LM2676SDX-5.0/NOPB - National Semiconductor

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

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

μA

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

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

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

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

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

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

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

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

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

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

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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