NSC LM2679

LM2679
SIMPLE SWITCHER ® 5A Step-Down Voltage Regulator
with Adjustable Current Limit
General Description
Features
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
5A 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 3.3V, 5V and 12V 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 (260KHz) 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 softstart 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
guaranteed to a ± 2% tolerance. The clock frequency is controlled to within a ± 11% tolerance.
n Efficiency up to 92%
n Simple and easy to design with (using off-the-shelf
external components)
n Resistor programmable peak current limit over a range
of 3A to 7A.
n 120 mΩ DMOS output switch
n 3.3V, 5V and 12V fixed output and adjustable (1.2V to
37V ) versions
n ± 2%maximum output tolerance over full line and load
conditions
n Wide input voltage range: 8V to 40V
n 260 KHz fixed frequency internal oscillator
n Softstart capability
n −40 to +125˚C operating junction temperature range
Applications
n Simple to design, high efficiency ( > 90%) step-down
switching regulators
n Efficient system pre-regulator for linear voltage
regulators
n Battery chargers
Typical Application
DS100847-3
SIMPLE SWITCHER ® is a registered trademark of National Semiconductor Corporation.
© 2000 National Semiconductor Corporation
DS100847
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LM2679 SIMPLE SWITCHER 5A Step-Down Voltage Regulator with Adjustable Current Limit
March 2000
LM2679
Connection Diagram and Ordering Information
TO-263 Package
Top View
DS100847-1
Order Number
LM2679S-3.3, LM2679S-5.0,
LM2679S-12 or LM2679S-ADJ
See NSC Package Number TS7B
TO-220 Package
Top View
DS100847-2
Order Number
LM2679T-3.3, LM2679T-5.0,
LM2679T-12 or LM2679T-ADJ
See NSC Package Number TA07B
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2
Storage Temperature Range
Soldering Temperature
Wave
Infrared
Vapor Phase
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Input Supply Voltage
Softstart Pin Voltage
Switch Voltage to Ground
Boost Pin Voltage
Feedback Pin Voltage
Power Dissipation
ESD (Note 2)
45V
−0.1V to 6V
−1V to VIN
VSW + 8V
−0.3V to 14V
Internally Limited
2 kV
−65˚C to 150˚C
4 sec, 260˚C
10 sec, 240˚C
75 sec, 219˚C
Operating Ratings
Supply Voltage
Junction Temperature Range (TJ)
8V to 40V
−40˚C to 125˚C
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. RADJ = 5.6KΩ
LM2679-3.3
Symbol
Parameter
Conditions
Typical
Min
Max
(Note 3)
(Note 4)
(Note 4)
3.234/3.201
3.366/3.399
VOUT
Output Voltage
VIN = 8V to 40V, 100mA ≤ IOUT ≤ 5A
3.3
η
Efficiency
VIN = 12V, ILOAD = 5A
82
Units
V
%
LM2679-5.0
Symbol
Parameter
Conditions
Typical
Min
Max
(Note 3)
(Note 4)
(Note 4)
4.900/4.850
5.100/5.150
VOUT
Output Voltage
VIN = 8V to 40V, 100mA ≤ IOUT ≤ 5A
5.0
η
Efficiency
VIN = 12V, ILOAD = 5A
84
Units
V
%
LM2679-12
Symbol
Parameter
Conditions
Typical
Min
Max
(Note 3)
(Note 4)
(Note 4)
11.76/11.64
12.24/12.36
VOUT
Output Voltage
VIN = 15V to 40V, 100mA ≤ IOUT ≤ 5A
12
η
Efficiency
VIN = 24V, ILOAD = 5A
92
Units
V
%
LM2679-ADJ
Symbol
Parameter
Conditions
VFB
Feedback
Voltage
VIN = 8V to 40V, 100mA ≤ IOUT ≤ 5A
VOUT Programmed for 5V
η
Efficiency
VIN = 12V, ILOAD = 5A
Typ
Min
Max
(Note 3)
(Note 4)
(Note 4)
1.21
1.186/1.174
1.234/1.246
84
3
Units
V
%
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LM2679
Absolute Maximum Ratings (Note 1)
LM2679
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
Typ
Min
Max
Units
6
mA
DEVICE PARAMETERS
IQ
Quiescent
Current
VFEEDBACK = 8V
4.2
For 3.3V, 5.0V, and ADJ Versions
VFEEDBACK = 15V
For 12V Versions
VADJ
Current Limit
Adjust Voltage
ICL
Current Limit
IL
Output
Leakage
Current
RDS(ON)
Switch
On-Resistance
fO
D
1.21
1.181/1.169
1.229/1.246
V
RADJ = 5.6KΩ, (Note 5)
6.3
5.5/5.3
7.6/8.1
A
VIN = 40V, Softstart Pin = 0V
VSWITCH = 0V
VSWITCH = −1V
1.0
6
1.5
15
ISWITCH = 5A
0.12
0.14/0.225
Ω
Oscillator
Frequency
Measured at Switch Pin
260
280
kHz
Duty Cycle
Maximum Duty Cycle
91
Minimum Duty Cycle
0
%
VFEEDBACK = 1.3V
ADJ Version Only
85
nA
IBIAS
Feedback Bias
Current
VSFST
Softstart
Threshold
Voltage
0.63
ISFST
Softstart Pin
Current
Softstart Pin = 0V
θJA
Thermal
Resistance
T Package, Junction to Ambient
θJA
225
mA
mA
%
0.53
3.7
0.74
V
6.9
µA
65
(Note 6)
T Package, Junction to Ambient
45
(Note 7)
θJC
T Package, Junction to Case
2
θJA
S Package, Junction to Ambient
56
˚C/W
(Note 8)
θJA
S Package, Junction to Ambient
35
(Note 9)
θJA
S Package, Junction to Ambient
26
(Note 10)
θJC
S Package, Junction to Case
2
++
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings indicate conditions under which of the device is
guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test condition, see the electrical Characteristics tables.
Note 2: ESD was applied using the human-body model, a 100pF capacitor discharged through a 1.5 kΩ resistor into each pin.
Note 3: Typical values are determined with TA = TJ = 25˚C and represent the most likely norm.
Note 4: All limits are guaranteed 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 guaranteed via correlation using standard standard Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
Note 5: The peak switch current limit is determined by the following relationship: ICL =37,125/ RADJ.
Note 6: Junction to ambient thermal resistance (no external heat sink) for the 7 lead TO-220 package mounted vertically, with 1⁄2 inch leads in a socket, or on a PC
board with minimum copper area.
Note 7: Junction to ambient thermal resistance (no external heat sink) for the 7 lead TO-220 package mounted vertically, with 1⁄2 inch leads soldered to a PC board
containing approximately 4 square inches of (1 oz.) copper area surrounding the leads.
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Note 8: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board area of 0.136 square inches (the same size as the
TO-263 package) of 1 oz. (0.0014 in. thick) copper.
Note 9: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board area of 0.4896 square inches (3.6 times the area of
the TO-263 package) of 1 oz. (0.0014 in. thick) copper.
Note 10: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board copper area of 1.0064 square inches (7.4 times the
area of the TO-263 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.
5
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LM2679
(Continued)
LM2679
Typical Performance Characteristics
Normalized
Output Voltage
Line Regulation
Efficiency vs Input Voltage
DS100847-5
DS100847-6
DS100847-4
Efficiency vs ILOAD
Switch Current Limit
DS100847-7
Switching Frequency
DS100847-8
Feedback Pin Bias Current
DS100847-13
DS100847-12
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Operating Quiescent Current
6
DS100847-9
LM2679
Block Diagram
DS100847-14
* Active Inductor Patent Number 5,514,947
† Active Capacitor Patent Number 5,382,918
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LM2679
Typical Performance Characteristics
Continuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 5A
L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ
Discontinuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 500 mA
L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ
DS100847-15
A: VSW Pin Voltage, 10 V/div.
B: Inductor Current, 2 A/div
C: Output Ripple Voltage, 20 mV/div AC-Coupled
DS100847-16
A: VSW Pin Voltage, 10 V/div.
B: Inductor Current, 1 A/div
C: Output Ripple Voltage, 20 mV/div AC-Coupled
Horizontal Time Base: 1 µs/div
Horizontal Time Base: 1 µs//iv
Load Transient Response for Continuous Mode
VIN = 20V, VOUT = 5V
L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ
Load Transient Response for Discontinuous Mode
VIN = 20V, VOUT = 5V,
L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ
DS100847-17
DS100847-18
A: Output Voltage, 100 mV//div, AC-Coupled.
B: Load Current: 500 mA to 5A Load Pulse
A: Output Voltage, 100 mV/div, AC-Coupled.
B: Load Current: 200 mA to 3A Load Pulse
Horizontal Time Base: 100 µs/div
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Horizontal Time Base: 200 µs/div
8
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.
PIN 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, it is recommended that a broad ground plane be used to minimize
signal coupling throughout the circuit
PIN 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 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.
PIN 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 (3.3V, 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.
PIN 7 - 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.
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 National Semiconductor’s Internet site located at http://www.national.com.
PIN 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 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).
PIN 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 guaranteed 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.
PIN 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 conDESIGN CONSIDERATIONS
DS100847-23
FIGURE 1. Basic circuit for fixed output voltage applications.
9
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LM2679
duction losses in the power switch to maintain high efficiency. The recommended value for C Boost is 0.01µF.
Application Hints
LM2679
Application Hints
(Continued)
DS100847-24
FIGURE 2. Basic circuit for adjustable output voltage applications
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.
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 2.0), can also be used to provide instant
component selection, circuit performance calculations for
evaluation, a bill of materials component list and a circuit
schematic.
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
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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.
(Continued)
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.
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.
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.
The peak switch current is equal to a factor of 37,125 divided
by RADJ. A resistance of 5.6KΩ sets the current limit to typically 6.3A and an RADJ of 8.25KΩ reduces 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:
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
SIMPLE DESIGN PROCEDURE
11
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LM2679
Application Hints
LM2679
Application Hints
Using the nomographs and tables in this data sheet (or use
the available design software at http://www.national.com) a
complete step-down regulator can be designed in a few
simple steps.
Step 4: Use Table 3 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 2 provides the actual capacitor characteristics.
Any of the following choices will work in the circuit:
Step 1: Define the power supply operating conditions:
2 x 220µF/10V Sanyo OS-CON (code C5)
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
LM2679−ADJ
Step 3: Determine the inductor required by using one of the
four nomographs, Figure 3 through Figure 6. Table 1 provides a specific manufacturer and part number for the inductor.
Step 4: Using Table 3 (fixed output voltage) or Table 6 (adjustable output voltage), determine the output capacitance
required for stable operation. Table 2 provides the specific
capacitor type from the manufacturer of choice.
Step 5: Determine an input capacitor from Table 4 for fixed
output voltage applications. Use Table 2 to find the specific
capacitor type. For adjustable output circuits select a capacitor from 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 5. 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
Step 2: Select an LM2679T-3.3. The output voltage will have
a tolerance of
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 4 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 2 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 5 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
(Continued)
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.
Use a value of 6.2KΩ.
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
± 2% at room temperature and ± 3% over the full operating
temperature range.
Step 3: Use the nomograph for the 3.3V device ,Figure 3.
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.
www.national.com
12
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 2 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 adquate.
(Continued)
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 2). For the adjustable device the output voltage is set by
the following relationship:
Step 6: From Table 5 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.
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:
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 6, requires a calculation of the inductor
Volt • microsecond constant (E • T expressed in V • µS) from
the following formula:
Use a value of 7.15KΩ.
where VSAT is the voltage drop across the internal power
switch which is Rds(ON) times Iload. In this example this would
be typically 0.12Ω x 3.5A or 0.42V and VD is 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:
Using Figure 6, 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.
Step 4: Use Table 6 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 2 provides 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)
Important Note: When using the adjustable device in low
voltage applications (less than 3V output), if the nomograph,
Figure 6, selects an inductance of 22µH or less, Table 6 does
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 6.
13
www.national.com
LM2679
Application Hints
LM2679
Application Hints
(Continued)
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)
DS100847-19
DS100847-20
FIGURE 3. LM2679-3.3
FIGURE 4. LM2679-5.0
DS100847-22
DS100847-21
FIGURE 6. LM2679-ADJ
FIGURE 5. LM2679-12
www.national.com
14
LM2679
Application Hints
(Continued)
Table 1. Inductor Manufacturer Part Numbers
Renco
Pulse Engineering
Coilcraft
Inductor
Reference
Number
Inductance
(µH)
Current
(A)
Through Hole
Surface
Mount
Through
Hole
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
2.00
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
—
Surface Mount
—
Inductor Manufacturer Contact Numbers
Coilcraft
Coilcraft, Europe
Pulse Engineering
Phone
(800) 322-2645
FAX
(708) 639-1469
Phone
+44 1236 730 595
FAX
+44 1236 730 627
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
15
www.national.com
LM2679
Application Hints
(Continued)
Table 2. Input and Output Capacitor Codes
Capacitor
Reference
Code
Surface Mount
AVX TPS Series
Sprague 594D Series
WV
(V)
Irms
(A)
C1
330
6.3
1.15
120
6.3
1.1
C2
100
10
1.1
220
6.3
1.4
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
www.national.com
C
(µF)
WV
(V)
Irms
(A)
Kemet T495 Series
C
(µF)
16
C
(µF)
WV
(V)
Irms
(A)
100
6.3
0.82
220
6.3
1.1
LM2679
Application Hints
(Continued)
Table 2. Input and Output Capacitor Codes (continued)
Through Hole
Capacitor
Reference
Code
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
Capacitor Manufacturer Contact Numbers
Nichicon
Panasonic
AVX
Sprague/Vishay
Sanyo
Kemet
Phone
(847) 843-7500
FAX
(847) 843-2798
Phone
(714) 373-7857
FAX
(714) 373-7102
Phone
(845) 448-9411
FAX
(845) 448-1943
Phone
(207) 324-4140
FAX
(207) 324-7223
Phone
(619) 661-6322
FAX
(619) 661-1055
Phone
(864) 963-6300
FAX
(864) 963-6521
17
www.national.com
LM2679
Application Hints
(Continued)
Table 3. Output Capacitors for Fixed Output Voltage Application
Output
Voltage
(V)
3.3
5
12
Output
Voltage
(V)
3.3
5
12
Surface Mount
Inductance
(µH)
AVX TPS Series
Sprague 594D
Series
Kemet T495 Series
No.
C Code
No.
C Code
No.
C Code
10
5
C1
5
C1
5
C2
15
4
C1
4
C1
4
C3
22
3
C2
2
C7
3
C4
33
1
C1
2
C7
3
C4
10
4
C2
4
C6
4
C4
15
3
C3
2
C7
3
C5
22
3
C2
2
C7
3
C4
33
2
C2
2
C3
2
C4
47
2
C2
1
C7
2
C4
10
4
C5
3
C6
5
C9
15
3
C5
2
C7
4
C9
22
2
C5
2
C6
3
C8
33
2
C5
1
C7
3
C8
47
2
C4
1
C6
2
C8
68
1
C5
1
C5
2
C7
100
1
C4
1
C5
1
C8
Through Hole
Inductance
(µH)
Sanyo OS-CON SA
Series
Sanyo MV-GX
Series
Nichicon PL Series
No.
C Code
No.
C Code
No.
C Code
No.
C Code
10
2
C5
2
C6
1
C8
2
C6
15
2
C5
2
C5
1
C7
2
C5
22
1
C5
1
C10
1
C5
1
C7
33
1
C5
1
C10
1
C5
1
C7
10
2
C4
2
C5
1
C6
2
C5
15
1
C5
1
C10
1
C5
1
C7
22
1
C5
1
C9
1
C5
1
C5
33
1
C4
1
C5
1
C4
1
C4
47
1
C4
1
C4
1
C2
2
C4
10
2
C7
1
C10
1
C14
2
C4
15
1
C8
1
C6
1
C17
1
C5
22
1
C7
1
C5
1
C13
1
C5
33
1
C7
1
C4
1
C12
1
C4
47
1
C7
1
C3
1
C11
1
C3
68
1
C6
1
C2
1
C10
1
C3
100
1
C6
1
C2
1
C9
1
C1
No. represents the number of identical capacitor types to be connected in parallel
C Code indicates the Capacitor Reference number in Table 2 for identifying the specific component from the manufacturer.
www.national.com
Panasonic HFQ
Series
18
LM2679
Application Hints
(Continued)
Table 4. Input Capacitors for Fixed Output Voltage Application
(Assumes worst case maximum input voltage and load current for a given inductance value)
Surface Mount
Output
Voltage
(V)
3.3
5
12
Output
Voltage
(V)
3.3
5
12
Inductance
(µH)
AVX TPS Series
Sprague 594D
Series
Kemet T495 Series
No.
C Code
No.
C Code
No.
10
3
C7
2
C10
3
C Code
C9
15
*
*
3
C13
4
C12
22
*
*
2
C13
3
C12
33
*
*
2
C13
3
C12
10
3
C4
2
C6
3
C9
15
4
C9
3
C12
4
C10
22
*
*
3
C13
4
C12
33
*
*
2
C13
3
C12
47
*
*
1
C13
2
C12
10
4
C9
2
C10
4
C10
15
4
C8
2
C10
4
C10
22
4
C9
3
C12
4
C10
33
*
*
3
C13
4
C12
47
*
*
2
C13
3
C12
68
*
*
2
C13
2
C12
100
*
*
1
C13
2
C12
Through Hole
Inductance
(µH)
Sanyo OS-CON SA
Series
Sanyo MV-GX
Series
No.
C Code
Nichicon PL Series
No.
C Code
Panasonic HFQ
Series
No.
C Code
No.
C Code
10
2
C9
2
C8
1
C18
1
C8
15
*
*
2
C13
1
C25
1
C16
22
*
*
1
C14
1
C24
1
C16
33
*
*
1
C14
1
C24
1
C16
10
2
C7
2
C8
1
C25
1
C8
15
*
*
2
C8
1
C25
1
C8
22
*
*
2
C13
1
C25
1
C16
33
*
*
1
C14
1
C23
1
C13
47
*
*
1
C12
1
C19
1
C11
10
2
C10
2
C8
1
C18
1
C8
15
2
C10
2
C8
1
C18
1
C8
22
*
*
2
C8
1
C18
1
C8
33
*
*
2
C12
1
C24
1
C14
47
*
*
1
C14
1
C23
1
C13
68
*
*
1
C13
1
C21
1
C15
100
*
*
1
C11
1
C22
1
C11
* Check voltage rating of capacitors to be greater than application input voltage.
No. represents the number of identical capacitor types to be connected in parallel
C Code indicates the Capacitor Reference number in Table 2 for identifying the specific component from the manufacturer.
19
www.national.com
LM2679
Application Hints
(Continued)
Table 5. Schottky Diode Selection Table
Reverse
Voltage
(V)
3A
20V
SK32
30V
SK33
Surface Mount
Through Hole
5A or More
3A
5A or
More
1N5820
SR302
MBRD835L
30WQ03F
40V
1N5821
31DQ03
SK34
MBRD1545CT
1N5822
30BQ040
6TQ045S
1N5825
MBR340
MBR745
30WQ04F
31DQ04
80SQ045
MBRS340
SR403
6TQ045
MBRD340
50V or
More
SK35
MBR350
30WQ05F
31DQ05
SR305
Diode Manufacturer Contact Numbers
International Rectifier
Motorola
General
Semiconductor
Diodes, Inc.
www.national.com
Phone
(310) 322-3331
FAX
(310) 322-3332
Phone
(800) 521-6274
FAX
(602) 244-6609
Phone
(516) 847-3000
FAX
(516) 847-3236
Phone
(805) 446-4800
FAX
(805) 446-4850
20
(Continued)
Table 6. Output Capacitors for Adjustable Output Voltage Applications
Surface Mount
Output Voltage
(V)
1.21 to 2.50
2.5 to 3.75
3.75 to 5
5 to 6.25
6.25 to 7.5
7.5 to 10
10 to 12.5
12.5 to 15
15 to 20
20 to 30
30 to 37
Inductance
(µH)
AVX TPS Series
Sprague 594D
Series
Kemet T495 Series
No.
C Code
No.
C Code
No.
C Code
33*
7
C1
6
C2
7
C3
47*
5
C1
4
C2
5
C3
33*
4
C1
3
C2
4
C3
47*
3
C1
2
C2
3
C3
22
4
C1
3
C2
4
C3
33
3
C1
2
C2
3
C3
47
2
C1
2
C2
2
C3
22
3
C2
3
C3
3
C4
33
2
C2
2
C3
2
C4
47
2
C2
2
C3
2
C4
68
1
C2
1
C3
1
C4
22
3
C2
1
C4
3
C4
33
2
C2
1
C3
2
C4
47
1
C3
1
C4
1
C6
68
1
C2
1
C3
1
C4
33
2
C5
1
C6
2
C8
47
1
C5
1
C6
2
C8
68
1
C5
1
C6
1
C8
100
1
C4
1
C5
1
C8
33
1
C5
1
C6
2
C8
47
1
C5
1
C6
2
C8
68
1
C5
1
C6
1
C8
100
1
C5
1
C6
1
C8
33
1
C6
1
C8
1
C8
47
1
C6
1
C8
1
C8
68
1
C6
1
C8
1
C8
100
1
C6
1
C8
1
C8
33
1
C8
1
C10
2
C10
47
1
C8
1
C9
2
C10
68
1
C8
1
C9
2
C10
100
1
C8
1
C9
1
C10
33
2
C9
2
C11
2
C11
47
1
C10
1
C12
1
C11
68
1
C9
1
C12
1
C11
100
1
C9
1
C12
1
C11
10
4
C13
8
C12
15
3
C13
5
C12
2
C13
4
C12
33
1
C13
3
C12
47
1
C13
2
C12
68
1
C13
2
C12
22
No Values Available
21
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LM2679
Application Hints
LM2679
Application Hints
(Continued)
Table 6. Output Capacitors for Adjustable Output Voltage Applications
(continued)
Through Hole
Output Voltage
(V)
1.21 to 2.50
2.5 to 3.75
3.75 to 5
5 to 6.25
6.25 to 7.5
7.5 to 10
10 to 12.5
12.5 to 15
15 to 20
Inductance
(µH)
Sanyo OS-CON SA
Series
30 to 37
Nichicon PL Series
No.
C Code
No.
C Code
No.
C Code
No.
2
C3
5
C1
5
C3
3
C
47*
2
C2
4
C1
3
C3
2
C5
33*
1
C3
3
C1
3
C1
2
C5
47*
1
C2
2
C1
2
C3
1
C5
22
1
C3
3
C1
3
C1
2
C5
33
1
C2
2
C1
2
C1
1
C5
47
1
C2
2
C1
1
C3
1
C5
22
1
C5
2
C6
2
C3
2
C5
33
1
C4
1
C6
2
C1
1
C5
47
1
C4
1
C6
1
C3
1
C5
68
1
C4
1
C6
1
C1
1
C5
22
1
C5
1
C6
2
C1
1
C5
33
1
C4
1
C6
1
C3
1
C5
47
1
C4
1
C6
1
C1
1
C5
68
1
C4
1
C2
1
C1
1
C5
33
1
C7
1
C6
1
C14
1
C5
47
1
C7
1
C6
1
C14
1
C5
68
1
C7
1
C2
1
C14
1
C2
100
1
C7
1
C2
1
C14
1
C2
33
1
C7
1
C6
1
C14
1
C5
47
1
C7
1
C2
1
C14
1
C5
68
1
C7
1
C2
1
C9
1
C2
100
1
C7
1
C2
1
C9
1
C2
33
1
C9
1
C10
1
C15
1
C2
47
1
C9
1
C10
1
C15
1
C2
68
1
C9
1
C10
1
C15
1
C2
C Code
100
1
C9
1
C10
1
C15
1
C2
33
1
C10
1
C7
1
C15
1
C2
47
1
C10
1
C7
1
C15
1
C2
68
1
C10
1
C7
1
C15
1
C2
100
1
C10
1
C7
1
C15
1
C2
1
C7
1
C16
1
C2
47
No Values
1
C7
1
C16
1
C2
68
Available
C2
1
C7
1
C16
1
100
1
C7
1
C16
1
C2
10
1
C12
1
C20
1
C10
15
1
C11
1
C20
1
C11
22
No Values
1
C11
1
C20
1
C10
33
Available
1
C11
1
C20
1
C10
47
1
C11
1
C20
1
C10
68
1
C11
1
C20
1
C10
* Set to a higher value for a practical design solution. See Applications Hints section
No. represents the number of identical capacitor types to be connected in parallel
C Code indicates the Capacitor Reference number in Table 2 for identifying the specific component from the manufacturer.
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Panasonic HFQ
Series
33*
33
20 to 30
Sanyo MV-GX
Series
22
LM2679
Physical Dimensions
inches (millimeters) unless otherwise noted
TO-263 Surface Mount Power Package
Order Number LM2679S-3.3, LM2679S-5.0,
LM2679S-12 or LM2679S-ADJ
NS Package Number TS7B
23
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LM2679 SIMPLE SWITCHER 5A Step-Down Voltage Regulator with Adjustable Current Limit
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
TO-220 Power Package
Order Number LM2679T-3.3, LM2679T-5.0,
LM2679T-12 or LM2679T-ADJ
NS Package Number TA07B
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