LM2676
LM2676 SIMPLE SWITCHER High Efficiency 3A Step-Down Voltage Regulator
Literature Number: SNVS031H
February 29, 2008
LM2676
SIMPLE SWITCHER® High Efficiency 3A Step-Down
Voltage Regulator
General Description
The LM2676 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 3A
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 in-
ductors for use with the LM2676 are available from several
manufacturers to greatly simplify the design process.
The LM2676 series also has built in thermal shutdown, cur-
rent limiting and an ON/OFF control input that can power
down the regulator to a low 50μA quiescent current standby
condition. The output voltage is guaranteed to a ±2% toler-
ance. The clock frequency is controlled to within a ±11%
tolerance.
Features
■Efficiency up to 94%
■Simple and easy to design with (using off-the-shelf
external components)
■150 mΩ DMOS output switch
■3.3V, 5V and 12V fixed output and adjustable (1.2V to
37V ) versions
■50μA standby current when switched OFF
■±2%maximum output tolerance over full line and load
conditions
■Wide input voltage range: 8V to 40V
■260 KHz fixed frequency internal oscillator
■−40 to +125°C operating junction temperature range
Applications
■Simple to design, high efficiency (>90%) step-down
switching regulators
■Efficient system pre-regulator for linear voltage regulators
■Battery chargers
Typical Application
10091403
SIMPLE SWITCHER® is a registered trademark of National Semiconductor Corporation
© 2008 National Semiconductor Corporation 100914 www.national.com
LM2676 SIMPLE SWITCHER High Efficiency 3A Step-Down Voltage Regulator
Connection Diagrams and Ordering Information
TO-263 Package
Top View
10091401
Order Number
LM2676S-3.3, LM2676S-5.0,
LM2676S-12 or LM2676S-ADJ
See NSC Package Number TS7B
TO-220 Package
Top View
10091402
Order Number
LM2676T-3.3, LM2676T-5.0,
LM2676T-12 or LM2676T-ADJ
See NSC Package Number TA07B
Top View
10091441
LLP-14
See NS package Number SRC14A
Ordering Information for LLP Package
Output Voltage Order Information Package Marking Supplied As
12 LM2676SD-12 S0003LB 250 Units on Tape and Reel
12 LM2676SDX-12 S0003LB 2500 Units on Tape and Reel
3.3 LM2676SD-3.3 S0003NB 250 Units on Tape and Reel
3.3 LM2676SDX-3.3 S0003NB 2500 Units on Tape and Reel
5.0 LM2676SD-5.0 S0003PB 250 Units on Tape and Reel
5.0 LM2676SDX-5.0 S0003PB 2500 Units on Tape and Reel
ADJ LM2676SD-ADJ S0003RB 250 Units on Tape and Reel
ADJ LM2676SDX-ADJ S0003RB 2500 Units on Tape and Reel
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LM2676
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
ON/OFF Pin Voltage −0.1V to 6V
Switch Voltage to Ground (Note 12) −1V to VIN
Boost Pin Voltage VSW + 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 (TJ)−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.
LM2676-3.3
Symbol Parameter Conditions Typical Min Max Units
(Note 3) (Note 4) (Note 4)
VOUT Output Voltage VIN = 8V to 40V, 100mA ≤ IOUT ≤ 3A 3.3 3.234/3.201 3.366/3.399 V
ηEfficiency VIN = 12V, ILOAD = 3A 86 %
LM2676-5.0
Symbol Parameter Conditions Typical Min Max Units
(Note 3) (Note 4) (Note 4)
VOUT Output Voltage VIN = 8V to 40V, 100mA ≤ IOUT ≤ 3A 5.0 4.900/4.850 5.100/5.150 V
ηEfficiency VIN = 12V, ILOAD = 3A 88 %
LM2676-12
Symbol Parameter Conditions Typical Min Max Units
(Note 3) (Note 4) (Note 4)
VOUT Output Voltage VIN = 15V to 40V, 100mA ≤ IOUT ≤ 3A 12 11.76/11.64 12.24/12.36 V
ηEfficiency VIN = 24V, ILOAD = 3A 94 %
LM2676-ADJ
Symbol Parameter Conditions Typ Min Max Units
(Note 3) (Note 4) (Note 4)
VFB Feedback Voltage VIN = 8V to 40V, 100mA ≤ IOUT ≤ 3A
VOUT Programmed for 5V 1.21 1.186/1.174 1.234/1.246 V
ηEfficiency VIN = 12V, ILOAD = 3A 88 %
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LM2676
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 Ad-
justable versions and VIN=24V for the 12V version.
Symbol Parameter Conditions Typ Min 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
ISTBY Standby Quiescent
Current
ON/OFF Pin = 0V 50 100/150 μA
ICL Current Limit 4.5 3.8/3.6 5.25/5.4 A
ILOutput Leakage
Current
VIN = 40V, ON/OFF Pin = 0V
VSWITCH = 0V
VSWITCH = −1V 16
200
15
μA
mA
RDS(ON) Switch On-
Resistance
ISWITCH = 3A 0.15 0.17/0.29 Ω
fOOscillator
Frequency
Measured at Switch Pin 260 225 280 kHz
D Duty Cycle Maximum Duty Cycle 91 %
Minimum Duty Cycle 0 %
IBIAS Feedback Bias
Current
VFEEDBACK = 1.3V
ADJ Version Only
85 nA
VON/OFF ON/OFF
Threshold Voltage
1.4 0.8 2.0 V
ION/OFF ON/OFF Input
Current
ON/OFF Input = 0V 20 45 μA
θJA Thermal
Resistance
T Package, Junction to Ambient 65
(Note 5)
θJA T Package, Junction to Ambient 45
(Note 6)
θJC T Package, Junction to Case 2
θJA S Package, Junction to Ambient 56 °C/W
(Note 7)
θJA S Package, Junction to Ambient 35
(Note 8)
θJA S Package, Junction to Ambient 26
(Note 9)
θJC S Package, Junction to Case 2 ++
θJA SD Package, Junction to Ambient 55
°C/W
(Note 10)
θJA SD Package, Junction to Ambient 29
(Note 11)
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LM2676
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: 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.
Note 6: 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.
Note 7: 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 8: 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 9: 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 10: Junction to ambient thermal resistance for the 14-lead LLP mounted on a PC board copper area equal to the die attach paddle.
Note 11: 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.
Note 12: The absolute maximum specification of the 'Switch Voltage to Ground' applies to DC voltage. An extended negative voltage limit of -8V applies to a
pulse of up to 20 ns, -6V of 60 ns and -3V of up to 100 ns.
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LM2676
Typical Performance Characteristics
Normalized
Output Voltage
10091409
Line Regulation
10091410
Efficiency vs Input Voltage
10091411
Efficiency vs ILOAD
10091412
Switch Current Limit
10091404
Operating Quiescent Current
10091405
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LM2676
Standby Quiescent Current
10091440
ON/OFF Threshold Voltage
10091413
ON/OFF Pin Current (Sourcing)
10091414
Switching Frequency
10091415
Feedback Pin Bias Current
10091416
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LM2676
Typical Performance Characteristics
Continuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 3A
L = 33 μH, COUT = 200 μF, COUTESR = 26 mΩ
10091417
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
Discontinuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 500 mA
L = 10 μH, COUT = 400 μF, COUTESR = 13 mΩ
10091418
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//iv
Load Transient Response for Continuous Mode
VIN = 20V, VOUT = 5V
L = 33 μH, COUT = 200 μF, COUTESR = 26 mΩ
10091419
A: Output Voltage, 100 mV//div, AC-Coupled.
B: Load Current: 500 mA to 3A Load Pulse
Horizontal Time Base: 100 μs/div
Load Transient Response for Discontinuous Mode
VIN = 20V, VOUT = 5V,
L = 10 μH, COUT = 400 μF, COUTESR = 13 mΩ
10091420
A: Output Voltage, 100 mV/div, AC-Coupled.
B: Load Current: 200 mA to 3A Load Pulse
Horizontal Time Base: 200 μs/div
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LM2676
Block Diagram
10091406
* Active Inductor Patent Number 5,514,947
† Active Capacitor Patent Number 5,382,918
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LM2676
Application Hints
The LM2676 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 3A, and highly ef-
ficient operation.
The LM2676 is part of the SIMPLE SWITCHER family of
power converters. A complete 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 de-
signed quickly. The software is provided free of charge and
can be downloaded from National Semiconductor's Internet
site located at http://www.national.com.
SWITCH OUTPUT
This is the output of a power MOSFET switch connected di-
rectly to the input voltage. The switch provides energy to an
inductor, an output capacitor and the load circuitry under con-
trol 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 volt-
age 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 LM2676. 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 ca-
pacitor 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 con-
duction 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 LM2676, it is recom-
mended that a broad ground plane be used to minimize signal
coupling throughout the circuit
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 volt-
age. 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 LM2676.
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.
ON/OFF
This input provides an electrical ON/OFF control of the power
supply. Connecting this pin to ground or to any voltage less
than 0.8V will completely turn OFF the regulator. The current
drain from the input supply when OFF is only 50μA. Pin 7 has
an internal pull-up current source of approximately 20μA and
a protection clamp zener diode of 7V to ground. When elec-
trically driving the ON/OFF pin the high voltage level for the
ON condition should not exceed the 6V absolute maximum
limit. When ON/OFF control is not required pin 7 should be
left open circuited.
DAP (LLP 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 http://
power.national.com.
DESIGN CONSIDERATIONS
10091407
FIGURE 1. Basic circuit for fixed output voltage applications.
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LM2676
10091408
FIGURE 2. Basic circuit for adjustable output voltage applications
Power supply design using the LM2676 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 volt-
age and load current) of the LM2676. A simple design proce-
dure 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.
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 compo-
nents 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 as-
sume 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 select-
ed.
The inductors offered have been specifically manufactured to
provide proper operation under all operating conditions of in-
put 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 character-
istics.
Renco: ferrite stick core inductors; benefits are typically low-
est 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, be-
ing 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 capaci-
tor, 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 ca-
pacitor 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 ca-
pacitance and the ESR value create a zero. These frequency
response effects together with the internal frequency com-
pensation circuitry of the LM2676 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
LM2676, the output capacitor is selected to provide a unity
gain bandwidth of 40KHz maximum. Each recommended ca-
pacitor value has been chosen to achieve this result.
In some cases multiple capacitors are required either to re-
duce 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
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LM2676
to reduce the closed loop unity gain bandwidth (to less than
40KHz). When parallel combinations of capacitors are re-
quired 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 out-
put 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 tem-
perature 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 vari-
ations.
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 capac-
itance. 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 nor-
mal operation and should be considered when selecting an
input capacitor.
The input capacitor should be placed very close to the input
pin of the LM2676. Due to relative high current operation with
fast transient changes, the series inductance of input con-
necting 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 de-
signs to add a small valued (0.1μF to 0.47μF) ceramic type
capacitor in parallel with the input capacitor to prevent or min-
imize any ringing.
CATCH DIODE
When the power switch in the LM2676 turns OFF, the current
through the inductor continues to flow. The path for this cur-
rent is through the diode connected between the switch output
and ground. This forward biased diode clamps the switch out-
put 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 effi-
ciency 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.
ADDITIONAL APPLICATON 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 out-
put 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 pos-
sible 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 re-
quiring an increased charging current.
A simple test to determine if this condition might exist for a
suspect application is to apply a short circuit across the output
of the converter, and then remove the shorted output condi-
tion. In an application with properly selected external compo-
nents, the output will recover smoothly.
Practical values of external components that have been ex-
perimentally 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.
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 sim-
ple steps.
Step 1: Define the power supply operating conditions:
Required output voltage
Maximum DC input voltage
Maximum output load current
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LM2676
Step 2: Set the output voltage by selecting a fixed output
LM2676 (3.3V, 5V or 12V applications) or determine the re-
quired feedback resistors for use with the adjustable LM2676
−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 (ad-
justable 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 capac-
itor 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 Volt-
age rating must be greater than Vin max.
Step 7: Include a 0.01μF/50V capacitor for Cboost in the de-
sign.
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 2.5A.
Through-hole components are preferred.
Step 1: Operating conditions are:
Vout = 3.3V
Vin max = 16V
Iload max = 2.5A
Step 2: Select an LM2676T-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 (Vin max) and the 2.5A
vertical line (Iload max) indicates that L33, a 22μH inductor, is
required.
From Table 1, L33 in a through-hole component is available
from Renco with part number RL-1283-22-43 or part number
PE-53933 from Pulse Engineering.
Step 4: Use Table 3 to determine an output capacitor. With a
3.3V output and a 22μH inductor there are four through-hole
output capacitor solutions with the number of same type ca-
pacitors 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:
1 x 220μF/10V Sanyo OS-CON (code C5)
1 x 1000μF/35V Sanyo MV-GX (code C10)
1 x 2200μF/10V Nichicon PL (code C5)
1 x 1000μF/35V Panasonic HFQ (code C7)
Step 5: Use Table 4 to select an input capacitor. With 3.3V
output and 22μH there are three through-hole solutions.
These capacitors provide a sufficient voltage rating and an
rms current rating greater than 1.25A (1/2 Iload max). Again
using Table 2 for specific component characteristics the fol-
lowing choices are suitable:
1 x 1000μF/63V Sanyo MV-GX (code C14)
1 x 820μF/63V Nichicon PL (code C24)
1 x 560μF/50V Panasonic HFQ (code C13)
Step 6: From Table 5 a 3A Schottky diode must be selected.
For through-hole components 20V rated diodes are sufficient
and 2 part types are suitable:
1N5820
SR302
Step 7: A 0.01μF capacitor will be used for Cboost.
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 re-
quired is 2A maximum. It is also desired to implement the
power supply with all surface mount components.
Step 1: Operating conditions are:
Vout = 14.8V
Vin max = 28V
Iload max = 2A
Step 2: Select an LM2676S-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 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, Fig-
ure 6, requires a calculation of the inductor Volt•microsecond
constant (E•T expressed in V•μS) from the following formula:
where VSAT is the voltage drop across the internal power
switch which is Rds(ON) times Iload. In this example this would
be typically 0.15Ω x 2A or 0.3V 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 2A vertical line (Iload max) indicates that L38 , a 68μH in-
ductor, should be used.
13 www.national.com
LM2676
From Table 1, L38 in a surface mount component is available
from Pulse Engineering with part number PE-54038S.
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 68μH
inductor there are three surface mount output capacitor solu-
tions. 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 ei-
ther 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 1A (1/2 Iout
max). From Table 2 it can be seen that C12, a 33μF/35V ca-
pacitor from Sprague, has the required voltage/current rating
of the surface mount components.
Step 6: From Table 5 a 3A Schottky diode must be selected.
For surface mount diodes with a margin of safety on the volt-
age rating one of five diodes can be used:
SK34
30BQ040
30WQ04F
MBRS340
MBRD340
Step 7: A 0.01μF capacitor will be used for Cboost.
LLP PACKAGE DEVICES
The LM2676 is offered in the 14 lead LLP surface mount
package to allow for a significantly decreased footprint with
equivalent power dissipation compared to the TO-263. For
details on mounting and soldering specifications, refer to Ap-
plication Note AN-1187.
www.national.com 14
LM2676
Inductor Selection Guides
For Continuous Mode Operation
10091421
FIGURE 3. LM2676-3.3
10091422
FIGURE 4. LM2676-5.0
10091423
FIGURE 5. LM2676-12
10091424
FIGURE 6. LM2676-ADJ
15 www.national.com
LM2676
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
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
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
www.national.com 16
LM2676
Capacitor Selection Guides
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
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
17 www.national.com
LM2676
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
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 4 C2 3 C1 4 C4
15 4 C2 3 C1 4 C4
22 3 C2 2 C7 3 C4
33 2 C2 2 C6 2 C4
5
10 4 C2 4 C6 4 C4
15 3 C2 2 C7 3 C4
22 3 C2 2 C7 3 C4
33 2 C2 2 C3 2 C4
47 2 C2 1 C7 2 C4
12
10 4 C5 3 C6 5 C9
15 3 C5 2 C7 4 C8
22 2 C5 2 C6 3 C8
33 2 C5 1 C7 2 C8
47 2 C4 1 C6 2 C8
68 1 C5 1 C5 2 C7
100 1 C4 1 C5 1 C8
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 1 C3 1 C10 1 C6 2 C6
15 1 C3 1 C10 1 C6 2 C5
22 1 C5 1 C10 1 C5 1 C7
33 1 C2 1 C10 1 C13 1 C5
5
10 2 C4 1 C10 1 C6 2 C5
15 1 C5 1 C10 1 C5 1 C6
22 1 C5 1 C5 1 C5 1 C5
33 1 C4 1 C5 1 C13 1 C5
47 1 C4 1 C4 1 C13 2 C3
www.national.com 18
LM2676
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
12
10 2 C7 1 C5 1 C18 2 C5
15 1 C8 1 C5 1 C17 1 C5
22 1 C7 1 C5 1 C13 1 C5
33 1 C7 1 C3 1 C11 1 C4
47 1 C7 1 C3 1 C10 1 C3
68 1 C7 1 C2 1 C10 1 C3
100 1 C7 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.
Table 4. Input Capacitors for Fixed Output Voltage Application
(Assumes worst case maximum input voltage and load current for a given inductance value)
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 2 C5 1 C7 2 C8
15 3 C9 1 C10 3 C10
22 * * 2 C13 3 C12
33 * * 2 C13 2 C12
5
10 2 C5 1 C7 2 C8
15 2 C5 1 C7 2 C8
22 3 C10 2 C12 3 C11
33 * * 2 C13 3 C12
47 * * 1 C13 2 C12
12
10 2 C7 2 C10 2 C7
15 2 C7 2 C10 2 C7
22 3 C10 2 C12 3 C10
33 3 C10 2 C12 3 C10
47 * * 2 C13 3 C12
68 * * 2 C13 2 C12
100 * * 1 C13 2 C12
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 1 C7 2 C4 1 C5 1 C6
15 1 C10 1 C10 1 C18 1 C6
22 * * 1 C14 1 C24 1 C13
33 * * 1 C12 1 C20 1 C12
5
10 1 C7 2 C4 1 C14 1 C6
15 1 C7 2 C4 1 C14 1 C6
22 * * 1 C10 1 C18 1 C13
33 * * 1 C14 1 C23 1 C13
47 * * 1 C12 1 C20 1 C12
19 www.national.com
LM2676
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
12
10 1 C9 1 C10 1 C18 1 C6
15 1 C10 1 C10 1 C18 1 C6
22 1 C10 1 C10 1 C18 1 C6
33 * * 1 C10 1 C18 1 C6
47 * * 1 C13 1 C23 1 C13
68 * * 1 C12 1 C21 1 C12
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.
Table 5. Schottky Diode Selection Table
Reverse
Voltage
(V)
Surface Mount Through Hole
3A 5A or More 3A 5A or More
20V SK32 1N5820
SR302
30V SK33 MBRD835L 1N5821
30WQ03F 31DQ03
40V SK34 MBRB1545CT 1N5822
30BQ040 6TQ045S MBR340 MBR745
30WQ04F 31DQ04 80SQ045
MBRS340 SR403 6TQ045
MBRD340
50V or
More
SK35 MBR350
30WQ05F 31DQ05
SR305
Diode Manufacturer Contact Numbers
International Rectifier Phone (310) 322-3331
FAX (310) 322-3332
Motorola Phone (800) 521-6274
FAX (602) 244-6609
General
Semiconductor
Phone (516) 847-3000
FAX (516) 847-3236
Diodes, Inc. Phone (805) 446-4800
FAX (805) 446-4850
Table 6. Output Capacitors for Adjustable Output Voltage Applications
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
1.21 to 2.50 33* 7 C1 6 C2 7 C3
47* 5 C1 4 C2 5 C3
2.5 to 3.75 33* 4 C1 3 C2 4 C3
47* 3 C1 2 C2 3 C3
www.national.com 20
LM2676
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
3.75 to 5
22 4 C1 3 C2 4 C3
33 3 C1 2 C2 3 C3
47 2 C1 2 C2 2 C3
5 to 6.25
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
6.25 to 7.5
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
7.5 to 10
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
10 to 12.5
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
12.5 to 15
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
15 to 20
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
20 to 30
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
30 to 37
10 4 C13 8 C12
15 3 C13 5 C12
22 No Values Available 2 C13 4 C12
33
1 C13 3 C12
47 1 C13 2 C12
68 1 C13 2 C12
Output Capacitors for Adjustable Output Voltage Applications (continued)
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
1.21 to 2.50 33* 2 C3 5 C1 5 C3 3 C
47* 2 C2 4 C1 3 C3 2 C5
2.5 to 3.75 33* 1 C3 3 C1 3 C1 2 C5
47* 1 C2 2 C1 2 C3 1 C5
21 www.national.com
LM2676
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
3.75 to 5
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
5 to 6.25
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
6.25 to 7.5
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
7.5 to 10
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
10 to 12.5
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
12.5 to 15
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
100 1 C9 1 C10 1 C15 1 C2
15 to 20
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
20 to 30
33 1 C7 1 C16 1 C2
47 No Values 1 C7 1 C16 1 C2
68 Available 1 C7 1 C16 1 C2
100 1 C7 1 C16 1 C2
30 to 37
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.
www.national.com 22
LM2676
Physical Dimensions inches (millimeters) unless otherwise noted
TO-263 Surface Mount Power Package
Order Number LM2676S-3.3, LM2676S-5.0,
LM2676S-12 or LM2676S-ADJ
NS Package Number TS7B
23 www.national.com
LM2676
TO-220 Power Package
Order Number LM2676T-3.3, LM2676T-5.0,
LM2676T-12 or LM2676T-ADJ
NS Package Number TA07B
14-Lead LLP Package
NS Package Number SRC14A
www.national.com 24
LM2676
Notes
25 www.national.com
LM2676
Notes
LM2676 SIMPLE SWITCHER High Efficiency 3A Step-Down Voltage Regulator
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