ETC LM2676

LM2676
SIMPLE SWITCHER ® High Efficiency 3A Step-Down
Voltage Regulator
General Description
Features
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
inductors 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,
current 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% tolerance. The clock frequency is controlled to within a ± 11%
tolerance.
n Efficiency up to 94%
n Simple and easy to design with (using off-the-shelf
external components)
n 150 mΩ DMOS output switch
n 3.3V, 5V and 12V fixed output and adjustable (1.2V to
37V ) versions
n 50µA standby current when switched OFF
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 −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
10091403
SIMPLE SWITCHER ® is a registered trademark of National Semiconductor Corporation.
© 2002 National Semiconductor Corporation
DS100914
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LM2676 SIMPLE SWITCHER High Efficiency 3A Step-Down Voltage Regulator
August 2002
LM2676
Connection Diagram and Ordering Information
TO-263 Package
Top View
TO-220 Package
Top View
10091401
10091402
Order Number
LM2676S-3.3, LM2676S-5.0,
LM2676S-12 or LM2676S-ADJ
See NSC Package Number TS7B
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 LDC14A
Ordering Information for LLP Package
Output Voltage
Order Information
Package Marking
Supplied As
12
LM2676LD-12
S0000EB
250 Units on Tape and Reel
12
LM2676LDX-12
S0000EB
2500 Units on Tape and Reel
3.3
LM2676LD-3.3
S0000CB
250 Units on Tape and Reel
3.3
LM2676LDX-3.3
S0000CB
2500 Units on Tape and Reel
5.0
LM2676LD-5.0
S0000DB
250 Units on Tape and Reel
5.0
LM2676LDX-5.0
S0000DB
2500 Units on Tape and Reel
ADJ
LM2676LD-ADJ
S0000FB
250 Units on Tape and Reel
ADJ
LM2676LDX-ADJ
S0000FB
2500 Units on Tape and Reel
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2
Storage Temperature Range
(Note 1)
Soldering Temperature
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
10 sec, 240˚C
Vapor Phase
75 sec, 219˚C
Supply Voltage
−0.3V to 14V
8V to 40V
Junction Temperature Range (TJ)
Internally Limited
ESD (Note 2)
4 sec, 260˚C
Infrared
Operating Ratings
VSW + 8V
Feedback Pin Voltage
Power Dissipation
Wave
−1V to VIN
Boost Pin Voltage
−65˚C to 150˚C
−40˚C to 125˚C
2 kV
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
(Note 3)
(Note 4)
(Note 4)
3.234/3.201
3.366/3.399
VOUT
Output Voltage
VIN = 8V to 40V, 100mA ≤ IOUT ≤ 3A
3.3
η
Efficiency
VIN = 12V, ILOAD = 3A
86
Units
V
%
LM2676-5.0
Symbol
Parameter
Conditions
Typical
Min
Max
(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
η
Efficiency
VIN = 12V, ILOAD = 3A
88
Units
V
%
LM2676-12
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 ≤ 3A
12
η
Efficiency
VIN = 24V, ILOAD = 3A
94
Units
V
%
LM2676-ADJ
Symbol
Parameter
Conditions
VFB
Feedback
Voltage
VIN = 8V to 40V, 100mA ≤ IOUT ≤ 3A
VOUT Programmed for 5V
η
Efficiency
VIN = 12V, ILOAD = 3A
Typ
Min
Max
(Note 3)
(Note 4)
(Note 4)
1.21
1.186/1.174
1.234/1.246
88
3
Units
V
%
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LM2676
Absolute Maximum Ratings
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 Adjustable
versions and VIN =24V for the 12V version.
Symbol
Parameter
Conditions
Typ
Min
Max
Units
4.2
6
mA
50
100/150
µA
5.25/5.4
A
DEVICE PARAMETERS
IQ
Quiescent
Current
VFEEDBACK = 8V
For 3.3V, 5.0V, and ADJ Versions
VFEEDBACK = 15V
For 12V Versions
ISTBY
Standby
Quiescent
Current
ON/OFF Pin = 0V
ICL
Current Limit
IL
Output Leakage
Current
VIN = 40V, ON/OFF Pin = 0V
VSWITCH = 0V
VSWITCH = −1V
16
200
15
RDS(ON)
Switch
On-Resistance
ISWITCH = 3A
0.15
0.17/0.29
Ω
fO
Oscillator
Frequency
Measured at Switch Pin
260
280
kHz
D
Duty Cycle
Maximum Duty Cycle
91
%
Minimum Duty Cycle
0
%
VFEEDBACK = 1.3V
ADJ Version Only
85
nA
IBIAS
Feedback Bias
Current
VON/OFF
ON/OFF
Threshold
Voltage
4.5
1.4
ION/OFF
ON/OFF Input
Current
ON/OFF Input = 0V
θJA
Thermal
Resistance
T Package, Junction to Ambient
θJA
20
3.8/3.6
225
0.8
µA
mA
2.0
V
45
µA
65
(Note 5)
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
LD Package, Junction to Ambient
55
(Note 10)
θJA
LD Package, Junction to Ambient
29
(Note 11)
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++
˚C/W
LM2676
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 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 1⁄2 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 1⁄2 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.
5
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LM2676
Typical Performance Characteristics
Normalized
Output Voltage
Line Regulation
10091409
10091410
Efficiency vs Input Voltage
Efficiency vs ILOAD
10091411
10091412
Switch Current Limit
Operating Quiescent Current
10091404
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10091405
6
LM2676
Typical Performance Characteristics
(Continued)
Standby Quiescent Current
ON/OFF Threshold Voltage
10091440
10091413
ON/OFF Pin Current (Sourcing)
Switching Frequency
10091414
10091415
Feedback Pin Bias Current
10091416
7
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LM2676
Typical Performance Characteristics
Discontinuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 500 mA
L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ
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.
10091418
B: Inductor Current, 1 A/div
A: VSW Pin Voltage, 10 V/div.
C: Output Ripple Voltage, 20 mV/div AC-Coupled
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 Discontinuous Mode
VIN = 20V, VOUT = 5V,
L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ
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.
10091420
A: Output Voltage, 100 mV/div, AC-Coupled.
B: Load Current: 500 mA to 3A Load Pulse
B: Load Current: 200 mA to 3A Load Pulse
Horizontal Time Base: 100 µs/div
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
C Boost
Application Hints
A capacitor must be connected from pin 3 to the switch
output, pin 1. This capacitor boosts the gate drive to the
internal MOSFET above Vin to fully turn it ON. This minimizes conduction losses in the power switch to maintain high
efficiency. The recommended value for C Boost is 0.01µF.
Ground
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
efficient operation.
The LM2676 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.
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
recommended 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
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 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
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 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
capacitor located close to pin 2.
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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 electrically 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.
10
LM2676
Application Hints
(Continued)
DESIGN CONSIDERATIONS
10091407
FIGURE 1. Basic circuit for fixed output voltage applications.
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
voltage and load current) of the LM2676. A simple design
procedure using nomographs and component tables provided in this data sheet leads to a working design with very
little effort. Alternatively, the design software, LM267X Made
Simple (version 6.0), can also be used to provide instant
component selection, circuit performance calculations for
evaluation, a bill of materials component list and a circuit
schematic.
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 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
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LM2676
Application Hints
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.
(Continued)
maximum ripple current of 30% of the maximum load current. If the ripple current exceeds this 30% limit the next
larger value is selected.
The inductors offered have been specifically manufactured
to provide proper operation under all operating conditions of
input and output voltage and load current. Several part types
are offered for a given amount of inductance. Both surface
mount and through-hole devices are available. The inductors
from each of the three manufacturers have unique characteristics.
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.
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.
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 LM2676. 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 LM2676 turns OFF, the current
through the inductor continues to flow. The path for this
current is through the diode connected between the switch
output and ground. This forward biased diode clamps the
switch output to a voltage less than ground. This negative
voltage must be greater than −1V so a low voltage drop
(particularly at high current levels) Schottky diode is recommended. Total efficiency of the entire power supply is significantly impacted by the power lost in the output catch diode.
The average current through the catch diode is dependent
on the switch duty cycle (D) and is equal to the load current
times (1-D). Use of a diode rated for much higher current
than is required by the actual application helps to minimize
the voltage drop and power loss in the diode.
During the switch ON time the diode will be reversed biased
by the input voltage. The reverse voltage rating of the diode
should be at least 1.3 times greater than the maximum input
voltage.
BOOST CAPACITOR
The boost capacitor creates a voltage used to overdrive the
gate of the internal power MOSFET. This improves efficiency
by minimizing the on resistance of the switch and associated
power loss. For all applications it is recommended to use a
0.01µF/50V ceramic capacitor.
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.
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 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
capacitor value has been chosen to achieve this result.
In some cases multiple capacitors are required either to
reduce the ESR of the output capacitor, to minimize output
ripple (a ripple voltage of 1% of Vout or less is the assumed
performance condition), or to increase the output capacitance to reduce the closed loop unity gain bandwidth (to less
than 40KHz). When parallel combinations of capacitors are
required it has been assumed that each capacitor is the
exact same part type.
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
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12
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
following choices are suitable:
(Continued)
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
LM2676 (3.3V, 5V or 12V applications) or determine the
required 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.
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 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.
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 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
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:
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 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 alternator supply typically used to power electronic equipment
from single battery 12V vehicle systems. The load current
required 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,
Figure 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:
13
www.national.com
LM2676
Application Hints
LM2676
Application Hints
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.
(Continued)
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
capacitor from Sprague, has the required voltage/current
rating of the surface mount components.
Using Figure 6, the intersection of 27V • µS horizontally and
the 2A vertical line (Iload max) indicates that L38 , a 68µH
inductor, should be used.
Step 6: From Table 5 a 3A Schottky diode must be selected.
For surface mount diodes with a margin of safety on the
voltage rating one of five diodes can be used:
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 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)
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
Application Note AN-1187.
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-
www.national.com
14
LM2676
Inductor Selection Guides
For Continuous Mode Operation
10091421
10091423
FIGURE 3. LM2676-3.3
FIGURE 5. LM2676-12
10091422
FIGURE 4. LM2676-5.0
10091424
FIGURE 6. LM2676-ADJ
15
www.national.com
LM2676
Inductor Selection Guides For Continuous Mode Operation
(Continued)
Table 1. Inductor Manufacturer Part Numbers
Inductor
Inductance
Reference
(µH)
Number
Renco
Pulse Engineering
Current
(A)
Through Hole
Surface
Mount
Through
Hole
Surface
Mount
Coilcraft
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
Inductor Manufacturer Contact Numbers
Coilcraft
Coilcraft, Europe
Pulse Engineering
www.national.com
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
16
—
—
DO5022P-103HC
LM2676
Capacitor Selection Guides
Table 2. Input and Output Capacitor Codes
Capacitor
Reference
Code
Surface Mount
AVX TPS Series
C (µF) WV (V)
C1
330
6.3
C2
100
C3
220
Irms
(A)
Sprague 594D Series
C (µF) WV (V)
Irms
(A)
Kemet T495 Series
C (µF) WV (V)
1.1
100
6.3
Irms
(A)
1.15
120
6.3
0.82
10
1.1
220
6.3
1.4
220
6.3
1.1
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
Table 2. Input and Output Capacitor Codes (continued)
Through Hole
Capacitor
Sanyo OS-CON SA Series Sanyo MV-GX Series
Nichicon PL Series
Reference
Irms
Irms
Irms
Code
C (µF) WV (V)
(A)
C (µF) WV (V)
(A)
C (µF) WV (V)
(A)
Panasonic HFQ Series
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 Selection Guides
(Continued)
Capacitor Manufacturer Contact Numbers
Nichicon
Phone
Panasonic
AVX
Sprague/Vishay
Sanyo
Kemet
(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
Table 3. Output Capacitors for Fixed Output Voltage Application
Surface Mount
Output
Inductance
Voltage (V)
(µH)
3.3
5
12
AVX TPS Series
Sprague 594D
Series
Kemet T495 Series
No.
C Code
No.
C Code
No.
C Code
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
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
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
Through Hole
Output
Inductance
Voltage (V)
(µH)
3.3
5
www.national.com
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
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
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
18
LM2676
Capacitor Selection Guides
(Continued)
Through Hole
Output
Inductance
Voltage (V)
(µH)
12
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
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)
Surface Mount
Output
Inductance
Voltage (V)
(µH)
3.3
5
12
AVX TPS Series
Sprague 594D
Series
No.
C Code
No.
10
2
C5
15
3
C9
22
*
*
33
*
*
10
2
C5
15
2
C5
22
3
33
47
Kemet T495 Series
C Code
No.
C Code
1
C7
2
C8
1
C10
3
C10
2
C13
3
C12
2
C13
2
C12
1
C7
2
C8
1
C7
2
C8
C10
2
C12
3
C11
*
*
2
C13
3
C12
*
*
1
C13
2
C12
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
Through Hole
Output
Inductance
Voltage (V)
(µH)
3.3
5
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
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
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
Capacitor Selection Guides
(Continued)
Through Hole
Output
Inductance
Voltage (V)
(µH)
12
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
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
3A
20V
SK32
30V
SK33
Through Hole
5A or More
3A
5A or More
1N5820
SR302
MBRD835L
30WQ03F
40V
1N5821
31DQ03
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
Motorola
General
Semiconductor
Diodes, Inc.
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
Table 6. Output Capacitors for Adjustable Output Voltage Applications
Surface Mount
Output Voltage
(V)
1.21 to 2.50
www.national.com
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
20
LM2676
Capacitor Selection Guides
(Continued)
Table 6. Output Capacitors for Adjustable Output Voltage Applications (Continued)
Surface Mount
Output Voltage
(V)
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
Inductance
(µH)
AVX TPS Series
Kemet T495 Series
No.
C Code
No.
C Code
No.
C Code
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
4
C13
8
C12
10
15
30 to 37
Sprague 594D
Series
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
www.national.com
LM2676
Capacitor Selection Guides
(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
C Code
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
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
47
No Values
68
Available
1
C7
1
C16
1
C2
1
C7
1
C16
1
C2
1
C7
1
C16
1
C2
100
1
C7
1
C16
1
C2
10
1
C12
1
C20
1
C10
15
1
C11
1
C20
1
C11
1
C11
1
C20
1
C10
22
No Values
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
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
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LM2676
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
TO-220 Power Package
Order Number LM2676T-3.3, LM2676T-5.0,
LM2676T-12 or LM2676T-ADJ
NS Package Number TA07B
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24
inches (millimeters) unless otherwise noted (Continued)
14-Lead LLP Package
NS Package Number LDC14A
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LM2676 SIMPLE SWITCHER High Efficiency 3A Step-Down Voltage Regulator
Physical Dimensions