
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Block Diagram
* Active Inductor Patent Number 5,514,947
Active Capacitor Patent Number 5,382,918
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APPLICATION HINTS
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 5A,
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 pre-determined from a variety of manufacturers. Using either
this data sheet or a design software program called LM267X Made Simple (version 2.0) a complete switching
power supply can be designed quickly. The software is provided free of charge and can be downloaded from
Texas Instruments Internet site located at http://www.ti.com.
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 260KHz 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).
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 8V to 40V. For best performance of the power supply the input pin should always be
bypassed with an input capacitor located close to pin 2.
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 0.01μF.
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, it is recommended that a broad ground plane be used
to minimize signal coupling throughout the circuit
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 need 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.
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 (3.3V, 5V
and 12V 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.
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SOFTSTART
A capacitor connected from pin 7 to ground allows for a slow turn-on 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
softstart is not required this pin should be left open circuited. Please see the CSS SOFTSTART CAPACITOR
section for further information regarding softstart capacitor values.
DAP (VSON PACKAGE)
The Die Attach Pad (DAP) can and should be connected to PCB Ground plane/island. For CAD and assembly
guidelines refer to Application Note AN-1187 at www.ti.com/lsds/ti/analog/powermanagement/power_portal.page.
DESIGN CONSIDERATIONS
Figure 16. Basic Circuit for Fixed Output Voltage Applications
Figure 17. Basic Circuit for Adjustable Output Voltage Applications
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. Alternatively, the design software, LM267X Made Simple (version 6.0), can also be
used to provide instant component selection, circuit performance calculations for evaluation, a bill of materials
component list and a circuit schematic.
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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.
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, will 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.
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.
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 260KHz switching
frequency of the LM2679, the output capacitor is selected to provide a unity gain bandwidth of 40KHz 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 40KHz). When parallel
combinations of capacitors are required it has been assumed that each capacitor is the exact same part type.
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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 should 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.
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 should be rated to handle this. Paralleling multiple capacitors proportionally increases the
current rating of the total capacitance. The voltage rating should 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 and should be considered when selecting an input
capacitor.
The input capacitor should 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 (0.1μF to 0.47μF) ceramic type capacitor in parallel
with the input capacitor to prevent or minimize any ringing.
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 1V 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 will be reversed biased by the input voltage. The reverse voltage rating of
the diode should be at least 1.3 times greater than the maximum input voltage.
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 it is
recommended to use a 0.01μF/50V ceramic capacitor.
RADJ, ADJUSTABLE CURRENT LIMIT
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 7A or higher to accommodate 5A 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 7A which would be conducted under load fault conditions.
If an application only requires a load current of 3A or 4A 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.
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The peak switch current is equal to a factor of 37,125 divided by RADJ. A resistance of 5.6KΩsets the current
limit to typically 6.3A and an RADJ of 8.25KΩreduces the maximum current to approximately 4.4A. For
predictable control of the current limit it is recommended to keep the peak switch current greater than 3A. For
lower current applications a 3A 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 will turn back ON. If a load fault continues,
the switch will again exceed the threshold and switch back OFF. This will result in a low duty cycle pulsing of the
power switch to minimize the overall fault condition power dissipation.
CSS SOFTSTART CAPACITOR
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 softstart turn-on time is programmable by the selection of Css.
The formula for selecting a softstart capacitor is:
(1)
Where:
ISST = Softstart Current, 3.7μA typical
tSS = Softstart time, from design requirements
VSST = Softstart Threshold Voltage, 0.63V typical
VOUT = Output Voltage, from design requirements
VSCHOTTKY = Schottky Diode Voltage Drop, typically 0.5V
VIN = Maximum Input Voltage, from design requirements
If this feature is not desired, leave the Softstart pin (pin 7) open circuited
With certain softstart capacitor values and operating conditions, the LM2679 can exhibit an overshoot on the
output voltage during turn on. Especially when starting up into no load or low load, the softstart function may not
be effective in preventing a larger voltage overshoot on the output. With larger loads or lower input voltages
during startup this effect is minimized. In particular, avoid using softstart capacitors between 0.033µF and F.
ADDITIONAL APPLICATION INFORMATION
When the output voltage is greater than approximately 6V, and the duty cycle at minimum input voltage is greater
than approximately 50%, the designer should 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.
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 ‘large’, 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 (½CV2), thus requiring an increased charging
current.
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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 will recover smoothly.
Practical values of external components that have been experimentally found to work well under these specific
operating conditions are COUT = 47µF, L = 22µH. It should 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 24V and the set output voltage is 18V, then for
a desired maximum current of 1.5A, the current limit of the chosen switcher must be confirmed to be at least 3A.
Under extreme over-current 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 will be automatically reduced to protect the IC.
Frequency below 100 KHz is typical for an extreme short circuit condition.
SIMPLE 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
Step 2: Set the output voltage by selecting a fixed output LM2679 (3.3V, 5V or 12V 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 18 through Figure 21.
Table 1 provides a specific manufacturer and part number for the inductor.
Step 4: Using Table 6 and Table 7 (fixed output voltage) or Table 12 and Table 13 (adjustable output voltage),
determine the output capacitance required for stable operation. Table 3 and Table 4 provide the specific
capacitor type from the manufacturer of choice.
Step 5: Determine an input capacitor from Table 8 and Table 9 for fixed output voltage applications. Use Table 3
and Table 4 to find the specific capacitor type. For adjustable output circuits select a capacitor from Table 3 and
Table 4 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 10. The current rating of the diode must be greater than I load max and the
Reverse Voltage rating must be greater than Vin max.
Step 7: Include a 0.01μF/50V capacitor for Cboost in the design and then determine the value of a softstart
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 should be set to at least 50% greater than Iout max (1.5 x Iout max).
FIXED OUTPUT VOLTAGE DESIGN EXAMPLE
A system logic power supply bus of 3.3V is to be generated from a wall adapter which provides an unregulated
DC voltage of 13V to 16V. The maximum load current is 4A. A softstart delay time of 50mS is desired. Through-
hole components are preferred.
Step 1: Operating conditions are:
Vout = 3.3V
Vin max = 16V
Iload max = 4A
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Step 2: Select an LM2679T-3.3. The output voltage will have a tolerance of
±2% at room temperature and ±3% over the full operating temperature range.
Step 3: Use the nomograph for the 3.3V device, Figure 18. The intersection of the 16V horizontal line (Vin max)
and the 4A vertical line (Iload max) indicates that L46, a 15μH inductor, is required.
From Table 1, L46 in a through-hole component is available from Renco with part number RL-1283-15-43.
Step 4: Use Table 6 and Table 7 to determine an output capacitor. With a 3.3V output and a 15μH inductor there
are four through-hole output capacitor solutions with the number of same type capacitors to be paralleled and an
identifying capacitor code given. Table 3 and Table 4 provide the actual capacitor characteristics. Any of the
following choices will work in the circuit:
2 x 220μF/10V Sanyo OS-CON (code C5)
2 x 820μF/16V Sanyo MV-GX (code C5)
1 x 3900μF/10V Nichicon PL (code C7)
2 x 560μF/35V Panasonic HFQ (code C5)
Step 5: Use Table 8 and Table 9 to select an input capacitor. With 3.3V output and 15μH there are three
through-hole solutions. These capacitors provide a sufficient voltage rating and an rms current rating greater than
2A (1/2 Iload max). Again using Table 3 and Table 4 for specific component characteristics the following choices
are suitable:
2 x 680μF/63V Sanyo MV-GX (code C13)
1 x 1200μF/63V Nichicon PL (code C25)
1 x 1500μF/63V Panasonic HFQ (code C16)
Step 6: From Table 10, a 5A or more Schottky diode must be selected. For through-hole components only 40V
rated diodes are indicated and 4 part types are suitable:
1N5825
MBR745
80SQ045
6TQ045
Step 7: A 0.01μF capacitor will be used for Cboost. For the 50mS softstart delay the following parameters are to
be used:
ISST: 3.7μA
tSS: 50mS
VSST: 0.63V
VOUT: 3.3V
VSCHOTTKY: 0.5V
VIN: 16V
Using Vin max ensures that the softstart delay time will be at least the desired 50mS.
Using the formula for Css a value of 0.148μF is determined to be required. Use of a standard value 0.22μF
capacitor will produce more than sufficient softstart delay.
Step 8: Determine a value for RADJ to provide a peak switch current limit of at least 4A plus 50% or 6A.
(2)
Use a value of 6.2KΩ.
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ADJUSTABLE OUTPUT DESIGN EXAMPLE
In this example it is desired to convert the voltage from a two battery automotive power supply (voltage range of
20V to 28V, typical in large truck applications) to the 14.8VDC alternator supply typically used to power electronic
equipment from single battery 12V vehicle systems. The load current required is 3.5A maximum. It is also
desired to implement the power supply with all surface mount components. Softstart is not required.
Step 1: Operating conditions are:
Vout = 14.8V
Vin max = 28V
Iload max = 3.5A
Step 2: Select an LM2679S-ADJ. To set the output voltage to 14.9V two resistors need to be chosen (R1 and R2
in Figure 17). For the adjustable device the output voltage is set by the following relationship:
(3)
Where VFB is the feedback voltage of typically 1.21V.
A recommended value to use for R1 is 1K. In this example then R2 is determined to be:
(4)
R2 = 11.23KΩ
The closest standard 1% tolerance value to use is 11.3KΩ
This will set the nominal output voltage to 14.88V which is within 0.5% of the target value.
Step 3: To use the nomograph for the adjustable device, Figure 21, requires a calculation of the inductor
Volt•microsecond constant (E•T expressed in V•μS) from the following formula:
(5)
where VSAT is the voltage drop across the internal power switch which is Rds(ON) times Iload. In this example this
would be typically 0.12Ωx 3.5A or 0.42V and VDis the voltage drop across the forward bisased Schottky diode,
typically 0.5V. The switching frequency of 260KHz is the nominal value to use to estimate the ON time of the
switch during which energy is stored in the inductor.
For this example E•T is found to be:
(6)
(7)
Using Figure 21, the intersection of 27V•μS horizontally and the 3.5A vertical line (Iload max) indicates that L48 , a
47μH inductor, or L49, a 33μH inductor could be used. Either inductor will be suitable, but for this example
selecting the larger inductance will result in lower ripple current.
From Table 1, L48 in a surface mount component is available from Pulse Engineering with part number P0848.
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Step 4: Use Table 12 and Table 13 to determine an output capacitor. With a 14.8V output the 12.5 to 15V row is
used and with a 47μH inductor there are three surface mount output capacitor solutions. Table 3 and Table 4
provide the actual capacitor characteristics based on the C Code number. Any of the following choices can be
used:
1 x 33μF/20V AVX TPS (code C6)
1 x 47μF/20V Sprague 594 (code C8)
1 x 47μF/20V Kemet T495 (code C8)
NOTE
When using the adjustable device in low voltage applications (less than 3V output), if the
nomograph, Figure 21, selects an inductance of 22μH or less, Table 12 and Table 13 do
not provide an output capacitor solution. With these conditions the number of output
capacitors required for stable operation becomes impractical. It is recommended to use
either a 33μH or 47μH inductor and the output capacitors from Table 12 and Table 13.
Step 5: An input capacitor for this example will require at least a 35V WV rating with an rms current rating of
1.75A (1/2 Iout max). From Table 3 and Table 4, it can be seen that C12, a 33μF/35V capacitor from Sprague,
has the highest voltage/current rating of the surface mount components and that two of these capacitor in parallel
will be adequate.
Step 6: From Table 10, a 5A or more Schottky diode must be selected. For surface mount diodes with a margin
of safety on the voltage rating one of two diodes can be used:
MBRD1545CT
6TQ045S
Step 7: A 0.01μF capacitor will be used for Cboost.
The softstart pin will be left open circuited.
Step 8: Determine a value for RADJ to provide a peak switch current limit of at least 3.5A plus 50% or 5.25A.
(8)
Use a value of 7.15KΩ.
VSON PACKAGE DEVICES
The LM2679 is offered in the 14 lead VSON surface mount package to allow for a significantly decreased
footprint with equivalent power dissipation compared to the DDPAK.
The Die Attach Pad (DAP) can and should be connected to PCB Ground plane/island. For CAD and assembly
guidelines refer to Application Note AN-1187 at www.ti.com/lsds/ti/analog/powermanagement/power_portal.page.
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Inductor Selection Guides
For Continuous Mode Operation
Figure 18. LM2679-3.3 Figure 19. LM2679-5.0
Figure 20. LM2679-12 Figure 21. LM2679-ADJ
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Table 1. Inductor Manufacturer Part Numbers
Inductor Renco Pulse Engineering Coilcraft
Inductance Current
Reference Through Hole Surface Mount Through Hole Surface Mount Surface Mount
H) (A)
Number
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 2.00 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 5.60 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
Table 2. Inductor Manufacturer Contact Numbers
Coilcraft Phone (800) 322-2645
FAX (708) 639-1469
Coilcraft, Europe Phone +44 1236 730 595
FAX +44 1236 730 627
Pulse Engineering Phone (619) 674-8100
FAX (619) 674-8262
Pulse Engineering, Phone +353 93 24 107
Europe FAX +353 93 24 459
Renco Electronics Phone (800) 645-5828
FAX (516) 586-5562
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Capacitor Selection Guides
Table 3. Input and Output Capacitor Codes—Surface Mount
Surface Mount
Capacitor
Reference AVX TPS Series Sprague 594D Series Kemet T495 Series
Code C F) WV (V) Irms (A) C F) WV (V) Irms (A) C F) 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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