LM2679
SIMPLE SWITCHER®5A Step-Down Voltage Regulator
with Adjustable Current Limit
General 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
5A loads with excellent line and load regulation characteris-
tics. 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 adjust-
able 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.
Features
nEfficiency up to 92%
nSimple and easy to design with (using off-the-shelf
external components)
nResistor programmable peak current limit over a range
of 3A to 7A.
n120 mDMOS output switch
n3.3V, 5V and 12V fixed output and adjustable (1.2V to
37V ) versions
n±2%maximum output tolerance over full line and load
conditions
nWide input voltage range: 8V to 40V
n260 KHz fixed frequency internal oscillator
nSoftstart capability
n−40 to +125˚C operating junction temperature range
Applications
nSimple to design, high efficiency (>90%) step-down
switching regulators
nEfficient system pre-regulator for linear voltage
regulators
nBattery chargers
Typical Application
10084703
SIMPLE SWITCHER®is a registered trademark of National Semiconductor Corporation.
May 2002
LM2679 SIMPLE SWITCHER 5A Step-Down Voltage Regulator with Adjustable Current Limit
© 2002 National Semiconductor Corporation DS100847 www.national.com
Connection Diagrams and Ordering Information
TO-263 Package
Top View TO-220 Package
Top View
10084701
Order Number
LM2679S-3.3, LM2679S-5.0,
LM2679S-12 or LM2679S-ADJ
See NSC Package Number TS7B
10084702
Order Number
LM2679T-3.3, LM2679T-5.0,
LM2679T-12 or LM2679T-ADJ
See NSC Package Number TA07B
Top View
10084735
LLP-14
See NS package Number LDC14A
Ordering Information for LLP Package
Output Voltage Order Information Package Marking Supplied As
12 LM2679LD-12 S0000VB 1000 Units on Tape and Reel
12 LM2679LDX-12 S0000VB 4500 Units on Tape and Reel
3.3 LM2679LD-3.3 S0000TB 1000 Units on Tape and Reel
3.3 LM2679LDX-3.3 S0000TB 4500 Units on Tape and Reel
5.0 LM2679LD-5.0 S0000UB 1000 Units on Tape and Reel
5.0 LM2679LDX-5.0 S0000UB 4500 Units on Tape and Reel
ADJ LM2679LD-ADJ S0000XB 1000 Units on Tape and Reel
ADJ LM2679LDX-ADJ S0000XB 4500 Units on Tape and Reel
LM2679
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Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Input Supply Voltage 45V
Softstart Pin Voltage −0.1V to 6V
Switch Voltage to Ground −1V to V
IN
Boost Pin Voltage V
SW
+8V
Feedback Pin Voltage −0.3V to 14V
Power Dissipation Internally Limited
ESD (Note 2) 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
Operating Ratings
Supply Voltage 8V to 40V
Junction Temperature Range (T
J
) −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 T
A
=T
J
= 25˚C. R
ADJ
= 5.6K
LM2679-3.3
Symbol Parameter Conditions Typical Min Max Units
(Note 3) (Note 4) (Note 4)
V
OUT
Output Voltage V
IN
= 8V to 40V, 100mA I
OUT
5A 3.3 3.234/3.201 3.366/3.399 V
ηEfficiency V
IN
= 12V, I
LOAD
=5A 82 %
LM2679-5.0
Symbol Parameter Conditions Typical Min Max Units
(Note 3) (Note 4) (Note 4)
V
OUT
Output Voltage V
IN
= 8V to 40V, 100mA I
OUT
5A 5.0 4.900/4.850 5.100/5.150 V
ηEfficiency V
IN
= 12V, I
LOAD
=5A 84 %
LM2679-12
Symbol Parameter Conditions Typical Min Max Units
(Note 3) (Note 4) (Note 4)
V
OUT
Output Voltage V
IN
= 15V to 40V, 100mA I
OUT
5A 12 11.76/11.64 12.24/12.36 V
ηEfficiency V
IN
= 24V, I
LOAD
=5A 92 %
LM2679-ADJ
Symbol Parameter Conditions Typ Min Max Units
(Note 3) (Note 4) (Note 4)
V
FB
Feedback
Voltage V
IN
= 8V to 40V, 100mA I
OUT
5A
V
OUT
Programmed for 5V 1.21 1.186/1.174 1.234/1.246 V
ηEfficiency V
IN
= 12V, I
LOAD
=5A 84 %
LM2679
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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 T
A
=T
J
= 25˚C. Unless otherwise specified V
IN
=12V for the 3.3V, 5V and
Adjustable versions and V
IN
=24V for the 12V version.
Symbol Parameter Conditions Typ Min Max Units
DEVICE PARAMETERS
I
Q
Quiescent
Current V
FEEDBACK
= 8V 4.2 6 mA
For 3.3V, 5.0V, and ADJ Versions
V
FEEDBACK
= 15V
For 12V Versions
V
ADJ
Current Limit
Adjust Voltage 1.21 1.181/1.169 1.229/1.246 V
I
CL
Current Limit R
ADJ
= 5.6K, (Note 5) 6.3 5.5/5.3 7.6/8.1 A
I
L
Output Leakage
Current V
IN
= 40V, Softstart Pin = 0V
V
SWITCH
=0V
V
SWITCH
= −1V 1.0
61.5
15
mA
mA
R
DS(ON)
Switch
On-Resistance I
SWITCH
= 5A 0.12 0.14/0.225
f
O
Oscillator
Frequency Measured at Switch Pin 260 225 280 kHz
D Duty Cycle Maximum Duty Cycle 91 %
Minimum Duty Cycle 0 %
I
BIAS
Feedback Bias
Current V
FEEDBACK
= 1.3V
ADJ Version Only 85 nA
V
SFST
Softstart
Threshold
Voltage 0.63 0.53 0.74 V
I
SFST
Softstart Pin
Current Softstart Pin = 0V 3.7 6.9 µA
θ
JA
Thermal
Resistance T Package, Junction to Ambient 65
(Note 6)
θ
JA
T Package, Junction to Ambient 45
(Note 7)
θ
JC
T Package, Junction to Case 2
θ
JA
S Package, Junction to Ambient 56 ˚C/W
(Note 8)
θ
JA
S Package, Junction to Ambient 35
(Note 9)
θ
JA
S Package, Junction to Ambient 26
(Note 10)
θ
JC
S Package, Junction to Case 2 ++
θ
JA
LD Package, Junction to Ambient 55
˚C/W
(Note 11)
θ
JA
LD Package, Junction to Ambient 29
(Note 12)
LM2679
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All Output Voltage Versions
Electrical Characteristics (Continued)
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 kresistor into each pin.
Note 3: Typical values are determined with TA=T
J= 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=T
J= 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.
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.
Note 11: Junction to ambient thermal resistance for the 14-lead LLP mounted on a PC board copper area equal to the die attach paddle.
Note 12: Junction to ambient thermal resistance for the 14-lead LLP 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.
LM2679
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Typical Performance Characteristics
Normalized
Output Voltage Line Regulation Efficiency vs Input Voltage
10084704 10084705 10084706
Efficiency vs I
LOAD
Switch Current Limit Operating Quiescent Current
10084707 10084708 10084709
Switching Frequency Feedback Pin Bias Current
10084712 10084713
LM2679
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Block Diagram
10084714
* Active Inductor Patent Number 5,514,947
Active Capacitor Patent Number 5,382,918
LM2679
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Typical Performance Characteristics
Continuous Mode Switching Waveforms
V
IN
= 20V, V
OUT
= 5V, I
LOAD
=5A
L = 10 µH, C
OUT
= 400 µF, C
OUT
ESR=13m
Discontinuous Mode Switching Waveforms
V
IN
= 20V, V
OUT
= 5V, I
LOAD
= 500 mA
L = 10 µH, C
OUT
= 400 µF, C
OUT
ESR=13m
10084715
Horizontal Time Base: 1 µs/div
A: VSW Pin Voltage, 10 V/div.
B: Inductor Current, 2 A/div
C: Output Ripple Voltage, 20 mV/div AC-Coupled
10084716
Horizontal Time Base: 1 µs//iv
A: VSW Pin Voltage, 10 V/div.
B: Inductor Current, 1 A/div
C: Output Ripple Voltage, 20 mV/div AC-Coupled
Load Transient Response for Continuous Mode
V
IN
= 20V, V
OUT
=5V
L = 10 µH, C
OUT
= 400 µF, C
OUT
ESR=13m
Load Transient Response for Discontinuous Mode
V
IN
= 20V, V
OUT
= 5V,
L = 10 µH, C
OUT
= 400 µF, C
OUT
ESR=13m
10084717
Horizontal Time Base: 100 µs/div
A: Output Voltage, 100 mV//div, AC-Coupled.
B: Load Current: 500 mA to 5A Load Pulse
10084718
Horizontal Time Base: 200 µs/div
A: Output Voltage, 100 mV/div, AC-Coupled.
B: Load Current: 200 mA to 3A Load Pulse
LM2679
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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. Acomplete design uses a minimum num-
ber 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 Semi-
conductor’s Internet site located at http://www.national.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 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.
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 mini-
mizes 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 applica-
tions such as those implemented with the LM2679, it is
recommended that a broad ground plane be used to mini-
mize 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 appli-
cation. 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 re-
quirements.
A resistor connected from pin 5 to ground establishes a
current (I
(pin 5)
= 1.2V / R
ADJ
) that sets the peak current
through the power switch. The maximum switch current is
fixed at a level of 37,125 / R
ADJ
.
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.
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.
DESIGN CONSIDERATIONS
10084723
FIGURE 1. Basic circuit for fixed output voltage applications.
LM2679
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Application Hints (Continued)
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 pro-
vided 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.
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 compari-
son of component specifications, equivalent devices from
other manufacturers could be substituted for use in an ap-
plication.
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 re-
quired for a given set of operating conditions. The nomo-
graphs 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 cur-
rent. 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 charac-
teristics.
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 fre-
quency 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
10084724
FIGURE 2. Basic circuit for adjustable output voltage applications
LM2679
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Application Hints (Continued)
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 capaci-
tance 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 typi-
cal 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 maxi-
mum 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 tempera-
ture 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 capaci-
tance. The voltage rating should also be selected to be 1.3
times the maximum input voltage. Depending on the unregu-
lated 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 recom-
mended. Total efficiency of the entire power supply is signifi-
cantly 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.
R
ADJ
, 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 7Awhich 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 maxi-
mum 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 R
ADJ
. A resistance of 5.6Ksets the current limit to
typically 6.3A and an R
ADJ
of 8.25Kreduces 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 fre-
quency 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 in-
creases 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:
I
SST
= Softstart Current, 3.7µA typical
t
SS
= Softstart time, from design requirements
V
SST
= Softstart Threshold Voltage, 0.63V typical
V
OUT
= Output Voltage, from design requirements
LM2679
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Application Hints (Continued)
V
SCHOTTKY
= Schottky Diode Voltage Drop, typically 0.5V
V
IN
= Maximum Input Voltage, from design requirements
If this feature is not desired, leave the Softstart pin (pin 7)
open circuited
SIMPLE DESIGN PROCEDURE
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 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
LM2679−ADJ
Step 3: Determine the inductor required by using one of the
four nomographs,
Figure 3
through
Figure 6
. Table 1 pro-
vides a specific manufacturer and part number for the induc-
tor.
Step 4: Using Table 3 (fixed output voltage) or Table 6
(adjustable output voltage), determine the output capaci-
tance 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 capaci-
tor 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 R
ADJ
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 I
out
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 volt-
age of 13V to 16V. The maximum load current is 4A. A
softstart delay time of 50mS is desired. Through-hole com-
ponents 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% 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 (V
in
max) and the
4A vertical line (I
load
max) indicates that L46, a 15µH induc-
tor, 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 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:
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 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 I
load
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 Table5a5Aormore Schottky diode must be
selected. For through-hole components only 40V rated di-
odes 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:
I
SST
: 3.7µA
t
SS
: 50mS
V
SST
: 0.63V
V
OUT
: 3.3V
V
SCHOTTKY
: 0.5V
V
IN
: 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 R
ADJ
to provide a peak switch
current limit of at least 4A plus 50% or 6A.
Use a value of 6.2K.
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 alter-
nator supply typically used to power electronic equipment
from single battery 12V vehicle systems. The load current
LM2679
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Application Hints (Continued)
required is 3.5Amaximum. 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 2
). For the adjustable device the output voltage is set
by the following relationship:
Where V
FB
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 = 11.23K
The closest standard 1% tolerance value to use is 11.3K
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
Voltmicrosecond constant (ET expressed in VµS) from
the following formula:
where V
SAT
is the voltage drop across the internal power
switch which is R
ds(ON)
times I
load
. In this example this would
be typically 0.12x 3.5Aor 0.42V and V
D
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 ET is found to be:
Using
Figure 6
, the intersection of 27VµS horizontally and
the 3.5A vertical line (I
load
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 so-
lutions. 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 condi-
tions 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.
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.
Step 6: From Table5a5Aormore 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 R
ADJ
to provide a peak switch
current limit of at least 3.5A plus 50% or 5.25A.
LM2679
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Application Hints (Continued) Use a value of 7.15K.
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)
Table 1. 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 2.00 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
10084719
FIGURE 3. LM2679-3.3
10084720
FIGURE 4. LM2679-5.0
10084721
FIGURE 5. LM2679-12
10084722
FIGURE 6. LM2679-ADJ
LM2679
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Application Hints (Continued)
Table 1. Inductor Manufacturer Part Numbers (Continued)
Inductor
Reference
Number
Inductance
(µH) Current
(A)
Renco Pulse Engineering Coilcraft
Through Hole Surface
Mount Through
Hole Surface
Mount Surface Mount
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
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
Table 2. Input and Output Capacitor Codes
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
LM2679
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Application Hints (Continued)
Table 2. Input and Output Capacitor Codes (continued)
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
Capacitor Manufacturer Contact Numbers
Nichicon Phone (847) 843-7500
FAX (847) 843-2798
Panasonic Phone (714) 373-7857
FAX (714) 373-7102
AVX Phone (845) 448-9411
FAX (845) 448-1943
Sprague/Vishay Phone (207) 324-4140
FAX (207) 324-7223
Sanyo Phone (619) 661-6322
FAX (619) 661-1055
Kemet Phone (864) 963-6300
FAX (864) 963-6521
LM2679
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Application Hints (Continued)
Table 3. Output Capacitors for Fixed Output Voltage Application
Output
Voltage (V) Inductance
(µH)
Surface Mount
AVX TPS Series Sprague 594D
Series Kemet T495 Series
No. C Code No. C Code No. C Code
3.3
10 5C15C15C2
15 4C14C14C3
22 3C22C73C4
33 1C12C73C4
5
10 4C24C64C4
15 3C32C73C5
22 3C22C73C4
33 2C22C32C4
47 2C21C72C4
12
10 4C53C65C9
15 3C52C74C9
22 2C52C63C8
33 2C51C73C8
47 2C41C62C8
68 1C51C52C7
100 1C41C51C8
Output
Voltage (V) Inductance
(µH)
Through Hole
Sanyo OS-CON SA
Series Sanyo MV-GX Series Nichicon PL Series Panasonic HFQ
Series
No. C Code No. C Code No. C Code No. C Code
3.3
10 2C52C61C82C6
15 2C52C51C72C5
22 1C51C101C51C7
33 1C51C101C51C7
5
10 2C42C51C62C5
15 1C51C101C51C7
22 1C51C91C51C5
33 1C41C51C41C4
47 1C41C41C22C4
12
10 2 C7 1 C10 1 C14 2 C4
15 1C81C61C171C5
22 1C71C51C131C5
33 1C71C41C121C4
47 1C71C31C111C3
68 1C61C21C101C3
100 1C61C21C91C1
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.
LM2679
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